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Pharmacological Research 63 (2011) 157–164

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Pharmacological Research

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Perspective

Pharmacological use of l-carnitine in uremic anemia: Has its full potential beenexploited?�

Mario Bonominia,∗, Victor Zammitb, Charles D. Puseyc, Amedeo De Vecchid, Arduino Arduinie,∗

a Institute of Nephrology, Department of Medicine, G. d’Annunzio University, Chieti, Italyb Clinical Sciences Research Institute, Warwick Medical School, CV7, 4AL, UKc Department of Renal Medicine, Imperial College London, Hammersmith Hospital, London, UKd UO Nephrology and Dialysis, Policlinico Hospital, Milano, Italye Department of Research and Development, CoreQuest, c/o Tecnopolo, Via della Posta 10, 6934 Bioggio, Switzerland

a r t i c l e i n f o

Article history:Received 25 October 2010Accepted 17 November 2010

Keywords:AnemiaDrug metabolismErythropoietinKidney failureHemodialysisl-Carnitine

a b s t r a c t

Anemia is one of the most frequent complications of kidney failure and a common cause of morbidity indialysis patients. A number of clinical studies have suggested that l-carnitine (LC), a naturally occurringcompound involved in bioenergetic processes, may alleviate the anemia of hemodialysis patients. SinceLC deficiency is commonly present in dialysis patients, LC has been described as a “conditional vitamin”.However, the use of LC supplementation in dialysis remains controversial as well as its mode of actionin preventing anemia. Recent literature shows that to fully exploit its pharmacological potential, LC mayhave to be administered at doses that achieve supra-physiological levels of LC in plasma and targetorgans. However, this concept has not been fully investigated in uremic patients. In this article, we willreview the use of LC in dialysis patients, and provide a rationale for the anti-anemic action of LC basedon biophysical, metabolic and antiapoptotic effects of this compound on erythropoiesis and the functionof mature, circulating erythrocytes. The discussion will be focused on experimental and clinical data thatsupport the concept that supra-physiological concentrations of LC may improve the anemic condition ofdialysis patients, as would be expected from a “conditional drug” rather than a “conditional vitamin”.

© 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Anemia is a common feature in patients suffering from chronicrenal failure, and represents one of the leading causes of increasedcardiovascular morbidity and mortality in these patients. It usu-ally becomes manifest when the creatinine clearance drops toapproximately 40 ml/min/1.73 m2 of body surface area, thereafterworsening with progressive deterioration in renal function [1].Although many factors may contribute, absolute or relative ery-thropoietin (EPO) deficiency is the predominant defect responsiblefor the anemia of chronic renal failure [2].

The introduction of EPO analogues, also termed erythropoiesis-stimulating agents (ESAs), such as epoetin-alpha and -beta,darbepoetin-alpha, and methoxy polyethylene glycol-epoetin beta,has revolutionized the care of patients suffering from renal anemia.The use of ESAs has almost completely eradicated the severe ane-

� Perspective articles contain the personal views of the authors who, as experts,reflect on the direction of future research in their field.

∗ Corresponding authors. Tel.: +41 79 7878312; fax: +41 91 2102211.E-mail addresses: m.bonomini@nephro.unich.it (M. Bonomini),

a.arduini@corequest.ch (A. Arduini).

mia of end-stage renal disease (ESRD) [3], mediating an increasein hemoglobin (Hb) levels without the risk of recurrent bloodtransfusions and iron overload. Administration of ESAs is generallyeffective and well tolerated, and is associated with several clinicalbenefits [4,5]. However, despite an increase in the use and aver-age dose of ESAs administered, and the attainment of a Hb levelabove the recommended target of 11 g/dl during erythropoietictherapy [6,7], it is evident from clinical studies that a significantproportion of uremic patients still fail to reach target Hb levels[8,9]. It is unknown how many of these patients are truly ESA resis-tant and how many are inadequately treated. Two recent papersraise the possibility that high doses of ESAs, particularly in patientsunable to reach target Hb levels, may be at least partly linked toincreased mortality [10,11]. A more recent randomized, double-blind, placebo-controlled, clinical trial (TREAT: Trial to ReduceCardiovascular Events with Aranesp Therapy) failed to show thatanemia-reversing therapy with darbepoetin may reduce the riskof cardiovascular morbidity and mortality in anemic patients withdiabetes and chronic kidney disease not undergoing dialysis [12].Thus, questions remain unanswered regarding the optimal thera-peutic use of ESAs in chronic kidney disease [13–15].

