METABOLISM OF 3'-AZIDO-3'-DEOXYTHYMIDINE (AZT) IN HUMAN PLACENTAL TROPHOBLASTS AND HOFBAUER CELLS

3'-Azido-3'-deoxythymidine (AZT) is currently under clinical investiga tion to assess its potential to inhibit maternal-fetal HIV transmissio n. To determine the activation of AZT to its phosphorylated metabolite s by placental cells, we characterized the intracellular phosphorylati on of AZT in two major cell types of the placenta, namely trophoblasts and Hofbauer cells. Although phosphorylation of AZT in trophoblast an d Hofbauer cells is 50- to 100-fold lower than that in human lymphocyt ic cell lines or activated lymphocytes, both cell types are capable of activating AZT to AZT triphosphate (AZTTP) at a level comparable to t hat of resting lymphocytes. We found that AZT monophosphate (AZTMP) wa s the major phosphorylated AZT metabolite, while AZT diphosphate (AZTD P) and AZTTP constituted less than 4% of the intracellular phosphoryla ted AZT pool. This result was independent of AZT concentration and exp osure time in both types of placental cells. The rate-limiting step in the conversion of AZT to AZTTP was determined to be thymidylate kinas e-catalyzed conversion of AZTMP to AZTDP. Trophoblasts and Hofbauer ce lls exhibited different timecourse and concentration-dependent profile s of intracellular AZT phosphorylation, suggesting that these two plac ental cells may have anabolic or catabolic enzymes of different compos ition or efficiency. AZTTP decayed in both trophoblasts and Hofbauer c ells with a half-life of 4-6 hr. These results should be useful in rat ionally designing AZT dosage regimens to treat HIV-infected women for prevention of maternal-fetal HIV transmission.