Tubular transporters and clearance of adefovir

Effect of probenecid on blood levels and renal elimination of furosemide and endogenous compounds in rats: Discovery of putative organic anion transporter biomarkers

Transporter-mediated drug-drug interactions (DDIs) are assessed using probe drugs and in vitro and in vivo models during drug development. The utility of endogenous metabolites as transporter biomarkers is emerging for prediction of DDIs during early phases of clinical trials. Endogenous metabolites such as pyridoxic acid and kynurenic acid have shown potential to predict DDIs mediated by organic anion transporters (OAT1 and OAT3). However, these metabolites have not been assessed in rats as potential transporter biomarkers. We carried out a rat pharmacokinetic DDI study using probenecid and furosemide as OAT inhibitor and substrate, respectively. Probenecid administration led to a 3.8-fold increase in the blood concentrations and a 3-fold decrease in renal clearance of furosemide. High inter-individual and intra-day variability in pyridoxic acid and kynurenic acid, and no or moderate effect of probenecid administration on these metabolites suggest their limited utility for prediction of Oat-mediated DDI in rats. Therefore, rat blood and urine samples were further analysed using untargeted metabolomics. Twenty-one m/z features (out of >8000 detected features) were identified as putative biomarkers of rat Oat1 and Oat3 using a robust biomarker qualification approach. These m/z features belong to metabolic pathways such as fatty acid analogues, peptides, prostaglandin analogues, bile acid derivatives, flavonoids, phytoconstituents, and steroids, and can be used as a panel to decrease variability caused by processes other than Oats. When validated, these putative biomarkers will be useful in predicting DDIs caused by Oats in rats.

Tenofovir Alafenamide Rescues Renal Tubules in Patients with Chronic Hepatitis B

Nucles(t)ide analogs (NAs) are effective for chronic hepatitis B (CHB). NAs suppress hepatic decompensation and hepatocarcinogenesis, leading to a dramatic improvement of the natural course of patients with CHB. However, renal dysfunction is becoming an important issue for the management of CHB. Renal dysfunction develops in patients with the long-term treatment of NAs including adefovir dipivoxil and tenofovir disoproxil fumarate. Recently, several studies have reported that the newly approved tenofovir alafenamide (TAF) has a safe profile for the kidney due to greater plasma stability. In this mini-review, we discuss the effectiveness of switching to TAF for NAs-related renal tubular dysfunction in patients with CHB.

Short-term efficacy after switching from adefovir dipivoxil and tenofovir disoproxil fumarate therapy to tenofovir alaferamide for chronic hepatitis B

The aim of the present study was to evaluate the effects of switching to tenofovir alafenamide (TAF) in patients who had received a nucleos(t)ide analog (NA) for the treatment of chronic hepatitis B (CHB). The data from 33 Japanese patients with CHB who received TAF therapy after using NA [adefovir dipivoxil (ADV) and/or tenofovir disoproxil fumarate (TDF)] were retrospectively analyzed. Specifically, the biochemical and virological markers from the start of the TAF treatment to 6 months later were assessed. Comparative evaluation was performed by dividing patients into two groups: Long-term (n=19) and short-term administration groups (n=14), with a cutoff administration duration of 10 years. In all 33 patients, the levels of serum hepatitis B surface antigen (HBsAg; 1,126±1,724 to 1,001±1,591 IU/ml; P<0.0001), serum alkaline phosphatase (ALP) (320±126 to 283±124 U/l; P=0.028), serum bone specific alkaline phosphatase (19.7±9.0 to 17.7±8.0 µg/l; P=0.0006) and urine β2-microglobulin-creatinine ratio (U-BMG/Cr; 5,224±17,471 to 3,547±14,652 µg/g·Cre; P=0.002) significantly decreased from baseline after 6 months. Serum HBsAg, serum ALP and U-BMG/Cr showed a significant reduction in both groups. In conclusion, switching from ADV or TDF to TAF resulted in a decrease in serum HBsAg and improvement in serum ALP and U-BMG/Cr after 6 months of treatment in patients regardless of history of treatment with NA.

