Antifertility effect of pseudolaric acid B

Pseudolaric acid B, a novel diterpenoid, was isolated from the root of Pseudolarix kaempferi Gorden. Pseudolaric acid B was injected sc 25 mg/kg or ig 15 mg/kg in rats and sc 30 mg/kg or ig 40 mg/kg in rabbits daily on d 7-9 after mating. In dogs, pseudolaric acid B was injected ig at 5 mg/kg for 3 d within 2 wks after mating. Early pregnancies were terminated in all treated animals. Pseudolaric acid B was given ig 10 mg/kg daily to 10♀rats on d 10-12 after mating. Midterm pregnancies of all rats were terminated. However, implantation was not prevented in rats when pseudolaric acid B 40 mg/kg was injected sc or ig daily on d 1-3 after mating. Pseudolaric acid B showed no estrogenic activity, but caused severe decidual hemorrhage and necrosis. It lowered the plasma progesterone level after d 5 of administration at antifertility dose levels. Progesterone did not antagonize the effects of pseudolaric acid B on early pregnancy in rats.