Vinblastine and sulfinpyrazone export by the multidrug resistance protein MRP2 is associated with glutathione export.
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The multidrug resistance proteins MRP1 and MRP2 are members of the same subfamily of ATP-binding cassette transporters. Besides organic molecules conjugated to negatively charged ligands, these proteins also transport cytotoxic drugs for which no negatively charged conjugates are known to exist. In polarized MDCKII cells, MRP1 routes to the lateral plasma membrane, and MRP2 to the apical plasma membrane. In these cells MRP1 transports daunorubicin, and MRP2 vinblastine; both transporters export reduced glutathione (GSH) into the medium. We demonstrate that glutathione transport in MDCKII-MRP1 cells is inhibited by the inhibitors of organic anion transporters sulfinpyrazone, indomethacin, probenecid and benzbromarone. In MDCKII-MRP2 cells, GSH export is stimulated by low concentrations of sulfinpyrazone or indomethacin, whereas export is inhibited down to control levels at high concentrations. We find that unmodified sulfinpyrazone is a substrate for MRP2, also at concentrations where GSH export is inhibited. We also show that GSH export in MDCKII-MRP2 cells increases in the presence of vinblastine, and that the stoichiometry between drug and GSH exported is between two and three. Our data indicate that transport of sulfinpyrazone and vinblastine is associated with GSH export. However, at high sulfinpyrazone concentrations this compound is transported without GSH. Models of MRP action are discussed that could explain these results. (+info)
Effect of urate-lowering therapy on the velocity of size reduction of tophi in chronic gout.
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OBJECTIVE: The optimal serum urate levels necessary for elimination of tissue deposits of monosodium urate in patients with chronic gout is controversial. This observational, prospective study evaluates the relationship between serum urate levels during therapy and the velocity of reduction of tophi in patients with chronic tophaceous gout. METHOD: Sixty-three patients with crystal-confirmed tophaceous gout were treated with allopurinol, benzbromarone, or combined therapy to achieve serum uric acid levels less than the threshold for saturation of urate in tissues. The tophi targeted for evaluation during followup were the largest in diameter found during physical examination. RESULTS: Patients taking benzbromarone alone or combined allopurinol and benzbromarone therapy achieved faster velocity of reduction of tophi than patients taking allopurinol alone. The velocity of tophi reduction was linearly related to the mean serum urate level during therapy. The lower the serum urate levels, the faster the velocity of tophi reduction. CONCLUSION: Serum urate levels should be lowered enough to promote dissolution of urate deposits in patients with tophaceous gout. Allopurinol and benzbromarone are equally effective when optimal serum urate levels are achieved during therapy. Combined therapy may be useful in patients who do not show enough reduction in serum urate levels with single-drug therapy. (+info)
Effects of aspirin and/or salicylate on hydrolysis and glucuronidation of indomethacin in rat erythrocytes and hepatocytes.
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This study was conducted to explore the mechanism of the pharmacokinetic interaction between aspirin (ASP) and indomethacin (IND) using rat erythrocytes (RBCs) and hepatocytes. ASP was hydrolyzed to salicylic acid (SA) in both the RBCs and hepatocytes. Within RBCs, aspirin and/or salicylate (ASP/SA) increased the concentration of IND, accompanied by a constant hydrolysis of IND. In hepatocytes, a low dose of IND was subjected to glucuronidation rather than hydrolysis, and ASP/SA inhibited both the acylglucuronidation of IND and hydrolysis of IND glucuronide. A high dose of IND underwent hydrolysis with about double the glucuronidation, and ASP/SA decreased the ratio of hydrolysis to glucuronidation, accompanied by a loss of ASP, IND and their metabolites from the medium. Collectively, the results provide metabolic insight into the mechanism of drug-drug interaction between ASP/SA and IND in the hepatocytes and RBCs. (+info)
A new class of CYP2C9 inhibitors: probing 2C9 specificity with high-affinity benzbromarone derivatives.
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Noncovalent forces, other than hydrophobic interactions, are important determinants of substrate bias exhibited by some cytochromes P450. The CYP2C9 pharmacophore is proposed to include either an anionic group or hydrogen bond donor in addition to its hydrophobic groups. By constructing analogs of benzbromarone, evidence supporting the existence of a 2C9 anion-binding site was revealed. A nonsubstituted phenol analog was determined to have a pKa of 8.4 and a Ki of 414 nM whereas those with dihalogenated benzoyl phenols had pKa values between 4.2 to 5.2 and Ki values as low as 1 nM. The nonhalogenated, nonionizable analog is the poorest binder at 796 nM. The Ki range covers around three orders of magnitude with even the weakest binder being a more potent inhibitor than 2C9 substrate phenytoin. Thus, benzbromarone derivatives represent a class of molecules with the potential to reveal more structural details of the 2C9 active site. (+info)
A case of exercise-induced acute renal failure in a patient with idiopathic renal hypouricemia developed during antihypertensive therapy with losartan and trichlormethiazide.
