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As with hOCT1 and hOCT2, hOCT3 is a low‐affinity, high‐capacity, bidirectional, poly‐specific transporter that transports a variety of endogenous and exogenous compounds [2]. Further, similar to its paralogs, hOCT3 mediates electrogenic transport of various cations with diverse chemical structures [2]. TEA, a model substrate for OCT1 and OCT2, is poorly transported by OCT3 [1]. A variety of naturally occurring molecules are substrates for OCT3 including micronutrients, neurotransmitters, and amino acids. In addition, the transporter transports a number of prescription drugs from many pharmacologic classes, including antiarrhythmics, antidepressants, and antacids. In addition, and distinct from its paralogs, OCT3 transports a number of hormones, including progesterone and testosterone. Model substrates for OCT3 include the cation MPP⁺, the fluorescent substrate ASP⁺, and the anti‐diabetic drug metformin (Table 2.2). Corticosterone serves as a model inhibitor for OCT3. Because the transporter is expressed at lower levels in comparison to its paralogs in organs of drug absorption (e.g., the intestine) and elimination (liver and kidney), OCT3 is not generally considered a target for DDIs.