PHARMACOKINETICS Important transporters located in intestinal, renal, and hepatic epithelia function in concert with metabolism of drugs in the selective absorption and elimination of endogenous substances and drugs (Figure 2-2). In addition, transporters mediate tissue-specific drug distribution (drug targeting); conversely, transporters also may serve as protective barriers to particular organs and cell types, controlling tissue distribution as well as the absorption and elimination of drugs.
PHARMACODYNAMICS: TRANSPORTERS AS DRUG TARGETS Membrane transporters are the targets of many drugs. For example, neurotransmitter transporters are the targets for drugs used in the treatment of neuropsychiatric disorders. SERT (SLC6A4) is a target for the selective serotonin reuptake inhibitors (SSRIs), a major class of antidepressant drugs. Other neurotrans-mitter reuptake transporters serve as drug targets for the tricyclic antidepressants, amphetamines (including amphetamine-like drugs used in the treatment of attention deficit disorder in children), and anticonvulsants. These transporters also may be involved in the pathogenesis of neuropsychi-atric disorders, including Alzheimer's and Parkinson's diseases. Transporters that are nonneuronal also may be potential drug targets (e.g., cholesterol transporters in cardiovascular disease, nucleo-side transporters in cancers, glucose transporters in metabolic syndromes, and Na+-H+ antiporters in hypertension).
DRUG RESISTANCE Membrane transporters play critical roles in the development of resistance to anticancer drugs, antiviral agents, and anticonvulsants. P-glycoprotein, which exports many chemotherapeutics from cells, is overexpressed in tumor cells after exposure to cytotoxic anticancer agents. Other transporters (e.g., breast cancer resistance protein [BCRP], organic anion transporters, and several nucleoside transporters) also have been implicated in resistance to anticancer drugs.
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