Therapy With Macrolides in Patients With Cystic Fibrosis

Allyson S. Gaylor, Pharm.D., Joan C. Reilly, Pharm.D.

Pharmacotherapy. 2002;22(2) 

In This Article

Mechanisms of Action

Mechanisms by which macrolide antibiotics are beneficial in DPB and possibly cystic fibrosis are not completely understood. Several theories have been postulated and tested. Macrolides are thought to have a wide range of effects on different pathways of the inflammatory process. They affect neutrophils, oxidative burst in phagocytes, and production of proinflammatory cytokines.[42,55,56] They also are thought to affect the host defense system, alter airway epithelial chloride transport, and inhibit virulence factors produced by P. aeruginosa.[2,42,55,57]

Erythromycin and other macrolides modulate neutrophil activity and function.[40,46] Preventing neutrophil migration and suppressing neutrophil chemotactic activity at inflammatory sites of the lungs is thought to contribute to the clinical effectiveness of macrolides in the treatment of chronic inflammatory airway disease.[39,58,59] One theory as to why the drugs are effective in reducing neutrophil activity is the high intracellular concentrations they achieve. Macrolides are neutral compounds in the blood and readily cross cell membranes. Once inside the cell they become protonated, and their lipophilicity is decreased. The drug then is trapped inside the cell, resulting in a high intracellular concentration, which may directly or indirectly alter neutrophil functions.[39,46,56,60]

Neutrophil chemotactic activity was evaluated in 13 patients with DPB before and after treatment with erythromycin.[39] All patients received erythromycin stearate 200 mg orally 3 times/day for a minimum of 6 months. Analysis of bronchoalveolar lavage (BAL) fluid revealed a substantial decrease in the total number of cells and percentage of neutrophils after treatment. Neutrophil chemotactic activity also was decreased. Patients had a significant increase in forced expiratory volume in 1 second (FEV1) percentage and percentage vital capacity.

Another study assessed BAL fluid in seven patients with DPB and five healthy volunteers before and after treatment with erythromycin.[49] On analysis of BAL fluid before treatment, the number of neutrophils was significantly higher in patients with DPB than in controls (37.7 ± 8.6 x 104/ml vs 0.5 ± 0.4 x 104/ml, p<0.01). Analysis after treatment showed a significant reduction in total cell count and number of neutrophils, which paralleled improvement in clinical signs and symptoms. Treatment with erythromycin also significantly improved FEV1 and forced vital capacity (FVC). Furthermore, changes in FEV1 correlated with the number of neutrophils in BAL fluid before and after treatment, indicating the number of neutrophils in the airway may play a role in airflow obstruction.

In contrast to these two studies, a prospective, single-blind trial with clarithromycin found no significant differences in absolute neutrophil numbers or FEV1 and FVC.[61]

Excessive oxidant generation is thought to be involved in cell and tissue damage associated with severe inflammatory reactions.[62] Possible antioxidant action was theorized to be partly responsible for the clinical effectiveness of macrolide antibiotics in inflammatory diseases.[46,56,58] Treatment with erythromycin significantly decreased intracellular oxidative capacity in neutrophils.[46] Substantial inhibition of superoxide generation by human neutrophils in vitro was seen after treatment with erythromycin and roxithromycin.[58]

The effect of four macrolide antibiotics on generation of various mediators and cytokines involved in the inflammatory process was assessed in vitro and in vivo.[56] Roxithromycin, clarithromycin, erythromycin, and azithromycin all caused dose-dependent reductions in nitric oxide (NO), prostaglandin E2, and TNF-a in pleural exudate of rats. In vitro studies using murine macrophages revealed a concentration-dependent decrease in production of TNF-a, IL-6, IL-1ß, and NO. Another study found that erythromycin at 1, 10, and 100 µg/ml significantly decreased IL-8 released by neutrophils (p<0.05) in vitro but did not significantly influence TNF-a production,[46] contrary to the previous study. Adhesion molecule expression on neutrophils, which is involved in mediation of chemotaxis, transepithelial migration, phagocytosis, and degranulation of neutrophils, also was significantly decreased (p<0.001).

