Advances in Pediatrics 55 (2008) 99–121

ADVANCES IN PEDIATRICS

Cystic Fibrosis: a Review of Pulmonaryand Nutritional Therapies

Reshma Amin, MD, Felix Ratjen, MD, PhD*Division of Respiratory Medicine, Department of Pediatrics, The Hospital for Sick Children,University of Toronto, 555 University Avenue, Toronto, ON, M5G 1X8, Canada

Cystic fibrosis (CF) is an autosomal recessive disease caused by a defectin the cystic fibrosis transmembrane conductance regulator (CFTR)gene. CFTR is expressed in the epithelium of several organs in the

body including the lungs, pancreas, gastrointestinal tract, reproductive tract,and skin, as well as the nasal mucosa. The lack of functional CFTR protein re-sults in disease manifestations in organs where CFTR has relevant physiologicfindings but respiratory disease is responsible for the shortened life span inmost patients. CF affects many organ systems and a thorough review of all as-pects of CF is beyond the scope of this review. Therefore, given the significantinterplay between pulmonary disease and nutritional status, this review willfocus on the pulmonary and nutritional aspects of CF.

CYSTIC FIBROSIS GENETICSThe CF gene was identified in 1989 and to date more than 1500 mutationshave been described [1]. The CFTR mutations can be grouped into six classes[2]. Class 1 mutations affect the transcription of CFTR by formation of prema-ture stop codons into the messenger RNA. In most cases the introduction ofa premature stop codon results in a truncated protein that will undergo degra-dation through nonsense-mediated decay, resulting in total absence of CFTRprotein. Class 2 mutations involve intracellular processing of the CFTR pro-tein. CFTR protein is produced but is misfolded, making it unstable; this leadsto degradation by the intracellular quality control machinery. An example ofa Class 2 mutation is delta F508, the most prevalent CFTR mutation through-out the world. Class 3 mutations alter the regulation of CFTR. CFTR proteinis successfully made and incorporated into the cell membrane surface. How-ever, activation of the channel is perturbed, reducing its opening probability.Class 4 mutations affect the chloride conductance of CFTR. The CFTR pro-tein channel can be activated to open and secrete chloride but the conductanceis reduced. Class 5 mutations result in the production of CFTR protein with

*Corresponding author. E-mail address: felix.ratjen@sickkids.ca (F. Ratjen).

0065-3101/08/$ – see front matterª 2008 Elsevier Inc. All rights reserved.doi:10.1016/j.yapd.2008.07.015

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normal chloride secretion but the number of CFTR channels is decreased.Finally, Class 6 mutations are the result of accelerated CFTR turnover fromthe cell surface.

The correlation between CFTR genotype and phenotype seems to corre-spond best with the pancreatic disease manifestations of CF. Class 1 and 2 mu-tations all result in the absence or severe reduction in functional CFTR and thiscorresponds with pancreatic insufficiency (seen in 85% of CF patients). Patientswith Class 3, 4, 5, and 6 mutations often have some residual CFTR functionthat corresponds to milder disease. These patients are usually pancreatic suffi-cient (seen in 15% of CF patients). However, the same principal does not applyto CF lung disease since the severity is highly variable even in patients carryingthe same CFTR mutation.

PATHOPHYSIOLOGY OF CYSTIC FIBROSISPULMONARY DISEASEThe pulmonary manifestations are the main cause of morbidity and mortalityin CF patients. CFTR is located on the apical surface of airway epithelial cellsand serosal cells of the submucosal glands. A lack of CFTR activity leads todecreased chloride secretion as well as sodium hyperabsorption since one ofthe physiologic functions of CFTR is to inhibit the epithelial sodium channel[3]. This results in a decreased airway surface liquid volume, which leads tocollapse of respiratory cilia, impaired mucociliary clearance, and mucus reten-tion on the lower airways. Inhaled microorganisms cannot be efficiently clearedfrom the CF airway, which predisposes CF patients to chronic bacterial infec-tion and recurrent pulmonary exacerbations.

CF airway infection is limited to a relatively small spectrum of bacteria. Ininfants and younger children, Staphylococcus aureus and Haemophilus influenzae pre-dominate, but a significant proportion already are infected with Pseudomonasaeruginosa, the most common CF pathogen. The prevalence of P aeruginosaincreases with age and to date most adult patients will be chronically infectedwith this organism. First infection occurs with a nonmucoid type of P aeruginosa.If left untreated, P aeruginosa converts to a mucoid form that produces alginate,making it less amenable to antibiotic therapy. Chronic P aeruginosa infection isassociated with a reduction in lung function and a poor prognosis [4]. Otherbacteria such as Burkholderia cepacia, Stenotrophomonas maltophilia, and Achromobacterxylosoxidans have become more prevalent. Their role in CF lung disease is lesswell established with the exception of B cepacia, which can cause a rapid declinein lung function once a patient is infected [5].

Neutrophilic inflammation plays a key role in the pathogenesis and progres-sion of CF lung disease [6]. Pulmonary infections attract neutrophils to the air-way and trigger an inflammatory cascade that is excessive and persists overtime. The airways’ antiprotease defense system is overwhelmed resulting inhigh levels of free elastase. Bronchoalveolar lavage (BAL) studies in CF infantsidentified by newborn screening have demonstrated elevated levels of neutro-phils and free elastase in CF airways as compared with controls [7]. Elastase

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digests elastin and other structural proteins, increases mucus secretion, releasesinterleukin-8 causing more neutrophilic recruitment, and cleaves opsonins,which prevents bacterial killing by neutrophils [6]. This results in a vicious cy-cle of airways infection and inflammation resulting in a decline in lung function(Fig. 1).

