Transfer of zidovudine (AZT) by human placenta

The transfer of zidovudine (AZT) across human placenta was studied using an in vitro perfu-sion system with independent maternal and fetal circulations. AZT is transferred toward the fetus more rapidly than t-glucose (transfer index 1.5), a water-soluble molecule smaller than AZT (267 vs. 180Da) that passively diffuses across the placenta. The transfer rate is proportional to the concentration in the maternal perfusate over a range of 0.03-300 p.M. Transfer rate in the reverse direction, toward the maternal perfusate, also exceeds that of L-glucose and fails to show saturability. These observations are consistent with simple diffusion. The partition of AZT and glucose between perfusion buffer and octanol is 1.04and 0.013, respectively, indicating that AZT is more lipophilic and providing a reasonable explanation for the more rapid transfer. AZT is extensively metabolized by the placenta to more polar and as yet unidentified metabolites that are not released into the perfusate. The possibility that the placental accumulation of such metabo-lite(s) may exert an antiviral action and may also affect placental function must be considered. The prevalence of human immunodeficiency virus (HIV) infection in children is rapidly increasing. It is estimated that there are>1000 cases of pediatric AIDS in the USA at pres