Relatively little attention has been paid to the fact that reducederythropoiesis in chronic renal failure is accompanied by a short-

1043-6618/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.phrs.2010.11.006

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ened red blood cell (RBC) half life. It is important to appreciate thaterythrocyte lifespan plays an important role in the erythropoieticresponse to recombinant human erythropoietin [16]. The uremicenvironment, combined with the impaired quality of RBCs, reducethe survival of mature erythrocytes by 30–70% of the expected nor-mal half life of the cells. Although measurement of the Hb contentin the circulation helps to evaluate the anemic status of patients,it does not provide adequate information on erythrocyte functionor dynamic changes (RBC turnover) occurring in anemic patientstreated with ESAs. Indeed, whilst ESA treatment increases RBC pro-duction (rate of RBC appearance in the blood stream), it may notaffect RBC half life or the rate of RBC disappearance from the bloodstream significantly. For example, it has been demonstrated thaterythrocyte life span of current ESA-treated dialysis patients is notdifferent from that reported some fifty years ago before the avail-ability of ESA [17].

An issue seldom addressed in the clinical debate around thetherapeutic action of ESAs is the fact that, although these agents arecapable of increasing the number of circulating red cells and henceHb levels, the quality of red cells reaching the blood stream maynot be significantly affected by ESA treatment. The quality of circu-lating red cells may best be represented by their ability to ensurecapillary perfusion, and efficient oxygen uploading in the lungsand offloading in peripheral tissues. Critical components of these“quality” attributes are the rheological properties of circulating ery-throcytes. It is also well known that RBC disappearance from theblood stream is strongly dependent on their visco-elastic properties(see also below). The disturbance of the visco-elastic properties ofRBCs would not only affect their mechanical ability to pass throughsmall capillaries, but also be expected to alter their ability to releaseATP and, hence, NO-dependent calibre vasodilation, in response toreduced oxygen tension [18]. Therefore, it is possible that whereasHb levels are elevated by ESA treatment, tissue oxygenation maybe still impaired due to the altered visco-elastic properties of RBCsin uremic patients.

2. l-Carnitine and red blood cell physiopathology

LC is a naturally occurring compound and its involvement inintermediary metabolism is essential to mammalian bioenergeticprocesses [19]. Indeed, a major role of LC is the transfer of activatedlong-chain fatty acids across the mitochondrial membranes, a con-ditio sine qua non for the mitochondrial �-oxidation of long-chainfatty acids [20,21]. Administration of LC is increasingly practised inESRD, since dialytic losses and reduced renal synthesis are known toresult in LC deficiency in these patients [22]. Clinical studies suggestthat LC treatment alleviates the anemic condition of hemodialysispatients. This intriguing pharmacological action of LC, which is dif-ficult to reconcile with the role played by this compound in cellularmetabolism, has been the subject of intensive debate [23,24].

In the following discussion, we will present evidence thatsupra-physiological concentrations of LC may favourably affect cer-tain physiopathological attributes of circulating RBCs. In turn themetabolic effects of such high concentrations of LC may help tothe observed anti-anemic action of LC in several clinical studieson dialysis patients (see below). In the interests of simplifying thediscussion, we classify LC actions into biophysical, metabolic andantiapoptotic.