TREATMENT OF CYSTIC FIBROSIS LUNG DISEASETreatment of CF lung disease can be broadly divided into two categories basedon their primary target within the cascade of CF pathophysiology (see Fig. 1)into symptomatic treatment that addresses downstream effects of the CFTRgene defect and those targeting the underlying abnormality. Symptomatic ther-apy includes anti-infective and anti-inflammatory therapies as well as agentsthat improve mucus clearance. Therapies that target the CFTR defect includeairway surface fluid hydration, ion transport modulators, and gene therapy, aswell as CFTR pharmacotherapy.

Symptomatic therapyAnti-infective treatment

Pulmonary infections are the central component of CF lung disease and devel-opments in strategies that target pulmonary infections are responsible for mostof the improvement in lifespan for CF patients in the past 2 decades. This re-view focuses on therapies targeted against Staphylococcus aureus and Pseudomonasaeruginosa, as these two bacteria are the most relevant for CF patients.

Inflammation

Mucus obstruction

Infection

Defective ion transport

Defective mucociliary clearance

Airway surface liquid depletion

CFTR gene defect

CF PathophysiologyCF Pathophysiology

Fig. 1. CF pathophysiology.

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CF patients are not infected right at birth but rather become infected withincreasing age. The best strategy to preserve an uninfected airway remainsunclear. As mentioned above, S aureus is usually the first bacterium causingairway infection in CF patients. There is little argument that CF patientswith S aureus should be treated with antistaphylococcal antibiotics if theydevelop a pulmonary exacerbation. However, what is unclear is whetherpatients should be treated if they are newly infected with S aureus but remainasymptomatic.

Continuous antistaphylococcal regimens for CF infants from the time ofdiagnosis onwards have previously been studied in clinical trials. Treatedpatients had a decreased number of respiratory cultures positive for S aureus,less cough, and fewer admissions to hospital [8]. However, two clinical trialsdemonstrated that antistaphylococcus treatment is associated with an increasedprevalence of P aeruginosa in the first 6 years of life [9,10]. However, most pa-tients in these studies received cephalosporins and there is ongoing discussionas to whether a narrow spectrum anti-staphylococcal antibiotic such as cloxacil-lin would lead to the same increased risk of early pseudomonas. Currently,continuous antistaphylococcal antibiotics are not standard of care.

P aeruginosa, the most prevalent CF pathogen, significantly contributes to CFlung disease. The transformation from nonmucoid to mucoid P aeruginosa is as-sociated with a decline in pulmonary function. Aggressive antibiotic therapy isunable to eradicate mucoid Pseudomonas; this is partially because alginate pre-vents antibiotic penetration into aerobic mucus plugs and provides a nidusfor the rapid development of resistant strains [1,11]. Given chronic infectionwith Pseudomonas leads to a decline in lung function, many early aggressive anti-pseudomonal strategies to prevent infection with pseudomonas have been stud-ied and all studies demonstrate a significant microbiological effect [12–15].Only one study has reported a beneficial effect on lung function [16]. However,these results are questionable given the comparison with historical controls.There are currently two large trials under way for CF patients with first Paeruginosa infection. The outcomes for these two trials, The Early PseudomonasInfection Control (EPIC) trial and Early Intervention Tobi Eradication(ELITE) trial, include pulmonary function and longitudinal monitoring ofthe evolution of airway infection [17]. These studies will likely shed some lighton the both clinical effectiveness as well as the optimal strategy for early erad-ication therapy in CF.

Once patients are chronically infected with P aeruginosa, inhaled antibioticshave been tried as ongoing therapy for chronic bacterial suppression. Interestin long-term inhaled antipseudomonal therapy first started 25 years ago be-cause of the potential for high intrapulmonary concentrations with minimalsystemic toxicity [18]. The original formulations for tobramycin were off-labelintravenous preparations. The doses were chosen historically based on the vialsize of the antibiotic. The first study of low-dose tobramycin (80 mg twicea day) was encouraging and demonstrated improved lung function [19]; how-ever, this study had small numbers and was not controlled.

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Ramsey and colleagues [20] conducted the largest randomized trial to dateusing tobramycin (300 mg) a preservative-free tobramycin solution formu-lated for inhalation. This was a placebo-controlled study of 300 mg ofinhaled tobramycin administered twice daily for 28 days alternating with28 days off treatment. Treatment resulted in a significant increase in FEV1

(forced expiratory volume in 1 second) as well as in a 36% reduction inthe use of intravenous antibiotics for pulmonary exacerbations [20]. A recentCochrane review confirmed the benefits of inhaled antipseudomonal antibi-otics on lung function and pulmonary exacerbations and their use is sup-ported by the CF pulmonary guidelines [21,22]; however, the ideal dosingregimen remains unclear. There has been only one study to date that com-pared two inhaled tobramycin dosing strategies [23]. This was an open-labelcrossover study for two, 3-month treatment periods that failed to show anysuperiority in efficacy of tobramycin (300 mg) [23]. However, the study wasnot blinded and a post-hoc analysis suggested that the study was not ade-quately powered.

