[Pharmacological studies of 4-ethoxy-2-methyl-5-morpholino-3(2H)-pyridazinone (M73101). (1). Analgesic activity (author's transl)].

The analgesic activity and the mode of action of M73101, a new analgesic anti-inflammatory agent, were investigated in mice and rats and compared to those of reference drugs. M73101 showed a marked analgesic activity against noxious stimuli induced by pressure (Haffner method and Randall-Selitto method), thermal (hot plate method), electric and chemical (acetic acid-stretching method and bradykinin-induced responses) stimulation, in a manner similar to those of basic anti-inflammatory drugs (BAD) such as aminopyrine, mepirizole, tiaramide HCl, and benzydamine HCl. The activities of M73101 were equal to and/or more potent than those of BAD, and more potent than those of acidic anti-inflammatory drugs. In addition, the analgesic activity of M73101 was not decreased by the pretreatment with levallorphan and showed no cross-tolerance to morphine in mice, indicating that the analgesic properties of M73101 differ from those of morphine. On the other hand, the analgesic activity of M73101 as well as BAD was decreased by repeated oral administration in mice, and the potency was weaker than mepirizole. Furthermore, the muscle relaxant effect of M73101 obtained by inclined screen test in mice was the weakest among the BAD, suggesting that analgesic activity of M73101 is not related to the muscle relaxant property. These results indicate that M73101 may be useful for clinical application as an anti-inflammatory drug possessing remarkable analgesic activity.