Background & Aims

Gamma-aminobutyric acid type A receptors (GABAARs) are the primary inhibitory receptors in the brain, responsible for fast inhibitory neurotransmission. GABAARs are also expressed in the peripheral nervous system, and emerging evidence suggests that activation of GABAARs expressed on dorsal root ganglia (DRG) neurons filter nociceptive signals from the periphery to the central nervous system (CNS) (1). Neuroactive steroid (NAS) GABAAR positive allosteric modulators (PAMs) are a drug class that have demonstrated clinical efficacy in a range of neurological and neuropsychiatric conditions, but the impact of modulating peripheral GABAARs with NAS GABAAR PAMs has not been evaluated. The aim of this study was to assess the analgesic efficacy of a NAS GABAAR PAM with low brain penetrance to determine whether targeting peripheral GABAARs could be an effective therapeutic strategy for pain disorders.

Methods

GABAAR PAM activity was evaluated using automated patch clamp electrophysiology with recombinant cell lines that stably expressed the alpha 1/beta 2/gamma 2 or alpha 4/beta 3/delta GABAAR subtype. Pharmacokinetics studies in rodents were performed to determine the brain/plasma ratio. An electroencephalogram (EEG) study in Sprague-Dawley rats was conducted to determine whether test compounds increase beta frequency (13-29Hz) band power, a translational measure of CNS target engagement, like other NAS GABAAR PAMs (2). Finally, to evaluate analgesic potential, the spared nerve injury (SNI) assay in C57Bl/6 mice was conducted, in which the tibial and peroneal branches of the sciatic nerve were severed while the sural branch was left intact, and mechanical allodynia was assessed using von Frey testing.

Results

Compound PG1 displayed potent and efficacious PAM activity at alpha 1/beta 2/gamma 2 or alpha 4/beta 3/delta GABAARs, and produced a leftward shift in the GABA concentration-response at both receptor subtypes. In both rats and mice, PG1 had an area-under-the-curve (AUC) brain/plasma ratio of <0.2, indicating that the compound had low brain permeability. At higher doses that produced significant brain concentrations, PG1 had pharmacological effects consistent with a GABAAR PAM mechanism in the CNS, such as increased beta frequency band power in the EEG study, a translationally relevant biomarker. PG1 was analgesic in the SNI assay, dose-dependently reversing mechanical allodynia produced by the neuropathic insult. Importantly, the brain concentration at the minimally effective dose was at near negligible levels (12 ng/g), suggesting that the analgesic effect was driven at least in part by a peripheral mechanism.

Conclusions

A NAS GABAAR PAM with low brain permeability demonstrated efficacy in a preclinical neuropathic pain model at near negligible brain exposure, supporting the hypothesis that modulating GABAARs in the periphery is a potentially effective therapeutic strategy for pain.

References

(1) Hao H, Ramli R, Wang C, Liu C, Shah S, Mullen P, Lall V, Jones F, Shao J, Zhang H, Jaffe DB, Gamper N, Du X. Dorsal root ganglia control nociceptive input to the central nervous system. PLoS Biol. 2023; 21:e3001958

(2) Althaus AL, Ackley MA, Belfort GM, Gee SM, Dai J, Nguyen DP, Kazdoba TM, Modgil A, Davies PA, Moss SJ, Salituro FG, Hoffmann E, Hammond RS, Robichaud AJ, Quirk MC, Doherty JJ. Preclinical characterization of zuranolone (SAGE-217), a selective neuroactive steroid GABAA receptor positive allosteric modulator. Neuropharmacology. 2020; 181:108333.

Presenting Author

Jacob Beckley

Poster Authors

Jacob Beckley

PhD

Sage Therapeutics

Lead Author

E-mail

Carolyn Johnson

PhD

Sage Therapeutics

Lead Author

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Amber Plante

PhD

Sage Therapeutics

Lead Author

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Jennifer Hoyt

B.S.

Sage Therapeutics

Lead Author

E-mail

Aaron Stewart

M.S.

Sage Therapeutics

Lead Author

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Steven Gee

PhD

Sage Therapeutics

Lead Author

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Matthew Hill

PhD

Sage Therapeutics

Lead Author

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Michael Quirk

PhD

Sage Therapeutics

Lead Author

E-mail

Topics

  • Treatment/Management: Pharmacology: Novel Targets