Current Medicinal Chemistry, 2011, 18, 3695-3706 3695

0929-8673/11 $58.00+.00 © 2011 Bentham Science Publishers Ltd.

Xanthine Derivatives in the Heart: Blessed or Cursed?

A. J. Szentmiklósi*,#,1, Á. Cseppent�#,1, R. Gesztelyi2, J. Zsuga3, Á. Körtvély4 , G. Harmati4 and P.P. Nánási4

1Department of Pharmacology and Pharmacotherapy, University of Debrecen, Medical and Health Science Center, Debrecen, H-4012, Hungary 2Department of Pharmacodynamics, University of Debrecen, Medical and Health Science Center, Debrecen, H-4012, Hungary 3Department of Neurology, University of Debrecen, Medical and Health Science Center, Debrecen, H-4032, Hungary 4Department of Physiology, University of Debrecen, Medical and Health Science Center, Debrecen, H-4012, Hungary

Abstract: Methylxanthines, such as theophylline, have been used to treat cardiorespiratory disorders, whereas caffeine is the most widely consumed psychoactive agent in various soft drinks. Because of the worldwide use of these drugs and the recently synthesized xanthine derivatives, an intensive research on the cardiac actions of these substances is under progress. This review focuses on the molecular mechanisms involved in the actions of xanthine derivatives with special reference to their adenosine receptor antagonistic properties. The main basic and human studies on the action of xanthines on impulse initiation and conduction, as well as the electrophysiological and mechanical activity of the working myocardium will be overviewed. The potential beneficial and harmful actions of the methylxanthines will be discussed in light of the recent experimental and clinical findings. The pharmacological features and clinical observations with adenosine receptor subtype-specific xanthine antagonists are also the subject of this paper. Based on the adenosine receptor-antagonistic activity of these compounds, it can be raised that xanthine derivatives might inhibit the cardioprotective action of endogenous adenosine on various subtypes (A1, A2A, A2B and A3) of adenosine receptors. Adenosine is an important endogenous substance with crucial role in the regulation of cardiac function under physiological and pathological conditions (preconditioning, postconditioning, ische-mia/reperfusion injury). Recent clinical studies show that acute administration of caffeine or theophylline can inhibit various types of preconditioning in human subjects. There are no human studies, however, for the cardiovascular actions of long-term administration of these drugs. Upregulation of adenosine receptors and increased effectiveness of adenosine receptor-related cardiovascular functions have been observed after long-lasting treatment with methylxanthines. In addition, there are data indicating that blood adenosine level in-creases after long-term caffeine administration. Since the salutary actions (and also the adverse reactions) of a number of xanthine deriva-tives are repeatedly shown, the main goal is the development of novel structures that mimic the actions of the conventional methylxanthi-nes as lead compounds, but their adenosine receptor subtype-specificity is higher, their water solubility is optimal, and the unwanted re-actions are minimized.

Keywords: Methylxanthines, xanthine derivatives, adenosine, adenosine receptors, subtype-selectivity, risk/benefit ratio.

1. INTRODUCTION

The commonly used methylxanthines theophylline, caffeine and theobromine are naturally occurring compounds in various plants. These substances are components of most widely consumed bever-ages, coffee, tea and cocoa. Theophylline and its salts (aminophyl-line, choline theophyllinate) or derivatives (enprophylline, doxofyl-line, bamiphylline) - depicted in Fig. (1) - are used in the therapy of bronchial asthma and chronic obstructive pulmonary disease (COPD), whereas pentoxyphylline in the treatment of peripheral vascular diseases and in the management of cerebrovascular insuf-ficiency, as well as in diabetic neuropathy [1]. Occasionally theo-phylline is applied to prevent sleep apnea in adults. Caffeine is an analeptic and central nervous system stimulant drug being generally used for enhancement of arousal and vigilance, suppression of fa-tigue, as well as to shorten reaction time of the motor system. In addition, caffeine has long been used as an adjuvant in combina-tions to potentiate the pharmacological actions of various analgesics such as acetaminophen and acetylsalicylic acid [2, 3].

Methylxanthines, like theophylline or caffeine, are also widely used in cardiology, since these drugs are appropriate medicaments for treatment of various bradyarrhythmias and disturbances of atrioventricular conduction. In some selected cases, theophylline may be used as classical “ino-dilator” and diuretic [4].

The mechanism of cellular action of methylxanthines is quite complex and their action depends on the concentration applied. According to the most conventional views, methylxanthines inhibit

*Address correspondence to this author at the Department of Pharmacology and Phar-macotherapy, University of Debrecen, Medical and Health Science Center, H-4012 Debrecen, Hungary; Tel: +36 52 411717 ext. 55018; Fax: +36 52 427899; E-mail:ajszm@king.pharmacol.dote.hu #These authors contributed equally to this work.

phosphodiesterase enzyme and induce mobilization of calcium from the sarcoplasmic reticulum. It is now becoming clearer that the mainstream of the site of actions of methylxanthines are the adeno-sine receptors, at least, in the therapeutically relevant concentra-tions of 20-50 �M [4, 5]. Accordingly, in this paper we discuss the cardiac actions of methylxanthines when applied in both therapeutic and toxic concentrations. We emphasize the cardiac electrophysi-ological effects of xanthines with special reference to antagonistic actions on adenosine receptors.

2. MECHANISM OF ACTION OF METHYLXANTHINES AND OTHER XANTHINE DERIVATIVES: BASIC STUDIES

2.1. Basic Studies on Acute Actions of Methylxanthines and other Xanthine Derivatives

Caffeine in millimolar concentrations has been shown to induce various electrophysiological effects on pacemaker fibers and work-ing myocardium. These effects display wide interspecies depend-ence. Thus, in rabbit sinoatrial nodal cells, caffeine (1 to10 mM) induced a concentration-dependent, atropine-resistant decrease in sinoatrial pacemaker activity. Caffeine increased the slow inward current, reduced the delayed rectifying outward current, and ele-vated the cytoplasmic Ca2+ concentration. Some of the preparations displayed arrhythmias resulting from the caffeine-induced calcium overload. The observed sinus bradycardia can also be related to this latter mechanism [6]. In canine cardiac Purkinje fibers, caffeine (0.5 to 3 mM) induced an oscillatory potential, which could be modu-lated by altering intracellular calcium concentration. This oscilla-tory potential explained the arrhythmias observed in Purkinje fibers [7]. In human atrial fibers theophylline (0.1 to 1 mM) was shown to increase the rate of phase 4 depolarization and the amplitude of the oscillatory potential during diastole and induced the development of spontaneous slow action potentials. These findings represent the

3696 Current Medicinal Chemistry, 2011 Vol. 18, No. 24 Szentmiklósi et al.

major electrophysiological basis helping to understand the theo-phylline-induced atrial ectopic activity [8]. In working myocardium caffeine stimulated the L-type Ca2+-current (ICa) [9, 10]. This effect can be ascribed to the ability of caffeine to activate the ryanodine channels in the sarcoplasmic reticulum. Caffeine is known to in-crease the calcium-induced calcium release, amplifying thus the slow inward current [11]. In contrast, there are studies indicating an inhibitory action of caffeine on ICa [12, 13]. In rat ventricular myo-cytes, Varro et al. [14] found that high concentration (20 mM) of caffeine reduced the amplitude of the inward rectifier potassium and inward calcium currents. Zhang et al. [15] described a caffeine-activated large-conductance plasma membrane cation channel in cardiac myocytes. It was suggested that this can be an additional mechanism by which caffeine and theophylline may contribute to generation of cardiac arrhythmias.

It is important to take into account that these changes occur in a concentration range of 1 to 20 mM of methylxanthines. This range is well above those xanthine concentrations (20-50 �M) which can be reached during therapeutic interventions or by consuming caf-feine-containing soft drinks. The cardiac actions of methylxanthines could not be explained by their phosphodiesterase inhibiting action or by mobilization of intracellular calcium, since these effects of methylxanthines are very slight at concentrations below 100 �M. On the other hand, methylxanthines have been shown to antagonize adenosine receptors effectively within their therapeutic concentra-tion range [16]. Therefore, it is conceivable that cardiac activity of methylxanthines can be related to their antagonistic actions directed to endogenous adenosinergic signalization [17].

Adenosine is an endogenous, crucial signaling purine nucleo-side that plays prominent role in regulation of cardiovascular func-tion, having also an important role in pathogenesis of various car-diovascular diseases. Adenosine is much implicated in the control of local blood flow of various organs [18], regulation of cardiac pacemaker activity, AV conduction, myocardial contractility, and cardioprotection [19-23]. Adenosine acts through G protein-coupled receptors on the target cells. Until recently, four subtypes of adeno-sine, namely A1, A2A, A2B and A3, have been identified structurally, functionally and pharmacologically. These adenosine receptors were characterized on the basis of their specific pharmacological profile (rank order of potency of agonists and antagonists), G pro-tein coupling, and signaling mechanism involved in the subtype-

specific adenosine receptor activation. All subtypes of adenosine receptors are available in the heart [24]. A1 adenosine receptors are negatively coupled to adenylyl cyclase, and are responsible for the reduction of sinoatrial rate, AV-conduction, atrial contractile force, attenuation of the positive inotropic effect of catecholamines, as well as for the protection against myocardial ischemia/reperfusion injury. A2A receptors are positively coupled to adenylyl cyclase. They are expressed in rat ventricular myocardium and are known to counteract some A1 adenosine receptor-mediated anti-adrenergic actions [25]. A2B adenosine receptors couple positively to adenylyl cyclase, and mitogen-activated protein kinase. These receptors were suggested to modulate cellular hypertrophy [26]. A3 receptors, like A1 adenosine receptors, are negatively linked to adenylyl cyclase. Their activation could inhibit the �-adrenergic receptor activation-induced enhancement of cAMP, as well as provides tolerance against hypoxic injury [27]. To understand all cardiac actions of methylxanthines, it is necessary to shortly survey the main effects of adenosine on the cardiac pacemakers, conduction systems and the working myocardium.

Regarding the action of adenosine on impulse initiation and conduction, the first described action of adenosine was slowing of the heart rate [28]. In spontaneously beating isolated right atrial myocardium, His bundles, and Purkinje fibers of guinea pigs, adenosine showed a characteristic suppressive effect on the sponta-neous firing rate in these specific pacemaker cells [29-35]. A1adenosine receptor activation enhanced the inward rectifier potas-sium current (IK-Ado), and consequently hyperpolarized the cell membrane resulting in reduction of the slope of phase 4 depolariza-tion [36-38]. Adenosine has also been reported to inhibit the pace-maker current (If) in sinoatrial and AV nodal cells [39-41]. In hu-man, Honey and his colleagues were the first to describe in 1930 [42] that adenosine induced sinus bradycardia. Endogenously re-leased adenosine might have a role in hypoxia- or ischemia-induced bradycardia, since the adenosine receptor antagonist aminopylline antagonized the hypoxia-induced suppression of sinoatrial pace-maker activity in spontaneously beating guinea pig right atrial preparations. As for humans, Watt has proposed that adenosine might be involved in bradyarrhythmias related to sick sinus syn-drome [43, 44].

Drury and Szent-Györgyi were the first to identify adenosine-induced heart block [28]. Later, Stafford observed that adenosine

N

HNN

N

O

O

CH3

H3C

Theophylline

N

NN

N

O

O

CH3

H3C

Caffeine

CH3

N

HNHN

N

O

O

Enprophylline

CH3

N

NN

N

O

O

CH3

Doxofylline

H3C O

O

N

NN

N

O

O

CH3

Pentoxyphylline

CH3

H3C

O

N

NN

N

O

O

CH3

Bamiphylline

H3C

NOH

CH3

Fig. (1). Chemical structures of most frequently used methylxanthine drugs.

Xanthine Derivatives in the Heart Current Medicinal Chemistry, 2011 Vol. 18, No. 24 3697

caused a concentration-dependent atrioventricular conduction block in guinea pig hearts [45]. Adenosine is also capable to induce an impaired or blocked of atrioventricular nodal conduction in humans [42, 46]. This is generally a short-lived, not too serious, adverse reaction, but prolonged complete heart block requiring intubation and temporary pacing has also been reported [47]. A1 adenosine receptors are abundantly expressed in the AV junction. Adenosine acts on the proximal part of atrioventricular junction (atrio-nodal region and nodal region), whereas no effect was observed on the nodal-His bundle [48]. In the negative dromotropic effect of adeno-sine, activation of the inward rectifier potassium current and the concomitant hyperpolarization of the cell membrane may have a crucial role. In addition, inhibition of L-type calcium channels has also been a suggested as underlying mechanism [39].

