Selective inhibitor of aldo-keto reductase 1C1 (AKR1C1; Ki = 4 nM) versus AKR1C2, AKR1C3, and AKR1C4 (Ki = 0.087, 4.2, and 18.2 μM, respectively); potently inhibits the metabolism of progesterone in endothelial cells overexpressing AKR1C1 (IC50 = 460 nM).