Interstitial lung diseases (ILDs), which can arise from a broad spectrum of distinct aetiologies, can manifest as a pulmonary complication of an underlying autoimmune and connective tissue disease (CTD-ILD), such as rheumatoid arthritis-ILD and systemic sclerosis (SSc-ILD). Patients with clinically distinct ILDs, whether CTD-related or not, can exhibit a pattern of common clinical disease behaviour (declining lung function, worsening respiratory symptoms and higher mortality), attributable to progressive fibrosis in the lungs. In recent years, the tyrosine kinase inhibitor nintedanib has demonstrated efficacy and safety in idiopathic pulmonary fibrosis (IPF), SSc-ILD and a broad range of other fibrosing ILDs with a progressive phenotype, including those associated with CTDs. Data from phase II studies also suggest that pirfenidone, which has a different—yet largely unknown—mechanism of action, may also have activity in other fibrosing ILDs with a progressive phenotype, in addition to its known efficacy in IPF. Collectively, these studies add weight to the hypothesis that, irrespective of the original clinical diagnosis of ILD, a progressive fibrosing phenotype may arise from common, underlying pathophysiological mechanisms of fibrosis involving pathways associated with the targets of nintedanib and, potentially, pirfenidone. However, despite the early proof of concept provided by these clinical studies, very little is known about the mechanistic commonalities and differences between ILDs with a progressive phenotype. In this review, we explore the biological and genetic mechanisms that drive fibrosis, and identify the missing evidence needed to provide the rationale for further studies that use the progressive phenotype as a target population.

Interstitial lung disease (ILD) secondary to drug-induced lung injury is an increasingly common cause of morbidity and mortality. The number of drugs associated with the development of ILD continues to rise, mainly due to the use of novel monoclonal antibodies and biologicals for neoplastic and rheumatological diseases, and includes, among others, chemotherapeutics, molecular targeting agents, immune checkpoint inhibitors, antibiotics, antiarrhythmics and conventional or biological disease-modifying antirheumatic drugs. Drug-induced ILD (DI-ILD) manifests with a variety of clinical patterns, ranging from mild respiratory symptoms to rapidly progressive respiratory failure and death. In most cases, there are no pathognomonic clinical, laboratory, radiological or pathological features and the diagnosis of DI-ILD is suspected in the presence of exposure to a drug known to cause lung toxicity and after exclusion of alternative causes of ILD. Early identification and permanent discontinuation of the culprit drug are the cornerstones of treatment with systemic glucocorticoids being used in patients with disabling or progressive disease. However, for certain drugs, such as checkpoint inhibitors, the frequency of lung toxicity is such that mitigation strategies are put in place to prevent this complication, and occurrence of DI-ILD is not necessarily synonymous with permanent drug discontinuation, particularly in the absence of valid therapeutic alternatives.