Many types of xenobiotic transporters have been identified. They generally exhibit multispecific recognition of various types of substrates, and mediate membrane permeation of therapeutic agents, thereby playing important roles in drug absorption and disposition. It has recently been proposed that protein-protein interactions involving the xenobiotic transporters may affect their function, localization and expression on plasma membranes. So-called adaptor proteins that directly interact with the transporters include PDZ domain-containing proteins (PSD95, Dlg and ZO1). These PDZ adaptors have multiple PDZ domains in their structure, and each PDZ domain can interact with the cytosolic region of the transporters, and so it has been hypothesized that transporters are localized within networks consisting of several transporters and adaptors. Interaction with a PDZ adaptor is essential for the cell-surface localization of at least some xenobiotic transporters, and therefore, such interaction could be required for efficiency and fidelity in the vectorial transport of xenobiotics and therapeutic agents in epithelial cells. This review article summarizes recent evidence on the interactions of xenobiotic transporters with adaptor proteins, and presents a working hypothesis concerning their pharmacological significance.
Nateglinide is an antidiabetic agent metabolized by CYP2C9 and CYP3A4; hence inhibitors of these CYP isozymes may interact with nateglinide. There are, however, only limited in vitro data on how to predict drug-drug interactions in vivo. We examined the effects of 18 drugs that may be prescribed together with nateglinide (metformin, buformin, aspirin, gemfibrozil, simvastatin, pioglitazone, rosiglitazone, carbamazepine, clarithromycin, gliclazide, clofibrate, fluconazole, bezafibrate, phenylbutazone, nifedipine, famotidine, ibuprofen and miconazole) on the conversion of nateglinide to its major metabolite (N-[trans-4-(1-hydroxy-1-methylethyl)-cyclohexanecarbonyl]-D-phenylalanine) using human liver microsomes. Eight compounds showed a<50% inhibitory effect and we estimated the Ki values for the remaining 10 compounds. Except for fluconazole and miconazole, 1+Iin, max, u/Ki calculated from the Ki values, was approximately 1 and thus the possibility of a drug-drug interaction was considered low. The value for fluconazole suggested the risk of interaction and agreed with the results of clinical studies in which the AUC of nateglinide increased by 48% when it was co-administered with fluconazole. The present study showed that nateglinide metabolism would hardly be affected by the drugs used in this study, except for miconazole and fluconazole that are potent inhibitors of multiple isoforms of CYPs.
We previously reported that MEEK, a generic product of vancomycin hydrochloride (VCM), was less nephrotoxic than a conventional preparation (S-VCM) in normal rats at a nephrotoxic dose (400 mg/kg) of VCM.1) To infer the clinical significance of this finding, we compared the risk of nephrotoxicity of these two formulations in rats with chronic renal failure in this study. MEEK or S-VCM was given intravenously to two weeks post-5/6 nephrectomy rats, and the pharmacokinetic profile of VCM and pathological evaluation were compared. There were no differences at the daily clinical dose (40 mg/kg), but at the twice the daily clinical dose (80 mg/kg), the mean plasma concentration of VCM was higher after S-VCM administration than after MEEK and the CLtot and CLr decreased to approximately 60% of those after MEEK. The renal tissue concentration of VCM was 1.5-fold higher at 24hr after S-VCM administration than after MEEK. Pathologically, no marked differences between the findings were observed at 24hr after administration of each formulation. These findings suggest that MEEK reduces renal damage caused by VCM and prevents the iatrogenic aggravation of nephrotoxicity. These results hold out hope that MEEK will permit high-dose administration of VCM, while revealing clinical significance of the nephrotoxicity-reduction by MEEK.
Since porphyrins are regarded as endogenous substrates for the ATP-binding cassette (ABC) transporter ABCG2, it is hypothesized that functional impairment owing to genetic polymorphisms or inhibition of ABCG2 by drugs may result in a disruption of cellular porphyrin homeostasis. In the present study, we expressed ABCG2 genetic variants, i.e., V12M, Q141K, S441N, and F489L, as well as the wild type (WT) in Flp-In-293 cells to examine the hypothesis. Cells expressing S441N and F489L variants exhibited high levels of both cellularly accumulated pheophorbide a and photosensitivity, when those cells were incubated with pheophorbide a and irradiated with visible light. To further elucidate the significance of ABCG2 in cellular porphyrin homeostasis, we observed cellular accumulation and compartmentation of porphyrin and pheophorbide a by means of a new fluorescence microscopy technology, and found that accumulation of porphyrin and pheophorbide a in the cytoplasm compartment was maintained at low levels in Flp-In-293 cells expressing ABCG2 WT, V12M, or Q141K. When ABCG2 was inhibited by imatinib or novobiocin, however, those cells became sensitive to light. Based on these results, it is strongly suggested that certain genetic polymorphisms and/or inhibition of ABCG2 by drugs can enhance the potential risk of photosensitivity.