Adenosine is thought to be exerted a strong depressant effect on atrial myocardial contractility. The negative inotropic effect of adenosine has been described in electrically driven rat [49], guinea pig, and human atrial preparations [50-53]. The negative inotropic effect in atrial myocardium is related to the marked shortening of action potentials [22, 49, 51, 54, 55]. Based on microelectrode re-cordings and 42K flux measurements, Jochem and Nawrath were the first to report that adenosine activated a potassium conductance [55]. Belardinelli and Isenberg, using voltage-clamp technique in single atrial myocytes, showed that the adenosine-activated conduc-tance was an inwardly rectifying potassium current [36]. It was also revealed that the adenosine- and acetylcholine-activated conduc-tances are identical, and these channels are associated with the re-ceptors via a pertussis toxin-sensitive G-protein. Schrader et al.reported that adenosine reduced the rate of rise and amplitude of the slow action potential in guinea pig atrial preparations [56]. Now it is clear that the mechanism of the depressant effects of adenosine on the atrial myocardium is related to activation of the inwardly-rectifying potassium current (IK-Ado) and a blockade of the slow inward current mediated by L-type Ca2+ channels [57].

Adenosine exerts quite different actions on contractility in atrial and ventricular myocardium. Under baseline conditions, adenosine had no negative inotropic action in ventricles - it displayed rather a moderate positive inotropic effect [58-64]. Importantly, adenosine had no effect on action potential duration or baseline L-type cal-cium current [65]. In contrast, in ventricular myocardium when adenylate cyclase was previously activated using various com-pounds, including isoprenaline, histamine and dopamine, adenosine displayed a negative inotropic action [58, 63, 64, 66-68]. Under these conditions adenosine or adenosine analogues attenuated all catecholamine-stimulated outward and inward currents. With the exception of rat and ferret, no adenosine-activated potassium cur-rent can be demonstrated in ventricular myocardium [39]. In addi-tion, the anti-� adrenergic action of adenosine is due to an inhibi-tion of adenylate cyclase activity with the concomitant reduction of cAMP content [10, 28, 29, 45]. On the other hand, adenosine has been shown to exert significant anti-adrenergic effect on myocar-dial preparations [64, 66]. Accordingly, there is a concept that adenosine acts as a negative feedback modulator of �-adrenoceptor-mediated responses in the heart [64, 66]. Although ventricular myo-cardium contains primarily A1 adenosine receptors, there are studies showing that A2A and A3 adenosine receptors are also expressed in ventricular myocytes [24].

Accordingly, methylxanthines are capable to antagonize the ac-tions of both exogenous and endogenous adenosine. The competi-tive antagonism between methylxanthines and exogenous adenosine in various cardiac functions has long been established [69-71].

A great mass of experimental evidence is available for estab-lishing the ability of methylxanthines to antagonize the effects of endogenous adenosine accumulated in certain conditions, like hy-poxia, ischemia, or increased metabolic activity. Regarding atriov-entricular conduction, Belardinelli et al. [72] demonstrated that

adenosine, like hypoxia, exerted a concentration-dependent prolon-gation of AV conduction in perfused guinea pig heart. Actions on conduction delay induced by both hypoxia and adenosine could be antagonized by aminophylline, an adenosine receptor antagonist. Belardinelli et al. [72] finally suggested that endogenously released adenosine might play an important role in AV conduction distur-bances also in case of acute hypoxia. Similar conclusions have been drawn in other studies [73, 74].

In our previously reported experiments [75], sinoatrial pace-maker activity of spontaneously beating guinea pig left atria was recorded using extracellular electrodes. During moderate hypoxia, the sinoatrial firing rate gradually decreased. This bradycardia was effectively antagonized by aminophylline, while enhanced in the presence of the adenosine deaminase inhibitor coformycin and the membrane purine transport uptake blocker dipyridamole, indicating the crucial role of the accumulated adenosine in the sinus bradycar-dia occurring under hypoxic conditions.

A similar experiment is presented in Fig. (2), using another xan-thine-type adenosine receptor antagonist, 8-p-(sulphophenyl)theo-phylline. This xanthine prevented the moderate hypoxia-induced decrease in sinoatrial pacemaker activity practically completely. As this compound acts on only at extracellular sites, therefore an addi-tional inhibitory action of enzyme phosphodiesterase can be ex-cluded.

Fig. (2). Action of 50 �M 8-p-(sulphophenyl)theophylline (8-SPT) on the moderate hypoxia-induced decrease in sinoatrial pacemaker activity of spontaneously beating guinea pig right atria. Asterisks indicate significant differences between the 8-SPT treated (n=6) and the untreated control (n=9) groups.

In electrically stimulated guinea pig atria, under normoxic con-ditions, theophylline induced a significant increase of contractile force within a concentration range of 0.1-1 mM, which is higher than usual adenosine receptor-antagonist concentrations [76]. Ac-cording to Collis et al. [77], the positive inotropic actions of theo-phylline and enprophylline were not due to adenosine receptor blockade, but rather to inhibition of phosphodiesterase. According to experiments of Bellemann and Scholz [78], the positive inotropic actions of high concentration (0.6 mM) of theophylline can be asso-ciated with an increase of sarcolemmal calcium influx and an action on intracellular calcium binding or storage sites. In electrically driven left atrial preparations of guinea pigs, adenosine reduced the contractile force and shortened the duration of intracellularly re-

0 10 20 30 40 50 60 70 80 9020

40

60

80

100

120

untreated

8-SPT

* ****

**

minutes during hypoxia

% o

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con

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3698 Current Medicinal Chemistry, 2011 Vol. 18, No. 24 Szentmiklósi et al.

corded action potentials. The similarity between the adenosine- and the hypoxia-induced alterations of myocardial electrical and me-chanical activities is striking, since both the adenosine- and hy-poxia-induced changes could readily be prevented by aminophyl-line. Extreme changes in action potential morphology due to long-lasting hypoxia could also be reversed by aminophylline. On the basis of these results it is assumed that the increased tissue level of endogenous adenosine in acute myocardial hypoxia might contrib-ute to functional impairment of atrial myocardium [21]. In nor-moxic conditions, methylxanthines, in therapeutically applied con-centrations, do not alter or minimally affect the functional activity of myocardium and pacemaker tissues. It can be supposed that stimulatory actions of methylxanthines applied in therapeutic con-centrations occur only under pathological conditions (e.g. hypoxia, ischemia), when endogenous tissue adenosine levels are substan-tially elevated.

2.2. Basic Studies on Chronic Actions of Xanthine Derivatives

In contrast to the extensive studies performed on the regulation of CNS adenosine receptors, there is only limited information available on the effect of chronic methylxanthine treatment on the cardiovascular action of adenosine. In the study by Wu et al. [79]the action of a 2-week theophylline treatment on myocardial A1adenosine receptors was investigated. Cardiac adenosine receptors were upregulated by chronic exposure to theophylline and this upregulation was accompanied by an increased sensitivity of ven-tricular myocytes to the electrophysiological and biochemical ef-fects of adenosine receptor agonists. Similarly, chronic (2 weeks) theophylline treatment resulted in upregulation of atrial and ven-tricular A1 adenosine receptors in rats [80]. It must be noted, how-ever, that only the indirect, antiadrenergic, negative inotropic and chronotropic effects of adenosine receptor agonist on atrial muscle were sensitized, but the direct cardioinhibitory actions were not altered. Chronic caffeine consumption (3 weeks) potentiated the A2adenosine receptor-related hypotensive action of adenosine in rats [81]. According to Varani et al. [82], repeated caffeine administra-tion upregulated human platelet adenosine A2A receptors, which is accompanied with an increased antiaggregatory action of the A2Aadenosine receptor agonist 2-hexenyl-5’-N-ethylcarboxamido-adenosine. Similar results were obtained also in rat platelets [83].Interestingly, in an experiment with repeated (23 days) adminis-tration of a selective A1 adenosine receptor antagonist KW-3902 (8-(noradamantan-3-yl)-1,3-dipropylxanthine) Kusaka and Karasawa [84] failed to observe any alteration in the A1 adenosine-receptor-related bradycardic action of 5’-N-ethylcarboxamidoadenosine (NECA).

In our experiments, the actions of long-term oral caffeine treat-ment were studied on the electrical and mechanical activity of A1adenosine receptor selective N6-cyclopentyladenosine (CPA) on guinea pig atrial myocardium, as well as on the antagonistic action of methylxanthines on the CPA-induced decrease of mechanical activity in atrial myocardium. Slow action potentials were gener-ated in potassium-depolarized (25 mM) guinea pig atrial muscles treated by 1 �M isoproterenol. CPA decreased the rate of rise and the duration of slow action potentials (Fig. 3, Table 1) in a concen-tration-dependent manner. A very strong sensitization in the CPA-induced electrophysiological changes was observed after a 2 week caffeine treatment.

On the basis of these experiments it can be suggested that after long-lasting caffeine treatment a strong sensitization develops in the CPA-induced depression of slow Ca2+ influx and outward K+ out-flow. It is thought that under these conditions Vmax represents the slow inward Ca2+ current, whereas duration of slow action potential is related to the outward potassium current [56]. In these experi-ments, guinea pigs were fed with caffeine for 10 weeks, their aver-age serum caffeine concentration was 42.3 ± 3.6 �M. In electrically driven left atrial preparations, sensitization to CPA-induced de-crease of mechanical activity developed. Its maximum was ob-served at week 2 of caffeine treatment, then the potency ratio mod-erately diminished (Table 1). From week 2, the Emax values were also elevated in atrial muscles of caffeine-treated groups. It is note-worthy, that at week 10 of caffeine treatment a moderate tolerance developed in the adenosine-antagonistic action of various methylx-anthines (e.g. theophylline, caffeine, and DPCPX), i.e. their anti-adenosine pA2 values were reduced. From these studies, an upregu-lation of A1 adenosine receptors due to chronic caffeine treatment, resulting in alterations in electrical and mechanical parameters of atrial myocardium, can be concluded.

3. HUMAN STUDIES

There are a number of early reports, indicating that the resting sinus frequency or the increase during exercise fails to alter signifi-cantly after therapeutic doses of caffeine [85]. Interestingly, in vari-ous pathological states or in healthy volunteers at various sub-maximal and maximal exercise intensities, when tissue concentra-tion of endogenous adenosine elevates, methylxanthines exert no-ticeable effects on the sinoatrial pacemaker. It is suggested by Watt [43] that sick sinus syndrome is an adenosine-mediated disease. In this syndrome, oral theophylline proved to be efficacious in increas-ing sinus rate at rest and during exercise, and also to reduce cardiac pauses [86, 87]. In a randomized, controlled trial (The THEOPACE

Fig. (3). CPA-induced, concentration-dependent depression of maximum velocity of depolarization (Vmax) and shortening of the duration of slow action poten-tials generated in guinea pig atrial trabeculae depolarized by 25 mM potassium and treated with 1 �M isoproterenol. The preparations were stimulated at a frequency of 0.5 Hz, transmembrane potentials were recorded using conventional sharp microelectrodes.

0 50 100 150 200-50

-40

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-10

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40

control

CPA 30 nM

CPA 100 nM

CPA 300 nM

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ial (

mV)

Time (ms)0 50 100 150 200

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CPA 1 nM

CPA 10 nM

CPA 30 nM

SOLVENT-TREATED CAFFEINE-TREATED

Xanthine Derivatives in the Heart Current Medicinal Chemistry, 2011 Vol. 18, No. 24 3699

Study) it was, however, shown that oral theophylline reduced only the minor symptomes of sick sinus syndrome, while was not effec-tive in patients suffering from severe forms of the disorder [88].Human electrophysiological studies are also available to demon-strate that oral theophylline suppressed the symptoms in sick sinus syndrome [89, 90]. Benditt et al. [90] reported that theophylline treatment was useful in young patients with recurrent symptomatic bradyarrhythmias. Furthermore, it was suggested that theophylline might be a reasonable alternative strategy in the treatment of chronic symptomatic bradycardia in the elderly [91, 92].

