The antinociceptive activity of flupirtine: a structurally new analgesic.

The antinociceptive activity of flupirtine was measured in various test procedures predictive of analgesic activity. In the electrostimulated pain test in mice the oral ED50 for flupirtine was 25.7 mg/kg p.o. Thus, flupirtine was approximately 31.7 times more potent than paracetamol (ED50: 814 mg/kg p.o.) and as potent as pentazocine (ED50: 38.5 mg/kg p.o.). Morphine (ED50: 16.8 mg/kg p.o.) was 1.5 times and buprenorphine (ED50: 2.6 mg/kg p.o.) 9.9 times more potent than flupirtine. In the hot plate test (mice) flupirtine (ED50: 32 mg/kg p.o.) was approximately half as potent as morphine (ED50: 15.5 mg/kg p.o.). The oral and intravenous antinociceptive activity (ED50) of flupirtine in the electrical tooth pulp stimulation test in conscious dogs was 3.5 mg/kg p.o. and 0.7 mg/kg i.v. which was similar to that of pentazocine (ED50: 4.2 mg/kg p.o. and 0.5 mg/kg i.v.). Buprenorphine had, as expected, stronger antinociceptive activity (ED50: 1.0 mg/kg p.o. and 0.04 mg/kg i.v.). Fifteen minutes after oral administration of 40 mg/kg flupirtine, the pain threshold in the electrostimulated pain test was increased by 54%. The maximal antinociceptive effect was observed 30 minutes after dosing. The analgesia lasted at least 75 minutes. Codeine significantly elevated the pain threshold 15 minutes after dosing. Its maximal effect was also reached 30 min after application but the antinociceptive activity wore off earlier than after flupirtine. The intracerebroventricular and intrathecal administration of flupirtine also caused dose dependent antinociceptive activity in dose ranges which, when applied systematically, did not produce analgesia in rats. The antinociceptive activity of flupirtine was not abolished by naloxone whether given orally or by the intraventricular or intrathecal routes. In opiate receptor binding studies flupirtine had no affinity for mu, delta or kappa opiate receptors at the highest concentration used (10(-5) M). Whereas buprenorphine and tramadol showed a striking similarity in the pharmaco-electroencephalogram recorded from different parts of the brain (frontal cortex, thalamus, striatum and the mesencephalic reticular formation) of the freely moving rat, flupirtine was clearly different in action. It produced dose dependent increases in nearly all frequency bands but its effects were different from those of the minor tranquillizer diazepam and the anticonvulsant phenobarbitone. These findings show that the central antinociceptive activity of flupirtine is not based on an opiate mechanism and is not comparable with that of diazepam and phenobarbitone.