Lost in translation; from animal models of pulmonary fibrosis to human disease

Pulmonary fibrosis affects over 5 million people worldwide. Of the diseases associated with pulmonary fibrosis, the most frequently found are the idiopathic interstitial pneumonias; a range of disorders that include the inexorable progressive and invariably fatal condition idiopathic pulmonary fibrosis (IPF). Besides the idiopathic fibrosing lung conditions, a diverse range of other maladies may result in the development of lung fibrosis, including connective tissue disease (CTD), acute lung injury, chronic hypersensitivity pneumonitis (HP), sarcoidosis, diseases of industrial exposure, such as asbestosis and chronic lung infection, especially tuberculosis.1 While the mechanisms that drive the development of fibrosis in each of these conditions remain poorly understood, it is becoming increasingly clear that following an initial trigger (be it recurrent alveolar injury in IPF, or inflammation driven by chronic exposure to inhaled allergen as in HP or inhalation of asbestos fibres as in asbestosis), the development of fibrosis is driven by the persistent activation of multiple pathways that are normally involved in the process of wound repair.2 As such, conditions considered clinically to be separate entities actually share multiple common pathogenetic pathways. Consequently, when considering treatment of progressive fibrotic lung disease, the importance of these multiple pathways in driving fibrosis transcends precise diagnostic labels. When selecting therapies for patients with pulmonary fibrosis, clinicians are faced with an essential dichotomy between diseases in which inflammation is central to pathogenesis (e.g. chronic HP, sarcoidosis, CTD) and diseases in which inflammation is a minor component (e.g. IPF). Recent trials have showed that potent anti-inflammatory therapy alters disease progression in scleroderma-associated pulmonary fibrosis.3,4 By contrast, the administration of corticosteroids and immunosuppressants to patients with IPF has been called into question because of a lack of evidence of efficacy.5 The two treatments to show most promise in IPF, N-acetyl cysteine and pirfenidone, both show considerable pleiotropism in interfering with multiple pathways involved in wound healing.6,7 The last decade has seen a number of large international multi-centre treatment trials in IPF, the results of which have been mixed.8 Phase three clinical trials for fibrotic lung disease have proved to be enormously costly and each has taken several years to complete. For this reason, pre-clinical models of disease provide an essential means of exploring candidate therapies, before definitive evaluation in human disease. The rodent bleomycin model of pulmonary fibrosis is widely used in the study of fibrotic lung disease, and especially in IPF.9 Installation of bleomycin into rodent lung produces widespread alveolar epithelial injury and apoptosis, resulting in a marked early influx of inflammatory cells and release of pro-inflammatory cytokines.10,11 In this regard, bleomycin injury at time-points from 0 to 7 days resembles acute lung injury in humans. At time-points from 7 through to at least 28 days, rodents exposed to bleomycin develop progressive fibrosis with activation of many of the key pathways that have been identified in human fibrotic lung disease. Thus, the bleomycin model exemplifies the dichotomy observed in human fibrotic lung disease, with an early inflammatory phase resulting in a later fibroproliferative phase characterised by activation of multiple wound healing pathways. Consequently, treatments aimed at ameliorating fibrosis in the bleomycin model will have differing effects depending on whether they are administered during the inflammatory or the fibroproliferative phase of the bleomycin response.12 In the current issue of Respirology, Pan and colleagues report that Rolipram, a phosphodiesterase (PDE)4 inhibitor, attenuates bleomycin-induced fibrosis in rats.13 PDE4 inhibitors are potent anti-inflammatory agents that inhibit the metabolism of key cellular second messengers, such as cyclic adenosine monophosphate (AMP). In the paper by Pan et al., rats that had received intra-tracheal bleomycin were treated with daily intra-peritoneal injections of Rolipram (or vehicle alone) from day 1 through to day 28. By using a variety of methodologies, the authors were able to show a reduction in inflammation at day 7 post-bleomycin and in fibrosis at day 28 following bleomycin injury. Furthermore, animals administered Rolipram had reduced serum levels of the pro-inflammatory cytokine TNF-α, but, as might be expected, had increased serum levels of cyclic AMP. These results suggest that PDE-4 inhibition is able to modulate the inflammatory response to bleomycin and, in so doing, attenuates the subsequent development of fibrosis. What this study fails to fully investigate, however, is the effect that Rolipram has on the process of fibrogenesis alone. By administering Rolipram during both the inflammatory and fibrotic phases of the bleomycin-induced lung injury response, the authors have failed to dissect out whether PDE4 is important in inflammation alone or whether it also plays a role in directly modulating pathways involved in fibroproliferation. As such, it is difficult to assess the potential of Rolipram as a treatment for IPF, a disease in which inflammation is not a key pathogenetic factor. In an analysis of the utility of the rodent bleomycin model as a tool for investigating potential therapies for IPF, Moeller et al. identified 221 experimental studies in the literature in which treatments had been administered with the intention of ameliorating bleomycin-induced fibrosis in rodents (and in which sufficient experimental methodology was reported to determine the dosing protocols used).14 In 211 of these studies, the treatments were given in a preventative manner (i.e. were administered during the inflammatory phase of the bleomycin response or were even given before bleomycin challenge). Despite the stated intent to explore potential anti-fibrotic therapies, treatments were administered in a therapeutic manner (i.e. given exclusively during the fibrotic phase, at least 7 days, after bleomycin-induced lung injury) in only 10 of the 221 reports. The failure to distinguish between anti-inflammatory effects and the post hoc modulation of the fibrotic response to bleomycin-injury undoubtedly explains why compounds that are highly effective in rodents (e.g. corticosteroids, γ-interferon, cyclosporine A and imatinib) have proved disappointing in the clinic.14 It is noteworthy that among the 10 compounds shown to attenuate fibrosis, when administered exclusively in the fibrotic phase of the bleomycin model, was pirfenidone-a compound that has shown promise in recent clinical trials.15 IPF is a disease in urgent need of effective treatments. The bleomycin model of fibrosis, especially when coupled with targeted genetic mutations in mice, has been a powerful experimental tool for dissecting out the pathways involved in fibroproliferation. However, as a means for validating potential therapies in IPF, the model has frequently proved disappointing. Experimental studies carried out in the bleomycin model have all too frequently overlooked the importance and the inter-relationship of the early inflammatory and later fibrotic phases of the injury response. The majority of investigators rely on interventions that attenuate early inflammation, an approach that fails to acknowledge that the subsequent reduction in fibrosis does not equate to a direct anti-fibrotic drug effect. For a treatment to be considered to have anti-fibrotic effects, it must attenuate bleomycin-induced fibrosis only when administered during the fibrotic phase of the bleomycin injury-repair response. A more rigorous approach to the assessment of candidate molecules in the rodent bleomycin model can only improve the chances of finding more efficacious anti-fibrotic treatments.