Colistin is another inhaled antibiotic that has been used to chronically sup-press Pseudomonas. Nebulized colistin has been shown to reduce the decline inlung function in CF patients compared with placebo [24,25]. Hodson andcolleagues [26] compared colistin (1 million units) twice daily to tobramycin(300 mg) twice daily for 1 month; the within mean difference in FEV1 was6.33% (95% CI �0.04 to 12.70) in favor of tobramycin. However, patientswere treated with colistin before the start of the study, which may have under-estimated its treatment benefit. In clinical practice, the use of colistin is reservedfor patients deteriorating on alternating month tobramycin (300 mg) as an add-on therapy during off months.

Recently, another inhaled antibiotic, aztreonam lysinate, has emerged asa potential new treatment option for CF patients. A double-blind, placebo-controlled, dose-escalation trial to assess tolerability and pharmacokinetics ofaztreonam lysinate has been completed in 24 CF patients [27]. Twenty-threeof the 24 patients tolerated the medication; 1 patient had an asymptomaticFEV1 decrease of greater than 20% [27]. Aztreonam concentrations in sputumwere at or above the maximal inhibitory concentration at which 50% of theisolates are inhibited (MIC50) for at least 4 hours post-dose; these data supportthe continued development of aztreonam lysinate for treatment of pulmonaryinfections in CF [27].

CF patients chronically infected with P aeruginosa, similar to patients infectedwith other bacteria, go through periods of clinical stability interrupted by pul-monary exacerbations. A pulmonary exacerbation can be loosely defined asa constellation of clinical symptoms such as weight loss and increased cough,new physical examination findings, radiographic changes, and a drop in pulmo-nary function. Usually, patients are treated with oral antibiotics if they havemild symptoms. Since limited options exist for effective oral therapy, patientswith more pronounced symptoms are often treated with intravenous antibioticsfor 2 to 4 weeks. Another approach to the treatment of pulmonary infections in

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CF patients is intravenous antibiotics every 3 months—irrespective of respira-tory symptoms. Regular intravenous therapy is hypothesized to result in im-proved clinical status and improved life expectancy; this has not beensupported by a randomized clinical trial [28]. However, clinical trials suffi-ciently powered to compare on-demand versus regular intravenous antibioticstrategies are lacking [29].

Mucolytic therapy

Mucus accumulation in the lower airways is a key feature of CF lung disease; themajor component of mucus in CF is not mucin derived from mucus-producingcells but rather pus that includes viscous material such as polymerized DNAderived from degraded neutrophils [30]. Currently, there are two nebulized med-ications used as mucolytics for CF: N-acetyl-L-cysteine (NAC) and dornase alfa.

NAC disrupts the structure of the mucus polymer by substituting free sulf-hydryl groups for the disulfide bonds connecting mucin proteins, loweringboth the viscosity and elasticity of the mucus [30]. Despite in vitro mucolyticactivity, there are no data to support the use of inhaled NAC in lung disease[31]. One hypothesis for the lack of clinical efficacy may be that NAC selec-tively depolymerizes the essential mucin polymer but leaves the pathologicpolymers of DNA and F-actin intact [30].

Oral acetylcysteine has been shown to improve pulmonary function in somepatients with chronic suppurative lung disease including chronic obstructivepulmonary disease: exacerbation rates have been shown to decrease by 30%[32,33]. The clinical benefit of oral NAC is likely secondary to the antioxidantproperties of acetylcysteine and its effect on inflammation rather than itsmucolytic properties. CF patients have a deficiency in the antioxidant, glutathi-one, in neutrophils and oral acetylcysteine has been shown to replete glutathi-one stores in a phase 1 trial [34]. Given the limited evidence to support the useof both inhaled and oral N-acetylcystine, its use cannot be recommended atpresent.

DNA released in large amounts from disintegrating neutrophils increasesmucus viscosity in respiratory secretions [30]. Dornase alfa is a recombinantform of the human DNase 1 enzyme and digests extracellular DNA releasedfrom necrosed neutrophils [30]. This results in decreased mucus viscosityand improved mucus clearance. Inhalation of dornase alfa is indicated to re-duce the frequency of respiratory infections requiring parenteral antibioticsand to improve or preserve pulmonary function in CF patients [30]. The safetyand benefit of dornase alfa has been demonstrated across the entire spectrum ofCF lung disease severity: even in patients with forced vital capacity (FVC) lessthan 40% predicted [35–38]. The inhalation of dornase alfa is also associatedwith a decreased number of pulmonary exacerbations as well as a positiveeffect on inflammation in the CF airways [39]. Efficacy during acute pulmonaryexacerbations has not yet been shown [40].

This medication is very well tolerated but some side effects that were foundto be significant in the Phase 3 study include voice changes, pharyngitis, and

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laryngitis [35] Furthermore, this medication often induces coughing and shouldnot be given right before bed.

DNA and F-actin copolymerize in the sputum to form a rigid network in themucin gel [41,42]. Gelsolin, an actin-severing protein, has been shown toreduce mucus viscosity but cannot be nebulized [43]. Interestingly, thymosin,an agent that sequesters F-actin and decreases sputum polymerization, hasbeen shown to be more effective when combined with dornase alfa than eithersubstance alone [43,44]. At present, in vivo data are lacking to support the useof either one of these agents.

