Org 20599

The anaesthetic action and modulation of GABAA receptor activity by the novel water-soluble aminosteroid Org 20599.[Pubmed:9014136]

Neuropharmacology. 1996;35(9-10):1209-22.

The anaesthetic profile of a novel water-soluble aminosteroid, Org 20599 [(2 beta, 3 alpha, 5 alpha)-21-chloro-3-hydroxy-2-(4-morpholinyl)pregnan-20-one methanesulphonate], and the ability of the compound to allosterically regulate the activity of the GABAA receptor, have been studied in comparison to the properties of established intravenous general-anaesthetic agents. Intravenously administered Org 20599 produced a rapid onset, short duration loss of the righting reflex in mice. The anaesthetic potency of Org 20599 was comparable to that of the steroids 5 alpha-pregnan-3 alpha-ol-20-one or alphaxalone, and exceeded that of propofol, thiopentone or pentobarbitone. Org 20599 and the reference anaesthetic agents allosterically displaced the binding of [35S]-t-butylbicyclophosphorothionate (TBPS) from GABAA receptors of rat-brain membranes with the order of potency: 5 alpha-pregnan-3 alpha-ol-20-one > Org 20599 > alphaxalone > propofol > thiopentone > pentobarbitone. At human recombinant alpha 1, beta 2, gamma 2L subunit-containing GABAA receptors expressed in Xenopus laevis oocytes, the anaesthetic agents produced a concentration-dependent and reversible potentiation of the peak amplitude of GABA-evoked currents. A similar positive allosteric action of Org 20599 was observed for the GABAA receptors expressed by bovine adrenal chromaffin cells maintained in culture. The rank order of potency in the aforementioned assays was identical to that determined from the displacement of TBPS binding. At concentrations greater than those required for potentiation of GABA, the anaesthetics exhibited GABA-mimetic effects with a rank order of potency that paralleled their modulatory activity. Such direct agonism varied greatly in maximal effect between compounds. The modulatory and direct agonist actions of Org 20599 were additionally confirmed utilizing rat hippocampal neurones in culture. The results indicate Org 20599 to be a potent and short-acting intravenous anaesthetic agent in mice and suggest positive allosteric regulation of GABAA receptor function to be a plausible molecular mechanism of action for the drug.

The interaction of anaesthetic steroids with recombinant glycine and GABAA receptors.[Pubmed:15033889]

Br J Anaesth. 2004 May;92(5):704-11.

BACKGROUND: Anaesthetic steroids are established positive allosteric modulators of GABAA receptors, but little is known concerning steroid modulation of strychnine-sensitive glycine receptors, the principal mediators of fast, inhibitory neurotransmission in the brain stem and spinal cord. This study compared the modulatory actions of five anaesthetic pregnane steroids and two non-anaesthetic isomers at human recombinant alpha1 glycine and alpha1beta2gamma2L GABAA receptors. METHODS: Recombinant alpha1 glycine or alpha1beta2gamma2L GABAA receptors were expressed in Xenopus laevis oocytes and agonist-evoked currents recorded under voltage-clamp. Steroid modulation of currents evoked by GABA, or glycine, was quantified by determining the potency (EC50) and maximal effect of the compounds. RESULTS: The anaesthetics minaxolone (EC50=1.3 microM), Org20599 (EC50=1.1 microM) and alphaxalone (EC50=2.2 microM) enhanced currents mediated by GABAA receptors. The anaesthetics also enhanced currents mediated by glycine receptors, although with higher EC50 values (minaxolone 13.1 microM; Org20599=22.9 microM and alphaxalone=27.8 microM). The maximal enhancement (to 780-950% of control) produced by the three steroids acting at the GABAA receptor was similar, but currents evoked by glycine were potentiated with increasing effectiveness by alphaxalone (199%)

Neuroactive steroids differ in potency but not in intrinsic efficacy at the GABA(A) receptor in vivo.[Pubmed:12388643]

J Pharmacol Exp Ther. 2002 Nov;303(2):616-26.

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, 2beta-3alpha-5alpha-3-hydroxy-2-(2,2-dimethylmorpholin-4-yl)-pregnan-11,20-dione (ORG 21465), 2beta-3alpha-5alpha-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. ml(-1) for pregnanolone, 221 +/- 83 ng. ml(-1) for Org 20599, and 483 +/- 42 ng. ml(-1) for alphaxalone to 1619 +/- 208 ng. ml(-1) for ORG 21465. A statistically significant correlation was observed between the in vivo potency and the IC(50) in an in vitro [(35)S]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.