3. Biophysical actions of l-carnitine

Many membrane functions such as signal transduction, modula-tion of enzymatic and transport activities, and passive modulationof antigens are affected by the bulk biophysical properties of themembrane bilayer and its cytoskeleton network [25]. In addition,

in certain cells, such as erythrocytes, the biophysical propertiesof the membrane and its cytoskeletal network are fundamentalfor their survival in the blood stream. Indeed, erythrocytes mustsurvive a variety of chemical and physical insults during their lifes-pan. The physical challenges to erythrocytes can be illustrated bytheir need to deform from their resting diameter of about 8 �mto diameters as small as 1.5 �m as they pass through the capil-lary bed and splenic sinusoids. The visco-elastic properties of RBCsare remarkable; anecdotally, their elastic shear modulus (�), whichfor normal RBCs is equal to 0.6 × 10−2 dyn/cm, may be favourablecompared with that of softest latex rubber [26]. The loss of theseelastic properties of circulating RBCs may in turn compromise tis-sue oxygenation. The main determinants of the physico-elasticproperties of RBCs are represented by cell surface area or geom-etry [27], membrane cytoskeleton properties [26], and to a lesserdegree cytoplasmic viscosity [28].

Erythrocyte deformability in a flow field depends upon a num-ber of interrelated cellular properties. In certain disease states,such as uremia, circulating erythrocytes are submitted to addi-tional insults leading to significant shortening of their survivalin the blood stream. The potential chemical challenges to uremicRBCs include, oxidative stress, ATP depletion, and impairment ofion homeostasis [29–32]. Several studies indicate that uremic RBCsshow a marked impairment of deformability [33–36]. It is not clearfrom these reports which specific defect[s] leads to an alteration ofthe rheological properties of such erythrocytes, but it is likely thatthe elements mainly affected by chronic uremia are the membranecytoskeleton and cell surface area.

Red blood cells play a pivotal role in matching microvascularoxygen supply with local tissue oxygen demand via an oxygen-dependent release of the vasodilator ATP [18]. Alteration of thevisco-elastic properties of red blood cells has been shown to alterthis pathway, which might further contribute to oxygen perfu-sion abnormalities [37]. In addition, it has been suggested thatboth spectrin–actin cytoskeleton network and membrane viscositymainly contribute in triggering the mechanosensitive ATP releaseand controlling the rate of its release, respectively [38].

A number of reports show that LC may favourably affect theimpaired rheological properties of erythrocytes in HD patients[39–42]. For example, a study on RBC deformability conductedin 15 HD patients has shown that 3 months of intravenous LCsupplementation (30 mg/kg body wt/dialysis session) significantlyalleviated the abnormalities in RBC deformability, and in turn trans-lated into a significant increase in hematocrit [39]. It has also beenshown [42] that the reduced RBC osmotic resistance observed inHD patients may be normalised by chronic LC treatment (20 mg/kgbody wt/dialysis session).

As discussed below, it is routinely assumed that LC administra-tion in HD patients results in a higher plasma LC exposure and/orRBC LC content than in untreated patients. Although the majority ofclinical trials of LC in HD patients do not allow any firm conclusionsto be reached on the relationship between plasma LC levels andits efficacy in anemia, a dose-dependent effect of LC on the anemicstate of HD patients has been demonstrated [43]. Moreover, a recentstudy [44] in 87 chronic HD patients supports the critical impor-tance of LC plasma levels in the response to ESA, as examined bythe erythropoietin resistance index. All patients classified as ESA-resistant (erythropoietin resistance index > 0.02 �g/kg/week/g Hb)displayed lower LC and total LC levels than their low erythropoietinresistance index counterparts [44].

