Placental passage of oxazepam and its metabolism in mother and newborn

The transfer across the placenta and the maternal and neonatal kinetics of oxazepam and its conjugate were studied in 12 patients and their newborns. A single oral dose of 25 mg was given to the women 40 min to 23 hr before delivery. The concentration of oxazepam and oxazepam conjugate was determined by electron capture gas chromatography. Oxazepam was readily absorbed and peak plasma concentrations were in the same range as those in healthy males and nonpregnant females given the same dose. When meperidine was given within one hour of oxazepam the absorption was delayed without a change in bioavailability. The elimination half‐lives (t½) ranged from 5 to 8 hr, which is shorter than reported for nonpregnant subjects. The t½s of conjugates were in the same range as those of oxazepam. Oxazepam was found in the umbilical vein of all patients. The concentrations of the oxazepam conjugate were lower (in three cases unmeasurable) than those of oxazepam. The ratio between umbilical/maternal vein oxazepam concentrations reached a value of about 1.35 and remained constant beyond a dose‐delivery time of 3 hr. The corresponding ratios of the conjugate were lower. The t½ of oxazepam was 3‐ to 4‐fold as long (mean 21.9 hr) in the newborns as in the mothers (mean 6.5 hr). In three newborns the plasma concentration of oxazepam conjugate rose during the first 6 to 10 hr of extrauterine life, which indicates that the newborn is able to conjugate oxazepam. The Apgar score values were normal in all newborns.