Human renal organic anion transporter 1 (hOAT1) and its role in the nephrotoxicity of antiviral nucleotide analogs

Early kidney injury during long-term adefovir dipivoxil therapy for chronic hepatitis B

AIM: To evaluate urine β2-microglobulin (β2-M), retinol-binding protein (RBP) excretion, and renal impairment with adefovir dipivoxil (ADV) for chronic hepatitis B.

METHODS: We enrolled 165 patients with chronic hepatitis B infection who were treated with ADV monotherapy (n = 90) or ADV plus lamivudine combination therapy (n = 75). An additional 165 chronic hepatitis B patients treated with entecavir were recruited as controls. We detected serum creatinine, urine β2-M, and RBP levels, and estimated the glomerular filtration rate (eGFR) at the initiation of antiviral therapy and every 6 mo for a period of five years.

RESULTS: Urine β2-M abnormalities were observed in patients during the first (n = 3), second (n = 7), third (n = 11), fourth (n = 16), and fifth (n = 21) year of ADV treatment. Urinary RBP abnormalities were observed in patients during the first (n = 2), second (n = 8), third (n = 12), fourth (n = 15), and fifth (n = 22) year of ADV treatment. eGFR decreased 20%-30% from baseline in 20 patients, 30%-50% in 12 patients, and > 50% in 3 patients during the five years of treatment. Further analysis indicated that decreases in eGFR of ≥ 30% relative to the baseline level correlated significantly with urine RBP and β2-M abnormalities. In contrast, both serum creatinine and eGFR remained stable in patients treated with entecavir, and only one of these patients developed a urine β2-M abnormality, and two developed urine RBP abnormalities during the five years of treatment.

CONCLUSION: Urine RBP and β2-M are biomarkers of renal injury during long-term ADV treatment for chronic hepatitis B, and indicate when treatment should be switched to entecavir.

Differentiating HIV-associated nephropathy from antiretroviral drug-induced nephropathy: a clinical challenge

With the introduction of potent combination antiretroviral therapy (cART) into clinical practice, HIV-infected patients have garnered much benefit. However, kidney disease continues to be a potential complication in this group. Whereas HIV-associated nephropathy (HIVAN) was the major renal complication prior to cART, co-morbid diseases and adverse renal effects of various drugs, in particular cART, now complicate the landscape. Clinicians now must differentiate HIVAN from cART nephrotoxicity. While sometimes this is easy and relatively straightforward, often the clinician faces a difficult challenge distinguishing these two etiologies of kidney disease. This review will discuss HIVAN and cART-related kidney disease and review the clinical and laboratory data that may be useful in differentiating these processes. Often, however, kidney biopsy may be required to differentiate HIVAN from cART nephrotoxicity as well as other kidney lesions associated with concurrent co-morbidities, both infectious and non-infectious.

Nucleoside transporter proteins as biomarkers of drug responsiveness and drug targets

Nucleoside and nucleobase analogs are currently used in the treatment of solid tumors, lymphoproliferative diseases, viral infections such as hepatitis and AIDS, and some inflammatory diseases such as Crohn. Two gene families are implicated in the uptake of nucleosides and nucleoside analogs into cells, SCL28 and SLC29. The former encodes hCNT1, hCNT2, and hCNT3 proteins. They translocate nucleosides in a Na⁺ coupled manner with high affinity and some substrate selectivity, being hCNT1 and hCNT2 pyrimidine- and purine-preferring, respectively, and hCNT3 a broad selectivity transporter. SLC29 genes encode four members, being hENT1 and hENT2 the only two which are unequivocally implicated in the translocation of nucleosides and nucleobases (the latter mostly via hENT2) at the cell plasma membrane. Some nucleoside-derived drugs can also interact with and be translocated by members of the SLC22 gene family, particularly hOCT and hOAT proteins. Inter-individual differences in transporter function and perhaps, more importantly, altered expression associated with the disease itself might modulate the transporter profile of target cells, thereby determining drug bioavailability and action. Drug transporter pharmacology has been periodically reviewed. Thus, with this contribution we aim at providing a state-of-the-art overview of the clinical evidence generated so far supporting the concept that these membrane proteins can indeed be biomarkers suitable for diagnosis and/or prognosis. Last but not least, some of these transporter proteins can also be envisaged as drug targets, as long as they can show “transceptor” functions, in some cases related to their role as modulators of extracellular adenosine levels, thereby providing a functional link between P1 receptors and transporters.

Tenofovir disoproxil fumarate: toxicity, toxicokinetics, and toxicogenomics analysis after 13 weeks of oral administration in mice

Tenofovir disoproxil fumarate (TDF) is a prodrug of tenofovir that exhibits activity against HIV and hepatitis B. The goals of this study were to evaluate the molecular mechanism of TDF-induced toxicity in mice after 13 weeks of daily oral administration (50-1000 mg/kg) by correlating transcriptional changes with plasma drug levels and traditional toxicology end points. Plasma levels and systemic exposure of tenofovir increased less than dose proportionally and were similar on days 1 and 91. No overt toxicity was observed following the completion of TDF administration. The kidneys of TDF-treated mice were histopathologically normal. This result is consistent with the genomic microarray results, which showed no significant differences in kidney transcriptional levels between TDF-treated animals and controls. In liver, after 4 and 13 weeks, cytomegaly was observed in mice treated with 1000 mg/kg of TDF, but mice recovered from this effect following cessation of administration. Analysis of liver transcripts on day 91 reported elevated levels of Cdkn1a in TDF-treated animals compared with controls, which may have contributed to the inhibition of liver cell cycle progression.