Sinus node dysfunction, primarily sinus bradycardia, is fre-quently associated with heart transplantation [93]. Bertolet and his coworkers [94] observed that use of theophylline for posttransplan-tation bradyarrhythmias was efficient in increasing the heart rate and reduced the need for implantation of a permanent pacemaker. Reversibility of prolonged chronotropic dysfunction by theophyl-line and aminophylline following orthotopic cardiac transplantation was also reported by a number of investigators [95-97]. Oral or intravenous theophylline proved to be effective in not only sinus node disturbances, but also in abnormal impulse conduction. Thus, Alboni et al. [97] described that oral theophylline represents a valid therapy in most patients with atrial fibrillation associated with a slow ventricular response. Intravenous theophylline proved to be a useful method in the management of bradycardia secondary to AV block and offers a pharmacological alternative to patients where other pharmacological strategies or cardiac pacemaker implantation may be contraindicated [98]. An interesting observation was re-ported by Bertolet et al. [99], who demonstrated the effectiveness of theophylline in atrial fibrillation due to adenosine accumulation in acute myocardial infarction. After application of theophylline, a rapid (< 5 min) and long-term conversion of atrial fibrillation to sinus rhythm was observed. These findings highlighted the role of

endogenous adenosine in development of atrial fibrillation and the possibility of an adenosine antagonist-based therapy. As theophyl-line is not a specific adenosine receptor antagonist, it is hopeful that A1 adenosine receptor selective antagonists (e.g. synthetic methylx-anthines) can be more efficient drugs to terminate this type of atrial fibrillation [74]. It is noteworthy that after caffeine consumption different changes in sinus rhythm were observed in healthy humans exposed to various submaximal and maximal exercise intensities. Thus, some investigators reported that caffeine increases the heart rate under these circumstances [100-103]. Recently, McClaran and Wetter [104] reported that in non-habitual caffeine users, low doses of caffeine decreased the heart rate during submaximal exercise, but not at near maximal and maximal exercise. It is tempting to suppose that because of the imbalance between oxygen requirement and consumption in maximal physical exercise, endogenous adenosine release can substantially be increased. In this case caffeine might counterbalance the endogenous adenosine-related suppression of sinoatrial pacemaker activity.

There are conflicting reports available on the possible arrhyth-mogenic action of caffeine or theophylline. According to a meta-analysis, moderate consumption of caffeine does not increase the frequency or severity of cardiac arrhythmias in healthy persons, in patients with ischemic heart disease, or in ones having pre-existing serious ventricular ectopy. There is an increase, however, in the frequency of ventricular extrasystoles in heavy coffee consumers drinking 9 or more cups daily [105]. It was recently found that moderate tea intake can be associated with a lower prevalence, while higher coffee consumption with a slightly higher prevalence of ventricular arrhythmias in patients hospitalized with acute myo-cardial infarction [106]. According to the Danish Diet, Cancer, and Health Study [107], in which 47 949 subjects were participated, consumption of caffeine was not associated with an elevated risk of

Table 1. Effects of Long-Term Caffeine Treatment on the CPA-Induced Alterations in the Mechanical and Electrical Activity of Guinea Pig Atrial Myocardium

Parameter Solvent-treated Caffeine-treated

Week 1 pD2

Emax

nConcentration ratio

7.73 ± 0.10 89.5 ± 3.5 5

8.35 ± 0.06* 92.8 ± 2.5 64.17

Week 2 pD2

Emax

nConcentration ratio Vmax (% reduction) APD75 (% shortening) n

7.54 ± 0.24 83.5 ± 3.2 637.8 ± 4.8 31.0 ± 5.5 5

8.43 ± 0.04** 93.0 ± 3.2* 67.76 95.7 ± 1.8** 69.7 ± 9.6* 4

Week 6 pD2

Emax

nConcentration ratio

7.78 ± 0.10 90.0 ± 1.6 5

8.58 ± 0.08* 95.2 ± 1.8 66.31

Week 10 pD2

Emax

nConcentration ratio

7.86 ± 0.08 85.8 ± 2.4 5

8.55 ± 0.07* 97.3 ± 1.1* 44.90

Data are mean values ± S.E.M, EC50 = the concentration producing half-maximum response (its negative based logarithm: pD2), Emax = maximum effect expressed in % of pre-CPA contractile force, n = number of experiments, Vmax = rate of rise of depolarization, APD75 = duration of action potential at 75% of repolarization. Changes in Vmax and APD75 were measured at 10 nM CPA concentration. Concentration ratios denote the ratios of EC50 values of CPA concentration-response curves (solvent-treated versus caffeine-treated). Aster-isks indicate significant differences between the caffeine-treated and the solvent-treated control groups.

3700 Current Medicinal Chemistry, 2011 Vol. 18, No. 24 Szentmiklósi et al.

atrial fibrillation or flutter. Similarly, at modest dose of caffeine, no action on the incidence of ventricular arrhythmia could be observed in patients with ventricular tachycardia or fibrillation [108, 109].

Regarding the effects of methylxanthines on the working myo-cardium, the picture is quite complex. Conrad and Prosnitz [110]studied the cardiovascular actions of aminophylline-infusion in healthy male volunteers and found that aminophylline exerted a mild positive inotropic effect, but this action could be prevented by pretreatment with the �-adrenergic receptor antagonist metoprolol or propranolol. The hemodynamic action of intravenously applied aminophylline was also studied in healthy human subjects and no significant improvement was found on cardiac performance during tread-mill exercise after using of the compound [111]. These find-ings are in good agreement with the further study of Conradson using theophylline and enprophylline [112]. According to the hemo-dynamic studies of Parker et al. [113], aminophylline was effective to increase cardiac output in patients suffering from heart failure, but no change was observed in patients with cor pulmonale [114] or chronic obstructive pulmonary disease [115].

Theophylline and aminophylline were extensively used over the last 70 years to increase inotropy in patients suffering from heart failure [113, 116], and to treat patients with bronchial asthma [117].However, because of the narrow therapeutic ratio and due to its cardiac, gastroenteric and CNS side effects, theophylline has lost popularity in the last decades. According to current AHA and ESC guidelines, the two primary pharmacological approaches in symp- tomatic heart failure are ACE inhibitors/ARBs and diuretics. Theo- phylline exerts a slight positive inotropic action combined with a good vasodilator activity, therefore, it can be an ideal drug for com- bined inotropic and vasodilator treatment of heart failure. In addi- tion, theophylline and its derivatives have also moderate diuretic activity based on adenosine receptor antagonism [118], decreasing thus the circulating blood volume and preload of the heart. Endoge- nous adenosine was also shown to play an important role in patho- genesis of heart failure. Plasma adenosine levels are increased in patients with heart failure in parallel with the severity of the disease [119]. In the nephron, A1 and A2 adenosine receptors are widely distributed and they have significant functional role. Adenosine can influence the glomerular filtration rate, renal hemodynamics, and the tubuloglomerular feedback. This purine nucleoside reduces glomerular filtration by inducing selective vasoactive actions on the

postglomerular and preglomerular vessels. Adenosine constricts the preglomerular vessels by activation of A1 adenosine receptors, whereas exerts a vasodilator action on the postglomerular arterioles by stimulation of A2 receptors [120]. Activation of A1 receptors decreases renal perfusion pressure, glomerular filtration, and tubu- lar sodium reabsorption [121]. It is supposed that adenosine may play a role in the kidney-mediated fluid retention in heart failure [122]. In this case, use of selective A1 adenosine receptor antago- nists might provide therapeutic benefits in the setting of congestive heart failure. At present, one of the primary approaches of treatment of heart failure is diuretic therapy. On the basis of clinical experi- ences, however, it became clear that the generally applied diuretics (e.g. furosemide) may reduce glomerular filtration rate in patients suffering from heart failure [120, 123]. In contrast, selective block- ade of A1 adenosine receptors by CVT-124 was shown to reduce afferent arteriole pressure, increase the urinary flow and sodium excretion [124, 125]. It is logical to assume that treatment with se- lective A1 adenosine antagonists can be more effective than the conventional diuretic therapy.

The chemical structures of some novel A1 adenosine receptor selective compounds are presented in Fig. (4). The newest drug in this category is rolofylline (KW-3902). In a randomized, placebo- controlled, dose-finding study (PROTECT Pilot Study), it was ob- served that A1 adenosine receptor blockade with rolofylline could prevent renal impairment in patients with acute heart failure and positively affected the acute symptoms as well as the 60 days out- come [126]. The large clinical trial, PROTECT, began in May 2007 and was completed in January 2009. 2033 patients from 173 sites of 19 countries were recruited in this study. The results were disap- pointing as in the larger trial the benefits observed in the PROTECT Pilot Study could not be replicated. Rolofylline showed no differ- ence from placebo regarding the primary end-points in patients with acute heart failure. In the study, there was a trend toward a lower incidence of serious adverse cardiac side reactions, but the inci- dence of neurological events (seizures and strokes) were higher. As a consequence, the drug has been dropped from development [127-129]. The clinical trial with another A1 adenosine receptor-selective blocker, naxifylline was also discontinued [130]. At present, it is not clear whether the basic principle is a pitfall or there might be prob- lems with the physicochemical properties of the investigated drugs? The resolution of this issue should be very important for the future

N

HNN

N

O

O

DPCPX (CPX)

CH3

H3C

N

HNN

N

O

O

Rolofylline (KW3902, NAX)

CH3

H3C

N

HNN

N

O

O

Naxifylline (BG-9719, CVT-124)

CH3

H3C O

N

HNN

N

O

O

Toponafylline (BG-9928)

CH3

H3C

OHO

N

HNN

N

O

O

H3C

OH PSB-36

Fig. (4). Chemical structures of DPCPX and new A1 adenosine receptor selective antagonists.

Xanthine Derivatives in the Heart Current Medicinal Chemistry, 2011 Vol. 18, No. 24 3701

trends of drug development. Currently there is only one A1 recep- tor-selective xanthine derivative, toponafylline under clinical trial for treatment of heart failure [130].

4. CURRENT PROBLEMS AND FUTURE PERSPECTIVES

Independently of the above mentioned facts, it appears that theophylline receives a renewed attention as according to a number of studies. This drug - applied in relatively low doses - was able to exert significant anti-inflammatory action in patients with bronchial asthma and chronic obstructive pulmonary disease [117, 131-135].On the basis of this recognition, a new era of use of methylxanthi-nes may start, the significance of which could affect not only the treatment conceptions of cardiorespiratory diseases, but also the synthesis of new and more effective methylxanthines. When con-sidering the ongoing and future use of methylxanthines, we should take into account a number of issues. According to the title of this work, we should wage the benefits and risks of xanthine treatment.

The main facts are as follows. (1) caffeine is the most widely consumed and relatively safe psychoactive agent all over the world. (2) Although treatment guidelines have regulated theophylline to third-line treatment of bronchial asthma and chronic obstructive pulmonary disease [136, 137], the rate of theophylline users amongasthmatic and COPD patients is relatively high [138]. (3) The anti-inflammatory action of theophylline, observed in low dose, will likely facilitate the worldwide use of theophylline again [117, 135].(4) There is an intense, lengthy interdisciplinary endeavor for de-veloping new, more selective and more effective xanthine deriva-tives. Although, according to large studies the moderate consump-tion of caffeine-containing beverages does not associate with an increased cardiovascular mortality and morbidity, there are some concerns that the use of methylxanthines should be taken into ac-count in point of view of possible cardiovascular consequences. Thus, it was emerged that methylxanthines are able to eliminate the preconditioning actions of endogenous adenosine [139]. As adeno-sine receptors are highly implicated in ischemia/reperfusion proc-ess, the same suggestion might also be conceivable for treatment with various xanthines having adenosine receptor-antagonist prop-erties. As for the possible implication of theophylline and xanthine structures, we will focus exclusively on four interesting aspects of cardiac activity: possible implications for (1) preconditioning, (2) postconditioning, (3) ischemia/reperfusion injuries, and (4) epige-netic processes in the heart.