If uremic RBCs are exposed to supra-physiological levels of LC,what is the evidence for a beneficial effect of LC on RBC phys-iopathology? Studies in vitro showed that the addition of 2 mM LCto human RBC was able to reverse the membrane fluidity changesinduced by a membrane perturbing agent, palmitoyl-carnitine, thussuggesting a membrane stabilizing effect of LC [45]. LC addition

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Fig. 1. Potential metabolic and biophysical interventions of LC on circulating red blood cells. Membrane phospholipid repair (left side): When oxidative challenge of a polyun-saturated fatty acid esterified in membrane phospholipid (PLP) generates a phospholipid hydroperoxide (PLP-OOH), the deacylating activity of a phospholipase A2 (PLA)hydrolizes the fatty acid hydroperoxide (FA-OOH), which in turn may be further reduced to the corresponding non-reactive fatty acid alcohol (FA-OH) by a glutathione perox-idase (GSH-Px). This is followed by a parallel increase of the lysopholipid acyl-CoA transferase (LAT), which would regenerate the original PLP and limit the accumulation ofthe potentially toxic lysophospholipid (LPL). The increased LAT activity requires an increased supply of activated acyl units (acyl-CoA) from the activity of the ATP-dependentenzyme acyl-CoA synthetase (ACS). The combined activities of LAT and ACS generate a cycle, whose efficiency is strictly dependent on the extent of the enzymatic rates ofLAT and ACS. Thus, the presence of a rate-coupler, carnitine palmitoyltransferase (CPT), would allow the ACS-LAT cycle to keep the acyl-CoA/free CoA ratio constant. Indeed,given the kinetic properties of CPT, any alteration of the acyl-CoA/free CoA ratio would be promptly buffered by the fully reversible and mass action sensitive CPT reaction.The presence of supraphysiological concentration of carnitine (Cn) would support the buffering action of CPT. In addition, acyl-carnitine (acyl-Cn) represents a reservoir ofacyl-CoA units at no ATP cost. Membrane visco-elastic properties (right side): Major determinants of the visco-elastic properties of red blood cell membrane are the cytoskeletalnetwork and membrane phospholipid bilayer. The cytoskeletal network is an organization of supramolecular proteins lying beneath the inner monolayer of erythrocytemembrane bilayer. As discussed in the text, LC may improve the visco-elastic properties of red blood cells by exerting a stabilizing effect on the membrane via a specificinteraction with certain cytoskeletal components [spectrin/actin as indicated in the figure by the red circle]. LC would also affect the visco-elastic properties of red blood cellmembrane by strengthening the polar head packing of membrane phospholipids (see also the text).

also improved morphological changes induced this detergent-likecompound. In a series of electronic paramagnetic resonance andfluorescence studies, it has been shown that LC in the low mil-limolar range decreases the molecular packing of the polar headof erythrocyte membrane phospholipids [46]. This would indicatethat the positively charged quaternary ammonia group and the neg-ative carboxylic group present at the two ends of the LC carbonskeleton allow it to intercalate between adjacent phosphatidyl-choline polar heads. Since LC does not easily cross the membranebilayer, these biophysical findings can be ascribed to an interactionof LC with the outer surface of the RBC membrane, where it maycounteract the noxious action of detergent-like compounds and/orfavourably modulate discrete membrane phospholipid domainsenriched in phosphatidylcholine. LC in the high micromolar rangemay also favourably affect RBC membrane stability [47], possiblyvia specific interactions with cytoskeletal membrane proteins suchas spectrin and actin [48]. It should be noted that the rheologi-cal properties of the RBC are also dependent on the strength ofthe interactions of its cytoskeletal network [49,50]. Collectively,these findings support the notion that supra-physiological concen-trations of LC may improve the visco-elastic properties of RBCs byintervening on both the outer and the inner side of erythrocytemembranes (see also Fig. 1).