Acquired Fanconi syndrome due to long-term adefovir administration in a patient with IgG-kappa monoclonal gammopathy and kappa Bence-Jones protein

A 77-year-old man was admitted to our hospital for a right femoral neck fracture. He had been prescribed lamivudine for chronic hepatitis B infection for 11 years, and adefovir was added 5 years ago. After hospitalization, a right femoral head prosthesis was performed successfully, but an unknown hypokalemia was revealed. Hypophosphatemia, hypouricemia, glucosuria, and panaminoaciduria were also revealed, and multiple microfractures were detected by bone scintigraphy. We diagnosed him as 'osteomalacia associated with Fanconi syndrome,' which was likely due to the adefovir. Moreover, a monoclonal IgG-kappa and a kappa Bence-Jones protein were detected in his serum and urine, respectively. We switched from adefovir plus lamivudine to entecavir and started calcitriol. His excessive urinary β2-microglobulin excretion and glucosuria had decreased dramatically at 10 weeks after the modification of drugs; those of the phosphate, uric acid and total protein, however, continued. Renal biopsy specimens obtained at 10 weeks after discontinuation of adefovir revealed focal tubular atrophic changes with/without inflammatory cells, which were predominantly observed next to glomeruli. Kappa-dominant staining was not observed in either glomeruli or tubules with immunostaining by the enzyme-labeled antibody method. Electron microscopy revealed neither crystalline structures in the cytoplasm of proximal tubules nor electron-dense deposits. Because of the remarkable proportional reduction of other urinary protein fractions, urinary M-peak appeared 26 weeks after discontinuation of adefovir, but the net amounts of the fraction decreased gradually.

Insights into the mechanism of action of cidofovir and other acyclic nucleoside phosphonates against polyoma- and papillomaviruses and non-viral induced neoplasia

Acyclic nucleoside phosphonates (ANPs) are well-known for their antiviral properties, three of them being approved for the treatment of human immunodeficiency virus infection (tenofovir), chronic hepatitis B (tenofovir and adefovir) or human cytomegalovirus retinitis (cidofovir). In addition, cidofovir is mostly used off-label for the treatment of infections caused by several DNA viruses other than cytomegalovirus, including papilloma- and polyomaviruses, which do not encode their own DNA polymerases. There is considerable interest in understanding why cidofovir is effective against these small DNA tumor viruses. Considering that papilloma- and polyomaviruses cause diseases associated either with productive infection (characterized by high production of infectious virus) or transformation (where only a limited number of viral proteins are expressed without synthesis of viral particles), it can be envisaged that cidofovir may act as antiviral and/or antiproliferative agent. The aim of this review is to discuss the advances in recent years in understanding the mode of action of ANPs as antiproliferative agents, given the fact that current data suggest that their use can be extended to the treatment of non-viral related malignancies.

Tenofovir-induced nephrotoxicity: incidence, mechanism, risk factors, prognosis and proposed agents for prevention

OBJECTIVE

In this study, data regarding epidemiology, risk factors, pathogenesis and outcome of tenofovir-induced nephrotoxicity will be reviewed, and current and future approaches for prevention will be discussed.

METHOD

The data were collected by searching Scopus, PubMed, Medline, Science direct, Clinical trials and Cochrane database systematic reviews. The keywords used as search terms were "Tenofovir", "TDF", "NRTI", "Nephrotoxicity", "Renal failure", "Kidney damage", "HIV" and "AIDS".

RESULTS AND CONCLUSION

Several predisposing factors including elevated baseline SCr, concomitant nephrotoxic medications, low body weight, advanced age, tenofovir disoproxil fumarate (TDF) dose and duration of treatment and lower CD4 cell count were identified as risk factors for development of TDF-induced nephrotoxicity. Cellular accumulation through increased entry from the human organic anion transporters and decreased efflux into tubular lumen is main mechanism of nucleotide analogue antiviral induced nephrotoxicity. Renal function assessment and monitoring at baseline and during TDF treatment are the main approach of prevention of TDF-induced nephrotoxicity. Rosiglitazone may be helpful in patients presenting with TDF-induced nephrotoxicity. Pretreatment with melatonin prevented all known histological changes in proximal tubular mitochondira induced by TDF. Use of antioxidants with mitochondria-targeted properties such as MitoQ or Mito-CP may prevent proximal tubular mitochondrial against TDF damage. Vitamin E, ebselen, lipoic acid, plastoquinone, nitroxides, SOD enzyme mimetics, Szeto-Schiller (SS) peptides, and quercetin are other potential agents for prevention of TDF-induced nephrotoxicity. However, data regarding effectiveness of nephroprotective agents against TDF-induced nephrotoxicity are not conclusive. Before extrapolation of the preclinical evidence to clinical practice, these evidence should be confirmed in future human studies.

