Pharmacokinetics during pregnancy

Little information is available regarding the pharmacokinetics of individual cytotoxic agents in the pregnant patient. Alterations in drug distribution are expected in view of the physiologic changes that occur during pregnancy (alterations in renal and hepatic function, plasma flow and plasma protein binding, increases in plasma volume, and the third space of the amniotic sac).

High lipid solubility, low molecular weight, and loose binding to plasma proteins favor placental transfer of drugs from mother to fetus. Changes in gastrointestinal motility secondary to pregnancy alter the absorption of oral agents. Hemodynamic changes occurring in pregnancy may alter distribution and excretion of chemotherapeutic agents. The increase in blood volume, especially during the second and third trimesters, might affect the distribution of antineoplastic agents and hypoproteinemia can increase free drug concentration. The increase in glomerular filtration rate alters the rate of renally excreted agents. 

These processes are constantly evolving and definite algorithms are difficult to follow. Nevertheless, the same chemotherapy dosages are being used in the pregnant and the non-pregnant state since there are no current guidelines for dose modification. The first studies in humans confirm the hypothesis though larger series are needed to draw firm conclusions. As far as we have the evidence, these changes do not appear to worsen the maternal prognosis.