Proton-pump inhibitors (PPIs, e.g. omeprazole and rabeprazole) are often administered to transplant patients as a treatment or prophylaxis for ulcers after surgery. Since tacrolimus and PPIs share the CYP3A4 system for metabolism, pharmacokinetic interactions are anticipated when they are administered simultaneously. We present a Japanese male patient who underwent a living-donor kidney transplantation having received tacrolimus, mycophenolate mofetil, and prednisolone for immunosuppression. The concentration/dose (C/D) ratio for tacrolimus was markedly higher during the period of treatment with omeprazole than ranitidine or rabeprazole. The results of liver functional tests were within the normal range during the use of these three antacid drugs. Since the higher C/D ratio for tacrolimus when omeprazole was being administered did not result from a decrease in the elimination of tacrolimus due to hepatic dysfunction, drug interaction between omeprazole and tacrolimus was strongly suspected. The present case indicates that rabeprazole can be used safely in place of omeprazole in kidney transplant recipients receiving tacrolimus.
The effect of capsaicin on intestinal cefazolin absorption was examined by means of an in situ closed loop method in rats to clarify whether the vanilloid receptor (TRPV1) is involved in drug absorption driven by passive diffusion. In control experiments with 1 mg/mL cefazolin, the amount of cefazolin absorbed from the closed loop was 15.3±1.5 μg/cm in the rat jejunum. The absorption amount was increased to 22.8±0.9 and 23.4±2.4 μg/cm when capsaicin was applied with cefazolin at concentrations of 10 and 400 μM, respectively. The enhancing effect of capsaicin on cefazolin absorption was suppressed when ruthenium red, a non-selective inhibitor of transient receptor potential (TRP) cation channels, was intravenously infused into the rat during the experiment. Cefazolin accumulation in the intestinal tissue was not altered in the presence of capsaicin. Collectively, the mechanism accounting for the capsaicin-induced increase in the intestinal cefazolin absorption is probably that capsaicin associating with TRPV1 increases the intrinsic permeability of cefazolin in intestine.
Wilson's disease is an inherited, autosomal recessive disorder of copper accumulation and toxicity. Lifelong chelation therapy is essential in all Wilson's disease patients. Intestinal absorption of some compounds is limited partly because they are preferentially transported in the secretory direction. Several ATP-binding cassette (ABC) transporters are expressed in the apical membrane of the small intestine and secrete various drugs into the lumen. In this study, we investigated the characteristics of the intestinal efflux ABC transporters in LEC rats. We found that the expression of multidrug resistance-associated protein 2 (Mrp2) in the jejunum of Long-Evans Cinnamon (LEC) rats, an animal model for Wilson's disease, is decreased.
A liver-specific transporter organic anion transporting polypeptide 1B1 (OATP1B1, also known as OATP-C) is encoded by SLCO1B1 and mediates uptake of various endogenous and exogenous compounds from blood into hepatocytes. In this study, 15 SLCO1B1 exons (including non-coding exon 1) and their flanking introns were comprehensively screened for genetic variations in 177 Japanese subjects. Sixty-two genetic variations, including 28 novel ones, were found: 7 in the 5′-flanking region, 1 in the 5′-untranslated region (UTR), 13 in the coding exons (9 nonsynonymous and 4 synonymous variations), 5 in the 3′-UTR, and 36 in the introns. Five novel nonsynonymous variations, 311T>A (Met104Lys), 509T>C (Met170Thr), 601A>G (Lys201Glu), 1553C>T (Ser518Leu), and 1738C>T (Arg580Stop), were found as heterozygotes. The allele frequencies were 0.008 for 1738C>T (Arg580Stop) and 0.003 for the four other variations. Arg580Stop having a stop codon at codon 580 results in loss of half of transmembrane domain (TMD) 11, TMD12, and a cytoplasmic tail, which might affect transport activity. In addition, novel variations, IVS12-1G>T at the splice acceptor site and -3A>C in the Kozak motif, were detected at 0.003 and 0.014 frequencies, respectively. Haplotype analysis using -11187G>A, -3A>C, IVS12-1G>T and 9 nonsynonymous variations revealed that the haplotype frequencies for *1b, *5, *15, and *17 were 0.469, 0.000 (not detected), 0.037, and 0.133, respectively. These data would provide fundamental and useful information for pharmacogenetic studies on OATP1B1-transported drugs in Japanese.
The vitamin D receptor (VDR) is a transcriptional factor responsive to 1α,25-dihydroxyvitamin D3 and lithocholic acid, and induces expression of drug metabolizing enzymes CYP3A4, CYP2B6 and CYP2C9. In this study, the promoter regions, 14 exons (including 6 exon 1's) and their flanking introns of VDR were comprehensively screened for genetic variations in 107 Japanese subjects. Sixty-one genetic variations including 25 novel ones were found: 9 in the 5′-flanking region, 2 in the 5′-untranslated region (UTR), 7 in the coding exons (5 synonymous and 2 nonsynonymous variations), 12 in the 3′-UTR, 19 in the introns between the exon 1's, and 12 in introns 2 to 8. Of these, one novel nonsynonymous variation, 154A>G (Met52Val), was detected with an allele frequency of 0.005. The single nucleotide polymorphisms (SNPs) that increase VDR expression or activity, -29649G>A, 2T>C and 1592(*308)C>A tagging linked variations in the 3′-UTR, were detected at 0.430, 0.636, and 0.318 allele frequencies, respectively. Another SNP, -26930A>G, with reduced VDR transcription was found at a 0.028 frequency. These findings would be useful for association studies on VDR variations in Japanese.