Pharmacokinetic parameters and mechanisms of inhibition of rat type 1 and 2 steroid 5 alpha-reductases: determinants for different in vivo activitiesof GI198745 and finasteride in the rat

The interaction of baculovirus expressed rat steroid 5 alpha -reductase typ es 1 and 2 (r5AR1 and r5AR2) with 17 beta -N-(2,5-bis(trifluoromethyl)pheny l)carbamoyl-4-aza-5 alpha -androst-1-en-3-one (GI198745) was investigated a t pH 7 and 37 degrees. This 5 alpha -reductase inhibitor was found previous ly to be a time-dependent inhibitor of the two human 5a-reductase isozymes. In contrast, we demonstrate in the present study that although GI198745 is a potent time-dependent inhibitor of r5AR2, it is a classical rapid-equili brium inhibitor of r5ARL This type of behavior with human and rat 5 alpha - reductases has been shown for the inhibitor 17 beta-(N-tert-butylcarbamoyl) -4-aza-5 alpha -androst-1-en-3-one (finasteride), a current therapy for ben ign prostatic hyperplasia. Inhibition of r5AR1 by GI198745 was competitive with testosterone and followed Michaelis-Menten kinetics with a K-i value o f 0.3 +/- 0.02 nM. Data for the inhibition of r5AR2 by GI198745 were consis tent with a two-step mechanism, where K-i is the dissociation constant for an initial enzyme-inhibitor complex and k(3) is the rate constant for the s econd slow step. The pseudo-bimolecular rate constant (k(3)/K-i) for the as sociation of GI198745 with r5AR2 was (2.0 +/- 0.4) X 10(7) M-1 sec(-1). The high affinity of this inhibitor for r5AR2 was further demonstrated by the inability of the enzyme-inhibitor complex to dissociate after approximately 7 days of dialysis at 4 degrees. Both GI198745 and finasteride appear to i nactivate r5AR2 by apparent irreversible modification, but are classical, r eversible inhibitors of r5ARL Therefore, we hypothesize that because of its pharmacokinetic parameters and increased potency against r5AR1, GI198745 i s more effective than finasteride in preventing the growth of the rat prost ate. (C) 2001 Elsevier Science Inc. All rights reserved.