The significance of the IL-1ß and IL-1 receptor antagonist (IL-1Ra) was evaluated in the airways of patients with DPB.[49] Interleukin-1Ra is a member of the cytokine family produced by human monocytes, alveolar macrophages, and neutrophils. It inhibits binding of IL-1 to its receptor, thereby suppressing the action of IL-1. Interleukin-1ß is an important cytokine in promoting neutrophil-mediated airway damage because a variety of cells produce IL-8 in response to IL-1ß. Baseline levels of IL-1ß and IL-8 were substantially higher in patients with DPB than in healthy volunteers. Interleukin-1Ra levels were also higher in patients with DPB, although statistical significance was not achieved. Erythromycin reduced IL-1ß and IL-8 in BAL fluid of patients with DPB to levels comparable with those of healthy subjects. The level of IL-1Ra was decreased, but the difference was not statistically significant. The level of IL-1ß correlated significantly with the number of neutrophils in BAL fluid in addition to levels of IL-8 and IL-1Ra. Similar results were found by others.[63,64,65]

Another interesting clue regarding beneficial nonantibacterial effects of macrolides lies in a report of a patient with cystic fibrosis with a history of P. aeruginosa infection who developed a fibrosarcoma of the left femoral biceps.[66] The patient was treated with surgery, radiotherapy, and chemotherapy with cyclophosphamide and epirubicin. Lung function improved significantly, and the P. aeruginosa infection cleared after chemotherapy. Chemotherapy induced overexpression of multidrug-resistance protein (MDR) and multidrug resistance-associated protein (MRP). These proteins belong to the same family of transmembrane transporters as the CFTR protein. It is possible that the overexpression of MDR and MRP resulted in complementation of CFTR, which could explain the improved lung function. Nasal epithelial cells were collected from this patient and a control patient with cystic fibrosis never exposed to chemotherapy and analyzed for mRNA of MDR and MRP. The mRNAs of MDR and MRP were identified in the case patient but undetectable in the control. This raises the question whether the macrolide benefit could be due to upregulation of MDR and complementary CFTR.[67]

A placebo-controlled trial investigated clarithromycin's effect on sputum production in patients with chronic respiratory infections.[57] After therapy with clarithromycin 100 mg twice/day or placebo for 8 weeks, clarithromycin significantly decreased sputum production (51 ± 6 vs 24 ± 3 g/day, p<0.001). No changes in the total number of bacterial colony-forming units/g of sputum or bacterial flora in the sputum samples were observed. A decrease in exudate volume was shown in other studies.[44,56] A similar decrease in nasal secretions was seen in patients with purulent rhinitis after treatment with clarithromycin.[68] This study also reported improvements in physical properties of nasal mucus such as hydration, viscoelasticity, cohesion, and ciliary transportability.

Pseudomonas aeruginosa

Bacterial colonization with P. aeruginosa and increased adherence to epithelial cells are common findings in patients with cystic fibrosis. Suspicion that the increased adherence is reversible led to an investigation of whether azithromycin could improve host defense by supporting repair of the oropharyngeal barrier.[42] Buccal adherence to epithelial cells of P. aeruginosa was evaluated before and after treatment with azithromycin 250 mg twice/week for 3 months in 11 patients with cystic fibrosis. Adherence decreased in all patients after treatment by a mean of 70%. Before therapy the mean (SD) number of P. aeruginosa was 8 (4.8) bacteria/buccal epithelial cell. After treatment, the mean adherence was 2.4 (1.1) bacteria/cell (p=0.007). In an extension of the trial, adherence increased again relative to the decrease in four patients in whom a third test was taken after therapy was discontinued.

Macrolides also are suspected of having direct effects on P. aeruginosa by inhibiting various exoproducts produced by the organism or by interacting with mucoid biofilm.[3,40,54,59] An in vitro study of the effect of erythromycin on production of elastase by P. aeruginosa showed a dose-dependent decrease in elastase production with no effect on proliferation.[54] Complete inhibition of elastase production was achieved in approximately 80% of P. aeruginosa strains at concentrations of 0.125-64 µg/ml, although these concentrations seem relatively high. In another study, azithromycin not only decreased the amount of elastase produced by P. aeruginosa, but decreased exotoxin A, protease, and phospholipase C production without affecting the growth of the organism.[4] This effect was due to a decrease in production rather than intrinsic antiexoenzyme properties.