4.1. Possible Implications for Preconditioning

It is widely believed that adenosine is a highly effective en-dogenous cardioprotective agent, which plays important role in ischemic preconditioning. It has been observed that a short period of ischemia increases the tolerance against a subsequent sustained ischemia [140, 141]. This might be the basis of the experience that the infarct size is smaller in patients who had several anginal at-tacks before acute myocardial infarction [140]. Preconditioning may occur in two phases: an early phase, called also early precondition-ing, and a delayed phase often mentioned as a second window of protection. The delayed phase of ischemic preconditioning is sup-posed to implicate synthesis of cytoprotective proteins [142]. There is good evidence that A1 and A3 adenosine receptors have crucial role in adenosine-mediated preconditioning in both experimental animals [143-149] and human subjects [150, 151]. In preconditiong, the role of adenosine is supported by strong pharmacological evi-dence, indicating that various xanthine-type adenosine receptor blockers, like 8-p-(sulphophenyl)theophylline, 8-(4-carboxyeth-enylphenyl)-1,3-dipropylxanthine (BW A1433), and 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) effectively eliminated precondition-ing [147, 152]. Recently, Riksen et al. [139] and Meijer et al. [153]have shown that acute caffeine administration prevented protection in two different human models of ischemic preconditioning. Schae-

fer et al. [154], as well as Claeys and his associates [155] observed that blockade of adenosine receptors with aminophylline limited ischemic preconditiong in human beings. Tomai and his coworkers [150] described that adenosine receptor blockade by bamiphylline inhibited the ischemic preconditioning during coronary angioplasty. The same group [156] has also demonstrated that bamiphylline improved exercise-induced myocardial ischemia, increased the ischemic threshold, prolonged exercise duration, delayed the onset of anginal pain, and reduced the ST segment depression - possibly via selective blockade of A1 adenosine receptors.

4.2. Possible Implications for Postconditioning

Myocardial ischemia is always followed by reperfusion associ-ated myocardial necrosis and apoptosis. This reperfusion injury can be diminished by postconditioning, which was first described by Zhao et al. [157]. Postconditioning includes a brief ischemic epi-sode immediately at the onset of reperfusion after a prolonged ischemic episode. This intervention was shown to reduce the size of the infarct area [157-159]. Although, the literature data are conflict-ing, it seems likely that A2A and A2B adenosine receptors may play critical role in postconditioning [160-162], however, involvement of A1 and A3 adenosine receptors have also been emerged [163].

4.3. Possible Implications for Ischemia/Reperfusion Injury

Adenosine is known to exert a protective effect against ischemic injury. In the adenosinergic cardioprotection, practically, all subtypes of adenosine receptors contribute. A1 adenosine recep-tor-induced cardioprotection operates when the receptors are acti-vated prior to and during ischemia. During reperfusion, A1 adeno-sine receptor activation is not effective [163-167]. Although, in human, the expression of A3 adenosine receptors is relatively poor, there are pharmacological proofs to support the protective role of A3 receptor activation both in the pre- and post-ischemic period, as well as during ischemia [160, 168-170]. In contrast to A1 and A3adenosine receptors, A2A and A2B adenosine receptors do not play a significant role in the pre-ischemic and ischemic phase, but are highly effective during reperfusion [163, 164, 170-173]. Several experimental studies provide evidence that blockade of any of the known adenosine receptor subtypes could somehow diminish the cardioprotective action of adenosine [163, 174].

4.4. Possible Implications for Epigenetic Mechanisms

The term “epigenetics” describes all meiotically and mitotically heritable alterations in gene expression that are not coded in the DNA sequence itself [175]. Epigenetic mechanisms involves DNA methylation, RNA-associated silencing and histone modification. Histone modifications as epigenetic modifiers are post-translational modifications of histones, including acetylation and methylation of conserved lysine residues. Histone deacetylases (HDACs) regulate cellular signaling and gene expression. Until recently, distinct genes encoding 18 mammalian HDACs (grouped into Class I, II and III) have been identified [176]. Histone deacetylase 2 (HDAC2) is a critical factor for embryonic development and cytokine signaling relevant for immune responses. Pathologic activity of this enzyme can lead to various cardiac disturbances such as myocardial hyper-trophy or heart failure [177]. Thus, inhibitors of HDAC2 enzyme are considered as important cardiac drugs, to be applied e.g. for prevention of myocardial hypertrophy [176, 178, 179]. It is notice-able that Class IIa HDACs (HDAC 4, 5 and 9) repress myocardial hypertrophy and Class I and Class IIa enzymes have opposing roles in regulation of cardiac myocyte growth and heart diseases [180].The roles of various HDACs are quite complex and they might also be involved in pathogenesis of cardiac arrhythmias and acute coro-nary syndromes [179].

This epigenetic mechanism might be significant in patients suf-fering from bronchial asthma and COPD. It was repeatedly de-

3702 Current Medicinal Chemistry, 2011 Vol. 18, No. 24 Szentmiklósi et al.

scribed that low concentrations of theophylline activated HDAC and suppressed the activation of inflammatory genes [134, 181].This action could be observed at low (1 to 10 �M) theophylline concentrations, whereas is lost at higher theophylline levels (100�M). Theophylline activates different subtypes of HDACs via an intracellular site of action, but it appears that it has a relatively more selective action on Class I than on Class II HDACs [182]. To our best knowledge, there are no reports on the action of theophylline and other xanthine derivatives on the cardiac HDACs, but it should be strictly considered whether these low theophylline levels might affect also the physiological or pathological cardiac functions? At present, we do not know whether theophylline can be regarded as a friend or a foe on the heart in this regard. Because the actions of various HDACs are very complex, the outcome of activation of these enzymes can be either beneficial or harmful. Extensive clini-cal studies are necessary to reveal the significance of this issue.

5. CONCLUSIONS

The main goal of this paper was to review the current state of the medical use and perspectives of xanthine structures, such as theophylline, caffeine and some of the novel adenosine subtype-specific xanthine derivatives. It can be stated that the conventional methylxanthine theophylline is still frequently prescribed world-wide, but the frequency of adverse reactions and the relatively low efficacy of this drug have led to reduced use in the pulmonological practice. The recent recognition of the specific anti-inflammatory action of low theophylline doses as a HDAC activator will possibly significantly accelerate the worldwide application of theophylline. It is tempting to speculate that the anti-inflammatory property of theophylline or theophylline-derivatives could be used in not only in bronchial asthma and COPD, but also in ischemia/reperfusion injuries, such as heart transplantations or acute myocardial infarc-tion. In the reperfusion phase of these cardiovascular events the crucial role of the inflammatory processes are widely accepted [183-186]. It has been shown that pretreatment with aminophylline before coronary artery bypass grafting or heart transplantation im-proved the mechanical and biochemical functions of the heart in human subjects [187, 188]. Regarding caffeine, this drug possibly remains the most widely consumed psychoactive substance in the future. There are intensive endeavors to synthesize new adenosine subtype-selective xanthine derivatives. Two A1 subtype selective, xanthine-type adenosine receptor antagonists were under clinical trials (rolofylline, naxifylline), but these studies were discontinued. At present, only toponafylline (BG-9928) is in a phase IIb clinical

trial for treating heart failure (Fig. 4). Xanthine-type A2A antago-nists were also under clinical trial in Parkinson disease, e.g. istrade-fylline (KW-6002) and ST-1535. ST-1535 is under phase I trial, whereas the study with istradefylline was discontinued [130]. Two xanthine-type A2B adenosine receptor antagonists are under clinical development for bronchial asthma (ATL 844 and GS 6201) [130].

It is surprising that relatively few subtype-selective xanthine-type antagonists were chosen for human studies and approximately half of the trials were discontinued because of ineffectiveness or adverse reactions. It appears that we should consider primarily the cardiac actions of caffeine, theophylline and their derivatives. It would be a useful option to develop compounds, like 8-p-(sulpho-phenyl)theophylline (8-SPT; Fig. 5), 8-(4’-carboxymethyloxy-phenyl)-1,3-dipropylxanthine or 8-(4’-carboxymethyloxyphenyl)-1,3-dipropylxanthine-2-aminoethylamide as theophylline deriva-tives, which are not able or very poorly able to penetrate the blood brain barrier [189]. With the use of these types of compounds, the central nervous system effects, like seizures, could effectively be avoided.

The actual concern is that how xanthine structures could influ-ence pre- and postconditioning, as well as ischemic-reperfusion injuries. As highlighted above, there are clinical experiences indi-cating that methylxanthines can inhibit preconditioning and the question emerged, whether these adenosine receptor antagonists can blunt the cardioprotective activity of endogenous adenosines. First, these clinical observations refer exclusively to acute administration of methylxanthines. Until recently, there are no chronic human trials. Second, there are experimental findings that adenosine recep-tors are upregulated and the adenosine-induced physiological re-sponses are concomitantly increased under chronic exposure to methylxanthines [79-83]. In this paper, we clearly demonstrated that - at least in guinea pigs - the long-term administration of oral caf-feine administration may strongly enhance the A1 adenosine recep-tor-induced electrical and mechanical responses in electrically driven left atrial myocardium. These findings strongly suggest that under chronic administration of caffeine or theophylline, adenosine receptors participating in preconditioning or ischemia-reperfusion processes are upregulated. Therefore, it can be predicted that long-lasting exposures to these methylxanthines do not worsen, but rather improve the effectiveness of these endogenous cardioprotec-tive mechanisms. In addition, it should be taken into account that caffeine is capable of inducing a dose-related increase of plasma adenosine levels [190]. These factors together could likely ensure the integrity of endogenous cardioprotective mechanisms. At any rate, in the light of a number of large prospective studies it can be

N

NN

N

O

O

Istradefylline (KW-6002)

CH3

H3C

CH3

OCH3

O CH3

N

HNN

N

O

O

GS 6201 (CVT-6883)

CH3

N

N

H3C

FF

F

N

HNN

N

O

O

CH3

8-p-(sulphophenyl)theophylline

H3C

OH

O

S

O

Fig. (5). Chemical structures of istradefylline, GS 6201, and 8-SPT.

Xanthine Derivatives in the Heart Current Medicinal Chemistry, 2011 Vol. 18, No. 24 3703

ascertained that chronic, but moderate coffee consumption (which is the main source of caffeine intake) does not increase generally the risk of coronary disease and heart failure. There might be, of course, adverse reactions associated mainly to the cardiac and neu-rohumoral actions of acute caffeine exposition, but these reactions might occur in a specific vulnerable population, in elderly and hy-pertensive patients, or when the possible adverse reactions are ge-netically determined [4, 191, 192]. The extremely high coffee con-sumption, however, carries large cardiovascular risk.

In conclusion, at present we are in a position to apply rational drug design also in the platform of adenosinergic pharmacology as there is a progressively accumulating knowledge both on the bene-ficial and harmful actions of endogenous and exogenous adenosine. The main goal is the development of novel structures that mimic the actions of the conventional methylxanthines as lead compounds, but their adenosine receptor subtype-specificity are higher, their water solubility are optimal, and the frequency of their adverse reactions are minimal. Therefore, the search for potent and subtype-selective adenosine receptor antagonists has been one of the most highly studied areas in medicinal chemistry [130, 193-198]. An attractive approach of drug development could be to synthetize pharmaco-logically silent, clinically viable, site- and event-specific allosteric adenosine receptor antagonists with subtype selectivity, which could be an enormous challenge for both academic and industrial research groups.

REFERENCES

[1] Barnes, P. Theophylline. Pharmaceuticals, 2010, 3, 725-747. [2] Sawynok, J.; Yaksh, T.L. Caffeine as an analgesic adjuvant: a review of

pharmacology and mechanisms of action. Pharmacol. Rev., 1993, 45, 43-85. [3] Fredholm, B.B.; Battig, K.; Holmen, J.; Nehlig, A.; Zvartau, E.E. Actions of

caffeine in the brain with special reference to factors that contribute to its widespread use. Pharmacol. Rev., 1999, 51, 83-133.

[4] Riksen, N.P.; Smits, P.; Rongen, G.A. The cardiovascular effects of methylxanthines. Handb. Exp. Pharmacol., 2011, 200, 413-437.

[5] Fredholm, B.B. Cardiovascular and renal actions of methylxanthines. Prog. Clin. Biol. Res., 1984, 158, 303-330.

