Use of PRRT and osilodrostat for ectopic Cushing syndrome secondary to thymic neuroendocrine neoplasm.

Summary: We present a case of recurrent thymic neuroendocrine neoplasm (NEN) in a 43-year-old male, who developed ectopic Cushing syndrome (CS), 8 years after his initial diagnosis. Due to persistent biochemical hypercortisolism and clinical symptoms of CS, he was trialled on multiple adrenal steroidogenesis inhibitors with surgical resection and radiotherapy. Eventually, he was transitioned to osilodrostat, a new potent adrenal steroidogenesis inhibitor. Following multidisciplinary discussion, he underwent peptide receptor radionuclide therapy (PRRT). After 12 months of therapy, his cortisol levels have normalised. This is a novel case of functional thymic NEN that was biochemically responsive to osilodrostat and PRRT. Osilodrostat predominantly blocks 11-β hydroxylase leading to a reduction in biochemical hypercortisolism and improvement in clinical manifestations. Initial evidence supported its role in the setting of Cushing's disease. There is growing evidence for its use in the setting of ectopic CS as observed with our case. PRRT continues to have a role in the setting of functional NENs. Learning points: Ectopic CS is a rare, functional syndrome most often seen with neuroendocrine tumours. Management relies on treatment of the primary pathology as well as hypercortisolism, for which there are new novel agents available including osilodrostat. Directed therapies such as peptide receptor radionuclide therapy can have favourable outcomes in addressing metabolic and biochemical disease in the setting of ectopic CS.