Anti-inflammatory therapy

Neutrophilic inflammation plays a key role in the pathogenesis and progressionof CF lung disease [6]. Neutrophilic inflammation leads to the formation oftoxic oxygen free radicals and free elastase; their persistence leads to pulmo-nary destruction [7]. The pathophysiological relevance of airway inflammationfor CF lung disease therefore supports the use of anti-inflammatory therapy.

Corticosteroids have been studied via oral, inhaled, and intravenous routes.Long-term oral steroids were first shown to be effective more than a decade ago[45]. However, the side effects (eg, growth retardation, diabetes, gastritis) inboth single and multicenter studies are too great to recommend long-term ste-roid use [46,47]. A chest physicians’ survey administered to physicians in theUnited Kingdom demonstrated that all survey responders admitted to usingshort-term oral steroids to treat pulmonary exacerbations [48]. Interestingly,despite the apparent widespread use of steroids to treat exacerbations by prac-titioners, there have been only two trials to date to study the effect of oral ste-roids during pulmonary exacerbations [49,50]. Tepper and colleagues [50]studied 20 infants randomized to routine pulmonary exacerbation treatmentplus intravenous hydrocortisone or placebo for 10 days. They also found nochange in lung function at the end of admission but found improved lung func-tion at the postadmission follow-up visit, suggesting a long-term benefit to lungfunction [50]. Recently, a pilot study has been completed in older CF patients.The addition of steroids to conventional treatment for a pulmonary exacerba-tion did not significantly affect pulmonary function or sputum markers ofinflammation [49].

Lucidi and colleagues [51] have taken a novel approach to the delivery ofsystemic corticosteroids: delivery of low-dose intravenous steroids through in-fusion of autologous erythrocytes loaded with dexamethasone. A pilot studycompared nine patients receiving monthly intravenous infusions for 2 yearsto patients receiving standard therapy. The FEV1 in the experimental groupwas higher than in the placebo group and there were no steroid side effects re-ported [51]. However, there is no evidence to date that this strategy will reducethe negative side effects of corticosteroid therapy.

Despite the lack of evidence in the literature to support the use of inhaledcorticosteroids as a long-term anti-inflammatory therapy, many CF patientsare receiving this treatment [52]. There certainly is a role for inhaled steroids

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in CF patients who have a concomitant diagnosis of asthma with symptomaticwheezing; however, a diagnosis of asthma is challenging in the CF populationgiven that the clinical phenotype varies over time.

Balfour-Lynn and colleagues [53] showed that withdrawal of inhaled cortico-steroids in children and adults did not result in an increase in pulmonary exac-erbations, adverse effect on lung function, or an increased use of antibiotics orrescue bronchodilators. Although the Cochrane review concluded that therewere no adverse effects of inhaled steroids in CF patients, a recent studyshowed a decrease in growth velocity in patients using fluticasone (1000 lg/day) over 12 months; catch-up growth was not seen 1 to 2 years after medica-tion discontinuation [52,54]. Therefore, inhaled corticosteroids do have a rolefor CF patients with asthma but their use should be limited to this subgroup ofpatients and high doses should be avoided.

High-dose ibuprofen has been studied as an alternative to corticosteroid ther-apy. In high doses, ibuprofen inhibits the migration, adherence, and aggrega-tion of neutrophils [55–64]. High-dose ibuprofen has been used instead ofconventional doses of ibuprofen because low doses of ibuprofen paradoxicallyincrease the influx of neutrophils [65]. In a blinded, randomized trial, CFpatients with mild lung disease were shown to have an improvement in theirFEV1% predicted decline [66]. However, the side-effect profile of this therapyas well as the need for pharmacokinetic monitoring to achieve adequate serumlevels has prevented its widespread use [66,67]. A recent single-center study re-ported that 45% of children had to discontinue high-dose ibuprofen because ofadverse effects [68].

A multicenter, Canadian trial of high-dose ibuprofen in CF patients withmild lung disease (FEV1 > 60%) has recently been published [69] Patients inthe treatment group had fewer days in hospital in a post-hoc analysis and ex-perienced no adverse effects; however, a significant change in FEV1 was notseen [69]. Of note, the study recruitment fell far below the sample size calcu-lated to power the study: 142 patients rather than 440 [69]. A 2007 Cochranereview of oral anti-inflammatory therapies for CF identified six trials for review[70]. All showed evidence of improved lung function or decreased intravenousantibiotic use. No major adverse effects were reported, but the trials wereunderpowered to detect clinically important differences in the incidence ofadverse effects. Konstan and colleagues [71] published an observational studyafter the Cochrane review that compared the decline in FEV1 among CF pa-tients aged 6 to 17 years on ibuprofen therapy with those who did not receiveibuprofen. Treated patients had a slower rate of lung function decline, but werealso more likely to be hospitalized for a gastrointestinal bleed (relative risk 2.72,P < .001). In summary, high-dose ibuprofen does seem to have a treatmentbenefit but has not become standard of care for CF patients because of themixed results of trial data as well as the adverse event profile.

