Pharmacokinetics describes the changes in drug concentrations over time and are closely monitored for chemotherapeutics given their narrow therapeutic index. Drugs are removed from the systemic circulation by the organs that metabolize them, such as the liver or kidneys, and are subsequently excreted through the urine, bile, or stool. The resulting plasma concentrations of a drug may be significantly different from the concentrations of drug to which the cancer cell is exposed. Factors that contribute to this disparity are illustrated in the figure.