4. Metabolic actions of l-carnitine

More direct experimental evidence of the beneficial effect of LCon RBC survival derives from a RBC conservation study [51]. In apaired cross-over study, non-leukoreduced RBCs stored with 5 mMLC showed less hemolysis, higher RBC ATP content, improved invivo recovery and improved in vivo survival as compared to con-trol conditions. In this study, it was also shown that membranephospholipid fatty acid turnover, a key process in the repair ofoxidatively damaged phospholipids, was favourably affected byexpansion of the intra-erythrocyte LC pool [51]. Thus, in addi-tion to the biophysical effects discussed above, which may in partaccount for the improved quality of stored RBCs, LC is able to exerta favourable metabolic action in a cellular environment deprivedof any sub-cellular organelles. LC is known for its role in facilitat-ing the transport of long chain fatty acids across the mitochondrialmembrane. However, LC also plays a pivotal role in membranephospholipid fatty acid turnover [52,53], a metabolic pathwayinvolved in the repair process of oxidatively injured membranephospholipids [53]. The RBCs of uremic patients as well as RBCsstored in plastic containers are considered to undergo oxidativedamage (see above). Since oxidized phospholipids may severelyimpair membrane properties, remodelling them would be expected

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to improve membrane integrity and potentially reduce hemolysis(see also Fig. 1). Thus, despite the absence of mitochondria in mam-malian RBCs, evidence for a role of LC in red cell metabolism issuggested by the presence of LC and carnitine palmitoyltransferasein the red cell, and their involvement in membrane phospholipidfatty acid turnover. Evidence that this may be important in RBC ofHD patients was provided by de los Reyes et al. [54]. These authorsshowed that in uremic RBC an alteration of the long-chain acyl-CoA/free CoA ratio was associated with significant reduction of keyenzymes involved in membrane phospholipid fatty acid turnover.LC treatment partially reversed these metabolic impairments.

5. Antiapoptotic actions of l-carnitine

Several in vitro investigations suggest that LC is able to inhibitcell death pathways, some of which also present in circulatingRBC. This occurs by either preventing sphingomyelin breakdown,and consequent synthesis of ceramide [55], or inhibiting cas-pase 3 [56]. Ceramide and activation of caspase 3 may inducethe exposure of phosphatidylserine at the extracellular face ofthe erythrocyte membrane [57,58], a well known signal for theelimination of RBCs by macrophages in vivo [59]. Because thepercentage of phosphatidylserine-expressing erythrocytes is sig-nificantly increased in uremic patients [60], such an abnormalitymight partly explain the reduced lifespan of uremic RBC [61]. LCmight alleviate such an abnormality and hence improve RBC sur-

vival in uremia. In this regard, a recent randomized controlled trialon the effects of LC on erythrocyte survival in HD patients, evalu-ated by the 51Cr labelling procedure, a gold standard methodologyin RBC survival studies, showed a strong trend toward improvedRBC survival in the LC treated group (p = 0.058) [62].

The potential antiapoptotic action of LC seems to be exertedeven at the level of erythropoiesis. Recently, Matsumura et al.demonstrated that the addition of LC at high concentration(>200 �M) to erythroid colonies in cultures of cells from fetalmouse liver, a major location of erythropoiesis during the embry-onic period, resulted in a significant increase in such colonies [63].In an attempt to gain better insight into the potential action of LCon erythropoiesis, Kitamura et al. investigated the erythropoieticeffect of LC using cells from mouse bone marrow, where erythro-poiesis takes place only after birth [64]. The authors showed that,in the presence of 0.5 or 1.0 IU/ml of recombinant human EPO, LC atconcentrations higher than 200 �M significantly enhanced CFU-Ecolony formation in mouse bone marrow cell cultures.

Programmed cell death is known to occur during the differ-entiation of progenitor cells. Erythropoietin contributes to RBCmaturation by retarding apoptosis, thus allowing erythroid progen-itors to complete their differentiation programme. Indeed, a majornegative regulation of erythropoiesis is the caspase-mediatedcleavage of GATA-1 or other erythropoietic factors [65]. It has beenreported that LC at millimolar levels inhibits the activation of cas-pase at various point in the Fas ligation pathway in Jurkat cells

