A Likely Case of Abemaciclib-Induced Hyperpigmentation in a Patient With Metastatic Breast Cancer

We report a case of a 64-year-old female with a past medical history of invasive right breast adenocarcinoma presented with diffuse hyperpigmentation of her skin after admission to the hospital for an infected breast implant. She had no recollection of a similar cutaneous reaction in her past. The patient had been on a chronic regimen of anastrozole and abemaciclib for her metastatic breast cancer. A punch biopsy revealed results were highly suspicious for a drug-induced hyperpigmentation reaction. After a thorough review of the patient's current and past medication lists, it was determined that her abemaciclib was the most likely culprit of her hyperpigmentation. This case is significant because of the rarity of this possible specific cutaneous reaction to abemaciclib. The literature that exists on cyclin-dependent kinase 4 and 6 inhibitors (CDK 4/6) is minimal. And so, the importance of shedding light on its possible cutaneous side effects is not only helpful for clinician diagnosis but also essential for patients to make informed decisions. To our knowledge, there is no other published literature on likely abemaciclib-induced hyperpigmentation.