Antileukotriene therapy has been explored as a potential anti-inflammatorytreatment in CF. Leukotriene B4 plays a role in neutrophilic recruitment.Konstan and colleagues [71] compared the eicosanoid content of epithelial

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lining fluid (ELF) obtained by bronchoalveolar lavage (BAL) from 17 patientswith CF and 10 healthy subjects. LTB4 was the predominant eicosanoid in theCF airway and the data suggested that the CF airways contain sufficientamounts of LTB4 both to recruit additional neutrophils into the airways andto stimulate neutrophils to release their injurious products [72]. In fact, the re-duction of LTB4 is one of the proposed mechanisms of action of ibuprofen [73].Therefore, therapies aimed at interfering with the production or action ofLTB4 may be beneficial for CF lung disease. A large phase 2 trial of a leukotri-ene B4 antagonist (amelubant) in CF patients with mild to moderate diseasewas stopped prematurely because of an excess number of significant adverseevents (admissions for pulmonary exacerbations), raising the question ofwhether the anti-inflammatory effects were too potent thereby favoring bacterialinfection.

Cysteinyl leukotrienes have also been shown to be present in increasedamounts in the CF airways. In a randomized, double-blind, crossover, pla-cebo-controlled study, 16 children with mild CF were treated with montelukastor placebo for 21 days [74]. There was a significant reduction in serum eosin-ophil cationic protein levels and eosinophils but no significant differences inlung function tests or clinical symptom scores [74]. Therefore, some evidenceexists that montelukast reduces eosinophilic inflammation in CF patients buteffects on neutrophilic inflammation have not been demonstrated. Montelukastis therefore likely to be useful only in the subgroup of atopic CF patients.

The success of macrolides for panbronchiolitis, a disease with many similar-ities to CF, led to the study of macrolides in CF patients [75]. Macrolides havebeen shown to improve lung function and/or decrease the number of pulmo-nary exacerbations in more than 350 P aeruginosa–positive CF patients whoare 6 years of age and older [76–78]. Interestingly, a post-hoc analysis showedthat the reduction in exacerbations was not necessarily predicted by improvedpulmonary function for individual patients, suggesting a heterogeneousresponse to treatment [79].

Although the clinical efficacy of macrolides has been demonstrated, themechanism of action of macrolides remains unclear [47]. A recent study aimedto determine the mechanism of action of azithromycin explored three mecha-nisms: a direct effect on CFTR and multidrug resistance (MDR) gene productexpression, improved epithelial ion transport, and impaired P aeruginosa adher-ence and transport [80]. The authors felt that none of the three were likely andspeculated that the mechanism of action may be a result of the breakdown ofP aeruginosa biofilms, allowing penetration of the antibiotics [80].

More recently, azithromycin has been studied in younger CF patients whoare not necessarily P aeruginosa positive. In a multicenter trial conducted inFrance, only 25% of the patients were chronically infected with Pseudomonas[81]. There was a significant reduction in the number of pulmonary exacerba-tions and equal effectiveness was shown in both those infected and not infectedwith Pseudomonas [82]. This suggests that this treatment doesn’t need to be lim-ited to those infected with Pseudomonas. A multicenter trial is currently under

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way to determine the efficacy of azithromycin in Pseudomonas-negative CFpatients [83].

Recently, there has been some concerning evidence regarding the emergenceof macrolide resistance. Phaff and colleagues [84] reviewed sputum cultures formore than 150 CF patients over the 4 years after azithromycin was introduced.S aureus resistance to erythromycin increased from 7% to 54% and Haemophilussp resistance to clarithromycin increased from 4% to 38%; actual azithromycinresistance was not reported in the study [84]. One study that followed patientsfor 3 years after the start of an oral macrolide interestingly showed that theimprovement in lung function that was seen after the first year was not sus-tained for the next 2 years [85] The results of this study suggest that thelong-term effect of macrolides is an area that needs further clarification.

Three studies looked at the effect of a1-antitrypsin inhibitors in CF patients[86–88]. The first study was a proof-of-concept study that showed that aerosol-ized a1-antitrypsin inhibitors decreased free elastase in bronchoalveolar lavagesamples of CF patients [86]. The second study was a prospective, double-blinded, randomized, placebo-controlled phase 2 trial to determine the safetyand efficacy of a1-antitrypsin inhibitors for CF patients. Thirty-nine patientswere randomized to receive nebulized a1-antitrypsin inhibitors for 4 weeks fol-lowed by a 2- week washout period and then another 2 weeks of therapy [87].This study did not demonstrate a significant reduction in sputum elastaseconcentrations and no effect on airway inflammation or lung function couldbe demonstrated. Recently, a prospective trial was done of 52 CF patientswhom received nebulized a1-antitrypsin inhibitor for 4 weeks [88]. Inflamma-tory indices such as free elastase, neutrophils and pro-inflammatory cytokinesall significantly decreased but a change in FEV1 was not seen [88]. The differ-ing results may be partially attributed to the delivery systems, as a highly effi-cient nebulization system was used in the latter trial. The lack of effect onpulmonary function may require longer studies, as anti-inflammatory treatmentis more likely to affect lung function decline rather than increasing lung func-tion in treated patients [88]. Therefore, a large randomized trial is needed toassess the efficacy of antiprotease inhibitors in this population.

A number of other potential anti-inflammatory agents have been evaluatedin pilot studies. These include oral cyclosporin, intravenous immunoglobulin,simvastatin, pioglitazone, hydroxychloroquine, low-dose methotrexate, andoral glutathione [47,89] Further details about each of these drugs and the ongo-ing studies to evaluate them can be found on the Cystic Fibrosis FoundationWeb site [90].

