Pulmonary toxicity encompasses a diverse array of adverse respiratory effects induced by medications, ranging from mild bronchospasm to life-threatening pneumonitis and irreversible pulmonary fibrosis. The lungs are continuously exposed to bloodborne agents and, for inhaled drugs, directly exposed to high concentrations of the offending substance. The limited regenerative capacity of alveolar epithelium and the delicate structure of the alveolar-capillary interface make the lungs susceptible to injury that can progress rapidly and carry substantial morbidity and mortality.
Mechanisms of injury include direct cytotoxicity, oxidative stress, immune-mediated inflammation, and disruption of pulmonary surfactant. Direct cytotoxicity damages alveolar epithelial cells and capillary endothelial cells, leading to increased permeability and interstitial inflammation. Oxidative stress results from the generation of reactive oxygen species that overwhelm antioxidant defenses. Immune-mediated mechanisms involve recruitment of inflammatory cells and release of pro-fibrotic cytokines that drive collagen deposition and architectural distortion of lung parenchyma. Some agents produce injury through multiple pathways, while others have highly specific mechanisms.
Bleomycin, a chemotherapeutic agent used in lymphomas and germ cell tumors, is the prototypical cause of drug-induced pulmonary fibrosis. The mechanism involves bleomycin-induced DNA damage in lung cells, which have relatively low levels of the inactivating enzyme bleomycin hydrolase. Pulmonary toxicity occurs in approximately 10 to 25 percent of treated patients and is dose-dependent, with cumulative doses above 400 to 450 units carrying the highest risk. Risk factors include advanced age, preexisting lung disease, renal impairment, high inspired oxygen concentrations, and concurrent thoracic radiation. The presentation ranges from an early pneumonitis with cough, fever, and infiltrates to a late fibrotic phase that can progress even after drug discontinuation.
Amiodarone, a class III antiarrhythmic agent, causes pulmonary toxicity in up to 10 percent of patients, making it one of the most common non-chemotherapy causes of drug-induced lung disease. The drug accumulates in lung tissue due to its long half-life and lipophilic properties, where it triggers phospholipid accumulation and inflammatory injury. Presentation is insidious, with progressive dyspnea, nonproductive cough, weight loss, and bilateral interstitial infiltrates on imaging. The risk is related to cumulative dose and is higher with daily doses exceeding 400 mg. Discontinuation of the drug and corticosteroid therapy generally lead to improvement, though complete resolution may take months.
Methotrexate causes pneumonitis in approximately 1 to 5 percent of patients, particularly in those receiving high-dose regimens for rheumatic diseases or oncology. The reaction is typically hypersensitivity-mediated, presenting acutely with fever, cough, dyspnea, and bilateral pulmonary infiltrates. Eosinophilia may be present in peripheral blood or bronchoalveolar lavage fluid. Symptoms usually resolve rapidly upon drug discontinuation and treatment with corticosteroids. Nitrofurantoin, an antibiotic used for urinary tract infections, produces both acute and chronic pulmonary reactions. The acute form presents with fever, cough, dyspnea, and pulmonary infiltrates within days to weeks of initiation, while the chronic form resembles interstitial pneumonitis with insidious onset after months to years of use.
Other chemotherapeutic agents associated with pulmonary toxicity include cyclophosphamide, busulfan, carmustine, and gemcitabine. Targeted therapies including EGFR inhibitors and immune checkpoint inhibitors have introduced new pulmonary toxicity patterns. Checkpoint inhibitor pneumonitis occurs in 3 to 5 percent of patients and can be severe, requiring high-dose corticosteroids and permanent discontinuation of immunotherapy. Bronchospasm can be induced by beta-blockers, NSAIDs in aspirin-sensitive individuals, and cholinergic agents, particularly in patients with underlying airway hyperreactivity.
Diagnosis and monitoring require a high index of suspicion, thorough medication history, pulmonary function testing with diffusing capacity for carbon monoxide, and high-resolution computed tomography. Bronchoscopy with bronchoalveolar lavage may help exclude infection and identify inflammatory patterns. The key to management is early recognition and discontinuation of the offending agent, supplemented by corticosteroids for inflammatory reactions and supportive care including oxygen therapy.
