PROGESTERONE ANTAGONIST RU-486 ACCOMMODATES BUT DOES NOT INDUCE LABORAND DELIVERY IN PRIMATES

The hormonal mechanisms of parturition in primates remain controversia l. Even so, the well-known decrease of plasma progesterone concentrati on near term is considered by many as the 'labour inducer'. The proges terone antagonist RU 486, which blocks progesterone activity at the ce llular receptor level, appears to be a useful hormonal tool by which t o study this tissue. Here, we tested its capacity to induce labour and delivery. A total of 23 Cynomolgus monkeys (Macaca fascicularis), wit hin 9-17 days of expected term, were assigned to four different protoc ols to study various doses, routes and regimens of RU 486 administrati on. Observations included uterine contractile patterns, pharmacokineti cs of RU 486 in plasma and passage of RU 486 into breast milk. None of the protocols tested successfully induced labour resulting in vaginal delivery within 24 h. Instead, the data demonstrate that blockade of progesterone activity by the progesterone antagonist was not sufficien t by itself to achieve parturition in these primates. Uterine myometri al contractile activity under RU 486 exposure was not sufficient to in duce labour and delivery. Moreover, the progesterone antagonist concen tration in breast milk was very low, indicating little passage to suck ling newborn infants.