Adefovir accumulation and nephrotoxicity in renal interstitium: Role of organic anion transporters of kidney

Common characteristics of drug induced nephrotoxicity are renal tubular and interstitial injury. Many studies have only focused on renal tubular injury. However, less is known about the effects of drugs in the renal interstitium on the nephrotoxicity. The aim of this study was to investigate the pharmacokinetics of adefovir (ADV) and the nephrotoxicity in the renal interstitium. Rats were treated with ADV alone or in combination with probenecid for 1, 7, 14, or 28 days. The renal interstitial fluid was collected by renal microdialysis. The concentration of ADV was determined by HPLC-MS/MS. Nephrotoxicity was evaluated by biochemical parameters or histological analysis. The results showed that organic anion transporters (OATs) inhibitor probenecid significantly increased the area under concentration-time curves (AUC) and peak concentration (C_max) of ADV in the renal interstitium, while the clearance (CL) in the renal interstitium was decreased in the ADV plus probenecid group compared to the ADV groups. After long-term treatment, interstitial fibrosis was present in the ADV plus probenecid group, whereas no trace of that could be detected in the ADV groups. Furthermore, a decrease was observed in the expression of OATs/Oats, which was dependent upon the concentrations and time of ADV treatment. In conclusion, it is possible that ADV could be accumulated in the interstitium when Oats were inhibited, which could cause renal interstitial fibrosis. Simply reducing cell uptake in long-term treatment might not be an effective method to protect against chronic nephrotoxicity.

Clinical Aspects of Transporter-Mediated Drug-Drug Interactions

Drug transporters play an essential role in disposition and effects of multiple drugs. Plasma concentrations of the victim drug can be modified by drug-drug interactions occurring in enterocytes (e.g., P-glycoprotein), hepatocytes (e.g., organic anion-transporting polypeptide 1B1 (OATP1B1)), and/or renal proximal tubular cells (e.g., organic cation transporter 2 (OCT2)/multidrug and toxin extrusion 1 and 2-K (MATE1/MATE2-K)). In addition, transporter-mediated drug-drug interactions can cause altered local tissue concentrations and possibly altered effects/toxicity (e.g., in liver and kidneys). During drug development, there is now an intensive in vitro screening of new molecular entities as transporter substrates and inhibitors, followed if necessary by drug-drug interaction studies in healthy volunteers. Nevertheless, there are still unresolved issues, which will also be discussed in this review article (e.g., the clinical significance of transporter-mediated drug-drug interactions of particular relevance to the elderly who are prescribed multiple drugs, with additional impaired liver or kidney function, and the extent to which medication safety in real life could be improved by a reduction of those interactions).

Renal Drug Transporters and Drug Interactions

Transporters in proximal renal tubules contribute to the disposition of numerous drugs. Furthermore, the molecular mechanisms of tubular secretion have been progressively elucidated during the past decades. Organic anions tend to be secreted by the transport proteins OAT1, OAT3 and OATP4C1 on the basolateral side of tubular cells, and multidrug resistance protein (MRP) 2, MRP4, OATP1A2 and breast cancer resistance protein (BCRP) on the apical side. Organic cations are secreted by organic cation transporter (OCT) 2 on the basolateral side, and multidrug and toxic compound extrusion (MATE) proteins MATE1, MATE2/2-K, P-glycoprotein, organic cation and carnitine transporter (OCTN) 1 and OCTN2 on the apical side. Significant drug-drug interactions (DDIs) may affect any of these transporters, altering the clearance and, consequently, the efficacy and/or toxicity of substrate drugs. Interactions at the level of basolateral transporters typically decrease the clearance of the victim drug, causing higher systemic exposure. Interactions at the apical level can also lower drug clearance, but may be associated with higher renal toxicity, due to intracellular accumulation. Whereas the importance of glomerular filtration in drug disposition is largely appreciated among clinicians, DDIs involving renal transporters are less well recognized. This review summarizes current knowledge on the roles, quantitative importance and clinical relevance of these transporters in drug therapy. It proposes an approach based on substrate-inhibitor associations for predicting potential tubular-based DDIs and preventing their adverse consequences. We provide a comprehensive list of known drug interactions with renally-expressed transporters. While many of these interactions have limited clinical consequences, some involving high-risk drugs (e.g. methotrexate) definitely deserve the attention of prescribers.

