Placental transfer of drugs in ruminant animals

Abstract

The exchange of drugs and other xenobiotics between the maternal circulation and the conceptus is of clinical significance because of possible adverse effects of these chemical on development and function of the fetus in utero Following fertilization, the gestational period can be divided into three unequal segments of differing pharmacological significance: (1) the period from fertilization to implantation of the blastocyst, (2) implantation to the end of metamorphosis, and (3) period of growth. These periods comprise the following respective proportions of the gestation period: 12%, 4% and 84% in the cow; 10%, 11% and 79% in the doe and ewe. The transfer of drugs and the potential consequences of exposure of the conceptus will differ depending on the stage of pregnancy in which administration takes place. Exposure of the blastocyst to toxic materials prior to implantation will cause death of the conceptus and termination of pregnancy. after impnatation, the embryo develops and organogenesis takes place. It is during this time that the embryo is most vulnerable to teratogenic effects. After metamorphosis has been completed the fetus grows in size and the fetal organs become functional. In the latter three-quarters of pregnancy, drugs entering the fetal circulation may exert effects on maturation of the nervous system, withdrawal of the umbilical hernia and a few other aspects of organ development. The primary effects, however, are as pharmacological agents acting on placental or fetal functions. The purpose of this chapter is to discuss the processes by which xenobiotics enter the female genital tract and fetus.