Fig. 2. Potential anti-apoptotic intervention of LC. Schematic representation of negative regulation of erythropoiesis triggered by death receptor activation or by erythropoietindeprivation/resistance (left side). Both these conditions induce the apoptotic cascade, with the consequent activation of caspase 3 that cleaves the transcription factors GATA-1. The cleavage of GATA-1 is responsible for either the maturation arrest or apoptosis of erythroid cells. By inhibiting caspase 3, LC would favourably affect the maturation oferythroid cells. Other abbreviations: Epo, erythropoietin; FADD, Fas-associated death domain; Bcl, antiapoptotic protein; JAK, Janus kinase; CFU, colony forming unit. Ceramide-and caspase 3-dependent cell death pathways are also operative in circulating red blood cells and are known as erythroapoptosis (right side). Indeed, the activation of suchcell death pathways leads to phosphatidylserine (PS) exposure at the extracellular face of the erythrocyte membrane, a well known signal for the elimination of RBCsby macrophages in vivo. LC may counteract PS exposure by either preventing ceramide production or inhibiting caspase 3. Other abbreviations: SCR, scramblase; AMT,aminophospholipid translocase; SM, sphyngomyelinase; PLA, phospholipase A2; PAF, platelet activating factor.

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Table 1Summary of studies evaluating the effects of l-carnitine treatment on RBC indices (Hb, Hct, ESA dose) in adult chronic hemodialysis patients.

First author, year Ref. # Study design Patients (n) l-Carnitine route ofadministrationa

l-Carnitinedose

Duration oftreatment

Effects of l-carnitine

Control l-Carnitine

Albertazzi, 1982b [78] Open-label, no control group 23 0 PO (n = 12)D (n = 11)

PO = 1 gD = 100 �mol/l

6 months Increased Hct

Trovato, 1982b [79] Randomized,placebo-controlled

13 13c PO 1.65 g 52 weeks Increased Hct

Vacha, 1983b [80] Single-blind,placebo-controlled (4 monthsplacebo treatment followingl-carnitine treatment)

29 0 IV 20 mg/kg 4 months Increased Hct

Bellinghieri, 1983b [81] Randomized, double-blind,placebo-controlled, cross-over

7 7 PO 2 g 2 months Increased Hct

Nilsson-Ehle, 1985b [82] Randomized, double-blind,placebo-controlled

14 14 IV 2 g 6 weeks No change in Hb

Donatelli, 1987b [83] Open-label, no control group 10 0 PO 2 g 2 months Increased Hct and HbLabonia, 1995 [84] Randomized, double-blind,

placebo-controlled13 11 IV 1 g 6 months Decreased EPO dose

Boran, 1996 [85] Open-label, no control group 14 0 IV 40–60 mg/kg(weekly)

9 months Increased Hct

Kavadias, 1996 [86] Open-label, no control group 8 0 IV 2 g 5 months Increased Hct atreduced EPO dose

Trovato, 1998 [87] Open-label, not randomized 25 35 V and PO 1 g (daily) 3 years Decreased EPO doseCaruso, 1998 [88] Randomized, double-blind,

placebo-controlled15d 16 IV 1 g 6 months No change in Hct or

EPO dosee

Kletzmayr, 1999 [43] Randomized, double-blind,placebo-controlled

20f 20 IV 5 or 25 mg/kg 8 months No change in EPOdoseg

Nicolaos, 2000 [39] Open-label, no control group 15 0 IV 30 mg/kg 3 months Increased HctMatsumoto, 2001 [89] Open-label, no control group 14 0 PO 500 mg 3 months Increased HctVeselà, 2001 [90] Open-label, not randomized 12 12 IV 15 mg/kg 6 months Decreased EPO doseVaux, 2004 [91] Randomized, double-blind,

placebo-controlled13 13 IV 20 mg/kg 16 weeks No change in Hb or

EPO doseKadiroglu, 2005 [92] Open-label, not randomized 17 17 IV 1 g 16 weeks Decreased EPO doseMitwalli, 2005 [93] Randomized, single-blind,