Osteomalacia induced by long-term low-dose adefovir dipivoxil: Clinical characteristics and genetic predictors

CONTEXT

Adefovir dipivoxil (ADV) was an important cause of adult-onset hypophosphatemic osteomalacia. However, its clinical characteristics and mechanisms have not been well defined.

OBJECTIVE

The objective of the study was to summarize the clinical characteristics of ADV-induced osteomalacia and to explore the association between ADV-associated tubulopathy and polymorphisms in genes encoding drug transporters.

DESIGN, SETTING, PATIENTS, AND MAIN OUTCOME MEASURE

Seventy-six affected patients were clinically studied. The SLC22A6 and ABCC2 genes were screened and compared with healthy people from the HapMap.

RESULTS

Hypophosphatemia, high serum alkaline phosphatase (ALP) levels, hypouricemia, nondiabetic glycosuria, proteinuria, metabolic acidosis and high bone turnover markers were the main metabolic characteristics. Fractures and pseudofractures occurred in 39 patients. Stopping ADV administration, supplementing calcitriol and calcium was effective during the follow-up period. Single SNP analysis revealed a higher percentage of the G/A genotype at c.2934 in exon 22 of the ABCC2 gene (rs3740070) in patients than in healthy people (12% [7 of 58 patients] vs. 0% [0 of 45 patients]; P=0.017), while there was no subject with homozygosity for the A allele at c.2934.

CONCLUSIONS

ADV can be nephrotoxic at a conventional dosage. The G/A genotype at c.2934 of the ABCC2 gene may be a predictor of patients at greater risk for developing ADV-associated tubulopathy. Larger case-control studies are needed to further verify this finding.

Effect of nonalcoholic steatohepatitis on renal filtration and secretion of adefovir

BACKGROUND AND AIMS

Adefovir, an acyclic nucleotide reverse transcriptase inhibitor used to treat hepatitis B viral infection, is primarily eliminated renally through cooperation of glomerular filtration with active tubular transport. Nonalcoholic steatohepatitis is a variable in drug disposition, yet the impact on renal transport processes has yet to be fully understood. The goal of this study was to determine the effect of nonalcoholic steatohepatitis on the pharmacokinetics of adefovir in rats given a control or methionine and choline deficient diet to induce nonalcoholic steatohepatitis.

METHODS

Animals received a bolus dose of 7mg/kg (35μCi/kg) [(3)H] adefovir with consequent measurement of plasma and urine concentrations. Inulin clearance was used to determine glomerular filtration rate.

RESULTS

Methionine and choline deficient diet-induced nonalcoholic steatohepatitis prolonged the elimination half-life of adefovir. This observation occurred in conjunction with reduced distribution volume and hepatic levels of adefovir. Notably, despite these changes, renal clearance and overall clearance were not changed, despite markedly reduced glomerular filtration rate in nonalcoholic steatohepatitis. Alteration of glomerular filtration rate was fully compensated for by a significant increase in tubular secretion of adefovir. Analysis of renal transporters confirmed transcriptional up-regulation of Mrp4, the major transporter for adefovir tubular secretion.

CONCLUSIONS

This study demonstrates changes to glomerular filtration and tubular secretion that alter pharmacokinetics of adefovir in nonalcoholic steatohepatitis. Nonalcoholic steatohepatitis-induced changes in renal drug elimination processes could have major implications in variable drug response and the potential for toxicity.

