Pembrolizumab-Induced Myositis: Diagnostic and Therapeutic Challenges From Two Case Reports and a Narrative Review

Pembrolizumab, an anti-PD-1 immune checkpoint inhibitor, has transformed the treatment of multiple solid tumors. However, it can rarely cause immune-related myositis, which may involve limb, bulbar, and respiratory muscles, posing a risk of severe morbidity and mortality. Early recognition and management are essential. We report two cases of pembrolizumab-induced myositis diagnosed and treated at our institution. A literature search of MEDLINE/PubMed was conducted to identify English-language case reports, focusing on clinical presentation, diagnostics, management, and outcomes.

The first case involved a 74-year-old woman with advanced lung adenocarcinoma who developed grade 4 myositis affecting limb and phonatory muscles; intravenous methylprednisolone combined with IVIG led to full recovery. The second case involved a 78-year-old woman with advanced endometrial carcinoma who developed grade 3 limb-girdle myositis, successfully treated with oral prednisone. Literature review (11 case reports) indicated that most patients were older males with lung cancer, presenting with proximal muscle weakness and elevated CK. Bulbar or ocular involvement was linked to a higher risk of myocarditis and respiratory complications. Early corticosteroid therapy, with IVIG for severe cases, was consistently effective.

Pembrolizumab-induced myositis is a rare but potentially life-threatening adverse event. Recognition of characteristic clinical features, prompt immunosuppressive therapy, and multidisciplinary care are critical for favorable outcomes. Additional studies are needed to establish standardized diagnostic criteria and optimal treatment protocols.