Antiviral activity of genital tract secretions after oral or topical tenofovir pre-exposure prophylaxis for HIV-1

BACKGROUND

Surrogate markers of HIV-1 pre-exposure prophylaxis and microbicide efficacy are needed. One potential surrogate is the antiviral activity in cervicovaginal lavage (CVL) after exposure to candidate products. We measured CVL antiviral activity in women using oral or vaginal tenofovir-based pre-exposure prophylaxis and correlated activity with drug and immune mediator levels.

METHODS

Inhibitory activity against HIV-1 and herpes simplex virus (HSV)-2 and concentrations of interleukin (IL)-1β, IL-6, IL-8, interferon-γ, induced protein 10 (IP-10), macrophage inflammatory protein (MIP)-1α, MIP-3a, lactoferrin, secretory leukocyte protease inhibitor, and defensins were measured in CVL obtained from 60 women at baseline and after 6 weeks of a randomized sequence of oral and topical tenofovir. CVL tenofovir concentrations were measured by mass spectrometry.

RESULTS

The number of women with CVL anti-HIV activity ≥ 90% increased significantly from 5.0% at baseline to 89.1% after daily use of 1% tenofovir gel (relative risk = 17.85, P < 0.001), but there was no increase after daily oral tenofovir. The CVL anti-HIV activity correlated with drug levels (Spearman correlation coefficient 0.64 after tenofovir gel; P < 0.001) but not with the concentrations of mucosal immune mediators. No increase in CVL anti-HSV activity was observed after either drug regimen, an observation consistent with the higher concentrations of tenofovir needed to inhibit HSV-2 infection. The CVL anti-HSV activity correlated with lactoferrin, defensins, IP-10, IL-8, and detectable levels of MIP-1α but not with drug levels.

CONCLUSIONS

CVL may provide a surrogate for local but not systemic drug efficacy and a tool to better understand mucosal factors that modulate antiviral activity in genital tract secretions.

Tenofovir alafenamide is not a substrate for renal organic anion transporters (OATs) and does not exhibit OAT-dependent cytotoxicity

BACKGROUND

Tenofovir alafenamide (TAF) is a novel prodrug of tenofovir that shows enhanced antiretroviral effect and reduced plasma tenofovir exposures at approximately one-tenth the clinically approved dose of tenofovir disoproxil fumarate (TDF). Tenofovir released from TDF undergoes active renal secretion via organic anion transporters (OAT1 and OAT3), leading to higher exposure of renal proximal tubules to tenofovir and a potential for renal adverse effects in a small subset of TDF-treated patients. Here, we evaluate the interaction of TAF with OAT1 and OAT3 to assess the potential for its active accumulation in proximal tubules.

METHODS

OAT-mediated transport and cytotoxicity (CC50) of TAF and tenofovir were assessed in cells expressing OATs and compared with matched transporter-null cells.

RESULTS

While OAT1 and OAT3 expression increased tenofovir cellular uptake by >70-fold and 8.2-fold, respectively, the expression of either OAT did not significantly change TAF intracellular accumulation under identical conditions. In addition, although tenofovir was significantly more cytotoxic in OAT1- and OAT3-expressing cells (>21 and >3.6 fold change in CC50 values, respectively), TAF in vitro cytotoxicity showed little to no change upon overexpression of either renal transporter (0.5-3.5 fold change in CC50).

CONCLUSIONS

Unlike tenofovir, TAF does not interact with renal transporters OAT1 or OAT3 and exhibits no OAT-dependent cytotoxicity. TAF is thus unlikely to actively accumulate in renal proximal tubules in an OAT-dependent manner, supporting the potential for an improved renal safety profile.

Drug-induced renal Fanconi syndrome

A number of therapeutic drugs are toxic to the kidney proximal tubule (PT) and can cause the renal Fanconi syndrome (FS). The most frequently implicated drugs are cisplatin, ifosfamide, tenofovir, sodium valproate and aminoglycoside antibiotics, and the new oral iron chelator deferasirox has also recently been associated with FS. The incidence of full or partial FS is almost certainly under-estimated due to a lack of appropriate systematic studies, variations in definitions of tubular dysfunction and under-reporting of adverse events. The clinical features of FS are amino aciduria, low molecular weight proteinuria, hypophosphataemia, metabolic acidosis and glycosuria. The most serious complications are bone demineralization from urinary phosphate wasting and progressive decline in kidney function. Commonly used tests for kidney function such as estimated glomerular filtration rate and urine albumin/creatinine ratio are not sensitive markers of PT toxicity; patients at risk should thus be monitored with more appropriate tests, and drugs should be stopped or reduced in dose if toxicity occurs. Substantial recovery of PT function can occur after withdrawal of therapy, but this can take months and chronic damage may persist in some cases.

