Hypothesis

In rheumatoid arthritis, baseline subclinical interstitial lung abnormality on chest imaging plus early dyspnea trajectory will outperform smoking history alone for predicting clinically significant leflunomide-associated interstitial lung toxicity within the first 20 weeks of therapy.

Rationale

Leflunomide lung toxicity is uncommon, but the strongest signals in case series and cohort analyses cluster around pre-existing ILD, abnormal baseline imaging, and early exposure windows rather than generic smoking history alone. If true, a simple baseline pulmonary-risk framework could reduce avoidable exposure in vulnerable patients.

Testable prediction

In a prospective RA cohort starting leflunomide, a model containing:

• baseline ILD/interstitial lung abnormality on HRCT or chest imaging,
• early symptom slope (dyspnea/cough during the first 8-20 weeks), and
• oxygenation change, will show better discrimination for severe drug-related lung toxicity than a comparator model using age + smoking history alone.

Falsification strategy

This hypothesis is weakened if imaging abnormalities do not improve discrimination or if smoking history alone performs equivalently after adjustment for baseline lung disease.

Suggested study design

• Multicenter new-user cohort of RA patients starting leflunomide
• Baseline chest-imaging classification before exposure where available
• Time-updated symptom capture and oximetry
• Primary endpoint: hospitalization, drug discontinuation for suspected lung toxicity, or adjudicated ILD/pneumonitis event within 20 weeks
• Analysis: penalized logistic regression and time-updated survival modeling with bootstrap calibration

Limitations

Confounding by rheumatoid ILD progression and pulmonary infection is substantial; adjudication must be blinded and conservative.

References

1. Sawada T, Inokuma S, Sato T, et al. Leflunomide-induced interstitial lung disease: prevalence and risk factors in Japanese patients with rheumatoid arthritis. Rheumatology (Oxford). 2009;48(9):1069-1072. DOI: 10.1093/rheumatology/kep163
2. Suissa S, Bernatsky S, Hudson M. Antirheumatic drug use and the risk of hospitalization for interstitial lung disease in rheumatoid arthritis. Arthritis Rheum. 2006;54(5):1435-1439. DOI: 10.1002/art.21799
3. Roubille C, Haraoui B. Interstitial lung diseases induced or exacerbated by DMARDs and biologic agents in rheumatoid arthritis: a systematic literature review. Semin Arthritis Rheum. 2014;43(5):613-626. DOI: 10.1016/j.semarthrit.2013.09.005