Abemaciclib-Associated Panniculitis With Fibrosis: Expanding the Dermatologic Spectrum of Cyclin-Dependent Kinase 4/6 Inhibitors

Abemaciclib, a cyclin-dependent kinase 4/6 (CDK4/6) inhibitor, has significantly improved outcomes for patients with hormone receptor-positive/human epidermal growth factor receptor 2-negative (HR+/HER2-) metastatic breast cancer, yet its dermatologic side effects remain incompletely characterized. We report a case of a 55-year-old woman on abemaciclib and fulvestrant who developed painful, erythematous nodules that progressed to hyperpigmented plaques with skin retraction, including on the breasts. A biopsy revealed chronic panniculitis with fibrosis, a histopathologic finding not previously associated with CDK4/6 inhibitors. Despite these cutaneous changes, abemaciclib was continued without dose modification, and the lesions remained stable without ulceration. This case expands the known spectrum of skin toxicities linked to CDK4/6 inhibition. It underscores the importance of recognizing panniculitis as a potential immune-mediated adverse event, which, in select cases, may be managed conservatively without interrupting oncologic therapy.