Renal dysfunction and hypophosphatemia during long-term lamivudine plus adefovir dipivoxil therapy in patients with chronic hepatitis B

BACKGROUND

Renal dysfunction and Fanconi's syndrome associated with hypophosphatemia caused by long-term administration of low-dose adefovir dipivoxil (ADV) has been reported in recent years. The aim of this retrospective study was to determine the incidence and factors associated with renal dysfunction and hypophosphatemia in patients with hepatitis B infection on long-term treatment with ADV and lamivudine (LAM).

METHODS

The study subjects were 292 patients treated with 10 mg/day ADV and 100 mg/day LAM for more than 6 months. We evaluated estimated glomerular filtration rate (eGFR), serum creatinine and serum phosphate level at the start of ADV and every 6 months.

RESULT

During a median treatment duration of 64 months, 28 (9.6 %) patients developed renal impairment (defined as eGFR < 50 ml/min/1.73 m(2)), and 73 (27.1 %) developed hypophosphatemia, including 14 with persistent hypophosphatemia. The cumulative incidences of renal impairment at 1, 3, and 5 years were 1.4, 7.5, 10.5 %, respectively, and those of hypophosphatemia were 6.8, 20.6, 26.7 %, respectively. Multivariate analysis identified old age, liver cirrhosis and hypertension as determinants of renal impairment, and male sex, HCC, low baseline serum phosphate as determinants of hypophosphatemia. Three of the 14 patients with persistent hypophosphatemia developed Fanconi's syndrome; their serum creatinine level remained normal, but eGFR was lower than at baseline.

CONCLUSION

Long-term treatment of hepatitis B with low-dose (10 mg/day) ADV and LAM can potentially cause renal impairment and hypophosphatemia. We advocate regular monitoring of serum phosphate and evaluation of eGFR, in addition to serum creatinine, in such patients.

Review article: nucleos(t)ide analogues in patients with chronic hepatitis B virus infection and chronic kidney disease

BACKGROUND

The treatment of chronic hepatitis B (CHB) in patients with chronic kidney disease (CKD) is based on nucleoside (lamivudine, telbivudine, entecavir) or nucleotide (adefovir, tenofovir) analogues (NAs), but it may be complex and the information is scarce. Entecavir and tenofovir represent the currently recommended first-line NAs for NA-naive CHB patients, while tenofovir is the NA of choice for CHB patients with resistance to nucleosides.

AIM

To review the efficacy and safety of NAs in adult CHB patients with CKD and to provide reasonable recommendations for their optimal management.

METHODS

Literature search in PubMed/Medline and manual search of relevant articles, reviews and book chapters.

RESULTS

NAs are cleared by kidneys and their dosage should be adjusted in patients with creatinine clearance <50 mL/min. There are concerns about nephrotoxic potential of the nucleotides, particularly adefovir, while improvements of creatinine clearance have been reported under telbivudine. Most existing data in CHB patients with CKD are for lamivudine and, less frequently, for other NAs, mostly entecavir. Besides CHB, NA should be used in case of immunosuppressive therapy in any HBsAg-positive patient with CKD including renal transplant (RT) recipients and in anti-HBs-positive recipients of kidney grafts from HBsAg-positive donors.

CONCLUSIONS

Chronic hepatitis B patients with chronic kidney disease receiving nucleoside analogues should be followed carefully for treatment efficacy and renal safety. Despite the absence of strong data, entecavir and telbivudine seem to be the preferred options for nucleoside analogue-naive CHB patients with chronic kidney disease, depending on viraemia and severity of renal dysfunction. More studies are certainly needed in this setting.

Recurrent bone fractures due to tenofovir-induced renal phosphate wasting

A 42-y-old HIV-infected man suffered from several stress fractures due to tenofovir-induced proximal tubular injury. Laboratory examination revealed hypophosphatemia due to renal phosphate wasting. Therefore, more attention has to be paid to the monitoring of serum phosphate and alkaline phosphatase levels, since tenofovir-related nephrotoxicity increases the risk of osteomalacia.

Depletion of the cellular antioxidant system contributes to tenofovir disoproxil fumarate - induced mitochondrial damage and increased oxido-nitrosative stress in the kidney

BACKGROUND

Nephrotoxicity is a dose limiting side effect of tenofovir, a reverse transcriptase inhibitor that is used for the treatment of HIV infection. The mechanism of tenofovir nephrotoxicity is not clear. Tenofovir is specifically toxic to the proximal convoluted tubules and proximal tubular mitochondria are the targets of tenofovir cytotoxicity. Damaged mitochondria are major sources of reactive oxygen species and cellular damage is reported to occur after the antioxidants are depleted. The purpose of the study is to investigate the alterations in cellular antioxidant system in tenofovir induced renal damage using a rat model.

RESULTS

Chronic tenofovir administration to adult Wistar rats resulted in proximal tubular damage (as evidenced by light microscopy), proximal tubular dysfunction (as shown by Fanconi syndrome and tubular proteinuria), and extensive proximal tubular mitochondrial injury (as revealed by electron microscopy). A 50% increase in protein carbonyl content was observed in the kidneys of TDF treated rats as compared with the control. Reduced glutathione was decreased by 50%. The activity of superoxide dismutase was decreased by 57%, glutathione peroxidase by 45%, and glutathione reductase by 150% as compared with control. Carbonic Anhydrase activity was decreased by 45% in the TDF treated rat kidneys as compared with control. Succinate dehydrogenase activity, an indicator of mitochondrial activity was decreased by 29% in the TDF treated rat kidneys as compared with controls, suggesting mitochondrial dysfunction.