[6] Satoh, H. Caffeine depression of spontaneous activity in rabbit sino-atrial node cells. Gen. Pharmacol., 1993, 24, 555-563.

[7] Paspa, P.; Vassalle, M. Mechanism of caffeine-induced arrhythmias in canine cardiac Purkinje fibers. Am. J. Cardiol., 1984, 53, 313-319.

[8] Lin, C.I.; Chuang, I.N.; Cheng, K.K.; Chiang, B.N. Arrhythmogenic effects of theophylline in human atrial tissue. Int. J. Cardiol., 1987, 17, 289-297.

[9] Kimoto, Y. Effects of caffeine on the membrane potentials and contractility of the guinea pig atrium. Jpn. J. Physiol., 1972, 22, 225-238.

[10] Yatani, A.; Imoto, Y.; Goto, M. The effects of caffeine on the electrical properties of isolated, single rat ventricular cells. Jpn. J. Physiol., 1984, 34,337-349.

[11] Kamishima, T.; Quayle, J.M. Ca2+-induced Ca2+ release in cardiac and smooth muscle cells. Biochem. Soc. Trans., 2003, 31, 943-946.

[12] Eisner, D.A.; Lederer, W.J.; Noble, D. Caffeine and tetracaine abolish the slow inward calcium current in sheep cardiac Purkinje fibres [proceedings]. J. Physiol., 1979, 293, 76P-77P.

[13] Hughes, A.D.; Hering, S.; Bolton, T.B. The action of caffeine on inward barium current through voltage-dependent calcium channels in single rabbit ear artery cells. Pflugers Arch., 1990, 416, 462-466.

[14] Varro, A.; Hester, S.; Papp, J.G. Caffeine-induced decreases in the inward rectifier potassium and the inward calcium currents in rat ventricular myocytes. Br. J. Pharmacol., 1993, 109, 895-897.

[15] Zhang, Y.A.; Tuft, R.A.; Lifshitz, L.M.; Fogarty, K.E.; Singer, J.J.; Zou, H. Caffeine-activated large-conductance plasma membrane cation channels in cardiac myocytes: characteristics and significance. Am. J. Physiol. Heart Circ. Physiol., 2007, 293, H2448-2461.

[16] Sattin, A.; Rall, T.W. The effect of adenosine and adenine nucleotides on the cyclic adenosine 3', 5'-phosphate content of guinea pig cerebral cortex slices. Mol. Pharmacol., 1970, 6, 13-23.

[17] Fredholm, B.B. Are Methylxanthine Effects Due to Antagonism of Endogenous Adenosine. Trends Pharmacol. Sci., 1980, 1, 129-132.

[18] Berne, R.M.; Knabb, R.M.; Ely, S.W.; Rubio, R. Adenosine in the local regulation of blood flow: a brief overview. Fed. Proc., 1983, 42, 3136-3142.

[19] Belardinelli, L.; Linden, J.; Berne, R.M. The cardiac effects of adenosine. Prog. Cardiovasc. Dis., 1989, 32, 73-97.

[20] Lerman, B.B.; Belardinelli, L. Cardiac electrophysiology of adenosine. Basic and clinical concepts. Circulation, 1991, 83, 1499-1509.

[21] Szentmiklosi, A.J.; Nemeth, M.; Szegi, J.; Papp, J.G.; Szekeres, L. On the possible role of adenosine in the hypoxia-induced alterations of the electrical

and mechanical activity of the atrial myocardium. Arch. Int. Pharmacodyn. Ther, 1979, 238, 283-295.

[22] Szentmiklosi, A.J.; Nemeth, M.; Cseppento, A.; Szegi, J.; Papp, J.G.; Szekeres, L. Effects of hypoxia on the guinea-pig myocardium following inhibition of adenosine deaminase by coformycin. Arch. Int. Pharmacodyn. Ther., 1984, 269, 287-294.

[23] Vinten-Johansen, J.; Thourani, V.H.; Ronson, R.S.; Jordan, J.E.; Zhao, Z.Q.; Nakamura, M.; Velez, D.; Guyton, R.A. Broad-spectrum cardioprotection with adenosine. Ann. Thorac. Surg., 1999, 68, 1942-1948.

[24] Auchampach, J.A.; Bolli, R. Adenosine receptor subtypes in the heart: therapeutic opportunities and challenges. Am. J. Physiol., 1999, 276, H1113-1116.

[25] Norton, G.R.; Woodiwiss, A.J.; McGinn, R.J.; Lorbar, M.; Chung, E.S.; Honeyman, T.W.; Fenton, R.A.; Dobson, J.G., Jr.; Meyer, T.E. Adenosine A1 receptor-mediated antiadrenergic effects are modulated by A2a receptor activation in rat heart. Am. J. Physiol., 1999, 276, H341-349.

[26] Dubey, R.K.; Gillespie, D.G.; Jackson, E.K. Adenosine inhibits collagen and protein synthesis in cardiac fibroblasts: role of A2B receptors. Hypertension, 1998, 31, 943-948.

[27] Strickler, J.; Jacobson, K.A.; Liang, B.T. Direct preconditioning of cultured chick ventricular myocytes. Novel functions of cardiac adenosine A2a and A3 receptors. J. Clin. Invest., 1996, 98, 1773-1779.

[28] Drury, A.N.; Szent-Gyorgyi, A. The physiological activity of adenine compounds with especial reference to their action upon the mammalian heart. J. Physiol., 1929, 68, 213-237.

[29] Versprille, A. The chronotropic effect of adenine- and hypoxanthine derivatives on isolated rat hearts before and after removing the sino-auricular node. Pflugers Arch. Gesamte Physiol. Menschen Tiere, 1966, 291, 261-267.

[30] DiMarco, J.P.; Sellers, T.D.; Berne, R.M.; West, G.A.; Belardinelli, L. Adenosine: electrophysiologic effects and therapeutic use for terminating paroxysmal supraventricular tachycardia. Circulation, 1983, 68, 1254-1263.

[31] Szentmiklosi, A.J.; Nemeth, M.; Szegi, J.; Papp, J.G.; Szekeres, L. Effect of adenosine on sinoatrial and ventricular automaticity of the guinea pig. Naunyn Schmiedebergs Arch. Pharmacol., 1980, 311, 147-149.

[32] James, T.N. The chronotropic action of ATP and related compounds studied by direct perfusion of the sinus node. J. Pharmacol. Exp. Ther., 1965, 149,233-247.

[33] Chiba, S.; Hashimoto, K. Differences in chronotropic and dromotropic responses of the SA and AV nodes to adenosine and acetylcholine. Jpn. J. Pharmacol., 1972, 22, 273-274.

[34] Urthaler, F.; James, T.N. Effects of adenosine and ATP on AV conduction and on AV junctional rhythm. J. Lab. Clin. Med., 1972, 79, 96-105.

[35] Pelleg, A.; Hurt, C.; Miyagawa, A.; Michelson, E.L.; Dreifus, L.S. Differential sensitivity of cardiac pacemakers to exogenous adenosine in vivo. Am. J. Physiol., 1990, 258, H1815-1822.

[36] Belardinelli, L.; Isenberg, G. Isolated atrial myocytes: adenosine and acetylcholine increase potassium conductance. Am. J. Physiol., 1983, 244,H734-737.

[37] West, G.A.; Belardinelli, L. Correlation of sinus slowing and hyperpolarization caused by adenosine in sinus node. Pflugers Arch., 1985,403, 75-81.

[38] West, G.A.; Belardinelli, L. Sinus slowing and pacemaker shift caused by adenosine in rabbit SA node. Pflugers Arch., 1985, 403, 66-74.

[39] Belardinelli, L.; Shryock, J.C.; Song, Y.; Wang, D.; Srinivas, M. Ionic basis of the electrophysiological actions of adenosine on cardiomyocytes. FASEB J., 1995, 9, 359-365.

[40] Wang, D.; Belardinelli, L. Mechanism of the negative inotropic effect of adenosine in guinea pig atrial myocytes. Am. J. Physiol., 1994, 267, H2420-2429.

[41] Pelleg, A.; Belardinelli, L. Cardiac electrophysiology and pharmacology of adenosine: basic and clinical aspects. Cardiovasc. Res., 1993, 27, 54-61.

[42] Honey, R.M.R., W.T.; Thomson, W.A.R. The action of adenosine upon the human heart. Q. J. Med., 1930, 23, 485-490.

[43] Watt, A.H. Sick sinus syndrome and adenosine. Lancet, 1985, 1, 1340. [44] Watt, A.H. Sick sinus syndrome: an adenosine-mediated disease. Lancet,

1985, 1, 786-788. [45] Stafford, A. Potentiation of adenosine and the adenine nucleotides by

dipyridamole. Br. J. Pharmacol. Chemother., 1966, 28, 218-227. [46] Toft, J.; Mortensen, J.; Hesse, B. Risk of atrioventricular block during

adenosine pharmacologic stress testing in heart transplant recipients. Am. J. Cardiol., 1998, 82, 696-697, A699.

[47] Harvey, M.G.; Safih, S.; Wallace, M. Adenosine-induced complete heart block: not so transient. Emerg. Med. Australas, 2007, 19, 559-562.

[48] Clemo, H.F.; Belardinelli, L. Effect of adenosine on atrioventricular conduction. II: Modulation of atrioventricular node transmission by adenosine in hypoxic isolated guinea pig hearts. Circ. Res., 1986, 59, 437-446.

[49] Hollander, P.B.; Webb, J.L. Effects of adenine nucleotides on the contractility and membrane potentials of rat atrium. Circ. Res., 1957, 5, 349-353.

[50] Grossman, A.; Furchgott, R.F. The Effects of Various Drugs on Calcium Exchange in the Isolated Guinea-Pig Left Auricle. J. Pharmacol. Exp. Ther., 1964, 145, 162-172.

3704 Current Medicinal Chemistry, 2011 Vol. 18, No. 24 Szentmiklósi et al.

[51] Degubareff, T.; Sleator, W., Jr. Effects of Caffeine on Mammalian Atrial Muscle, and Its Interaction with Adenosine and Calcium. J. Pharmacol. Exp. Ther., 1965, 148, 202-214.

[52] Bohm, M.; Meyer, W.; Mugge, A.; Schmitz, W.; Scholz, H. Functional evidence for the existence of adenosine receptors in the human heart. Eur. J. Pharmacol., 1985, 116, 323-326.

[53] Collis, M.G. Evidence for an A1-adenosine receptor in the guinea-pig atrium. Br. J. Pharmacol., 1983, 78, 207-212.

[54] Szentmiklosi, A.J.; Nemeth, M.; Cseppento, A.; Szegi, J.; Papp, J.G.; Szekeres, L. Potentiation of the myocardial actions of adenosine in the presence of coformycin, a specific inhibitor of adenosine deaminase. Arch. Int. Pharmacodyn. Ther., 1982, 256, 236-252.

[55] Jochem, G.; Nawrath, H. Adenosine activates a potassium conductance in guinea-pig atrial heart muscle. Experientia, 1983, 39, 1347-1349.

[56] Schrader, J.; Rubio, R.; Berne, R.M. Inhibition of slow action potentials of guinea pig atrial muscle by adenosine: a possible effect on Ca2+ influx. J. Mol. Cell. Cardiol., 1975, 7, 427-433.

[57] Fassina, G.; de Biasi, M.; Ragazzi, E.; Caparrotta, L. Adenosine: a natural modulator of L-type calcium channels in atrial myocardium? Pharmacol. Res., 1991, 23, 319-326.

[58] Bohm, M.; Bruckner, R.; Hackbarth, I.; Haubitz, B.; Linhart, R.; Meyer, W.; Schmidt, B.; Schmitz, W.; Scholz, H. Adenosine inhibition of catecholamine-induced increase in force of contraction in guinea-pig atrial and ventricular heart preparations. Evidence against a cyclic AMP- and cyclic GMP-dependent effect. J. Pharmacol. Exp. Ther., 1984, 230, 483-492.

[59] Bohm, M.; Burmann, H.; Meyer, W.; Nose, M.; Schmitz, W.; Scholz, H. Positive inotropic effect of Bay K 8644: cAMP-independence and lack of inhibitory effect of adenosine. Naunyn Schmiedebergs Arch. Pharmacol., 1985, 329, 447-450.

