ReviewThe bleomycin animal model: A useful tool to investigate treatment options for idiopathic pulmonary fibrosis?
Introduction
Idiopathic pulmonary fibrosis (IPF) is a chronic progressive and ultimately fatal lung disease of unknown etiology. Its prognosis is poor and the outcome even worse than in many malignant diseases. IPF is one of the most frequent interstitial lung diseases and is characterized by the histological pattern of usual interstitial pneumonia (UIP) (ATS, 2000). The natural history of IPF is unknown and the onset of symptoms is gradual, starting usually with non-productive cough and exertional dyspnea. With involvement of larger areas of the lung, severe dyspnea at rest and signs of right heart failure develop (ATS, 2002). In some cases the clinical state is preserved for a period of several years, but the majority of patients deteriorate more rapidly. Mortality during acute exacerbation is high. The prevalence of IPF is estimated at 20/100,000 for males and 13/100,000 for females, and survival time from diagnosis ranges from 2 to 4 years (Kim, Collard, & King, 2006). Histological characteristics of UIP include remodeling of lung architecture with fibroblastic foci and “honeycombing”. The lung involvement is patchy with a predominantly basal and subpleural pattern of matrix deposition and tissue distortion (ATS, 2002). Most patients present at an advanced stage of disease. Treatment options for pulmonary fibrosis are limited. The clinical management focuses on treatment of complications (e.g. right heart failure, infections, etc.), supportive care and in few cases involves lung transplantation. Anti-inflammatory drugs such as prednisone may carry symptomatic relief, but they do not appear to halt progression of fibrosis, and their beneficial effects in IPF remain in question. Cytotoxic drugs (cyclophosphamide, azathioprin, etc.) have not been shown to improve lung function or life expectancy and may be associated with harmful side effects.
The last two decades have markedly improved the knowledge about underlying mechanisms of pulmonary fibrosis and helped to identify potential targets for novel therapies. However, despite the large number of antifibrotic drugs being described in experimental pre-clinical studies, the translation of these findings into clinical practice has not been accomplished yet. This review will focus on the bleomycin model of pulmonary fibrosis, highlight its undisputable contribution to investigation of basic pathomechanism of disease and critically reflect its usefulness in determining efficacy of antifibrotic drugs.
Section snippets
Animal models of pulmonary fibrosis
Animal models play an important role in the investigation of diseases, and many models are established to examine pulmonary pathobiology. Chronic diseases are more difficult to model. The situation with IPF is even more complicated, since the etiology and natural history of the disease is unclear and no single trigger is known that is able to induce “IPF” in animals. Different models of pulmonary fibrosis have been developed over the years. Most of them mimic some, but never all features of
Bleomycin
Bleomycin is a chemotherapeutic antibiotic, produced by the bacterium “Streptomyces verticillus” (Adamson, 1976, Umezawa, 1967). Its use in animal models of pulmonary fibrosis is based on the fact that fibrosis is one of the major adverse drug effects of bleomycin in human cancer therapy. Bleomycin plays an important role in the treatment of lymphoma, squamous cell carcinomas, germ cell tumors and malignant pleural effusion, where it is injected intrapleurally. It is believed that bleomycin
The bleomycin animal model
Bleomycin as an agent to induce experimental lung fibrosis was first described in dogs (Fleischman et al., 1971), later in mice (Adamson & Bowden, 1974), hamsters (Snider, Celli, Goldstein, O’Brien, & Lucey, 1978), and rats (Thrall, McCormick, Jack, McReynolds, & Ward, 1979). It causes inflammatory and fibrotic reactions within a short period of time, even more so after intratracheal instillation. The initial elevation of pro-inflammatory cytokines (interleukin-1, tumor necrosis factor-α,
Drug intervention studies in the bleomycin model
The bleomycin animal model is widely used in the assessment of potential antifibrotic agents. A large number of compounds have been shown to prevent fibrotic progression in this model and have been suggested to qualify for clinical use. We performed a Pub Med search and identified 232 papers published between 1980 and 2006 which discuss antifibrotic compounds in the bleomycin model. All these compounds were reported to be successful and antifibrotic, either as “preventive treatment” (that means
Selected examples of preventive compounds
Ginkgo biloba is a flavonoid-rich antioxidant, containing ginkgolides extracted from Ginkgo leaves. Clinically, this substrate is used as a memory enhancer, anti-vertigo agent and for intermittent claudication. Evidence exists that Gingko biloba improves blood flow, protects from free radicals and blocks platelet aggregation and blood clotting (Dubey, Shankar, Upadhyaya, & Deshpande, 2004; Ernst, 2002, Mahady, 2002). These properties appear to be potentially antifibrotic, and for that reason
Selected examples of therapeutic compounds
Pirfenidone is an orally active small molecule drug with anti-inflammatory, antioxidant and antifibrotic effects. It is known that Pirfenidone modifies the regulation of cytokines, including PDGF, and thereby inhibits fibroblast proliferation and extracellular matrix synthesis. It has also been shown to reduce the increase in TGF-β levels after bleomycin administration. The exact mechanism for the antifibrotic effect is not yet fully understood (Gurujeyalakshmi et al., 1999). Therapeutic
Clinical trials
Only a relatively small number of compounds considered as having “promising antifibrotic properties” in the bleomycin model were or currently are tested in clinical trials (Walter, Collard, & King, 2006). Some of them were retrospective analyses or case series only. Among the drugs tried or on trial are Etanercept, Imatinib, Prednisone (Daniil et al., 1999; Douglas et al., 1997; Douglas, Ryu, & Schroeder, 2000; Douglas et al., 1998; Nicholson, Dubois, Hansell, & Wells, 2000; Riha et al., 2002;
Selected examples of compounds in clinical trials
N-Acetylcysteine (NAC) is a precursor in the formation of the antioxidant glutathione and possesses the ability of reducing free radicals. This drug has been in clinical use as mucolytic therapy in a variety of respiratory diseases, in the management of acetaminophen overdose, and in the prevention of radiocontrast-induced nephropathy by augmenting glutathione reserves for binding of toxic metabolites. In the context of pulmonary fibrosis, is has shown effectiveness as preventive medication in
Summary
Major discrepancies between drug effects in animal models and in human trials have recently been pointed out, which may be due to design of the models, assessment tools for determination of drug efficacy, and timing of drug application (Perel et al., 2007). These facts have to be taken into consideration, especially for long-term drug intervention studies. In the context of pulmonary fibrosis and the bleomycin model it means that experimental findings have to be interpreted carefully, with the
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