CONCLUSION

Tenofovir- induced mitochondrial damage and increased oxidative stress in the rat kidneys may be due to depletion of the antioxidant system particularly, the glutathione dependent system and MnSOD.

Evaluation of the effect of cobicistat on the in vitro renal transport and cytotoxicity potential of tenofovir

A once-daily single-tablet antiretroviral regimen containing tenofovir (TFV) disoproxil fumarate, emtricitabine (FTC), elvitegravir (EVG), and cobicistat (COBI) is an approved combination for the treatment of patients infected with HIV. COBI and TFV have been reported to interact with distinct transporters in renal proximal tubules; while TFV is renally eliminated by a combination of glomerular filtration and tubular secretion via anion transporters OAT1, OAT3, and MRP4, COBI inhibits renal cation transporters, particularly MATE1, resulting in a measurable decrease in the tubular secretion of creatinine. To investigate the potential for a renal drug-drug interaction between TFV and COBI in vitro, the uptake of TFV in the presence and absence of COBI was determined in fresh human renal cortex tissue and in cells expressing the relevant renal transporters. At concentrations exceeding clinical protein-unbound plasma levels, COBI did not significantly inhibit the transport of TFV by the anion transporters OAT1, OAT3, and MRP4 (50% inhibitory concentrations [IC50s] of >15, 6.6, and 8.5 μM, respectively). Conversely, TFV had little or no effect on the cation transporters OCT2 and MATE1 (IC50 > 100 μM). Consistent with studies using individual transporters, no increase in the accumulation of TFV in freshly isolated human renal cortex tissue or renal proximal tubule cells (RPTECs) was observed in the presence of COBI. Finally, COBI alone or in combination with FTC and EVG did not affect the sensitivity to TFV of cultured primary RPTECs or cells coexpressing OAT1 and MRP4. These results illustrate that COBI and TFV interact primarily with distinct renal transporters and indicate a low potential for pharmacokinetic renal drug-drug interaction.

Severe recurrent rhabdomyolysis-induced acute kidney injury in a HIV-infected patient on antiretroviral therapy

Antiretroviral medications, specifically tenofovir, have been linked to acute tubular necrosis in humans with a suggested mechanism of direct tubular injury. Rhabdomyolysis has rarely been described in patients on highly active antiretroviral therapy (HAART). To the best of our knowledge, severe recurrent rhabdomyolysis-induced acute kidney injury (AKI) in a HIV-infected patient on two different triple antiretroviral regimens has not been reported. We present a HIV-positive patient who first developed heme pigment-induced oliguric AKI due to non-traumatic rhabdomyolysis, 5 days after initiation of triple antiretroviral therapy. Renal function normalized 2 months after discontinuation of antiretroviral therapy. Two weeks after reinitiating a different HAART regimen, our patient developed a recurrent episode of severe rhabdomyolysis-induced AKI. Both rhabdomyolysis and AKI resolved after discontinuation of the second antiretroviral regimen. First tenofovir and subsequently abacavir seem to be the likely culprits in our case. We also briefly discuss tenofovir nephrotoxicity followed by a literature review on rhabdomyolysis in HIV-infected patients.

Update on tenofovir toxicity in the kidney

Tenofovir (TFV) is a widely used and effective treatment for HIV infection. Numerous studies have shown that TFV exposure is associated with small but significant declines in estimated glomerular filtration rate (eGFR). However, TFV toxicity is targeted mainly at the proximal tubule (PT), and in severe cases can cause the renal Fanconi syndrome or acute kidney injury. Severe toxicity occurs in a minority of patients, but milder PT dysfunction is more common; the long-term significance of this on kidney and bone health is uncertain. Recent work suggests that changes in eGFR on TFV therapy might be explained by inhibition of PT creatinine secretion rather than actual alterations in glomerular function. Risk factors for nephrotoxicity include pre-existing kidney disease, increased age, and low body mass. Mitochondria in the PT are the targets of TFV toxicity, but the exact mechanisms remain unclear. Substantial improvement of renal function occurs in many patients with TFV toxicity upon stopping therapy, but function does not always return to baseline. In recent years, TFV usage has been extended to new clinical spheres, including pediatrics, resource-poor settings and treatment of hepatitis B infection; theoretical reasons exist as to why some of these patients might be at higher or lower risk of TFV toxicity. Finally, strategies have been proposed to prevent TFV toxicity or enhance recovery.