Development of an in vitro renal epithelial disease state model for xenobiotic toxicity testing

There is a growing impetus to develop more accurate, predictive and relevant in vitro models of renal xenobiotic exposure. As part of the EU-FP7, Predict-IV project, a major aim was to develop models that recapitulate not only normal tissue physiology but also aspects of disease conditions that exist as predisposing risk factors for xenobiotic toxicity. Hypoxia, as a common micro-environmental alteration associated with pathophysiology in renal disease, was investigated for its effect on the toxicity profile of a panel of 14 nephrotoxins, using the human proximal tubular epithelial RPTECT/TERT1 cell line. Changes in ATP, glutathione and resazurin reduction, after 14 days of daily repeat exposure, revealed a number of compounds, including adefovir dipivoxil with enhanced toxicity in hypoxia. We observed intracellular accumulation of adefovir in hypoxia and suggest decreases in the efflux transport proteins MRP4, MRP5, NHERF1 and NHERF3 as a possible explanation. MRP5 and NHERF3 were also down-regulated upon treatment with the HIF-1 activator, dimethyloxalylglycine. Interestingly, adefovir dependent gene expression shifted from alterations in cell cycle gene expression to an inflammatory response in hypoxia. The ability to investigate aspects of disease states and their influence on renal toxin handling is a key advantage of in vitro systems developed here. They also allow for detailed investigations into mechanisms of compound toxicity of potential importance for compromised tissue exposure.

Transporters and drug-drug interactions: important determinants of drug disposition and effects

Uptake and efflux transporters determine plasma and tissue concentrations of a broad variety of drugs. They are localized in organs such as small intestine, liver, and kidney, which are critical for drug absorption and elimination. Moreover, they can be found in important blood-tissue barriers such as the blood-brain barrier. Inhibition or induction of drug transporters by coadministered drugs can alter pharmacokinetics and pharmacodynamics of the victim drugs. This review will summarize in particular clinically observed drug-drug interactions attributable to inhibition or induction of intestinal export transporters [P-glycoprotein (P-gp), breast cancer resistance protein (BCRP)], to inhibition of hepatic uptake transporters [organic anion transporting polypeptides (OATPs)], or to inhibition of transporter-mediated [organic anion transporters (OATs), organic cation transporter 2 (OCT2), multidrug and toxin extrusion proteins (MATEs), P-gp] renal secretion of xenobiotics. Available data on the impact of nutrition on transport processes as well as genotype-dependent, transporter-mediated drug-drug interactions will be discussed. We will also present and discuss data on the variable extent to which information on the impact of transporters on drug disposition is included in summaries of product characteristics of selected countries (SPCs). Further work is required regarding a better understanding of the role of the drug metabolism-drug transport interplay for drug-drug interactions and on the extrapolation of in vitro findings to the in vivo (human) situation.

OAT1 and OAT3: targets of drug-drug interaction between entecavir and JBP485

Entecavir and JBP485 (a dipeptide) exhibit the antihepatitis activities and it is possible for the two drugs to be coadministered in the treatment of hepatitis. We aimed to elucidate whether entecavir was a substrate of OAT1, OAT3, OCT, and PEPT1 and to investigate the targets of drug-drug interactions between entecavir and JBP485. Plasma and urine concentrations of entecavir following intravenous and oral administration in vivo, uptake of entecavir in kidney slices and transfected cells in vitro, were determined by LC-MS/MS. Following intravenous co-administration of entecavir and JBP485 in rats, entecavir AUC increased 1.93-fold, t1/2β was prolonged 2.08-fold, CLP decreased 49%, CLR decreased 73%, and accumulated urinary excretion decreased 54%. However, following oral co-administration, the entecavir Tmax and Cmax were not affected; the degree of change in other pharmacokinetic parameters (AUC, t1/2β, CLP, and accumulated urinary excretion) was similar to that of intravenous administration. The uptake of entecavir was nearly identical in hPEPT1- as in vector-HELA cells. In rat kidney slices, uptake of entecavir was markedly inhibited by p-aminohippurate, benzylpenicillin, JBP485, and tetraethyl ammonium. In hOAT1- and hOAT3-HEK293 cells, uptake of entecavir was significantly higher compared to vector-HEK293 cells and was markedly inhibited by p-aminohippurate, benzylpenicillin, and JBP485. Km and Vmax values of entecavir were 250 μM and 0.83 nmol/mg protein/30s (OAT1) and 23 μM and 1.1 nmol/mg protein/30 s (OAT3), respectively. Entecavir is the substrate of OAT1, OAT3, and OCT. Moreover, OAT1 and OAT3 are the targets of DDI between entecavir and JBP485.