Pseudomonas aeruginosa recovered from the lungs of patients with cystic fibrosis often is encased in mucoid-alginate biofilm. The biofilm allows the organism to adhere closely to the airway surface and leave it more resistant to attack by antibacterial drugs and interaction with neutrophils. Interference with biofilm formation could play a part in the effect of macrolides in patients with cystic fibrosis. When P. aeruginosa was incubated in an alginate-promoting medium for 2 weeks, the viscosity of the medium increased secondary to alginate production.[53] When the same strain was incubated with azithromycin under the same conditions, the viscous fluid disappeared as a result of inhibited alginate production. Alginate production was inhibited in a dose-dependent manner by incubation with erythromycin, clarithromycin, or azithromycin. Enzymes in the alginate system activate the alginate production inside P. aeruginosa. Incubation with erythromycin, clarithromycin, roxithromycin, or azithromycin inhibited the enzymatic activity of guanosine diphospho-mannose dehydrogenate in the alginate biosynthetic pathway.[53]

Similar results were seen with azithromycin.[69] Azithromycin at 1/256 minimum inhibitory concentration (MIC; 0.39 µg/ml) significantly suppressed alginic acid production by mucoid P. aeruginosa (p<0.02). Glycocalyx production, produced by nonmucoid strains in the early stage of biofilm formation, was also significantly inhibited by azithromycin at 1/16 MIC (6.25 µg/ml; p<0.02). When viewed under the scanning electron microscope, a netlike granular substance was observed on the surface of mucoid P. aeruginosa when grown on drug-free medium. The substance was markedly decreased when grown on a medium with 1/4 MIC (25 µg/ml) of azithromycin, which suggests inhibition of alginic acid production. Similar results were seen with nonmucoid P. aeruginosa, suggesting inhibition of glycocalyx production and suppressed biofilm formation.

When the interaction of human neutrophils with P. aeruginosa biofilms was investigated, the neutrophil response to nonmucoid biofilm was stronger than that to mucoid biofilm.[3] When biofilm was treated with a macrolide before neutrophil incubation, the response was markedly enhanced compared with the untreated control. A dose-dependent effect was also noted. These findings suggest that biofilm is involved in impairment of phagocytosis of P. aeruginosa, and inhibiting its formation with macrolides may enhance clearance of bacteria by the body's own defense system.

With reports of efficacy of long-term macrolide antibiotics in patients with chronic P. aeruginosa infection, the effect of sub-MICs of macrolide antibiotics on the sensitivity and viability of P. aeruginosa was investigated.[2,70] The initial study showed that subinhibitory levels of erythromycin, clarithromycin, and azithromycin enhanced the susceptibility of the organism to serum bactericidal activity by altering cell surface structures.[70] Exposure time was the key factor. Serum susceptibility was unaffected at 12 and 24 hours but was significantly enhanced at 36 hours (p<0.05). A later study assessed the viability of P. aeruginosa after incubation periods exceeding 24 hours and reported similar results.[2] Azithromycin decreased viability of P. aeruginosa, the extent of which depended on both drug concentration and incubation time. Nonmucoid strains were more susceptible to inhibition than mucoid strains, and protein synthesis was inhibited. This was the first study to show direct antipseudomonal activity of macrolides at clinically achievable concentrations.

An exciting discovery was reported at the fourteenth annual North American Cystic Fibrosis Conference in November 2000. Dr. Ugo Pradal and colleagues from the Cystic Fibrosis Center in Verona, Italy, presented an abstract showing that long-term azithromycin therapy may restore chloride transport in some patients.[71] In 10 patients in whom azithromycin had shown good clinical benefit, the nasal potential difference was studied at baseline and after 1 month of treatment with azithromycin 500 mg/day. Before treatment all patients had abnormalities of ion transport typical of cystic fibrosis. Restoration of chloride secretion was observed in 6 of 10 patients after 1 month of treatment (p=0.02). The mechanism behind the increase is unknown.

Studies analyzed the synergistic effect of macrolides with antipseudomonal antibiotics. Murine models of biofilm-forming P. aeruginosa respiratory infection were treated with placebo, clarithromycin, levofloxacin, or both clarithromycin and levofloxacin for 10 days.[72] Treatment with levofloxacin or clarithromycin alone had no significant effect on the number of viable bacteria in the lungs; however, the two antibiotics together produced a significant decrease in that number (p<0.05). Another study showed approximately 85% of P. aeruginosa survived inside biofilm after incubation at twice the minimum bactericidal concentration of ciprofloxacin.[53] A remarkable killing effect was observed when ciprofloxacin was combined with clarithromycin or azithromycin. Others, however, reported no significant synergism between tobramycin, amikacin, piperacillin, ceftazidime, or imipenem and erythromycin or roxithromycin at concentrations achievable in blood.[58]

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