Potential effect of pharmacogenetics on maternal, fetal and infant antiretroviral drug exposure during pregnancy and breastfeeding

Mother-to-child-transmission rates of HIV in the absence of any intervention range between 20 and 45%. However, the provision of antiretroviral drugs (ARVs) during pregnancy, delivery and breastfeeding can reduce HIV transmission to less than 2%. Physiological changes during pregnancy can influence ARV disposition. Associations between SNPs in genes coding for metabolizing enzymes, and/or transporters, and ARVs disposition are well described; however, relatively little is known about the influence of these SNPs on ARV pharmacokinetics during pregnancy and lactation as well as their effect on distribution into the fetal compartment and breast milk excretion. Differences in maternal, fetal and infant ARV exposure due to SNPs may affect the efficacy and safety of ARVs used to prevent mother-to-child-transmission. The aim of this review is to provide an update on the effect of pregnancy-induced changes on the pharmacokinetics of ARVs and highlight the potential role of pharmacogenetics.

Systemic and Renal Pharmacokinetics of Adefovir and Tenofovir Upon Coadministration

Adefovir and tenofovir are nucleotide analogs that undergo renal secretion by the human renal organic anion transporter. The pharmacokinetics of tenofovir and adefovir following the administration of tenofovir disoproxil fumarate and adefovir dipivoxil alone and together were determined in 24 healthy subjects in an 8-day, open-label, fixed-sequence study. Subjects received oral doses of adefovir dipivoxil on days 1 and 8 and oral doses of tenofovir disoproxil fumarate on days 2 to 8. Pharmacokinetic sampling was performed on days 1, 7, and 8. The plasma pharmacokinetics of tenofovir and adefovir were unaltered upon coadministration. Furthermore, the renal clearances (CLrenal) of tenofovir and adefovir were unaffected by their coadministration. The plasma Cmax values of tenofovir and adefovir were 33-fold and 340-fold lower than their Km values for the human renal organic anion transporter. These results demonstrate that coadministration of tenofovir disoproxil fumarate and adefovir dipivoxil does not result in substantial changes to their individual pharmacokinetic profiles.

A perspective on progress and gaps in HIV prevention science

In the past few years, the transdisciplinary field of HIV prevention has reached several milestones. Topically applied tenofovir gel provided significant protection from sexual transmission of HIV in a large-scale clinical trial and oral Truvada (emtricitabine/tenofovir disoproxil fumarate) was recently approved for preexposure prophylaxis (PrEP) following two successful clinical trials in men and women. These achievements are tempered by the disappointing results of other clinical trials, which highlight the complexities of prevention research. In this perspective, we discuss scientific and developmental gaps for topical chemoprophylaxis of the sexual transmission of HIV, which depends on the complex interactions between the pharmacokinetics and pharmacodynamics of drugs, formulation and delivery systems, anatomic site of transmission, and host mucosal immune defenses. Despite the considerable time and resources devoted to unraveling the initial steps in sexual transmission of HIV, current knowledge is based on animal models and human explanted tissue, which may not fully recapitulate what happens clinically. Understanding these events, including the role that sex hormones, semen, and mucosal secretions play in transmission, and the interplay between innate immunity, the mucosal environment, and drug efficacy is paramount. This drives some of the most pressing questions in the field.

The antiviral activity and mechanism of action of (S)-[3-hydroxy-2-(phosphonomethoxy)propyl] (HPMP) nucleosides

One class of compounds that has shown promise as antiviral agents are the (S)-[3-hydroxy-2-(phosphonomethoxy)propyl] (HPMP) nucleosides, members of the broader class of acyclic nucleoside phosphonates. These HPMP nucleosides are nucleotide analogs and have been shown to be effective inhibitors of a wide range of DNA viruses. Prodrugs of these compounds, which achieve higher levels of the active metabolites within the cell, have an expanded activity spectrum that also includes RNA viruses and retroviruses. Because they are analogs of natural nucleotide substrates, HPMP nucleosides are predicted to target polymerases (DNA polymerases, RNA polymerases and reverse transcriptases), resulting in the inhibition of viral genome replication. Previous work using the replicative enzymes of different viruses including human cytomegalovirus (HCMV) and vaccinia virus DNA polymerases and human immunodeficiency virus type 1 (HIV-1) reverse transcriptase has shown that the activated forms of these compounds are substrates for viral polymerases and that incorporation of these compounds into either the primer strand or the template strand inhibits, but does not necessarily terminate, further nucleic acid synthesis. The activity of these compounds against other viruses that do not encode their own polymerases, like polyoma viruses and papilloma viruses, suggests that host cell DNA polymerases are also targeted. This complex mechanism of action and broad activity spectrum has implications for the development of resistance and host cell genome replication, and suggests these compounds may be effective against other viruses such as influenza virus, respiratory syncytial virus and Dengue virus. This class of nucleotide analogs also points to a potential avenue for the development of newer antivirals.

Practical Considerations For Developing Nucleoside Reverse Transcriptase Inhibitors

Nucleoside reverse transcriptase inhibitors (NRTI) remain a cornerstone of current antiretroviral regimens in combinations usually with a non-nucleoside reverse transcriptase inhibitor (NNRTI), a protease inhibitor (PI), or an integrase inhibitor (INI). The antiretroviral efficacy and relative safety of current NRTI results from a tight and relatively specific binding of their phosphorylated nucleoside triphosphates (NRTI-TP) with the HIV-1 reverse transcriptase which is essential for replication. The intracellular stability of NRTI-TP produces a sustained antiviral response, which makes convenient dosing feasible. Lessons learned regarding NRTI pharmacology screening, development, and use are discussed. NRTI and prodrugs currently under clinical development are outlined.

