Treatment strategies and safety of rechallenge in the setting of immune checkpoint inhibitors-related myositis: A national multicenter study.

OBJECTIVES The occurrence of immune-related myositis (irM) is increasing, yet there are no therapeutic guidelines. We sought to analyze the current therapeutic strategies of irM and evaluate the outcomes of immune checkpoint inhibitors (ICI) rechallenge. METHODS We conducted a nationwide retrospective study between April 2018 and March 2020 including irM without myocardial involvement. Depending on the presence of cutaneous signs or unusual histopathological features, patients were classified into two groups: typical or atypical irM. Therapeutic strategies were analyzed in both groups. The modalities and outcomes of ICI rechallenge were reviewed. RESULTS Among the 20 patients, 16 presented typical irM. Regardless of severity, most typical irM were treated with steroid monotherapy (n = 14/16) and all had a complete response within ≤ 3 weeks. The efficacy of oral steroids for non-severe typical irM (n = 10) was the same with low-dose (≤ 0.5 mg/kg/day) or high-dose (1 mg/kg/day). Severe typical irM were successfully treated with intravenous methylprednisolone. Atypical irM (n = 4) had a less favorable evolution, including one irM-related death, and required heavy immunosuppression. ICI were safely reintroduced in 9 patients presenting a moderate (n = 6) or a severe (n = 3) irM. CONCLUSION Our data highlight that steroid monotherapy is an effective treatment for typical irM, either with prednisone or with intravenous methylprednisone pulses depending on the severity. The identification of unusual features is important in determining the initial therapeutic strategy. The outcomes of rechallenged patients are in favor of a safe reintroduction of ICI following symptom resolution and CK normalization in moderate and severe forms of irM.