placebo-controlled18 18h IV 15 mg/kg 6 months Increased Hb and Hct

Savica, 2005 [72] Randomized,placebo-controlled

48 65 IV 20 mg/kg 6 months Increased Hb

Abbreviations: RBC, red blood cell; Hb, hemoglobin; Hct, hematocrit; ESA, erythropoiesis-stimulating agent; Ref, reference; n, number.a IV, intravenous after hemodialysis session unless otherwise specified; PO, per os/day; D, in the dialysate.b Before introduction of rHuEPO.c Only 11 patients evaluated as 2 were excluded.d Only 12 patients evaluated due to 3 dropouts.e Among the 12 evaluable treated patients, EPO dose was reduced in 7.f Only 19 patients evaluated due to 1 dropout.g Among the 19 evaluable treated patients, EPO dose was reduced in 8 (mean decrease 37%).h Only 13 patients evaluated due to 5 dropouts. In bold characters studies showing no effect of LC supplementation.

[56] and caspase activity in platelets from ESRD patients [66]. Inaddition, LC may induce haem-oxygenase-1 [67], a known anti-apoptotic molecule [68]. In summary, these reports suggest that LCmay influence erythropoiesis by inhibiting the apoptosis of progen-itor cells (see also Fig. 2).

Prevention of apoptosis might also be brought about bythe anti-inflammatory activity of LC [69]. Uremia is a state ofchronic microinflammation characterized by an increase in pro-inflammatory cytokines which can directly inhibit erythropoiesisand promote apoptosis of erythroid precursors [70,71]. Severalreports have provided evidence that LC infusion in HD patientsmay improve inflammatory indices [72–74]. LC supplementation-associated improvement in cellular defence against chronicinflammation in HD patients most likely occurs by modulation ofthe specific signal transduction cascade activated by overproduc-tion of pro-inflammatory cytokines and oxidative stress [74].

6. l-Carnitine in dialysis

Dialysis patients often suffer from altered hormonal and bio-chemical homeostasis secondary to loss of kidney function. Thus,anemia from impaired erythropoietin production, osteodystrophyfrom reduced vitamin D metabolism, and neuropathy from altered

folic acid metabolism represent some of the main abnormalitiesresulting from functional deficiencies. In all instances, their cor-rection requires the use of pharmacologic doses of the associatedhormones or nutrients.

An additional functional deficiency magnified by the dialytictreatment is the disturbance of LC homeostasis. Indeed, carnitinesupplementation has been extensively used in dialysis and one ofits most convincing therapeutic intervention seems to be directedtoward an improvement of anemia [75]. Moreover, based on theevidence available, and on the excellent safety profile associatedwith the compound, the need has been highlighted for clinicaltrials on the use of LC in uremic anemic patients with very highESA requirements and with negative investigations for reversiblecauses of anemia [76,77]. The results of such trials would enablethe reduction of ESA dosages currently used to improve Hb levels.

A summary of published clinical trials evaluating the effects ofLC treatment on RBC indices in adult chronic hemodialysis (HD)patients is reported in Table 1. Most studies, particularly in patientsnot treated with ESA, indicate a favourable effect on uremic ane-mia following LC administration. However, caution is needed ininterpreting the available results, due to relatively small samplesizes, analysis of “on treatment” rather than “intention to treat” out-comes, inadequate documentation or control of iron metabolism,

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Fig. 3. Multiple modes of action through which LC may affect the number and quality of red blood cells in the circulation. Inhibition of apoptosis by LC (A) is expected toincrease red blood cells number. On the other hand, by promoting phospholipid remodelling (B) and improving viscoelastic properties (C), LC is expected to affect red bloodcells quality, and hence, increasing their life span.

and differences between studies in LC administration method, dose,and duration of therapy. Caution is also warranted because pub-lished studies may exclude clinical trials that were not completedbecause of negative findings. Moreover, in two studies, LC admin-istration did not show any favourable action on anemia [82,91],whereas in two other studies LC administration seemed to be effi-cacious in improving the anemic condition only in a sub-groupof HD patients [43,88]. These sub-groups of responders were ret-rospectively identified according either to the higher increase ofLC plasma levels during active treatment [88] or higher baselineplasma and RBC LC levels [43]. As recently suggested by Reuteret al. [44], this apparent dichotomy of patients between respondersand non-responders may depend on the proportion of non-acetylacylcarnitines within the total carnitine pool. Thus, controversy andmisconceptions about the use of LC in hemodialysis patients persist[76,94,95].