Ion transport modifier therapy

Defective CFTR signaling results in impaired chloride secretion. Increased in-tracellular Ca2þ has been shown to stimulate chloride secretion across the CFairway through the alternative chloride channel [91]. As such, Ca2þ-dependentagonists have been proposed as therapeutic agents for CF. Mason and col-leagues [85] demonstrated that purine and pyrimidine nucleotide triphosphates

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(ATP, UTP), acting at P2Y2 receptors, stimulate chloride secretion in CF tra-cheal epithelium via an increase in intracellular Ca2þ. However, the naturalP2Y2 agonists, ATP and UDP, have very short half-lives, which is why deriv-atives with enhanced stability such as denufusol have been developed [92]. De-nufusol is a P2Y2-receptor agonist. Its main mechanism of action is to stimulatechloride ion secretion and inhibit sodium ion reabsorption, thus hydrating theairway lumen by fluid co-transport [93–96]. A safety and tolerability study ofdenufusol in CF patients with mild to moderate CF lung disease has been com-pleted [97]. Doses of up to 60 mg were inhaled, with good tolerance in mostpatients [97]. The most common adverse effects in children were cough andan acute, reversible drop in FEV1 [97]. Two phase 2 trials have been completedand demonstrate improved lung function in the experimental group [98,99]. Alarge multicenter phase 3 trial titled Transport of Ions to Generate EpithelialRehydration (TIGER) is currently under way to assess the efficacy of thiscompound in patients with mild lung disease as an early intervention strategyfor CF [92].

Moli1901 (duramycin) is a peptide that interacts with phospholipids in theplasma and cell membrane and also activates the alternative chloride channelby elevating intracellular calcium [92]. A phase 2 trial of inhaled Moli1901 in24 CF patients demonstrated that the medication was both safe and effective[100]. A multicenter, European efficacy study has been started this year [92].

Treatments that target cystic fibrosis transmembraneconductance regulatorAirway surface liquid hydration therapy

Effective mucociliary clearance depends on adequate volume of airway surfaceliquid [101]. In CF lung disease, impaired epithelial chloride secretion and so-dium hyperabsorption lead to depletion of airway surface fluid and subsequentimpaired mucus clearance [102]. This has led researchers to study osmotic sub-stances such as inhaled mannitol and hypertonic saline that can increase airwaysurface liquid volume.

Mannitol is a six-carbon monosaccharide that can be encapsulated to be a sta-ble dry powder for inhalation [103]. Mannitol is able to create an osmotic gra-dient that causes an influx of water into the CF airway and restores the volumeof the airway surface liquid [103]. The effect of mannitol on mucociliary clear-ance has been shown in a radioaerosol study where mucociliary clearance wassignificantly increased in the patients receiving mannitol (300 mg by inhalation)as compared with controls [104]. Charlton and colleagues [105] demonstratedthe short-term efficacy of mannitol in a clinical trial. The investigators random-ized 39 CF patients to inhaled mannitol (420 mg twice a day) or placebo for2 weeks, followed by a 2-week washout, in a 6-week crossover trial. A 7% in-crease in FEV1% predicted was seen in the group that received mannitol [105].However, the long-term benefit of inhaled mannitol remains unknown and isan area of further study.

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There is some concern that chronic mannitol inhalation can contribute to theproliferation of bacteria. Mannitol is a carbon source that can be metabolizedby bacteria such as Pseudomonas [105,106]. This potential adverse effect needsto be clarified by quantitative microbiologic cultures in future studies [104].Dry powder mannitol is not currently approved for clinical use as a mucociliaryclearance agent in patients with CF.

Before 2006, smaller studies using hypertonic saline showed promisingshort-term benefits including improved mucus transport, hydration of the air-way surface, and improved mucociliary clearance and lung function in CFpatients [107–112]. Hypertonic saline has been shown to directly improve mu-cociliary clearance [112]. The increase was concentration dependent, as muco-ciliary clearance continued to increase up to a concentration of 7%. Higherconcentrations (12%) did not stimulate mucociliary clearance further andwere poorly tolerated; therefore, 7% is the concentration that was used in sub-sequent clinical trials [112]. The sustained effect of hypertonic saline on airwaysurface liquid (ASL) volume was elegantly shown in a study by Donaldson andcolleagues [102]. The ASL volume increased fourfold after inhaled hypertonicsaline (7%) in normal airways and returned to baseline within 10 minutes. Incontrast, the effect was much greater and lasted longer in the CF airway. Ina randomized controlled trial there was also evidence of improved mucociliaryclearance for up to 8 hours after one dose and lung function improved within2 weeks [102].

In 2006, a large multicenter trial demonstrated for the first time the long-term benefit of inhaled hypertonic saline [113]. After 48 weeks of inhaledhypertonic saline (7%), patients had an improved FEV1 and fewer pulmonaryexacerbations as compared with controls [113]. There is not yet a study in theliterature assessing the effect of inhaled hypertonic saline on infants and youngchildren with CF. A single-center study recently showed that inhaled hyper-tonic saline can be used safely in this age group [114]. A multicenter trial toassess the effect of inhaled hypertonic saline in infants is planned for the startof 2008.

