KF20405) showed the maximum potency with an IC50 value of 0.48 +/- 0.086 nM, which was 20-fold higher potency than 1 (MK-906). Compound 16 effectively inhibited DHT production 4 h after a 3 mg/kg oral administration. Several potent indole derivatives, 1 and 2 ((+/-)-ONO-3805), were tested versus rat and human isozymes. Nonsteroidal inhibitors such as indole derivatives and 2 were 2-3 orders of magnitude