Chemico-physical investigation of tenofovir loaded polymeric nanoparticles

Tenofovir (PMPA), an acyclic nucleoside phosphonate analog, is one of the most important drugs used for the HIV treatment. Unfortunately, several adverse reactions are related to its i.v. administration owing to the saturation of an anionic renal transporter. In order to improve the drug administration, the PMPA was embedded into a new type of nanocarriers based on poly-(D,L-lactide-co-glycolide) (PLGA) and/or chitosan (CH). The strategies for the preparation of nanoparticles (Nps) with a more efficient drug loading respect to the one reported in the literature for PMPA nanoencapsulation were investigated. CH was added in the first inner emulsion or in the external phase during the second emulsion of water/oil/water (W/O/W) Nps. The addition of CH in the first inner emulsion was the most promising technique. The Nps have a Z-average of 230 nm, a Z-potential of -3 mV and an EE% of 15 that was 2.5-3 times higher than that obtained with PLGA Nps or CH Nps. In vitro release studies showed a limited control on drug release in phosphate buffer (pH 7.4) while an initial burst effect followed by a slow drug release was observed in acidic receiving phase (pH 4.6). These results suggest the PLGA/CH Nps should be an effective and attractive anti-HIV drug carrier to study the cellular uptake and drug delivery on target cells such as macrophages.

Benefits associated with antiviral treatment in kidney allograft recipients with chronic hepatitis B virus infection

BACKGROUND & AIMS

Hepatitis B virus (HBV) infection is more frequent in kidney recipients than in the general population with a higher rate of liver-related morbidity and mortality. We evaluated the benefit associated with HBV viral suppression by nucleos(t)ide analogues treatment in HBV-infected kidney recipients.

METHODS

This retrospective study included 42 HBsAg-positive kidney recipients, 33 males, 9 females, median age 54 years, followed up during a mean of 15.4±11.8 years after kidney transplantation. Mean treatment duration by single or combined nucleos(t)ide analogues was 6.8±4.3 years. Fibrosis, before treatment, according to Metavir score was: F4 for 6 patients, F3 for 10, F2 for 6, and F0-F1 for 20 patients. The primary end point, the patient survival, was defined as patient death or liver transplantation, the secondary end point was graft survival.

RESULTS

HBV DNA at the last evaluation was undetectable (<12 IU/ml) in 92.8% of patients. During the follow-up, 8 patients died (17.7%), death being related to hepatocellular carcinoma in 4 (9.5%), including 1 patient with baseline mild fibrosis, and to extrahepatic causes in 4. This mortality rate is strikingly lower than that previously reported in HBV-infected kidney recipients before oral antiviral therapies. Graft survival seems to be improved when compared to the former series.

CONCLUSIONS

Suppression of HBV replication associated with nucleo(s)tide analogues treatment improves the survival of HBV-infected kidney recipients. Viral suppression does not exclude regular follow-up given the risk of occurrence of hepatocellular carcinoma even in non-cirrhotic patients.

Renal toxicity of long-term therapy with tenofovir in HIV-infected patients

Data are sparse on long-term renal toxicity of tenofovir as measured by estimated glomerular filtration rate (eGFR) and progression to advanced stages of chronic kidney disease (CKD). The objective of the study is to determine the incidence of renal impairment associated with the use of tenofovir in HIV-infected patients, using abacavir as a control. In a single tertiary care center, all HIV-infected patients with baseline CKD stage 0 or 1 (CKD-1), who were started on either tenofovir or abacavir from 1998 to 2008 and had at least 1 follow-up eGFR measure on therapy, were included in this retrospective analysis. Progression to CKD stages 2 to 5 was compared using Kaplan-Meier analysis. Progression to CKD-2 and CKD-3 occurred more frequently in patients who received tenofovir than those receiving abacavir (CKD-2, 2-year actuarial frequency, 48.8% vs 23.7%; P < .001, log rank; CKD-3, 5.8% vs 0.0%; P = .028). Only 1 patient in the tenofovir group progressed to CKD-4 and none to CKD-5. Treatment with tenofovir was the only independent factor associated with progression to CKD-2 (hazard ratio [HR], 2.12; 95% confidence interval [CI]: 1.41-3.18; P < .001) and to CKD-3 (HR, 4.91; 95% CI, 1.02-23.7; P = .048). In HIV-infected patients, long-term therapy with tenofovir is associated with mild-to-moderate nephrotoxicity which is significantly higher than in abacavir-treated patients.

The role of transporters in the toxicity of nucleoside and nucleotide analogs

INTRODUCTION

Two families of nucleoside analogs have been developed to treat viral infections and cancer, but these compounds can cause tissue- and cell-specific toxicity related to their uptake and subcellular activity, which are dictated by host enzymes and transporters. Cellular uptake of these compounds requires nucleoside transporters that share functional similarities but differ in substrate specificity. Tissue-specific cellular expression of these transporters enables nucleoside analogs to produce their tissue-specific toxic effects, a limiting factor in the treatment of retroviruses and cancer.

