Disposition of 4-methylbenzoylglycine in rat isolated perfused kidney and effects of hippurates on renal mitochondrial metabolism

Uremic toxins inhibit transport by breast cancer resistance protein and multidrug resistance protein 4 at clinically relevant concentrations

During chronic kidney disease (CKD), there is a progressive accumulation of toxic solutes due to inadequate renal clearance. Here, the interaction between uremic toxins and two important efflux pumps, viz. multidrug resistance protein 4 (MRP4) and breast cancer resistance protein (BCRP) was investigated. Membrane vesicles isolated from MRP4- or BCRP-overexpressing human embryonic kidney cells were used to study the impact of uremic toxins on substrate specific uptake. Furthermore, the concentrations of various uremic toxins were determined in plasma of CKD patients using high performance liquid chromatography and liquid chromatography/tandem mass spectrometry. Our results show that hippuric acid, indoxyl sulfate and kynurenic acid inhibit MRP4-mediated [³H]-methotrexate ([³H]-MTX) uptake (calculated Ki values: 2.5 mM, 1 mM, 25 µM, respectively) and BCRP-mediated [³H]-estrone sulfate ([³H]-E1S) uptake (Ki values: 4 mM, 500 µM and 50 µM, respectively), whereas indole-3-acetic acid and phenylacetic acid reduce [³H]-MTX uptake by MRP4 only (Ki value: 2 mM and IC₅₀ value: 7 mM, respectively). In contrast, p-cresol, p-toluenesulfonic acid, putrescine, oxalate and quinolinic acid did not alter transport mediated by MRP4 or BCRP. In addition, our results show that hippuric acid, indole-3-acetic acid, indoxyl sulfate, kynurenic acid and phenylacetic acid accumulate in plasma of end-stage CKD patients with mean concentrations of 160 µM, 4 µM, 129 µM, 1 µM and 18 µM, respectively. Moreover, calculated Ki values are below the maximal plasma concentrations of the tested toxins. In conclusion, this study shows that several uremic toxins inhibit active transport by MRP4 and BCRP at clinically relevant concentrations.

Mechanisms and clinical implications of renal drug excretion

The body defends itself against potentially harmful compounds like drugs, toxic compounds, and their metabolites by elimination, in which the kidney plays an important role. Renal clearance is used to determine renal elimination mechanisms of a drug, which is the result of glomerular filtration, active tubular secretion and reabsorption. The renal proximal tubule is the primary site of carrier-mediated transport from blood to urine. Renal secretory mechanisms exists for, anionic compounds and organic cations. Both systems comprises several transport proteins, and knowledge of the molecular identity of these transporters and their substrate specificity has increased considerably in the past decade. Due to overlapping specificities of the transport proteins, drug interactions at the level of tubular secretion is an event that may occur in clinical situation. This review describes the different processes that determine renal drug handling, the techniques that have been developed to attain more insight in the various aspects of drug excretion, the functional characteristics of the individual transport proteins, and finally the implications of drug interactions in a clinical perspective.