Placental transfer of pinazepam and its metabolite N-desmethyldiazepam in women at term

Placental transfer of drugs administered to the mother

Drugs administered to mothers have the potential to cross the placenta and reach the fetus. Under particular circumstances, the comparison of the drug concentration in the maternal and fetal plasma may give an idea of the exposure of the fetus to the maternally administered drugs. In this review drugs are classified according to their type of transfer across the placenta. Several drugs rapidly cross the placenta and pharmacologically significant concentrations equilibrate in maternal and fetal plasma. Their transfer is termed 'complete'. Other drugs cross the placenta incompletely, and their concentrations are lower in the fetal than in maternal plasma. The majority of drugs fit into 1 of these 2 groups. A limited number of drugs reach greater concentrations in fetal than maternal plasma. It is said that these drugs have an 'exceeding' transfer. The impression prevails that suxamethonium chloride (succinylcholine chloride) and doxorubicin do not cross the placenta. However, a careful analysis of the literature suggests that this impression is wrong and that all drugs cross the placenta, although the extent transfer varies considerably. The following parameters were considered as possible factors determining the extent of placental transfer: (i) the molecular weight of the drug; (ii) the pKa (pH at which the drug is 50% ionised); and (iii) the extent of drug binding to the plasma protein. Drugs with molecular weights greater than 500D have an incomplete transfer across the human placenta. Strongly dissociated acid drug molecules should have an incomplete transfer, but this does not seem to be an absolute rule. For example, ampicillin and methicillin transfer completely and they are strongly dissociated at physiological pH. The extent of drug binding to plasma protein does not influence the type of drug transfer across the human placenta.

Placental transfer and distribution of pinazepam and its metabolite N-desmethyldiazepam in the maternal and fetal rabbit: effect of the stage of gestation

The distribution of pinazepam and its metabolite N-desmethyldiazepam was studied in fetuses of New Zealand rabbits on the 20th and 27th day of pregnancy. The concentrations of both compounds were also measured in the maternal brain, liver and uterus. Pregnant rabbits were sacrificed at 0.5, 2, 4 and 12 h after intravenous administration of pinazepam (5 mg/kg). The concentrations of pinazepam and N-desmethyldiazepam in various biological specimens were measured by a specific gas-chromatographic procedure. Pinazepam and N-desmethyldiazepam rapidly crossed the placenta. In 20 day old fetuses, comparable concentrations of pinazepam were found in the liver, brain, heart, lungs and kidneys. In contrast, the liver of 27 day old fetuses accumulated pinazepam at concentrations higher than the other tissues. The hepatic extraction of pinazepam, already described in adult rabbits (1), develops prenatally. A preferential accumulation of pinazepam rather than N-desmethyldiazepam was also observed in the maternal uterus. In this tissue the concentrations of pinazepam were 5-10 times higher on the 27th rather than the 20th day of pregnancy. The stage of pregnancy influences the distribution pattern of pinazepam in rabbit fetuses and their mothers.