Relationship between prolonged gestation and nifedipine pharmacokinetics in long-term tocolysis

In this study, we examined the pharmacokinetics of nifedipine and investigated the maternal and foetal background factors that prolong pregnancy in pregnant women undergoing long-term tocolysis. This prospective observational study included 38 pregnant women hospitalised for threatened preterm labour and treated with nifedipine extended-release tablets in combination with an intravenous ritodrine infusion. Maternal plasma nifedipine concentrations were determined using high-performance liquid chromatography. All patients were administered 20 or 40 mg/dose of nifedipine every 6 h at the time of blood sampling. The plasma trough concentration (C_trough ) was 22.6 ± 17.3 ng/mL, the maximum plasma concentration (C_max ) was 30.9 ± 15.3 ng/mL and the time to maximum concentration (T_max ) was 1.70 ± 1.10 h, as determined using noncompartmental analysis (NCA). The area under the curve for drug concentration (AUC_tau ) was 152.3 ± 91.8 mg/L・h, and oral clearance (CL/F) was 0.17 ± 0.08 L/h. Using logistic regression analyses, we identified the factors that predicted term delivery from 37 weeks to <42 weeks of gestation. Gestational age at admission and the AUC_tau of nifedipine can predict term delivery. The AUC_tau of nifedipine is a valuable regulatory predictor of term delivery in pregnant women undergoing long-term tocolysis.