Finasteride - A Slow-Binding 5-Alpha-Reductase Inhibitor

A microsomal preparation of human prostatic tissue was used to study the kinetics of interaction of steroid 5alpha-reductase with finasteride, a known 5alpha-reductase inhibitor. This molecule has been reported to reversibly bind 5alpha-reductase in a competitive manner to testosterone with a K(i) value in the 10 nM range. The results presented in this paper show that enzyme-inhibitor complex formation does not take place instantaneously as assumed in previous studies. At neutral pH and 37-degrees-C, the association of enzyme with inhibitor is governed by a rate constant, k(on), of 2.7 X 10(5) M-1 s-1. This low k(on) value, in combination with the high energy of activation of the association reaction (150 kJ mol-1), indicates that the association process is not diffusion controlled and may proceed through intermediate steps. However, such an intermediate was not detected kinetically under the inhibitor concentrations investigated. We therefore conclude that the equilibrium dissociation constant, K(i)*, for the initial binding of the enzyme to the inhibitor is higher than 1.5 X 10(-7) M. Even at inhibitor concentrations as low as 1 nM, the reaction was completely displaced to the EI complex and no residual activity detected once the equilibrium was reached. Hence, the interaction between finasteride and 5alpha-reductase can also be characterized by a very low overall equilibrium dissociation constant (K(i) << 10(-9) M), at least 1 order of magnitude lower than previously reported values. pH profiles of k, the pseudo-first-order rate constant for the association of enzyme with inhibitor, and V(m)/K(m) were similar, indicating that the same ionizable groups are involved in the interaction of the enzyme with both testosterone and finasteride. Our results show that finasteride can efficiently compete with testosterone for the binding to 5alpha-reductase since we demonstrate that K(i) << K(m). The data presented in this paper support what is observed in vivo, predicting that pseudoirreversible inhibition of steroid 5alpha-reductase will take place in the presence of testosterone.