Context: Metyrapone and ketoconazole, frequently used steroidogenesis inhibitors for treatment of Cushing's syndrome, can be associated with side effects and limited efficacy. Osilodrostat is a CYP11B1 and CYP11B2 inhibitor, with unknown effects on other steroidogenic enzymes.
Objective: To compare the effects of osilodrostat, metyrapone, and ketoconazole on adrenal steroidogenesis, and pituitary adenoma cells in vitro.
Methods: HAC15 cells, seventeen primary human adrenocortical cell cultures, and pituitary adenoma cells were incubated with osilodrostat, metyrapone, or ketoconazole (0.01-10 µM). Cortisol and ACTH were measured using chemiluminescence immunoassays, and steroid profiles by liquid chromatography-mass spectrometry.
Results: In HAC15 cells, osilodrostat inhibited cortisol production more potently (IC50: 0.035 µM) than metyrapone (0.068 µM; P<0.0001), and ketoconazole (0.621 µM; P<0.0001). IC50 values of osilodrostat and metyrapone for basal cortisol production varied with a 25-fold and 18-fold difference, respectively, with comparable potency. Aldosterone production was inhibited more potently by osilodrostat versus metyrapone and ketoconazole. Osilodrostat and metyrapone treatment resulted in strong inhibition of corticosterone and cortisol, 11-deoxycortisol accumulation, and modest effects on adrenal androgens. No pituitary-directed effects of osilodrostat were observed.
Conclusions: Under our study conditions, osilodrostat is a potent cortisol production inhibitor in human adrenocortical cells, comparable with metyrapone. All steroidogenesis inhibitors showed large variability in sensitivity between primary adrenocortical cultures. Osilodrostat might inhibit CYP11B1 and CYP11B2, in some conditions to a lesser extent CYP17A1 activity, and a proximal step in the steroidogenesis. Osilodrostat is a promising novel treatment option for Cushing's syndrome, and in vivo differences with metyrapone are potentially driven by pharmacokinetic differences.

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Journal Journal of Clinical Endocrinology and Metabolism
Creemers, S.G, Feelders, R.A, de Jong, F.H, G.J.H. Franssen, de Rijke, Y.B, van Koetsveld, P.M, & Hofland, L.J. (2019). Osilodrostat is a Potential Novel Steroidogenesis Inhibitor for the Treatment of Cushing's Syndrome. Journal of Clinical Endocrinology and Metabolism. doi:10.1210/jc.2019-00217