Challenges of New Approaches in Metastatic Merkel Cell Carcinoma

Merkel cell carcinoma is a rare and aggressive cutaneous tumor, and the use of checkpoint inhibitors immunotherapy is a recent indication in its metastatic setting, both first and second line. However, the widespread use of immunotherapy is associated with an increase of acute and late immune-mediated adverse events. We present a case of an elderly fit patient with metastatic Merkel cell carcinoma treated with pembrolizumab who developed diabetic ketoacidosis, a severe immune-mediated adverse event. A multidisciplinary approach was crucial to overcome the life-threatening event. Even with early treatment stop, the patient had a significant and durable response to the treatment for 15 months. Meanwhile, a progressive pan-cerebellar syndrome emerged, possible due to a paraneoplastic syndrome with a negative onco-neuronal antibody panel, although an autoimmune etiology associated with immunotherapy could not be excluded. Unfortunately, the situation was irreversible and refractory to immunomodulatory treatment. Despite the unpredictable toxicity, it is important to note the efficacy profile, with a progression-free survival of 15 months, which is higher than the one reported in reference clinical trials in this setting.