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Vol. 304, Issue 1, 88-101, January 2003
Division of Pharmacology, Leiden/Amsterdam Center for Drug
Research, Leiden University, Leiden, The Netherlands (S.A.G.V.,
F.L.C.W., J.M.G.-S., E.T., M.D.); Pfizer Global Research and
Development, Discovery Biology, Sandwich, Kent, United Kingdom
(P.H.v.G.); and Mathematical Institute, Leiden University, Leiden, The
Netherlands (L.A.P.)
A mechanism-based pharmacokinetic-pharmacodynamic (PK/PD) model for
neuroactive steroids, comprising a separate characterization of 1) the
receptor activation process and 2) the stimulus-response relationship,
was applied to various nonsteroidal GABAA receptor modulators. The EEG effects of nine prototypical GABAA
receptor modulators (six benzodiazepines, one imidazopyridine, one
cyclopyrrolone, and one
-carboline) were determined in rats in
conjunction with plasma concentrations. Population PK/PD modeling
revealed monophasic concentration-EEG effect relationships with large
differences in potency (EC50) and intrinsic activity
between the compounds. The data were analyzed on the basis of the
mechanism-based PK/PD model for (synthetic) neuroactive steroids on the
assumption of a single and unique stimulus-response relationship. The
model converged yielding estimates of both the apparent in vivo
receptor affinity (KPD) and the in vivo
intrinsic efficacy (ePD). The values of
KPD ranged from 0.41 ± 0 ng·ml
1 for bretazenil to 436 ± 72 ng·ml
1 for clobazam and the values for
ePD from
0.27 ± 0 for methyl 6,7-dimethoxy-4-ethyl-
-carboline-3-carboxylate to 0.54 ± 0.02 for diazepam. Significant linear correlations were observed
between KPD for unbound concentrations and
the affinity in an in vitro receptor bioassay (r = 0.93) and between ePD and the GABA-shift in
vitro (r = 0.95). The findings of this
investigation show that the in vivo effects of nonsteroidal
GABAA receptor modulators and (synthetic) neuroactive
steroids can be described on the basis of a single unique transducer
function. In this paradigm, the nonsteroidal GABAA receptor
modulators behave as partial agonists relative to neuroactive steroids.
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