Vol. 303, Issue 2, 616-626, November 2002

Neuroactive Steroids Differ in Potency but Not in Intrinsic Efficacy at the GABA_A Receptor in Vivo

S. A. G. Visser, W. W. F. T. Gladdines, P. H. van der Graaf, L. A. Peletier and M. Danhof

Division of Pharmacology, Leiden/Amsterdam Center for Drug Research, Leiden University, Leiden, The Netherlands (S.A.G.V., W.W.F.T.G., M.D.); Pfizer Global Research and Development, Discovery Biology, Kent, United Kingdom (P.H.v.d.G.); and Mathematical Institute, Leiden University, Leiden, The Netherlands (L.A.P.)

The objective of the present investigation was to characterize the in vivo EEG effects of (synthetic) neuroactive steroids on the basis of a recently proposed mechanism-based pharmacokinetic/pharmacodynamic (PK/PD) model. After intravenous administration, the time course of the EEG effect of pregnanolone, 2-3-5-3-hydroxy-2-(2,2-dimethylmorpholin-4-yl)-pregnan-11,20-dione (ORG 21465), 2-3-5-21-chloro-3-hydroxy-2-(4-morpholinyl)-pregnan-20-one (ORG 20599), and alphaxalone was determined in conjunction with plasma concentrations in rats. For each neuroactive steroid the PK/PD correlation was described on the basis of a two-compartment pharmacokinetic model with an effect compartment to account for hysteresis. The observed concentration EEG effect relationships were biphasic and characterized with a mechanism-based pharmacodynamic model, which is based on a separation between the receptor activation process and the stimulus-response relationship. A single unique biphasic stimulus-response relationship could be identified for all neuroactive steroids, which was successfully described by a parabolic function. The receptor activation process was described by a hyperbolic function. Estimates for the maximum activation (e_PD) were similar for the different neuroactive steroids but values of the potency estimate (K_PD) ranged from 157 ± 16 ng · ml¹ for pregnanolone, 221 ± 83 ng · ml¹ for ORG 20599, and 483 ± 42 ng · ml¹ for alphaxalone to 1619 ± 208 ng · ml¹ for ORG 21465. A statistically significant correlation was observed between the in vivo potency and the IC₅₀ in an in vitro [³⁵S]t-butylbicyclophosphorothionate binding assay (r = 0.91). It is concluded that the new PK/PD model constitutes a new mechanism-based approach to the quantification of the effects of (synthetic) neuroactive steroids in vivo effects. The results show that the neuroactive steroids differ in potency but not in intrinsic efficacy at the GABA_A receptor in vivo.