Pregnanolone emulsion. A new steroid preparation for intravenous anaesthesia: an experimental study in mice.

The anaesthetic activity of pregnanolone (a metabolite of progesterone) in emulsion formulation administered intravenously to male mice was compared with that of Althesin. The loss of righting reflex for 15 seconds was used to estimate the anaesthetic effect. The mean anaesthetic dose (AD50) for the pregnanolone emulsion was 5.25 mg/kg and for Althesin, 2.8 mg/kg. The mean lethal dose (LD50) was 44 mg/kg for pregnanolone and 54 mg/kg for Althesin. The sleeping time after pregnanolone was 2.5-3.5 times longer than after Althesin in dosages above 7.5 mg/kg. No signs of pain or local reaction from injection were observed for either of the drugs. The onset of action was fast for both drugs, with only minor signs of excitation, and recovery was rapid and without excitation. The results indicate that the anaesthetic properties of pregnanolone emulsion are very similar to those of Althesin. Further studies will show whether it can fill the major vacuum left in anaesthetic practice after the withdrawal of Althesin.