|
1: Biochem Pharmacol 2001
Oct 1;62(7):933-42
Pharmacokinetic parameters
and mechanisms of inhibition of rat type 1 and 2 steroid
5alpha-reductases: determinants for different in vivo
activities of GI198745 and finasteride in the rat.
Stuart JD, Lee FW, Simpson Noel D, Kadwell SH, Overton LK,
Hoffman CR, Kost TA, Tippin TK, Yeager RL, Batchelor KW,
Bramson HN.
Division of Biochemistry, Glaxo Wellcome Inc., 5 Moore Drive,
Research Triangle Park, NC 27709, USA. jds14989@gsk.com
The interaction of baculovirus expressed rat steroid
5alpha-reductase types 1 and 2 (r5AR1 and r5AR2) with
17beta-N-(2,5-bis(trifluoromethyl)phenyl)carbamoyl-4-aza-5alpha-androst-1-en-3-one
(GI198745) was investigated at pH 7 and 37 degrees.
This
5alpha-reductase inhibitor was found previously to be a
time-dependent inhibitor of the two human 5alpha-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-equilibrium inhibitor of r5AR1.
This type of behavior with human and rat 5alpha-reductases has
been shown for the inhibitor
17beta-(N-tert-butylcarbamoyl)-4-aza-5alpha-androst-1-en-3-one
(finasteride), a current therapy for benign prostatic
hyperplasia. Inhibition of r5AR1 by GI198745 was competitive
with testosterone and followed Michaelis-Menten kinetics with
a K(i) value of 0.3 +/- 0.02 nM.
Data for the inhibition of
r5AR2 by GI198745 were consistent 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
second slow step.
The pseudo-bimolecular rate constant (k(3)/K(i))
for the association 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 inactivate r5AR2 by apparent irreversible
modification, but are classical, reversible inhibitors of
r5AR1.
Therefore, we hypothesize that because of its
pharmacokinetic parameters and increased potency against
r5AR1, GI198745 is more effective than finasteride in
preventing the growth of the rat prostate.
|