The main assumption in using LC for the treatment of HDpatients relates to potential LC deficiency occurring due to thehemodialysis procedure. As a result of this observation, and studiesof other causes of primary and secondary LC deficiencies, this com-pound has been described as a “conditionally essential nutrient”or “conditional vitamin” [96,97]. The concept of a conditional vita-min requires the correction of LC deficiency through LC-treatment,assuming that the deficiency per se has a physiopathological cor-relate. This is certainly the case for some rare genetic disordersaffecting LC metabolism and function, such as those involvingdefects in the high-affinity plasmalemmal LC transporter [98].Indeed, LC treatment of these patients represents a life saving ther-apy [99]. More debatable seems to be the therapeutic potentialin correcting moderate LC deficiency such as that present in HDpatients. It should also be taken into account that reference inter-vals for plasma LC levels are generally calculated in large healthynon-uremic populations; therefore attempts to restore LC levels inpatients with ESRD to within the normal reference interval may bemisguided. We cannot exclude the possibility that such ‘normal’concentrations are not adequate for the particular metabolic needsof ESRD patients. For example, in uremic patients, ‘normal’ folateor B12 levels are often accompanied by high homocysteine lev-els. In these cases additional folate or B12 supplementation mayachieve a significant lowering of homocysteine, thus suggestingthe need for concentrations above the normal reference interval of

these vitamins [100]. Similarly, in dialysis patients, several investi-gators consider ‘normal’ parathyroid hormone levels as inadequateand it is routinely suggested that levels of up to 300 ng/ml shouldbe maintained to ensure adequate bone turnover.

However, since the most common LC dosage used to treat HDpatients [10–20 mg/kg administered intravenously after each dial-ysis session] results in free plasma LC levels much higher than thosefound in healthy individuals, it is not clear whether any beneficialeffect observed should be assigned to correction of the LC deficiencyor to attainment of supra-physiological plasma levels of LC. In thisregard, experimental and clinical data indicate that LC at concen-trations (low millimolar range) higher than those physiologicallypresent in the extra- and intra-cellular milieu (low micromolar tolow millimolar range) exerts in vitro and in vivo favourable pharma-cological actions [101]. In addition, very high plasma LC exposure inHD patients treated with LC for a prolonged period of time has beensafely achieved [102]. Moreover, as discussed in the previous sec-tions, supraphysiological levels of LC are needed to elicit favourableresponses in uremic red blood cells.

Thus, the use of LC in dialysis may be envisioned as a “conditionaldrug”; the concept a conditional drug requires the achievement ofsupra-physiological LC concentrations in plasma and target organin order to trigger a beneficial effect.

7. Conclusions

The therapeutic role of LC in the anemia of ESRD is still debated.A large body of recent literature shows that to exploit fully itspharmacological potential, the achievement of supra-physiologicallevels of LC in plasma and target organs may be necessary [101].Based on the experimental evidence discussed above, and consid-ering the excellent safety profile of LC even at high plasma exposure[102], a high LC concentration may have favourable effects on ure-mic anemia (Fig. 3). However, the concept of supra-physiological LCtherapy, i.e. using LC as a conditional drug, has not been fully inves-tigated in the uremic population. Consequently, the appropriateconcentration/exposure that needs to be achieved to fully exploitthe beneficial actions of LC in the overall population or in a par-ticular sub-group of HD patients remains to be determined. Giventhe relative ease with which supra-physiological plasma levels ofLC can be achieved in HD patients, owing to the absence of renal

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clearance, future clinical trials should address this issue thus help-ing to resolve the debate about the role of LC treatment in dialysispatients.

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