Tenofovir renal proximal tubular toxicity is regulated by OAT1 and MRP4 transporters

Tenofovir disoproxil fumarate (TDF) is an oral prodrug and acyclic nucleotide analog of adenosine monophosphate that inhibits HIV-1 (HIV) reverse transcriptase. A growing subset of TDF-treated HIV(+) individuals presented with acute renal failure, suggesting tenofovir-associated kidney-specific toxicity. Our previous studies using an HIV transgenic mouse model (TG) demonstrated specific changes in renal proximal tubular mitochondrial DNA (mtDNA) abundance. Nucleosides are regulated in biological systems via transport and metabolism in cellular compartments. In this study, the role(s) of organic anion transporter type 1 (OAT1) and multidrug-resistant protein type 4 (MRP4) in transport and regulation of tenofovir in proximal tubules were assessed. Renal toxicity was assessed in kidney tissues from OAT1 knockout (KO) or MRP4 KO compared with wild-type (WT, C57BL/6) mice following treatment with TDF (0.11 mg/day), didanosine (ddI, a related adenosine analog, 0.14 mg/day) or vehicle (0.1 M NaOH) daily gavage for 5 weeks. Laser-capture microdissection (LCM) was used to isolate renal proximal tubules for molecular analyses. mtDNA abundance and ultrastructural pathology were analyzed. mtDNA abundance in whole kidneys from both KO and WT was unchanged regardless of treatment. Renal proximal tubular mtDNA abundance from OAT1 KO also remained unchanged, suggesting prevention of TDF toxicity due to loss of tenofovir transport into proximal tubules. In contrast, renal proximal tubules from MRP4 KO exhibited increased mtDNA abundance following TDF treatment compared with WT littermates, suggesting compensation. Renal proximal tubules from TDF-treated WT and MRP4 KO exhibited increased numbers of irregular mitochondria with sparse, fragmented cristae compared with OAT1 KO. Treatment with ddI had a compensatory effect on mtDNA abundance in OAT1 KO but not in MRP4 KO. Both OAT1 and MRP4 have a direct role in transport and efflux of tenofovir, regulating levels of tenofovir in proximal tubules. Disruption of OAT1 activity prevents tenofovir toxicity but loss of MRP4 can lead to increased renal proximal tubular toxicity. These data help to explain mechanisms of human TDF renal toxicity.

Analysis of three-dimensional systems for developing and mature kidneys clarifies the role of OAT1 and OAT3 in antiviral handling

