Abstracts

Rationale: Treatment of status epilepicus requires rapid administration of antiseizure agents, which are typically delivered either by the intravenous (i.v.) or intramuscular (i.m.) routes. Allopregnanolone (3α-hydroxy-5α-pregnan-20-one; 5α,3α-P), an endogenous progesterone-derived steroid that is a positive allosteric modulator of GABAA receptors, is a powerful antiseizure agent with potential in the treatment of status epilepticus. The objective of this study was to determine the dosing of allopregnanolone to protect against seizures when delivered i.v. and i.m. Methods: The mouse 6 Hz and pentylenetetrazol seizure models were used. Solutions of 5α,3α-P were made in 6% (0.5 and 1.5 mg/ml) sulfobutylether-β-cyclodextrin sodium salt (Captisol®) in 0.9% saline. The solutions were injected i.v. or i.m. (1, 2 and 30 min or 2 and 30 min, respectively) prior to administration of the 6 Hz electrical stimulus or PTZ (80 mg/kg, i.p.). In case of the PTZ model, animals were observed for 30 min and times to myoclonic jerks and clonic and tonic seizures were recorded. Anticonvulsant activity was assessed by the delay in onset of seizure signs. Allopregnanolone plasma levels in rats were determined by LC-MS. Results: 5α,3α-P exhibited protective activity in the 6 Hz test 1-15 min after i.v. infusion (1.5 mg/kg) but was inactive at 30 min. In contrast, with i.m. administration (3 mg/kg) the onset of protective activity was slower (within 2 min) and lasted <2 h. At a dose of 0.1 mg/kg i.v. 5α,3α-P failed to significantly delay seizure onset in the PTZ model at all pretreatment times (1, 2 and 30 min) whereas a dose of 0.5 mg/kg administered 1 min before PTZ caused a marked delay for myoclonic jerks and clonic seizures and in 62.5% of animals prevented tonic seizures and mortality that invariably accompanies tonic seizures. When injected 2 min before PTZ 5α,3α-P (0.5 mg/kg) caused a similar increase in time to onset of seizures signs and prevented tonic seizures in 25% of animals. 5α,3α-P at a dose of 1.5 mg/kg completely prevented tonic seizures and mortality when injected i.v. 1 and 2 min before PTZ. When injected i.m. 2 min before PTZ, 0.25, 0.5 and 1.5 mg/kg 5α,3α-P protected 0%, 50% and 100%, respectively, of animals from tonic seizures. 5α,3α-P at the dose of 1.5 mg/kg i.m. provided significant protection against tonic seizures when injected 30 min before PTZ; the same dose injected i.v. 30 min before PTZ was inactive. In rats, an i.v. bolus dose of 0.5 and 1.0 mg/kg 5α,3α-P caused mean peak plasma levels (2 min) of 337 and 746 ng/ml, respectively; for both doses, the pooled mean two component half-times were 2 and 22 min. Conclusions: Our results demonstrate that i.v. 5α,3α-P provides very rapid but transitory anticonvulsant activity. When injected i.m., 5α,3α-P acts comparably quickly and has a longer duration of action. Parenteral 5α,3α-P may be useful for the acute treatment of seizures.