Seizures induced by theophylline and isoniazid in mice

Isoniazid-induced seizures respond poorly to anticonvulsants but well to pyridoxine (Vitamin B6); theophylline produces difficult-to-treat seizures with substantial morbidity and mortality. Theophylline therapy depresses plasma pyridoxal-5'-phosphate (PLP), the active metabolite of pyridoxine, suggesting that theophylline-induced seizures might be amenable to treatment with pyridoxine. Our study established the dose-response relationship for convulsions due to isoniazid and theophylline in mice and determined if pyridoxine antagonized such seizures. Female CD-1 outbred mice weighing 25 to 30 g were used. Clonic seizures had clonic activity lasting 5 sec; tonic seizures had loss of the righting reflex with tonic hindlimb extension. Groups of 10 mice received single doses of 50, 100, 150, 200, 250 or 300 mg aminophylline/kg i.p. or 100, 150, 200, 250, 300 or 350 mg isoniazid/kg i.p. and were observed for seizures or death. Pyridoxine or saline with aminophylline or isoniazid were administered simultaneously. The LD50 for aminophylline was 266 mg/kg; for isoniazid it was 160 mg/kg. Doses of 150 mg aminophylline/kg or 100 mg isoniazid/kg did not induce seizures. Pyridoxine with aminophylline or isoniazid did not alter the frequency or time of onset of seizures or death. This was unexpected because pyridoxine antagonizes theophylline-induced seizures in mice and reverses isoniazid-induced seizures in humans. We found no evidence that PLP depletion in mice is a mechanism for seizures induced by isoniazid or aminophylline in a fashion similar to isoniazid in humans.