MON-461 Drug Repurposing to Explore Novel Treatment for Cushing Disease

Abstract

Cushing Disease (CD), caused by adrenocorticotropic hormone (ACTH)-secreting pituitary adenomas, leads to excess production of adrenal-derived cortisol. Current medical treatments for CD include adrenal steroidogenesis inhibitors (Ketoconazole, Metyrapone), the somatostatin receptor ligand Pasireotide, and the glucocorticoid receptor antagonist (Korlym). Although Pasireotide is tumor directed, its effect on tumor growth is modest, and it frequently causes hyperglycemia. Additionally, long-term compliance with these drugs is < 30% - either due to escape from control caused by increased tumor-derived ACTH or drug side effects. Clearly there is a large unmet need for improved medical treatments for CD. We hypothesize that an ideal drug would act on the tumor itself, potently and selectively inhibit corticotroph tumor-derived ACTH production to attain sustained eucorticolism and inhibit corticotroph tumor growth. Using our recently developed ACTH quantification assay (ACTH AlphaLISA), we screened an annotated kinase inhibitor library (KIL, n=430) at 10μM, 1μM and 100nM doses. The KIL contains inhibitors of PI3K/AKT/mTOR (n=95), protein tyrosine kinases (n=87), MAPK (n=45), angiogenesis (n=44), cell cycle (n=44), JAK/STAT (n=26), and others (n=89). Out of 430 compounds screened, 6, 20 and 115 compounds exhibited >50% ACTH AlphaLISA inhibition and 36, 105 and 263 compounds exhibited >50% nuclei inhibition in murine corticotroph tumor AtT20 cells at 100nM, 1μM and 10μM respectively. Three out of the 6 compounds that exhibited efficacy >50% at 100nM are currently being evaluated in Phase II clinical trials in a variety of solid tumors. Given the involvement of PI3K/AKT pathway in pituitary tumor pathogenesis and recent findings of aberrant USP8 mutant-associated EGFR activation in corticotroph tumors, we have thus far performed dose-response curves for one of the PI3K inhibitors using two separate assays of ACTH secretion (AlphaLISA and ELISA) and cell proliferation (Hoechst nuclei staining and CellTilter Glow assay). Both demonstrated comparable inhibition rates with an EC50 of 246nM for ACTH inhibition determined by AlphaLISA assay, and LC50 of 18 nM for cytotoxicity. Experiments to elucidate the mode of action of the PI3K inhibitor on ACTH synthesis and/or secretion, demonstrated it inhibited POMC mRNA expression by 40% at 300nM and 87% at 1μM (p<0.001), indicating the drug may block activation of POMC regulatory proteins such as the known PI3K targets AKT and mTOR. Given the already defined drug safety and efficacy profiles of many of these drugs, and their potent action at low doses to inhibit not only ACTH secretion but additionally corticotroph tumor cell proliferation, some may be suitable for drug repurposing studies to develop novel CD therapies.