Racemic ketoconazole (RK) is a steroidogenesis inhibitor used for treatment of Cushing's syndrome. Levoketoconazole (COR-003), the pure 2S,4R enantiomer, is potentially more potent and safe compared to RK. We compared in vitro effects of levoketoconazole and RK on adrenocortical and pituitary adenoma cells.HAC15 cells and 15 primary human neoplastic adrenocortical cultures (+/- ACTH), and murine (AtT20) and human corticotroph adenoma cultures were incubated with levoketoconazole or RK (0.01-10µM). Cortisol and ACTH were measured using a chemiluminescence immunoassay system, and steroid profiles by liquid chromatography-tandem mass spectrometry (LC-MS/MS).In HAC15, levoketoconazole inhibited cortisol at lower concentrations (IC50: 0.300µM) compared to RK (0.611µM; P<0.0001). IC50 values of levoketoconazole for basal cortisol production in primary adrenocortical cultures varied over a 24-fold range (0.00578µM to 0.140µM), with in two patients a higher sensitivity of levoketoconazole versus RK (2.1- and 3.7-fold). LC-MS/MS analysis in selected cases revealed more potent inhibition of cortisol and other steroid profile components by levoketoconazole versus RK. In AtT20, levoketoconazole inhibited cell growth and ACTH secretion (10µM: -54% and -38%, respectively), and levoketoconazole inhibited cell number in one of two primary human corticotroph pituitary adenoma cultures (-44%, P<0.001).Levoketoconazole potently inhibits cortisol production in adrenocortical cells, with a variable degree of suppression between specimens. Levoketoconazole inhibits adrenal steroid production more potently compared to RK and might also inhibit ACTH secretion and growth of pituitary adenoma cells. Together with previously reported potential advantages, this indicates that levoketoconazole is a promising novel pharmacotherapy for Cushing's syndrome.