Inhaled hypertonic saline (7%), 4 mL, as used in the phase 3 Australianstudy would add on 30 minutes of therapy time for CF patients using contem-porary nebulization systems. Nebulization systems with a more efficient nebu-lizer system such as the eFlow Rapid Nebulizer (Pari, Germany) have beenshown to be both time saving and well tolerated [115]. These systems willhopefully be available for clinical use shortly. Hypertonic saline is associatedwith bronchospasm and patients should be pretreated with bronchodilators.In addition, pre- and postnebulization spirometry should be done with the firstdose of the medication. Although, there are known side effects such as saltytaste, nausea, dyspnea, and chest pain, it has been well tolerated in the largerstudies [102,113]. Unlike many other CF therapies, it is a very inexpensivemedication. While its clinical effectiveness is clear in the short term, thelong-term effects of hypertonic saline on pulmonary infection and inflammationremain unclear.

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Gene therapy

The gene defect causing CF is known and as such CFTR gene therapy hasbeen explored by researchers as a curative treatment. Although several vectorshave been studied in human patients, adenoviruses, adeno-associated viruses(AAV), and cationic lipids seem to be the most promising [116]. CFTR canbe successfully delivered to the airway epithelium but its effects are short-livedwhich makes repeated dosing essential [92]. Unfortunately, this presents a ma-jor challenge to the use of adenovirus vectors. Although they have a highertransfection rate than lipid vectors, they are immunogenic, which limits theability for multiple dosing [92]. Furthermore, Tosi and colleagues [117] re-ported an increase in antiadenovirus immune response in mice when hostshave a preexisting infection with pseudomonas. Although similar studiesneed to be repeated in humans, chronic infection may be a significant hurdlefor gene therapy.

Adeno-associated virus vectors have subsequently been developed with thehope for decreased antiadenovirus immunogenicity. Moss and colleagues[118] completed a phase 2 trial assessing the safety and efficacy of repeateddoses of gene therapy using AAV vectors. The investigators showed both a sig-nificant improvement in FEV1 as well as a reduction in sputum interleukin-8(IL-8) [118]. Based on these results, Targeted Genetics Corporation initiateda study powered to detect changes in lung function; however, the trial didnot meet its outcome measure and the program has been discontinued [119].The disappointing results may be attributed to one or a combination of (1)the vector’s transfection inefficiency, (2) the promoter used to drive CFTRexpression, or (3) antiviral immunogenicity [119].

Recent attention has focused on the cationic lipid vector. The UK CysticFibrosis Gene Therapy Consortium has a double-blind, placebo-controlledgene therapy study planned for 2009 [120]. The intervention will be adminis-tered over a 1-year time period. Although gene therapy is not an imminentlyavailable therapy, it is a promising future intervention.

Cystic fibrosis transmembrane conductance regulator pharmacotherapy

Unlike the previously mentioned interventions, these therapies are CFTR mu-tation class specific. As such, these interventions are tailored to a patient’s classof genetic defect (see Fig. 1) [121]. Class 1 mutations, stop mutations, are theresult of the insertion of a premature stop codon (PTC). Aminoglycosides,an example of a correcting agent, have been shown in proof-of-concept studiesto induce read-through of a PTC enabling the formation of a full-length andfunctionally active CFTR protein [122–124]. Phase 2 trials are currently underway to assess a second-generation agent known as PTC124 [17].

CFTR pharmacotherapeutic agents for other classes of CFTR mutations canbe classified as agents that correct the CFTR trafficking defect or agents thatpotentiate CFTR function. VX-770, a compound developed by Vertex Phar-maceuticals Inc (Cambridge, Massachusetts) is a promising CFTR potentiator.VX-770 increases cyclic AMP–dependent chloride secretion in cell cultures

112 AMIN & RATJEN

expressing the dF508, R117H, or G551d- CFTR mutations [92]. VX-770 is cur-rently being studied in CF patients with G551D mutations; if the results arepositive, this compound could then be used in CF patients with other muta-tions [92]. Flavenoids are another known CFTR potentiator, and studies arecurrently under way to assess their effectiveness for CF patients [125].

GROWTH AND NUTRITIONMalnutrition is a key feature of CF. Defective CFTR in the pancreatic epithe-lium results in ductal obstruction by proteinaceous secretions and acinardestruction. CF patients become pancreatic insufficient when more than 95%of total pancreatic exocrine function is lost [126]. These patients develop fat,protein, and carbohydrate malabsorption and are at risk for fat soluble vitamindeficiencies (ie, vitamins A, D, E, and K) as well as other mineral deficiencies.

Nutrition and lung function are intertwined. Poor nutritional status and poorsomatic growth secondary to pancreatic insufficiency likely affects lung growthas well as the ability to repair lung disease [127]. Similarly, pulmonary diseaseaffects linear growth through appetite suppression and increased energy expen-ditures (see Fig. 2) [127]. Corey and colleagues [4] showed that a poor weight incystic fibrosis patients was associated with lung disease and suggested that mal-nutrition may be part of the causal pathway leading to mortality. Although, thisrelationship seems intuitive for CF patients with severe lung disease, it wasn’tknown until recently if this holds true for younger patients with milder disease.Prospective data were used to determine if weight and height percentiles forpatients between 3 and 6 years of age predicted pulmonary function at age 6.Pulmonary function was found to be highest in those whose weight for ageremained greater than the 10th percentile from ages 3 to 6 and lowest in thosewho remained less than the 10th percentile [127]. These findings reinforce theimportance of good nutrition for CF patients starting from a young age.