[60] Bruckner, R.; Fenner, A.; Meyer, W.; Nobis, T.M.; Schmitz, W.; Scholz, H. Cardiac effects of adenosine and adenosine analogs in guinea-pig atrial and ventricular preparations: evidence against a role of cyclic AMP and cyclic GMP. J. Pharmacol. Exp. Ther., 1985, 234, 766-774.

[61] Buckley, N.M. Cardiac effects of nucleosides after propranolol treatment of isolated hearts. Am. J. Physiol., 1970, 218, 1399-1405.

[62] Chiba, S.; Himori, N. Different inotropic responses to adenosine on the atrial and ventricular muscle of the dog heart. Jpn. J. Pharmacol., 1975, 25, 489-491.

[63] Schrader, J.; Baumann, G.; Gerlach, E. Adenosine as inhibitor of myocardial effects of catecholamines. Pflugers Arch., 1977, 372, 29-35.

[64] Dobson, J.G., Jr. Mechanism of adenosine inhibition of catecholamine-induced responses in heart. Circ. Res., 1983, 52, 151-160.

[65] Isenberg, G.; Belardinelli, L. Ionic basis for the antagonism between adenosine and isoproterenol on isolated mammalian ventricular myocytes. Circ. Res., 1984, 55, 309-325.

[66] Dobson, J.G., Jr. Reduction by adenosine of the isoproterenol-induced increase in cyclic adenosine 3',5'-monophosphate formation and glycogen phosphorylase activity in rat heart muscle. Circ. Res., 1978, 43, 785-792.

[67] Endoh, M.; Yamashita, S. Adenosine antagonizes the positive inotropic action mediated via beta-, but not alpha-adrenoceptors in the rabbit papillary muscle. Eur. J. Pharmacol., 1980, 65, 445-448.

[68] Hughes, P.R.; Stone, T.W. Inhibition by purines of the inotropic action of isoprenaline in rat atria. Br. J. Pharmacol., 1983, 80, 149-153.

[69] Collis, M.G.; Palmer, D.B.; Saville, V.L. Comparison of the potency of 8-phenyltheophylline as an antagonist at A1 and A2 adenosine receptors in atria and aorta from the guinea-pig. J. Pharm. Pharmacol., 1985, 37, 278-280.

[70] Griffith, S.G.; Meghji, P.; Moody, C.J.; Burnstock, G. 8-phenyltheophylline: a potent P1-purinoceptor antagonist. Eur. J. Pharmacol., 1981, 75, 61-64.

[71] Collis, M.G.; Jacobson, K.A.; Tomkins, D.M. Apparent affinity of some 8-phenyl-substituted xanthines at adenosine receptors in guinea-pig aorta and atria. Br. J. Pharmacol., 1987, 92, 69-75.

[72] Belardinelli, L.; Belloni, F.L.; Rubio, R.; Berne, R.M. Atrioventricular conduction disturbances during hypoxia. Possible role of adenosine in rabbit and guinea pig heart. Circ. Res., 1980, 47, 684-691.

[73] Xu, J.; Tong, H.; Wang, L.; Hurt, C.M.; Pelleg, A. Endogenous adenosine, A1 adenosine receptor, and pertussis toxin sensitive guanine nucleotide binding protein mediate hypoxia induced AV nodal conduction block in guinea pig heart in vivo. Cardiovasc. Res., 1993, 27, 134-140.

[74] Hayes, E.S. Adenosine receptors and cardiovascular disease: the adenosine-1 receptor (A1) and A1 selective ligands. Cardiovasc. Toxicol., 2003, 3, 71-88.

[75] Cseppento, A.; Szentmiklosi, A.J.; Meszaros, J.; Szegi, J. Hypoxia-Induced Changes in the Automaticity of Sino-Atrial Node. Acta Physiol. Acad. Sci. Hung., 1979, 53, 157-157.

[76] Brown, L.; Erdmann, E. Concentration-response curves of positive inotropic agents before and after ouabain pretreatment. Cardiovasc. Res., 1985, 19,288-298.

[77] Collis, M.G.; Keddie, J.R.; Torr, S.R. Evidence that the positive inotropic effects of the alkylxanthines are not due to adenosine receptor blockade. Br. J. Pharmacol., 1984, 81, 401-407.

[78] Bellemann, P.; Scholz, H. Relationship between theophylline uptake and inotropic effect in the guinea-pig heart. Br. J. Pharmacol., 1974, 52, 265-274.

[79] Wu, S.N.; Linden, J.; Visentin, S.; Boykin, M.; Belardinelli, L. Enhanced sensitivity of heart cells to adenosine and up-regulation of receptor number

after treatment of guinea pigs with theophylline. Circ. Res., 1989, 65, 1066-1077.

[80] Lee, H.T.; Thompson, C.I.; Linden, J.; Belloni, F.L. Differential sensitization of cardiac actions of adenosine in rats after chronic theophylline treatment. Am. J. Physiol., 1993, 264, H1634-1643.

[81] von Borstel, R.W.; Wurtman, R.J.; Conlay, L.A. Chronic caffeine consumption potentiates the hypotensive action of circulating adenosine. Life Sci., 1983, 32, 1151-1158.

[82] Varani, K.; Portaluppi, F.; Gessi, S.; Merighi, S.; Ongini, E.; Belardinelli, L.; Borea, P.A. Dose and time effects of caffeine intake on human platelet adenosine A(2A) receptors : functional and biochemical aspects. Circulation, 2000, 102, 285-289.

[83] Zhang, Y.; Wells, J.N. The effects of chronic caffeine administration on peripheral adenosine receptors. J. Pharmacol. Exp. Ther., 1990, 254, 757-763.

[84] Kusaka, H.; Karasawa, A. Effects of repeated administration of KW-3902, a novel adenosine A1-receptor antagonist, on its pharmacological actions. Jpn. J. Pharmacol., 1993, 63, 513-519.

[85] Newcombe, P.F.; Renton, K.W.; Rautaharju, P.M.; Spencer, C.A.; Montague, T.J. High-dose caffeine and cardiac rate and rhythm in normal subjects. Chest, 1988, 94, 90-94.

[86] Alboni, P.; Ratto, B.; Cappato, R.; Rossi, P.; Gatto, E.; Antonioli, G.E. Clinical effects of oral theophylline in sick sinus syndrome. Am. Heart J., 1991, 122, 1361-1367.

[87] Saito, D.; Matsubara, K.; Yamanari, H.; Obayashi, N.; Uchida, S.; Maekawa, K.; Sato, T.; Mizuo, K.; Kobayashi, H.; Haraoka, S. Effects of oral theophylline on sick sinus syndrome. J. Am. Coll. Cardiol., 1993, 21, 1199-1204.

[88] Alboni, P.; Menozzi, C.; Brignole, M.; Paparella, N.; Gaggioli, G.; Lolli, G.; Cappato, R. Effects of permanent pacemaker and oral theophylline in sick sinus syndrome the THEOPACE study: a randomized controlled trial. Circulation, 1997, 96, 260-266.

[89] Alboni, P.; Rossi, P.; Ratto, B.; Pedroni, P.; Gatto, E.; Antonioli, G.E. Electrophysiologic effects of oral theophylline in sinus bradycardia. Am. J. Cardiol., 1990, 65, 1037-1039.

[90] Benditt, D.G.; Benson, D.W., Jr.; Kreitt, J.; Dunnigan, A.; Pritzker, M.R.; Crouse, L.; Scheinman, M.M. Electrophysiologic effects of theophylline in young patients with recurrent symptomatic bradyarrhythmias. Am. J. Cardiol., 1983, 52, 1223-1229.

[91] Ling, C.A.; Crouch, M.A. Theophylline for chronic symptomatic bradycardia in the elderly. Ann. Pharmacother., 1998, 32, 837-839.

[92] Kragie, L.; Sekovski, B. Theophylline--an alternative therapy for bradyarrhythmia in the elderly. Pharmacotherapy, 1992, 12, 324-330.

[93] Heinz, G.; Hirschl, M.; Buxbaum, P.; Laufer, G.; Gasic, S.; Laczkovics, A. Sinus node dysfunction after orthotopic cardiac transplantation: postoperative incidence and long-term implications. Pacing Clin. Electrophysiol., 1992, 15, 731-737.

[94] Bertolet, B.D.; Eagle, D.A.; Conti, J.B.; Mills, R.M.; Belardinelli, L. Bradycardia after heart transplantation: reversal with theophylline. J. Am. Coll. Cardiol., 1996, 28, 396-399.

[95] Rothman, S.A.; Jeevanandam, V.; Seeber, C.P.; Pina, I.L.; Hsia, H.H.; Bove, A.A.; Miller, J.M. Electrophysiologic effects of intravenous aminophylline in heart transplant recipients with sinus node dysfunction. J. Heart Lung Transplant., 1995, 14, 429-435.

[96] Ellenbogen, K.A.; Szentpetery, S.; Katz, M.R. Reversibility of prolonged chronotropic dysfunction with theophylline following orthotopic cardiac transplantation. Am. Heart J., 1988, 116, 202-206.

[97] Alboni, P.; Ratto, B.; Scarfo, S.; Rossi, P.; Cappato, R.; Paparella, N. Dromotropic effects of oral theophylline in patients with atrial fibrillation and a slow ventricular response. Eur. Heart J., 1991, 12, 630-634.

[98] Cawley, M.J.; Al-Jazairi, A.S.; Stone, E.A. Intravenous theophylline--an alternative to temporary pacing in the management of bradycardia secondary to AV nodal block. Ann. Pharmacother., 2001, 35, 303-307.

[99] Bertolet, B.D.; Hill, J.A.; Kerensky, R.A.; Belardinelli, L. Myocardial infarction related atrial fibrillation: role of endogenous adenosine. Heart, 1997, 78, 88-90.

[100] Bell, D.G.; Jacobs, I.; Zamecnik, J. Effects of caffeine, ephedrine and their combination on time to exhaustion during high-intensity exercise. Eur. J. Appl. Physiol. Occup. Physiol., 1998, 77, 427-433.

[101] Bell, D.G.; McLellan, T.M. Exercise endurance 1, 3, and 6 h after caffeine ingestion in caffeine users and nonusers. J. Appl. Physiol., 2002, 93, 1227-1234.

[102] McNaughton, L.R.; Lovell, R.J.; Siegler, J.; Midgley, A.W.; Moore, L.; Bentley, D.J. The effects of caffeine ingestion on time trial cycling performance. Int. J. Sports Physiol. Perform., 2008, 3, 157-163.

[103] Sasaki, H.; Takaoka, I.; Ishiko, T. Effects of sucrose or caffeine ingestion on running performance and biochemical responses to endurance running. Int. J. Sports Med., 1987, 8, 203-207.

[104] McClaran, S.R.; Wetter, T.J. Low doses of caffeine reduce heart rate during submaximal cycle ergometry. J. Int. Soc. Sports Nutr., 2007, 4, 11.

[105] Myers, M.G. Caffeine and cardiac arrhythmias. Ann. Intern. Med., 1991, 114,147-150.

[106] Mukamal, K.J.; Alert, M.; Maclure, M.; Muller, J.E.; Mittleman, M.A. Tea consumption and infarct-related ventricular arrhythmias: the determinants of myocardial infarction onset study. J. Am. Coll. Nutr., 2006, 25, 472-479.

Xanthine Derivatives in the Heart Current Medicinal Chemistry, 2011 Vol. 18, No. 24 3705

[107] Frost, L.; Vestergaard, P. Caffeine and risk of atrial fibrillation or flutter: the Danish Diet, Cancer, and Health Study. Am. J. Clin. Nutr., 2005, 81, 578-582.

[108] Graboys, T.B.; Blatt, C.M.; Lown, B. The effect of caffeine on ventricular ectopic activity in patients with malignant ventricular arrhythmia. Arch. Intern. Med., 1989, 149, 637-639.

[109] Chelsky, L.B.; Cutler, J.E.; Griffith, K.; Kron, J.; McClelland, J.H.; McAnulty, J.H. Caffeine and ventricular arrhythmias. An electrophysiological approach. JAMA, 1990, 264, 2236-2240.

[110] Conrad, K.A.; Prosnitz, E.H. Cardiovascular effects of theophylline. Partial attenuation by beta-blockade. Eur. J. Clin. Pharmacol., 1981, 21, 109-114.

