Targeting γ1-containing GABA-A receptors spares γ2-benzodiazepine mediated neurophysiological impairments in human volunteers

INTRODUCTION: Benzodiazepines modulate GABA-A receptors containing the γ2 subunit. Although effective, their clinical utility is limited by γ2-mediated side effects, including sedation and ataxia. While the γ2 subunit is prevalent throughout the brain, by comparison, γ1 subunits are enriched in the amygdala and connected nuclei that process emotion, fear, and anxiety-related behaviors. We hypothesized that a γ1-selective positive allosteric modulator (PAM) could preferentially engage anxiety-relevant circuitry while sparing cortical and motor functions associated with ubiquitous γ2 subunit.

AIMS: To evaluate the selectivity, target engagement, and neurophysiological profile (NeuroCart) of NTX-1955, a γ1-selective PAM, in human volunteers.

METHODS: The selectivity of NTX-1955 for γ1 vs γ2 was measured by radioligand binding in vitro. In human volunteers, repeated neurophysiological measures of saccadic peak velocity (SPV), smooth pursuit, adaptive tracking, body sway, and subjective alertness were evaluated in a randomized, double-blind, placebo-controlled, within-subject crossover study on Days 1 and 7 of dosing. Separate cohorts of N=12 participants were randomized to treatment sequences of placebo, 5, 10, and 15 mg, and N=6 participants to sequences of placebo, 25 mg and alprazolam (1 mg IR BID). Target engagement was quantified using PET imaging with the γ1-selective tracer [11C]-RO7285378 and the γ2-selective tracer [11C]-flumazenil in separate cohorts of N=7 and 6 participants, respectively.

RESULTS: NTX-1955 demonstrated strong selectivity in vitro favoring γ1 over γ2 (Ki γ1=20 nM; Ki γ2=2000 nM; Kiγ2/Kiγ1=100; log(Kiγ2/Kiγ1)=2.0), whereas alprazolam preferentially bound γ2 (Ki γ1=4000 nM; Ki γ2=20 nM; Kiγ2/Kiγ1=0.005; log(Kiγ2/Kiγ1)=−2.3). Alprazolam produced the expected effects on all neurophysiological measures (for example SPV -88.5 LSMean difference from placebo of -81 with 95% CI of −110 to −51 deg/sec on Day 1), consistent with reported historical data with benzodiazepines in these assays. In contrast, NTX-1955 (5, 10, and 15 mg) did not substantially impair any neurophysiological measures vs placebo, on either Day 1 or Day 7. At 25 mg effects were limited to adaptive tracking on Day 1 (-6.3% LSMean; -9.3 to -3.3 95% CI), with a magnitude like alprazolam (-6.6%; -9.8 to -3.4), supporting tolerability up to 25 mg with impairment limited to adaptive tracking. Plasma concentrations of NTX-1955 accumulated approximately 2-fold to steady-state, indicating an effective half-life of 24 hours. All doses of NTX-1955 were well tolerated, with fewer sedation- or somnolence-related adverse events than alprazolam and rates comparable to placebo. In human PET studies, NTX-1955 demonstrated significant reductions in [11C]-RO7285378 binding, consistent with occupancy of the γ1-GABA-A receptor (range 5% to 45%), while no reduction in [11C]-flumazenil binding was observed, consistent with no detectable γ2 occupancy.

CONCLUSIONS: NTX-1955 demonstrated a clear pharmacological dissociation from the γ2-benzodiazepine profile. These results establish that selective γ1 target engagement is achievable in humans without inducing classic γ2-benzodiazepine-like CNS impairment, thereby defining a dose- and exposure-window appropriate for evaluating the efficacy and safety of NTX-1955 in the treatment of anxiety disorders.

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