Growth and nutritional management is a central component of CF therapy.Pancreatic enzymes and fat-soluble vitamin supplements are standard of care

•Increased Caloric Needs•CFTR Defect•Persistent Infection andInflammation

CaloricSupplyCaloricSupply

CaloricExpenditure

CaloricExpenditure

Malnutrition in Cystic Fibrosis

•Decreased Caloric Intake•Anorexia•Repeated Pulmonary Exacerbations

•Enteral Loss due to pancreaticinsufficiency

•CFRD

Fig. 2. Factors contributing to malnutrition in a cystic fibrosis patient.

113CYSTIC FIBROSIS

for pancreatic-insufficient patients. However, even with exogenous enzymesupplementation, malabsorption is rarely completely resolved. In addition,CF patients have an increased energy expenditure. Therefore, CF patientsare to consume between 20% and 50% more calories than non-CF patients.Despite this increased caloric intake, some CF patients fail to thrive. WhenCF patients show signs of growth retardation according to their percentiles,patients will be started on oral nutritional supplements; if they fail to improve,a gastrostomy tube will be inserted for nutritional supplementation.

Height is an independent risk factor for poor pulmonary function and is ofparticular concern because even patients with good nutritional status do notachieve full target height [127–129]. Chronically elevated inflammatory cyto-kine levels have been shown to both affect the production and the secretionof growth hormone (GH) and induce GH resistance at the tissue level[130,131]. As such, GH therapy has been explored as a treatment option forCF patients. Multiple small studies using recombinant human growth hormone(rhGH) in CF children with height and weight less than the 10th percentilehave shown improvements in somatic growth [132–138]. A randomized, con-trolled, crossover trial designed to determine both the efficacy of growthhormone after 1 year of therapy and to assess the sustained effects after treat-ment cessation has been completed. Patients who received rhGH had greatergains in height and weight, increased lean mass, fewer exacerbations, andthe benefits were sustained for 1 year after the rhGH was stopped [139]. How-ever, there was no difference in pulmonary function between the two groups[139]. Schnabel and colleagues [140] conducted the only double-blind pla-cebo-controlled study to assess the efficacy and safety of two dosages of GHin CF. While a significant increase in growth was observed in treated patients,increases in weight failed to be significant, since patients in the placebo groupalso showed significant improvement. Interestingly, weight gain was limited topatients with better lung function, raising the question of whether an earlier in-tervention would be more efficacious. No effects on pulmonary function intreated patients were observed, which was similar to the Hardin and colleaguestrial [137]. The maximal oxygen uptake during peak exercise increased signif-icantly in treated patients. This suggests that GH may have a positive effect onphysical activity, which has been previously shown to be related to lung func-tion decline [141]. However, rhGH is not a benign intervention. It is adminis-tered by subcutaneous injection, weekly. In addition, there is a theoretic risk ofmalignancy and diabetes. As such, there is currently insufficient evidence torecommend the routine use of recombinant human growth hormone therapyfor patients with CF.

CYSTIC FIBROSIS–RELATED DIABETESCystic fibrosis–related diabetes (CFRD) can develop in CF patients as a conse-quence of the pancreatic pathology. The prevalence increases with age; theaverage age of onset is between 18 and 21 years [142]. It rarely develops in

114 AMIN & RATJEN

patients younger than age 10 [143]. Therefore, annual screening for CFRDwith an oral glucose tolerance test starts at age 10.

The onset of CFRD is often insidious and the diagnosis is usually precededby a decline in pulmonary function [144,145]. Therefore, delaying the diagno-sis can result in a preventable decline in clinical status. CFRD can be chronic orintermittent. In CF patients with intermittent CFRD, the hyperglycemia oftenoccurs during times of stress (ie, pulmonary exacerbations) or while on steroidtherapy. CF patients with CFRD may develop microvascular complications,but macrovascular complications are rare. This may be a result of a shortenedlifespan in CF patients or a result of fat malabsorption [146].

The goal of treatment in CFRD is to achieve optimal nutrition and avoidmetabolic derangement by maintaining normoglycemia [146]. Insulin is thetreatment of choice. The decision to treat with insulin is a clinical decisionbased on patient status as well as glucose levels measured at home or in thehospital [146]. Intermittent CFRD can be treated with insulin during theseepisodes of hyperglycemia as compared with patients with chronic CFRDwhom require daily insulin therapy. There is a broad range of different insulinregimens and the treatment regimen chosen can be tailored to the individualpatient’s needs. In addition, studies are currently under way to determine iforal antiglycemic agents are an effective intervention for patients with newlydiagnosed CFRD [147,148].

SUMMARYIn summary, there is a significant interplay between the pulmonary manifesta-tions and nutritional status of CF patients. The advances in CF clinical care inthe past 2 decades are mainly attributed to anti-infective therapy as well as ag-gressive nutritional management. Currently, there are multiple therapeuticagents that are in clinical trial that target either the underlying CFTR defector the downstream effects of CFTR. The broad spectrum of therapeutic agentsbeing studied as well as the advances in therapies that target the underlyingCFTR defect are exciting, making it likely that at least one of the treatmentswill make a major difference in how we will treat CF in the future.

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