[111] Elliott, C.G.; Nietrzeba, R.M.; Adams, T.D.; Crapo, R.O.; Jensen, R.L.; Yanowitz, F.G. Effect of intravenous aminophylline upon the incremental exercise performance of healthy men. Respiration, 1985, 47, 260-266.

[112] Conradson, T.B. Bronchodilating plasma concentrations of enprofylline and theophylline have minor cardiovascular effects in man. Acta Pharmacol. Toxicol. (Copenh.), 1986, 58, 204-208.

[113] Parker, J.O.; Kelly, G.; West, R.O. Hemodynamic effects of aminophylline in heart failure. Am. J. Cardiol., 1966, 17, 232-239.

[114] Parker, J.O.; Kelkar, K.; West, R.O. Hemodynamic effects of aminophylline in cor pulmonale. Circulation, 1966, 33, 17-25.

[115] Parker, J.O.; Ashekian, P.B.; Di Giorgi, S.; West, R.O. Hemodynamic effects of aminophylline in chronic obstructive pulmonary disease. Circulation, 1967, 35, 365-372.

[116] Howarth, S.; Mc, M.J.; Sharpey-Schafer, E.P. The circulatory action of theophylline ethylene diamine. Clin. Sci. (Lond.), 1947, 6, 125-135.

[117] Cosio, B.G.; Soriano, J.B. Theophylline again? Reasons for believing. Eur. Respir. J., 2009, 34, 5-6.

[118] Collis, M.G.; Baxter, G.S.; Keddie, J.R. The adenosine receptor antagonist, 8-phenyltheophylline, causes diuresis and saliuresis in the rat. J. Pharm. Pharmacol., 1986, 38, 850-852.

[119] Funaya, H.; Kitakaze, M.; Node, K.; Minamino, T.; Komamura, K.; Hori, M. Plasma adenosine levels increase in patients with chronic heart failure. Circulation, 1997, 95, 1363-1365.

[120] Gottlieb, S.S. Renal effects of adenosine A1-receptor antagonists in congestive heart failure. Drugs, 2001, 61, 1387-1393.

[121] Shryock, J.C.; Belardinelli, L. Adenosine and adenosine receptors in the cardiovascular system: biochemistry, physiology, and pharmacology. Am. J. Cardiol., 1997, 79, 2-10.

[122] Lucas, D.G., Jr.; Hendrick, J.W.; Sample, J.A.; Mukherjee, R.; Escobar, G.P.; Smits, G.J.; Crawford, F.A., Jr.; Spinale, F.G. Cardiorenal effects of adenosine subtype 1 (A1) receptor inhibition in an experimental model of heart failure. J. Am. Coll. Surg., 2002, 194, 603-609.

[123] Tenstad, O.; Williamson, H.E. Effect of furosemide on local and zonal glomerular filtration rate in the rat kidney. Acta Physiol. Scand., 1995, 155,99-107.

[124] Gottlieb, S.S.; Skettino, S.L.; Wolff, A.; Beckman, E.; Fisher, M.L.; Freudenberger, R.; Gladwell, T.; Marshall, J.; Cines, M.; Bennett, D.; Liittschwager, E.B. Effects of BG9719 (CVT-124), an A1-adenosine receptor antagonist, and furosemide on glomerular filtration rate and natriuresis in patients with congestive heart failure. J. Am. Coll. Cardiol., 2000, 35, 56-59.

[125] Gottlieb, S.S.; Brater, D.C.; Thomas, I.; Havranek, E.; Bourge, R.; Goldman, S.; Dyer, F.; Gomez, M.; Bennett, D.; Ticho, B.; Beckman, E.; Abraham, W.T. BG9719 (CVT-124), an A1 adenosine receptor antagonist, protects against the decline in renal function observed with diuretic therapy. Circulation, 2002, 105, 1348-1353.

[126] Cotter, G.; Dittrich, H.C.; Weatherley, B.D.; Bloomfield, D.M.; O'Connor, C.M.; Metra, M.; Massie, B.M. The PROTECT pilot study: a randomized, placebo-controlled, dose-finding study of the adenosine A1 receptor antagonist rolofylline in patients with acute heart failure and renal impairment. J. Card. Fail., 2008, 14, 631-640.

[127] Massie, B.M.; O'Connor, C.M.; Metra, M.; Ponikowski, P.; Teerlink, J.R.; Cotter, G.; Weatherley, B.D.; Cleland, J.G.; Givertz, M.M.; Voors, A.; DeLucca, P.; Mansoor, G.A.; Salerno, C.M.; Bloomfield, D.M.; Dittrich, H.C. Rolofylline, an adenosine A1-receptor antagonist, in acute heart failure. N. Engl. J. Med., 2010, 363, 1419-1428.

[128] Ponikowski, P.; Mitrovic, V.; O'Connor, C.M.; Dittrich, H.; Cotter, G.; Massie, B.M.; Givertz, M.M.; Chen, E.; Murray, M.; Weatherley, B.D.; Fujita, K.P.; Metra, M. Haemodynamic effects of rolofylline in the treatment of patients with heart failure and impaired renal function. Eur. J. Heart Fail., 2010, 12, 1238-1246.

[129] Weatherley, B.D.; Cotter, G.; Dittrich, H.C.; DeLucca, P.; Mansoor, G.A.; Bloomfield, D.M.; Ponikowski, P.; O'Connor, C.M.; Metra, M.; Massie, B.M. Design and rationale of the PROTECT study: a placebo-controlled randomized study of the selective A1 adenosine receptor antagonist rolofylline for patients hospitalized with acute decompensated heart failure and volume overload to assess treatment effect on congestion and renal function. J. Card. Fail., 2010, 16, 25-35.

[130] Muller, C.E.; Jacobson, K.A. Recent developments in adenosine receptor ligands and their potential as novel drugs. Biochim. Biophys. Acta, 2010.

[131] Lim, S.; Tomita, K.; Caramori, G.; Jatakanon, A.; Oliver, B.; Keller, A.; Adcock, I.; Chung, K.F.; Barnes, P.J. Low-dose theophylline reduces eosinophilic inflammation but not exhaled nitric oxide in mild asthma. Am. J. Respir. Crit. Care Med., 2001, 164, 273-276.

[132] Hirano, T.; Yamagata, T.; Gohda, M.; Yamagata, Y.; Ichikawa, T.; Yanagisawa, S.; Ueshima, K.; Akamatsu, K.; Nakanishi, M.; Matsunaga, K.; Minakata, Y.; Ichinose, M. Inhibition of reactive nitrogen species production in COPD airways: comparison of inhaled corticosteroid and oral theophylline. Thorax, 2006, 61, 761-766.

[133] Cosio, B.G.; Iglesias, A.; Rios, A.; Noguera, A.; Sala, E.; Ito, K.; Barnes, P.J.; Agusti, A. Low-dose theophylline enhances the anti-inflammatory effects of steroids during exacerbations of COPD. Thorax, 2009, 64, 424-429.

[134] Ito, K.; Lim, S.; Caramori, G.; Cosio, B.; Chung, K.F.; Adcock, I.M.; Barnes, P.J. A molecular mechanism of action of theophylline: Induction of histone deacetylase activity to decrease inflammatory gene expression. Proc. Natl. Acad. Sci. U S A., 2002, 99, 8921-8926.

[135] Barnes, P.J. Theophylline: new perspectives for an old drug. Am. J. Respir. Crit. Care. Med., 2003, 167, 813-818.

[136] Kroegel, C. Global Initiative for Asthma (GINA) guidelines: 15 years of application. Expert Rev. Clin. Immunol., 2009, 5, 239-249.

[137] Celli, B.R.; MacNee, W. Standards for the diagnosis and treatment of patients with COPD: a summary of the ATS/ERS position paper. Eur. Respir. J., 2004, 23, 932-946.

[138] Wieshammer, S.; Dreyhaupt, J.; Basler, B. Theophylline and cardiac stress in patients with dyspnea: an observational study. Pharmacology, 2010, 86, 189-195.

[139] Riksen, N.P.; Zhou, Z.; Oyen, W.J.; Jaspers, R.; Ramakers, B.P.; Brouwer, R.M.; Boerman, O.C.; Steinmetz, N.; Smits, P.; Rongen, G.A. Caffeine prevents protection in two human models of ischemic preconditioning. J. Am. Coll. Cardiol., 2006, 48, 700-707.

[140] Yellon, D.M.; Downey, J.M. Preconditioning the myocardium: from cellular physiology to clinical cardiology. Physiol. Rev., 2003, 83, 1113-1151.

[141] Murry, C.E.; Jennings, R.B.; Reimer, K.A. Preconditioning with ischemia: a delay of lethal cell injury in ischemic myocardium. Circulation, 1986, 74,1124-1136.

[142] Sommerschild, H.T.; Kirkeboen, K.A. Adenosine and cardioprotection during ischaemia and reperfusion--an overview. Acta Anaesthesiol. Scand., 2000, 44, 1038-1055.

[143] Liu, G.S.; Thornton, J.; Van Winkle, D.M.; Stanley, A.W.; Olsson, R.A.; Downey, J.M. Protection against infarction afforded by preconditioning is mediated by A1 adenosine receptors in rabbit heart. Circulation, 1991, 84,350-356.

[144] Tsuchida, A.; Miura, T.; Miki, T.; Shimamoto, K.; Iimura, O. Role of adenosine receptor activation in myocardial infarct size limitation by ischaemic preconditioning. Cardiovasc. Res., 1992, 26, 456-461.

[145] Auchampach, J.A.; Gross, G.J. Adenosine A1 receptors, KATP channels, and ischemic preconditioning in dogs. Am. J. Physiol., 1993, 264, H1327-1336.

[146] Tracey, W.R.; Magee, W.; Masamune, H.; Kennedy, S.P.; Knight, D.R.; Buchholz, R.A.; Hill, R.J. Selective adenosine A3 receptor stimulation reduces ischemic myocardial injury in the rabbit heart. Cardiovasc. Res., 1997, 33, 410-415.

[147] Liu, G.S.; Richards, S.C.; Olsson, R.A.; Mullane, K.; Walsh, R.S.; Downey, J.M. Evidence that the adenosine A3 receptor may mediate the protection afforded by preconditioning in the isolated rabbit heart. Cardiovasc. Res., 1994, 28, 1057-1061.

[148] Wang, J.; Drake, L.; Sajjadi, F.; Firestein, G.S.; Mullane, K.M.; Bullough, D.A. Dual activation of adenosine A1 and A3 receptors mediates preconditioning of isolated cardiac myocytes. Eur. J. Pharmacol., 1997, 320,241-248.

[149] Hill, R.J.; Oleynek, J.J.; Magee, W.; Knight, D.R.; Tracey, W.R. Relative importance of adenosine A1 and A3 receptors in mediating physiological or pharmacological protection from ischemic myocardial injury in the rabbit heart. J. Mol. Cell. Cardiol., 1998, 30, 579-585.

[150] Tomai, F.; Crea, F.; Gaspardone, A.; Versaci, F.; De Paulis, R.; Polisca, P.; Chiariello, L.; Gioffre, P.A. Effects of A1 adenosine receptor blockade by bamiphylline on ischaemic preconditioning during coronary angioplasty. Eur. Heart J., 1996, 17, 846-853.

[151] Mitchell, C.H.; Peterson-Yantorno, K.; Carre, D.A.; McGlinn, A.M.; Coca-Prados, M.; Stone, R.A.; Civan, M.M. A3 adenosine receptors regulate Cl- channels of nonpigmented ciliary epithelial cells. Am. J. Physiol., 1999, 276,C659-666.

[152] Downey, J.M.; Liu, G.S.; Thornton, J.D. Adenosine and the anti-infarct effects of preconditioning. Cardiovasc. Res., 1993, 27, 3-8.

[153] Meijer, P.; Oyen, W.J.; Dekker, D.; van den Broek, P.H.; Wouters, C.W.; Boerman, O.C.; Scheffer, G.J.; Smits, P.; Rongen, G.A. Rosuvastatin increases extracellular adenosine formation in humans in vivo: a new perspective on cardiovascular protection. Arterioscler. Thromb. Vasc. Biol., 2009, 29, 963-968.