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Exercise-induced acute renal failure (ARF) developed in a 45-year-old man during antihypertensive therapy with losartan and trichlormethiazide. The antihypertensive therapy was stopped and marked hypouricemia became apparent during improvement of his renal function. The daily urinary excretion of uric acid was normal and an increased fractional excretion of uric acid was observed. Renal biopsy revealed that the kidney was recovering from acute tubular necrosis with interstitial fibrosis. Based on the results of pyrazinamide and benzbromarone tests, we classified this case as one of presecretory reabsorption defect of uric acid. Antihypertesive therapy with benidipine and candesartan was initiated, and the patient has not had any ARF episodes since. Because idiopathic renal hypouricemia can be associated with exercise-induced ARF and chronic renal dysfunction, careful antihypertensive therapy and follow-up evaluation of renal function might be necessary for hypertensive patients with idiopathic renal hypouricemia. (+info)
Circadian rhythm of plasma uric acid and handling stress-induced hyperuricemia in conscious cebus monkeys.
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An apparent circadian rhythm of plasma uric acid and the effect of handling stress on plasma uric acid level in conscious cebus monkeys were demonstrated. The lowest level of plasma uric acid in the circadian rhythm occurred early in the morning and the highest, before bedtime at night. With experimental handling stress, the plasma uric acid level rose to much more than the maximum level of the circadian rhythm. Stress-induced hyperuricemia could be inhibited without an increase of urinary uric acid excretion by the minor tranquilizer diazepam at doses of more than 1 mg/kg, p.o. On the other hand, benzbromarone at 20 mg/kg, p.o. significantly inhibited the hyperuricemia with a hyperuricosuric effect, while probenecid at 50 mg/kg, p.o. had no effect on either the increased plasma uric acid or urinary uric acid excretion. Accordingly, it is concluded that the plasma uric acid level in conscious cebus monkeys easily fluctuates with experimental conditions and that the animals can be utilized to evaluate the hypouricemic and hyperuricosuric property of benzbromarone-like agents. (+info)
CYP2C9 genotype-dependent effects on in vitro drug-drug interactions: switching of benzbromarone effect from inhibition to activation in the CYP2C9.3 variant.
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The CYP2C9.3 variant exhibits marked decreases in substrate turnover compared with the wild-type enzyme, but little is known regarding the effect this variant form may have on the occurrence of drug-drug interactions. To examine this possibility, the effect of the potent CYP2C9 inhibitor, benzbromarone, was studied with regard to CYP2C9.1- and CYP2C9.3-mediated flurbiprofen metabolism to evaluate whether the variant enzyme exhibits differential inhibition kinetics. Although benzbromarone inhibited CYP2C9.1 activity as expected, CYP2C9.3-mediated flurbiprofen 4'-hydroxylation was activated in the presence of benzbromarone. T1 relaxation studies revealed little change in distances of flurbiprofen protons from the heme iron of either CYP2C9.1 or CYP2C9.3 in the presence of benzbromarone compared with flurbiprofen alone. Spectral binding studies were also performed to investigate whether benzbromarone affected substrate binding, with the addition of benzbromarone having little effect on flurbiprofen-binding affinity in both CYP2C9.1 and CYP2C9.3. Docking studies with the 2C9.1 structure crystallized with a closed active site identified multiple but overlapping subsites with sufficient space for benzbromarone binding in the enzyme when flurbiprofen was positioned closest to the heme. If the closed conformation of 2C9.3 is structurally similar to 2C9.1, as expected for the conservative I359L mutation, then the dynamics of benzbromarone binding may account for the switching of drug interaction effects. In conclusion, the I359L amino acid substitution found in CYP2C9.3 not only reduces metabolism compared with CYP2C9.1 but can also dramatically alter inhibitor effects, suggesting that differential degrees of drug inhibition interactions may occur in individuals with this variant form of CYP2C9. (+info)
Prediction of CYP2C9-mediated drug-drug interactions: a comparison using data from recombinant enzymes and human hepatocytes.
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The IC50 values of 14 drugs were determined in recombinantly expressed CYP2C9 (rCYP2C9) and human hepatocytes and the data used to simulate clinical area under the plasma concentration-time curve (AUC) changes upon coadministration with prototypic CYP2C9 substrates. There was an excellent correlation between IC(50, apparent) values determined using diclofenac and naproxen as CYP2C9 substrates (r2 = 0.82, p < 0.0001), with values being generally higher in the naproxen assay. After correcting for nonspecific binding, the IC(50, unbound) values were similar between the assays, for the majority of compounds. Two compounds, amiodarone and benzbromarone, demonstrated substrate-specific differences, activating naproxen O-demethylase to approximately 250% of control activity at 1 mM and 1 microM, respectively, while inhibiting diclofenac 4'-hydroxylation with IC(50, apparent) values of 3 microM and 0.04 microM, respectively. CYP2C9 IC(50, apparent) values generated in human hepatocytes were systematically higher than those determined with rCYP2C9. After correcting for nonspecific binding, there was an excellent correlation of IC(50, unbound) values generated in the different milieu (r2 = 0.88, p < 0.0001). The ratio of inhibitor concentration at the entrance to the liver to the inhibition constant ([I]in/Ki) was used to simulate clinical deltaAUC changes and compared with that observed in vivo. Where [I]in, total/Ki, apparent) was used, there were zero false negatives (observed deltaAUC >or=2, predicted deltaAUC <2), eight correct assignations, and seven false positives (observed deltaAUC 2. Where [I]in, unbound/Ki, unbound was used, there was one false negative, 14 correct assignations, and zero false positives. In summary, the data presented here suggest that for CYP2C9 interactions, the use of total liver inhibitor concentrations may indeed avoid false negatives, but more realistic predictions may be achieved using unbound liver inhibitor concentrations and unbound in vitro inhibition parameters. (+info)