ABCG2 transporter: therapeutic and physiologic implications in veterinary species

Drug transporters significantly influence drug pharmacokinetics and pharmacodynamics. While P-glycoprotein, the product of the MDR1 (ABCB1) gene, is the most well-characterized ABC transporter, the pharmacological importance of a related transporter, ABCG2, is starting to be realized in veterinary medicine. Based primarily on human and rodent studies, a number of clinically relevant, structurally and functionally unrelated drugs are substrates for ABCG2. ABCG2 is expressed by a variety of normal tissues including the intestines, renal tubular cells, brain and retinal capillary endothelial cells, biliary canalicular cells, and others, where it functions to actively extrude substrate drugs. In this capacity, ABCG2 limits oral absorption of substrate drugs and restricts their distribution to privileged sites such as the brain and retina. ABCG2 is also expressed by tumor cells where it functions to limit the intracellular accumulation of cytotoxic agents, contributing to multidrug resistance. Several ABCG2 polymorphisms have been described in human patients, some of which result in altered drug disposition, increasing susceptibility to adverse drug reactions. Additionally, ABCG2 polymorphisms in humans have been associated with disease states such as gout. Feline ABCG2 has recently been demonstrated to have several amino acid differences at conserved sites compared with 10 other mammalian species. These amino acid differences adversely affect transport function of feline ABCG2 relative to that of human ABCG2. Furthermore, these differences appear to be responsible for fluoroquinolone-induced retinal toxicity in cats and may play a role in acetaminophen toxicity as well. Studies in rodents and sheep have determined that ABCG2 expressed in mammary tissue is responsible for the secretion of many compounds (both therapeutic and toxic) into milk. Finally, data in rodent models suggest that ABCG2 may play an important role in regulating a number of physiologic pathways involved in protecting erythrocytes from oxidative damage.