Antibiotic yields anxiolytic

Benzodiazepines such as diazepam are commonly prescribed to control anxiety, and act by enhancing the action of the neurotransmitter GABA (γ-aminobutyric acid) at the brain GABA_A receptor. But drugs of this class have sedative side effects, an undesirable feature that has spurred a search for alternatives by Kelvin Gee and colleagues. In a Nature Medicine advanced online publication, the team describe a first-in-class anxiolytic that is free of sedative side effects.

The search strategy devised by Gee et al. was based on the hypothesis that mitigation of side effects might be achieved by targeting non-benzodiazepine-binding sites of GABA_A receptors. Compounds related to the fluoroquinolone antibiotic norfloxacin were chosen as screening candidates for two reasons: first, a small proportion of patients taking this antibiotic experience anxiety as a side effect; second, norfloxacin is known to antagonize GABA_A receptors at an undefined site. So, chemical modification of norfloxacin could generate compounds with the opposite, GABA-enhancing anxiolytic effect.

In a standard assay for GABA modulation, several norfloxacin-related chemotypes inhibited the binding of the convulsant TBPS to GABA_A receptors. Analysis of the most active product, dubbed compound 4, showed that this compound does not act directly on the GABA_A receptor-binding sites for TBPS, norfloxacin, benzodiazepines or GABA.

To confirm the GABA-modulatory action of compound 4, electrophysiological studies were conducted on HEK293 cells expressing different human GABA_A receptor subtypes. Compound 4 enhanced GABA-mediated currents in a dose-dependent manner, but only in those cells that expressed the α₂ subunit of the receptor. Failure of the benzodiazepine binding-site antagonist flumazenil to block this potentiation confirmed that compound 4 binds to a non-benzodiazepine site. So structural manipulation of norfloxacin did produce a compound that binds to an alternative GABA_A receptor site, but did this modification result in anxiolytic rather than anxiety-inducing effects?

In two rodent models of anxiety, treatment with compound 4 relieved anxiety-related behaviour. Notably, the magnitude of this effect was comparable to that afforded by diazepam. But the real breakthrough came from assessments of motor performance using the rotarod test — diazepam caused motor impairment whereas compound 4 did not, even at the maximal soluble dose. Compound 4 might therefore be at the forefront of a new approach to the design of robust anxiolytics with improved side-effect profiles.