[154] Schaefer, S.; Correa, S.D.; Valente, R.J.; Laslett, L.J. Blockade of adenosine receptors with aminophylline limits ischemic preconditioning in human beings. Am. Heart J., 2001, 142, E4-E4.

[155] Claeys, M.J.; Vrints, C.J.; Bosmans, J.M.; Conraads, V.M.; Snoeck, J.P. Aminophylline inhibits adaptation to ischaemia during angioplasty. Role of adenosine in ischaemic preconditioning. Eur. Heart J., 1996, 17, 539-544.

[156] Gaspardone, A.; Crea, F.; Iamele, M.; Tomai, F.; Versaci, F.; Pellegrino, A.; Chiariello, L.; Gioffre, P.A. Bamiphylline improves exercise-induced

3706 Current Medicinal Chemistry, 2011 Vol. 18, No. 24 Szentmiklósi et al.

myocardial ischemia through a novel mechanism of action. Circulation, 1993, 88, 502-508.

[157] Zhao, Z.Q.; Corvera, J.S.; Halkos, M.E.; Kerendi, F.; Wang, N.P.; Guyton, R.A.; Vinten-Johansen, J. Inhibition of myocardial injury by ischemic postconditioning during reperfusion: comparison with ischemic preconditioning. Am. J. Physiol. Heart Circ. Physiol., 2003, 285, H579-588.

[158] Staat, P.; Rioufol, G.; Piot, C.; Cottin, Y.; Cung, T.T.; L'Huillier, I.; Aupetit, J.F.; Bonnefoy, E.; Finet, G.; Andre-Fouet, X.; Ovize, M. Postconditioning the human heart. Circulation, 2005, 112, 2143-2148.

[159] Lemoine, S.; Puddu, P.E.; Durand, C.; Lepage, O.; Babatasi, G.; Ivascau, C.; Massetti, M.; Gerard, J.L.; Hanouz, J.L. Signaling pathways involved in postconditioning-induced cardioprotection of human myocardium, in vitro.Exp. Biol. Med. (Maywood), 2010, 235, 768-776.

[160] Kin, H.; Zatta, A.J.; Lofye, M.T.; Amerson, B.S.; Halkos, M.E.; Kerendi, F.; Zhao, Z.Q.; Guyton, R.A.; Headrick, J.P.; Vinten-Johansen, J. Postconditioning reduces infarct size via adenosine receptor activation by endogenous adenosine. Cardiovasc. Res., 2005, 67, 124-133.

[161] Morrison, R.R.; Tan, X.L.; Ledent, C.; Mustafa, S.J.; Hofmann, P.A. Targeted deletion of A2A adenosine receptors attenuates the protective effects of myocardial postconditioning. Am. J. Physiol. Heart Circ. Physiol., 2007, 293, H2523-2529.

[162] Eckle, T.; Krahn, T.; Grenz, A.; Kohler, D.; Mittelbronn, M.; Ledent, C.; Jacobson, M.A.; Osswald, H.; Thompson, L.F.; Unertl, K.; Eltzschig, H.K. Cardioprotection by ecto-5'-nucleotidase (CD73) and A2B adenosine receptors. Circulation, 2007, 115, 1581-1590.

[163] Headrick, J.P.; Peart, J.N.; Reichelt, M.E.; Haseler, L.J. Adenosine and its receptors in the heart: Regulation, retaliation and adaptation. Biochim. Biophys. Acta, 2010.

[164] Thornton, J.D.; Liu, G.S.; Olsson, R.A.; Downey, J.M. Intravenous pretreatment with A1-selective adenosine analogues protects the heart against infarction. Circulation, 1992, 85, 659-665.

[165] Headrick, J.P. Ischemic preconditioning: bioenergetic and metabolic changes and the role of endogenous adenosine. J. Mol. Cell. Cardiol., 1996, 28, 1227-1240.

[166] Zhao, Z.Q.; Nakanishi, K.; McGee, D.S.; Tan, P.; Vinten-Johansen, J. A1 receptor mediated myocardial infarct size reduction by endogenous adenosine is exerted primarily during ischaemia. Cardiovasc. Res., 1994, 28,270-279.

[167] Louttit, J.B.; Hunt, A.A.; Maxwell, M.P.; Drew, G.M. The time course of cardioprotection induced by GR79236, a selective adenosine A1-receptor agonist, in myocardial ischaemia-reperfusion injury in the pig. J. Cardiovasc. Pharmacol., 1999, 33, 285-291.

[168] Carr, C.S.; Hill, R.J.; Masamune, H.; Kennedy, S.P.; Knight, D.R.; Tracey, W.R.; Yellon, D.M. Evidence for a role for both the adenosine A1 and A3 receptors in protection of isolated human atrial muscle against simulated ischaemia. Cardiovasc. Res., 1997, 36, 52-59.

[169] Auchampach, J.A.; Ge, Z.D.; Wan, T.C.; Moore, J.; Gross, G.J. A3 adenosine receptor agonist IB-MECA reduces myocardial ischemia-reperfusion injury in dogs. Am. J. Physiol. Heart Circ. Physiol., 2003, 285,H607-613.

[170] Maddock, H.L.; Mocanu, M.M.; Yellon, D.M. Adenosine A(3) receptor activation protects the myocardium from reperfusion/reoxygenation injury. Am. J. Physiol. Heart Circ. Physiol., 2002, 283, H1307-1313.

[171] Yang, Z.; Day, Y.J.; Toufektsian, M.C.; Xu, Y.; Ramos, S.I.; Marshall, M.A.; French, B.A.; Linden, J. Myocardial infarct-sparing effect of adenosine A2A receptor activation is due to its action on CD4+ T lymphocytes. Circulation, 2006, 114, 2056-2064.

[172] Rork, T.H.; Wallace, K.L.; Kennedy, D.P.; Marshall, M.A.; Lankford, A.R.; Linden, J. Adenosine A2A receptor activation reduces infarct size in the isolated, perfused mouse heart by inhibiting resident cardiac mast cell degranulation. Am. J. Physiol. Heart Circ. Physiol., 2008, 295, H1825-1833.

[173] Kuno, A.; Critz, S.D.; Cui, L.; Solodushko, V.; Yang, X.M.; Krahn, T.; Albrecht, B.; Philipp, S.; Cohen, M.V.; Downey, J.M. Protein kinase C protects preconditioned rabbit hearts by increasing sensitivity of adenosine A2b-dependent signaling during early reperfusion. J. Mol. Cell. Cardiol., 2007, 43, 262-271.

[174] Headrick, J.P.; Hack, B.; Ashton, K.J. Acute adenosinergic cardioprotection in ischemic-reperfused hearts. Am. J. Physiol. Heart Circ. Physiol., 2003,285, H1797-1818.

[175] Egger, G.; Liang, G.; Aparicio, A.; Jones, P.A. Epigenetics in human disease and prospects for epigenetic therapy. Nature, 2004, 429, 457-463.

[176] Bush, E.W.; McKinsey, T.A. Targeting histone deacetylases for heart failure. Expert Opin. Ther. Targets, 2009, 13, 767-784.

[177] Kramer, O.H. HDAC2: a critical factor in health and disease. Trends Pharmacol. Sci., 2009, 30, 647-655.

[178] McKinsey, T.A. Isoform-selective HDAC inhibitors: Closing in on translational medicine for the heart. J. Mol. Cell. Cardiol., 2010.

[179] Kee, H.J.; Kook, H. Roles and targets of class I and IIa histone deacetylases in cardiac hypertrophy. J. Biomed. Biotechnol., 2011, 2011, 928326.

[180] Zhang, C.L.; McKinsey, T.A.; Chang, S.; Antos, C.L.; Hill, J.A.; Olson, E.N. Class II histone deacetylases act as signal-responsive repressors of cardiac hypertrophy. Cell, 2002, 110, 479-488.

[181] Barnes, P.J. Theophylline in chronic obstructive pulmonary disease: new horizons. Proc. Am. Thorac. Soc., 2005, 2, 334-339; discussion 340-331.

[182] Cosio, B.G.; Tsaprouni, L.; Ito, K.; Jazrawi, E.; Adcock, I.M.; Barnes, P.J. Theophylline restores histone deacetylase activity and steroid responses in COPD macrophages. J. Exp. Med., 2004, 200, 689-695.

[183] Lutz, J.; Thurmel, K.; Heemann, U. Anti-inflammatory treatment strategies for ischemia/reperfusion injury in transplantation. J. Inflamm. (Lond.), 2010,7, 27.

[184] Walsh, M.C.; Bourcier, T.; Takahashi, K.; Shi, L.; Busche, M.N.; Rother, R.P.; Solomon, S.D.; Ezekowitz, R.A.; Stahl, G.L. Mannose-binding lectin is a regulator of inflammation that accompanies myocardial ischemia and reperfusion injury. J. Immunol., 2005, 175, 541-546.

[185] Blancke, F.; Claeys, M.J.; Jorens, P.; Vermeiren, G.; Bosmans, J.; Wuyts, F.L.; Vrints, C.J. Systemic inflammation and reperfusion injury in patients with acute myocardial infarction. Mediators Inflamm., 2005, 2005, 385-389.

[186] Werns, S.W.; Lucchesi, B.R. Inflammation and myocardial infarction. Br. Med. Bull., 1987, 43, 460-471.

[187] Kaplan, S.; Ozisik, K.; Morgan, J.A.; Dogan, R. Measurement of Troponin T and I to detect cardioprotective effect of aminophylline during coronary artery bypass grafting. Interact. Cardiovasc. Thorac. Surg., 2003, 2, 310-315.

[188] Luo, W.J.; Qian, J.F.; Jiang, H.H. Pretreatment with aminophylline reduces release of Troponin I and neutrophil activation in the myocardium of patients undergoing cardioplegic arrest. Eur. J. Cardiothorac. Surg., 2007, 31, 360-365.

[189] Fredholm, B.B.; Jacobson, K.A.; Jonzon, B.; Kirk, K.L.; Li, Y.O.; Daly, J.W. Evidence that a novel 8-phenyl-substituted xanthine derivative is a cardioselective adenosine receptor antagonist in vivo. J. Cardiovasc. Pharmacol., 1987, 9, 396-400.

[190] Conlay, L.A.; Conant, J.A.; deBros, F.; Wurtman, R. Caffeine alters plasma adenosine levels. Nature, 1997, 389, 136.

[191] Riksen, N.P.; Rongen, G.A.; Smits, P. Acute and long-term cardiovascular effects of coffee: implications for coronary heart disease. Pharmacol. Ther., 2009, 121, 185-191.

[192] El-Sohemy, A.; Cornelis, M.C.; Kabagambe, E.K.; Campos, H. Coffee, CYP1A2 genotype and risk of myocardial infarction. Genes Nutr., 2007, 2,155-156.

[193] Moro, S.; Gao, Z.G.; Jacobson, K.A.; Spalluto, G. Progress in the pursuit of therapeutic adenosine receptor antagonists. Med. Res. Rev., 2006, 26, 131-159.

[194] Baraldi, P.G.; Tabrizi, M.A.; Gessi, S.; Borea, P.A. Adenosine receptor antagonists: translating medicinal chemistry and pharmacology into clinical utility. Chem. Rev., 2008, 108, 238-263.

[195] Jacobson, K.A.; Gao, Z.G. Adenosine receptors as therapeutic targets. Nat. Rev. Drug Discov., 2006, 5, 247-264.

[196] Kalla, R.V.; Zablocki, J.; Tabrizi, M.A.; Baraldi, P.G. Recent developments in A2B adenosine receptor ligands. Handb. Exp. Pharmacol., 2009, 99-122.

[197] Kalla, R.V.; Zablocki, J. Progress in the discovery of selective, high affinity A(2B) adenosine receptor antagonists as clinical candidates. Purinergic Signal., 2009, 5, 21-29.

[198] Yang, M.; Soohoo, D.; Soelaiman, S.; Kalla, R.; Zablocki, J.; Chu, N.; Leung, K.; Yao, L.; Diamond, I.; Belardinelli, L.; Shryock, J.C. Characterization of the potency, selectivity, and pharmacokinetic profile for six adenosine A2A receptor antagonists. Naunyn Schmiedebergs Arch. Pharmacol., 2007, 375, 133-144.

Received: April 13, 2011 Revised: July 08, 2011 Accepted: July 10, 2011