Nifedipine concentration in maternal and umbilical cord blood after nifedipine GITS tablets for tocolysis

Aims: Nifedipine is used “off-label“ for tocolysis. Since many years we use nifedipine GITS (gastro-intestinal therapeutic system) tablets. However, there is no safe information about the nifedipine concentrations in maternal and corresponding umbilical cord blood.

Objective: To determine nifedipine concentration in maternal plasma at steady state and in maternal / umbilical cord plasma after tocolysis with nifedipine GITS tablets 30–150mg/d.

Study design: Prospective study in pregnant women treated for threatening preterm labor, measuring nifedipine in maternal blood at steady state and in maternal (M) and corresponding umbilical blood (U) at delivery.

Results: Dose dependent nifedipine concentrations (mean±SE) at steady state were 53.86±6.49µg/l (all doses, n=31), 37.86±7 98 (dose 60mg/d, n=13) and 91.89±11.72 (dose 150mg/d, n=7) (P<0.002) respectively. Nifedipine in M and U declined both in a ln linear regression with t1/2 of 17.4h (M) and 20.4h (U) reaching less than 1 μg/l >80 hours after the last GITS administration. U and M concentrations correlated in a linear regression and the U/M ratio was 0.77±0.1 (n=21, mean±SE).

Conclusions: Steady state plasma concentrations after GITS in pregnant women with preterm labour are predominantly <100 μg/l and cross the placenta for 77%.