Drug repurposing: halogenated salicylanilides inhibit USP8 catalytic activity and ACTH release by pituitary cells

Ubiquitin-specific protease 8 (USP8) has emerged as a crucial regulator of various cellular processes including membrane dynamics and receptor trafficking. Additionally, USP8 gain-of-function variants are frequently observed in Cushings disease, a severe condition characterized by a pituitary adenoma and dysregulated adrenocorticotropic hormone (ACTH) secretion. Moreover, USP8 overexpression contributes to resistance to chemotherapy in cancer. Here, we report the identification of Closantel, a halogenated salicylanilide, as a potent and reversible inhibitor of USP8 catalytic activity. We defined a screening approach in buffering condition counteracting the selection of oxidizing compounds and identified Closantel as a potent inhibitor of USP8 among a library of 2,240 FDA-approved drugs and compounds under clinical development. We demonstrated further that Closantel treatment led to a dose-dependent reduction of ACTH secretion by pituitary cells and resulted in the decrease in POMC gene expression, the precursor of ACTH, highlighting its potential therapeutic efficacy in Cushings disease. In addition to Closantel, we identified several other halogenated salicylanilides, including Niclosamide that is subjected to several clinical investigations in cancer treatment, as inhibitors of both USP8 activity and ACTH secretion by pituitary cells. Our findings underscore the repurposing potential of halogenated salicylanilides in targeting USP8-mediated cellular dysfunctions in Cushings disease and cancer.