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Pharmacokinetics of Fetal and Neonatal Exposure to Drugs

From the Department of Pharmaceutics, School of Pharmacy, State University of New York at Buffalo, Amherst, New York

Abstract

Essentially all drugs taken by a pregnant woman are transferred across the placenta to the fetus. Fetal drug exposure, as reflected by the unbound (free) drug concentration versus time profile in fetal plasma, is determined by 1) the amount of drug taken by or administered to the mother and the absorption rate constant or rate of intravenous injection of the drug; 2) the distribution and elimination kinetics of the drug in the mother; 3) the transfer rate constants of the drug across the placenta in both directions; 4) possible "first-pass" biotransformation in the placenta and fetal liver (which appears to be of quantitatively minor importance in most cases); and 5) the distribution and elimination kinetics of the drug in the fetus. Exposure to a transplacentally acquired drug is often greater in the newborn infant than in the fetus due to the relatively slow clearance of the drug from the newborn's body caused by the functional immaturity of biotransformation and excretion processes. Fetal drug elimination is mediated primarily by the mother's metabolic and excretory processes and therefore does not usually depend on the functionality of these processes in the fetus. The physician can minimize unnecessary fetal and neonatal exposure to drugs by 1) educating women of childbearing age and pregnant women appropriately; 2) giving careful consideration to the risk-benefit aspects of administering drugs to pregnant patients; and 3) being aware of (and using drugs in accordance with) the pharmacokinetic principles that govern the concentration time profiles of drugs in the mother and fetus.