Synthesis and Evaluation of Analogues of the Partial Agonist 6-(Propyloxy)-4-(methoxymethyl)-β-carboline-3-carboxylic Acid Ethyl Ester (6-PBC) and the Full Agonist 6-(Benzyloxy)-4-(methoxymethyl)-β- carboline-3-carboxylic Acid Ethyl Ester (Zk 93423) at Wild Type and Recombinant GABA_A Receptors

Abstract

A pharmacophore and an alignment rule have previously been reported for BzR agonist ligands. The design and synthesis of 6-(propyloxy)-4-(methoxymethyl)-beta-carboline-3-carboxylic acid ethyl ester (6-PBC, 24, IC50 = 8.1 nM) was based on this pharmacophore. When evaluated in vivo this ligand exhibited anticonvulsant/anxiolytic activity but was devoid of the muscle relaxant/ataxic effects of "classical" 1,4-benzodiazepines (i.e., diazepam). Significantly, 6-PBC 24 also reversed diazepam-induced muscle relaxation in mice. The 3-substituted analogues 40-46 and 48 of 6-PBC 24 and Zk 93423 27(IC50 = 1 nM) were synthesized and evaluated in vitro to determine what affect these modifications would have on the binding affinity at recombinant BzR subtypes. With the exception of the 3-amino ligands 40 and 41, all the beta-carbolines were found to exhibit high binding affinity at BzR sites. The 3-propyl ether derivative 45 was also evaluated in vivo and found to be devoid of any proconvulsant or anticonvulsant activity at doses up to 40 mg/kg. The 6-(1-naphthylmethyloxy) and 6-octyloxy analogues 25, 26, 28, and 29 of 6-PBC 24 were synthesized to further evaluate the proposed alignment of agonists vs inverse agonists in the pharmacophore of the BzR. In addition, ligands 26 and 29 were designed to probe the dimensions of lipophilic pocket L3 at the agonist site. The activity of 29 was evaluated in vivo; however, this analogue elicited no pharmacological effects at doses up to 80 mg/kg. These and other related beta-carbolines were also examined in five recombinant GABAA receptor subtypes. Ligands 52-61 all exhibited moderate to high affinity at GABAA receptors containing alpha1 subunits. These ligands will be useful in further defining the pharmacophore at alpha1 beta3 gamma2 receptors.

Benzodiazepines and heightened aggressive behavior in rats: reduction by GABAA/?1 receptor antagonists

“…A 10-fold selectivity may appear rather modest, but considering that the α 1 subunit appears in more than 50% of GABA A receptors, even a small degree of selectivity translates to a significant degree of receptor occupancy. β-CCt also binds competitively with BZ agonists over a range of doses in various in vivo and in vitro assays (Shannon et al 1984;Carroll et al 1991;Cox et al 1998;Griebel et al 1999;Paronis et al 2001;Rowlett et al 2001). It is not surprising that this mixed inverse agonist/ antagonist produces varied results according to experimental preparation.…”

Dopamine and Benzodiazepine-Dependent Mechanisms Regulate the EtOH-Enhanced Locomotor Stimulation in the GABAA α1 Subunit Null Mutant Mice

“…Recombinant receptor studies show that bCCt exhibits a 410-fold selectivity for the GABA a1 over the a2 and a3 receptors, and a 4110-fold selectivity for the a1-over the a5 subtype (Cox et al, 1995). Thus, bCCt exhibits the greatest binding selectivity of the currently available a1 receptor ligands McKernan et al, 2000;Cox et al, 1998). In contrast, flumazenil is a nonselective BDZbinding antagonist at the diazepam-sensitive sites (Huang et al, 2000).…”

“…When necessary, some drug treatments were sonicated. bCCt was synthesized by several of the authors (WY and JMC) using previously published procedures (see Cox et al, 1998;June et al, 2003), while Ro15-1788 (flumazenil) was a gift from Hoffman La Roche (Nutley, NJ). The a1-selective mixed agonistantagonist bCCt (Griebel et al, 1999;June et al, 2003) was selected since it has been shown to block the reinforcing, and the locomotor depressant actions of EtOH .…”

GABAA and Opioid Receptors of the Central Nucleus of the Amygdala Selectively Regulate Ethanol-Maintained Behaviors

“…However, bCCt also exhibits some affinity (albeit lower) for both the a2 and a3 receptors (Cox et al, 1995;Carroll et al, 2001;Harvey et al, 2002) (for a review, see Harvey et al, 2002). Behavioral studies in several species (eg, rats, mice, primates) show that bCCt is a BDZ antagonist exhibiting competitive binding site interaction with BDZ agonists over a broad range of doses (Shannon et al, 1984;Griebel et al, 1999;Cox et al, 1998;Carroll et al, 2001Carroll et al, , 1991Rowlett et al, 2001;Paronis et al, 2001). Other studies show that bCCt produces anxiolytic effects in rodents ) and potentiates the anticonflict response induced by a1 subtype agonists in primates (Paronis et al, 2001).…”