Characterization of the anticonvulsant properties of Ganaxolone (CCD 1042; 3α-hydroxy-3β-methyl-5α-pregnan-20-one), a selective, high-affinity, steroid modulator of the γ-aminobutyric acid(A) receptor

Ganaxolone (CCD 1042) is a 3β-methyl-substituted analog of the endogenous neuroactive steroid 3α-hydroxy-5α-pregnan-20-one. Ganaxolone inhibited binding of the γ-aminobutyric acid (GABA)(A) receptor-chloride channel ligand t-[³⁵S]butylbicyclophosphorothionate (IC₅₀ of 80 nM) and enhanced binding of the benzodiazepine site ligand [³H]flunitrazepam (EC₅₀ of 125 nM) and the GABA site ligand [³H]muscimol (EC₅₀ of 86 nM), consistent with activity as a positive allosteric modulator of the GABA(A) receptor. Electrophysiological recordings showed that, whereas nanomolar concentrations of ganaxolone potentiated GABA-evoked chloride currents in Xenopus oocytes expressing the human GABA(A) receptor subunits α1 β1 γ2(L), α2β1γ2(L) or α3β1 γ2(L), direct activation of chloride flux occurred to a limited extent only at micromolar concentrations. Ganaxolone was effective in nontoxic doses against clonic convulsions induced by s.c. pentylenetetrazol administration in mice and rats (ED₅₀ values of 4.3 and 7.8 mg/kg i.p., respectively). Ganaxolone also exhibited potent anticonvulsant activity against seizures induced by s.c. bicuculline (ED₅₀ of 4.6 mg/kg i.p.), i.p. TBPS (ED₅₀ of 11.7 mg/kg i.p.) and i.p. aminophylline (ED₅₀ of 11.5 mg/kg i.p.) in mice. Although ganaxolone effectively blocked tonic seizures induced by maximal electroshock in mice (ED₅₀ of 29.7 mg/kg i.p.), it did so only at doses that produced ataxia on the Rotorod (TD₅₀ of 33.4 mg/kg i.p.). Conversely, ganaxolone was a potent anticonvulsant against fully kindled stage 5 seizures induced by corneal kindling in rats (ED₅₀ of 4.5 mg/kg i.p.), producing these effects at doses well below those that resulted in ataxia (TD₅₀ of 14.2 mg/kg i.p.). The seizure threshold, as determined by an increase in the dose of i.v. infused pentylenetetrazol required to induce clonus, was also significantly elevated by nontoxic doses of ganaxolone in mice. In summary, these data indicate that ganaxolone is a high-affinity, stereoselective, positive allosteric modulator of the GABA(A) receptor complex that exhibits potent anticonvulsant activity across a range of animal procedures. The profile of anticonvulsant activity obtained for ganaxolone supports clinical evaluation of this drug as an antiepileptic therapy with potential utility in the treatment of generalized absence seizures as well as simple and complex partial seizures.