The organic anion transporters OAT1 (SLC22A6, originally identified by us as NKT) and OAT3 (SLC22A8) are critical for handling many toxins, metabolites, and drugs, including antivirals (Truong, D. M., Kaler, G., Khandelwal, A., Swaan, P. W., and Nigam, S. K. (2008) J. Biol. Chem. 283, 8654-8663). Although microinjected Xenopus oocytes and/or transfected cells indicate overlapping specificities, the individual contributions of these transporters in the three-dimensional context of the tissues in which they normally function remain unclear. Here, handling of HIV antivirals (stavudine, tenofovir, lamivudine, acyclovir, and zidovudine) was analyzed with three-dimensional ex vivo functional assays using knock-out tissue. To investigate the contribution of OAT1 and OAT3 in various nephron segments, the OAT-selective fluorescent tracer substrates 5-carboxyfluorescein and 6-carboxyfluorescein were used. Although OAT1 function (uptake in oat3(-/-) tissue) was confined to portions of the cortex, consistent with a proximal tubular localization, OAT3 function (uptake in oat1(-/-) tissue) was apparent throughout the cortex, indicating localization in the distal as well as proximal nephron. This functional localization indicates a complex three-dimensional context, which needs to be considered for metabolites, toxins, and drugs (e.g. antivirals) handled by both transporters. These results also raise the possibility of functional differences in the relative importance of OAT1 and OAT3 in antiviral handling in developing and mature tissue. Because the HIV antivirals are used in pregnant women, the results may also help in understanding how these drugs are handled by developing organs.

Fanconi's Syndrome Associated with Prolonged Adefovir Dipivoxil Therapy in a Hepatitis B Virus Patient

Adefovir dipivoxil (ADV) is commonly used as an antiviral agent in the treatment of chronic hepatitis B or human immunodeficiency virus infection. Nephrotoxicity has been shown to occur at daily dosages of 60-120 mg. Fanconi's syndrome is a generalized dysfunction of the renal proximal tubular cells, which is usually accompanied by complications. Here we report a case of Fanconi's syndrome in a chronic hepatitis B patient who had been treated with a prolonged regimen of ADV at 10 mg/day. A 47-year-old man complained of severe back and chest-wall pain. He had chronic hepatitis B and had been treated with ADV at a daily dose of 10 mg for 38 months. He was hospitalized because of severe bone pain, and laboratory and radiologic findings suggested a diagnosis of Fanconi's syndrome with osteomalacia. After discontinuation of the ADV, he recovered and was discharged from hospital. His laboratory findings had normalized within 2 weeks. This case indicates that Fanconi's syndrome can be acquired by a chronic hepatitis B patient taking ADV at a conventional dosage of 10 mg/day. Therefore, patients treated with long-term ADV should be checked regularly for the occurrence of ADV-induced Fanconi's syndrome.

Substrate overlap between Mrp4 and Abcg2/Bcrp affects purine analogue drug cytotoxicity and tissue distribution

The use of probe substrates and combinations of ATP-binding cassette (ABC) transporter knockout (KO) animals may facilitate the identification of common substrates between apparently unrelated ABC transporters. An unexpectedly low concentration of the purine nucleotide analogue, 9-(2-(phosphonomethoxy)ethyl)-adenine (PMEA), and up-regulation of Abcg2 in some tissues of the Mrp4 KO mouse prompted us to evaluate the possibility that Abcg2 might transport purine-derived drugs. Abcg2 transported and conferred resistance to PMEA. Moreover, a specific Abcg2 inhibitor, fumitremorgin C, both increased PMEA accumulation and reversed Abcg2-mediated PMEA resistance. We developed Mrp4 and Abcg2 double KO mice and used both single KOs of Abcg2 and Mrp4 mice to assess the role of these transporters in vivo. Abcg2 contributed to PMEA accumulation in a variety of tissues, but in some tissues, this contribution was only revealed by the concurrent absence of Mrp4. Abcg2 also transported and conferred resistance to additional purine analogues, such as the antineoplastic, 2-chloro-2'-deoxyadenosine (cladribine) and puromycin, a protein synthesis inhibitor that is often used as a dominant selectable marker. Purine analogues interact with ABCG2 by a site distinct from the prazosin binding site as shown by their inability to displace the substrate analogue and photoaffinity tag [(125)I]iodoarylazidoprazosin. These studies show that Abcg2, like Mrp4, transports and confers resistance to purine nucleoside analogues and suggest that these two transporters work in parallel to affect drug cytotoxicity and tissue distribution. This new knowledge will facilitate an understanding of how Abcg2 and Mrp4, separately and in combination, protect against purine analogue host toxicity as well as resistance to chemotherapy.