LAUSR

e14118 Background: A rare and often fatal spectrum of NM immune-related adverse events (irAEs) includes myositis, myocarditis, and myasthenia gravis (MG). A deeper understanding of their clinical and immunologic features may improve outcomes. Methods: Sixteen patients (pts) with genitourinary cancers who were treated with ICI and developed myositis, myocarditis, MG, or overlap syndrome (≥2) were identified. Diagnostic criteria included serum CK (n=16), aldolase (n=14), troponin (n=12), BNP (n=15), autoantibodies (n=16), and cytokines (n=9). Biopsies of skeletal muscle (n=13) and endomyocardium (n=4) were also obtained. Immune profiling is being performed on blood and tissue for pts consented to an institutional-approved laboratory protocol. Results: ICI-associated NM irAEs consisted of isolated myositis (n=8, 50%), myocarditis (n=1, 6%), MG (n=1, 6%), or overlap syndrome (n=6, 38%), with the most common presenting symptom being weakness (n=9, 56%). CK levels were elevated in 15 (94%) pts, and autoantibodies were detected in 9 (56%). Muscle biopsies showed predominantly T cell infiltration. All 16 pts received corticosteroids, 12 (75%) were treated with other immunosuppressive agents, and 14 (88%) underwent plasmapheresis. Five pts (50%) with isolated irAEs achieved symptom resolution, and none in the overlap group. (Table) Conclusions: NM irAEs occurred more frequently with combination ICI, and were associated with weakness, and elevated CK level and serum autoantibodies. Both T and B cells appear to drive disease pathogenesis. The presence of one toxicity on the spectrum warrants evaluation for associated irAEs. Compared to isolated NM irAEs, the overlap syndrome appears to be more treatment refractory. Current guidelines require refinement to identify effective treatments to improve outcomes. [Table: see text]