A novel, a‐2, 3‐selective GABAA receptor modulator with improved central penetrability and associated antiepileptic and anti‐pain efficacy
Authors: Witkin Jeffrey M , Methuku K R , Schkeryantz J M , Gleason S D , Okun A , Porreca F , Cramer J W , Jones T M , Catlow J T , Li X , Poe M M , Li G , Arnold A , Cook J M 
Published: November 2023

BackgroundPositiveallosteric modulators of GABAA receptors transduce a host ofbeneficial effects but can have motor and sedative liabilities. A benzodiazepine, HZ‐166, had previously been characterized as an alpha 2,3‐selective GABAA receptor modulator with anticonvulsant, anxiolytic, and anti‐nociceptive properties but reduced motor effects. However, the ester moiety in HZ‐166 quickly metabolizes to the carboxylic acid leading to variable exposures of ester in plasma and brain. Moreover, the acid of HZ‐166 possesses significant invitro activity and may well contribute to efficacy seen in in vivo models. We have identified five membered ringheterocycles (oxazoles, dioxazoles) as competent ester replacements which lend themselves to comparable in vitro potency, reduced metabolic complications, consequently leading to an overall increase in unbound central concentration and thus increased efficacy in animal models of pain and epilepsy.MethodsPlasmaand brain exposures of compounds were assessed after i.p. dosing. Compounds were evaluated for efficacy inseveral rodent seizure‐induction models. We also evaluated the anti‐nociceptive effects of one compound (KRM‐II‐81)in rats exposed to chronic pain induced by spinal nerve ligation (SNL) (L5/6)by the Von Frey test. Finally,assessment of motor impairment was evaluated on the inverted screen test.ResultsKRM‐II‐81which utilizes a bioisosteric oxazole as a five membered ring heterocycle replacementof the ester in HZ‐166 did not negatively affect selectivity for α 3(0.97 μM) over α 1 (> 20 μM). However, KRM‐II‐81 demonstrated enhancedbrain exposure versus HZ‐166 after systemic administration. Accordingly, unbound mouse brain exposure exhibited by KRM‐II‐81 after 1 hr (10 mg/kg, i.p.) was 978 nM. In contrast, HZ‐166 possessed >100 folddecrease in mouse unbound brain concentration (6.4 nM) when dosed under the same conditions. Increased central exposure translated to increased efficacy without induction of motor side effects. KRM‐II‐81 (30 mg/kg, i.p.) was fully protective against pentylenetetrazole (PTZ)‐induced clonic convulsions in rats (ED50 = 10.2 mg/kg), whereas HZ‐166 was only partially effective. Further, whereas HZ‐166 (up to 60 mg/kg, i.p.) was ineffective in increasing the dose of i.v.‐infused PTZ required for convulsions, KRM‐II‐81 was efficacious at a dose as low as 10 mg/kg. HZ‐166 was also not active against electroshock‐induced convulsions in mice while KRM‐II‐81 showed efficacy at 30 mg/kg. KRM‐II‐81 also increased pain thresholds by ~2.5 times, an effect equivalent to gabapentin without observable changes in sedation or motor impairment. Brains from SNL rats exhibited decreases in K+‐evoked glutamate release.ConclusionsSubstitution of an oxazole bioisostere for the ester in HZZ‐166 did not affect selectivity α subtype selectivity but significantly increased mouse brain exposure after systemic dosing. Increased brain exposure translated to increased potency and efficacy in rodent seizure‐induction models as well as efficacy against nociceptive challenge in a rodent chronic pain model. In addition, motor side‐effects were not exacerbated by KRM‐II‐81. Thus, KRM‐II‐81 represents a significant advance in the pharmacology of the GABAa receptor.