Effect of pregnancy, mode of administration and neonatal age on the pharmacokinetics of zalcitabine (2',3'-dideoxycytidine) in the pigtailed macaque (Macaca nemestrina)

Our objective was to determine the effect of pregnancy, mode of administration and neonatal age on the pharmacokinetics of the anti-HIV drug zalcitabine (2',3'-dideoxycytidine; ddC) in the pigtailed macaque (Macaca nemestrina). Zalcitabine was administered as an i.v. bolus dose to pregnant dams (n = 3) at term and at 6 weeks post-partum. No significant differences were found between the pre- and post-partum systemic plasma clearance, steady-state volume of distribution or terminal plasma half-life of zalcitabine, indicating that pregnancy does not affect the pharmacokinetics of the drug in the macaque. The observed maternal plasma, fetal plasma and amniotic fluid concentration-time profiles were compared with profiles that were simulated using pharmacokinetic parameter estimates obtained in an earlier constant i.v. infusion study in pregnant macaques. The fetal:maternal ratio of the area under the simulated zalcitabine plasma concentration-time profile after an i.v. bolus dose (0.58) was close to the earlier observed fetal:maternal steady-state plasma concentration ratio after i.v. infusion of the drug (0.58 +/- 0.05). The excellent agreement between observed and simulated fetal:maternal ratio of zalcitabine demonstrates that the steady-state infusion experimental design can be used to estimate the drug exposure to the fetus after a single dose. To determine the influence of age on the pharmacokinetics of zalcitabine, the drug was administered as a single i.v. bolus dose to four infant macaques serially at the ages of 1-2 weeks, 1 month and 4 months. The systemic plasma clearance of zalcitabine was significantly smaller and the terminal plasma half-life significantly longer at age 1-2 weeks than at 1 and 4 months of age. If replicated in humans, these substantial age-dependent changes in the pharmacokinetics of zalcitabine would warrant smaller and less frequent dosing with zalcitabine in HIV-infected neonates than in older children and adults.