Drug Repurposing in Cancer and Cushing’s Disease: Halogenated Salicylanilides Inhibit USP8 Catalytic Activity and ACTH Release by Pituitary Cells

Ubiquitin-specific protease 8 (USP8) is a key regulator of various cellular processes including membrane dynamics and receptor trafficking. USP8 overexpression contributes to chemotherapy resistance in cancer. Furthermore, USP8 gain-of-function variants are commonly found in patients with Cushing’s disease, which is characterized by a pituitary adenoma and dysregulated adrenocorticotropic hormone (ACTH) secretion. In this study, we report that closantel, a halogenated salicylanilide, is a potent and reversible inhibitor of USP8 catalytic activity. Using a screening approach under buffered conditions containing l -cysteine but lacking dithiothreitol, which prevented the selection of oxidizing compounds, we identified closantel from a library of 2240 U.S. Food and Drug Administration (FDA)-approved drugs and compounds under clinical investigation. We further demonstrated that closantel treatment led to a dose-dependent reduction of ACTH secretion by pituitary cells and a decreased expression of the POMC gene, which encodes the precursor of ACTH. In addition to closantel, we showed that other halogenated salicylanilides, including niclosamide, a compound currently under investigation in several clinical trials for cancer treatment, act as inhibitors of both USP8 activity and ACTH secretion by pituitary cells. Our findings emphasize the potential of repurposing halogenated salicylanilides to treat Cushing’s disease and cancer.