AREAS COVERED

This review discusses the families of nucleoside transporters and how they mediate cellular uptake of nucleoside analogs. Specific focus is placed on examples of known cases of transporter-mediated cellular toxicity and classification of the toxicities resulting. Efflux transporters are also explored as a contributor to analog toxicity and cell-specific effects.

EXPERT OPINION

Efforts to modulate transporter uptake/clearance remain long-term goals of oncologists and virologists. Accordingly, subcellular approaches that either increase or decrease intracellular nucleoside analog concentrations are eagerly sought and include transporter inhibitors and targeting transporter expression. However, additional understanding of nucleoside transporter kinetics, tissue expression and genetic polymorphisms is required to design better molecules and better therapies.

First pharmacokinetic and safety study in humans of the novel lipid antiviral conjugate CMX001, a broad-spectrum oral drug active against double-stranded DNA viruses

CMX001 is a novel, broad-spectrum lipid antiviral conjugate (LAC) that produces high intracellular levels of the active antiviral agent cidofovir diphosphate (CDV-PP). Study CMX001-102 was a randomized, double-blind, placebo-controlled, parallel group, dose-escalating study in healthy volunteers. The objectives of the study were to evaluate the safety and pharmacokinetic parameters of CMX001 after single and multiple doses. Single doses ranging from 0.25 to 2.0 mg/kg of body weight and multiple doses ranging from 0.1 to 1.0 mg/kg (3 total doses, administered every 6 days) were given orally. Safety was assessed using comprehensive clinical and laboratory evaluations, including enhanced monitoring for potential gastrointestinal (GI) effects using wireless capsule endoscopy (WCE). Serial plasma and pooled urine samples were collected to estimate pharmacokinetic parameters for both CMX001 and cidofovir (CDV). No adverse events occurred that prevented dose escalation. No clinically significant drug-related changes in blood chemistry, hematology, renal function, or intraocular pressure were observed. No CMX001-related gastrointestinal mucosal changes were observed by WCE. CMX001 was absorbed rapidly, with maximum plasma concentrations observed 2 to 3 h postdose. Maximum plasma drug concentration and systemic exposure of CMX001 increased approximately in proportion to dose following single and multiple doses; no significant accumulation of CMX001 or CDV was observed following multiple doses. We conclude that CMX001 is orally bioavailable and well tolerated in healthy volunteers at doses up to 2 mg/kg, approximately 140 mg in a typical adult. This is the first demonstration of the use of phospholipid conjugation technology to achieve plasma drug exposures that are expected to result in activity against multiple double-stranded DNA viruses.

Nephrotoxicity of HAART

Highly active antiretroviral therapy (HAART) and other medical therapies for HIV-related infections have been associated with toxicities. Antiretroviral therapy can contribute to renal dysfunction directly by inducing acute tubular necrosis, acute interstitial nephritis, crystal nephropathy, and renal tubular disorders or indirectly via drug interactions. With the increase in HAART use, clinicians must screen patients for the development of kidney disease especially if the regimen employed increases risk of kidney injury. It is also important that patients with chronic kidney disease (CKD) are not denied the best combinations, especially since most drugs can be adjusted based on the estimated GFR. Early detection of risk factors, systematic screening for chronic causes of CKD, and appropriate referrals for kidney disease management should be advocated for improved patient care. The interaction between immunosuppressive therapy and HAART in patients with kidney transplants and the recent endorsement of tenofovir/emtricitabine by the Centers for Disease Control (CDC) for preexposure prophylaxis bring a new dimension for nephrotoxicity vigilance. This paper summarizes the common antiretroviral drugs associated with nephrotoxicity with particular emphasis on tenofovir and protease inhibitors, their risk factors, and management as well as prevention strategies.

Genetic variants of ABCC10, a novel tenofovir transporter, are associated with kidney tubular dysfunction

BACKGROUND

Tenofovir (TFV) causes kidney tubular dysfunction (KTD) in some patients, but the mechanism is poorly understood. Genetic variants in TFV transporters are implicated; we explored whether ABCC10 transports TFV and whether ABCC10 single-nucleotide polymorphisms (SNPs) are associated with KTD.

METHODS

TFV accumulation was assessed in parental and ABCC10-transfected HEK293 cells (HEK293-ABCC10), CD4(+) cells and monocyte-derived macrophages (MDMs). Substrate specificity was confirmed by cepharanthine (ABCC10 inhibitor) and small interfering RNA (siRNA) studies. Fourteen SNPs in ABCC10 were genotyped in human immunodeficiency virus-positive patients with KTD (n = 19) or without KTD (controls; n = 96). SNP and haplotype analysis was performed using Haploview.

RESULTS

TFV accumulation was significantly lower in HEK293-ABCC10 cell lines than in parental HEK293 cells (35% lower; P = .02); this was reversed by cepharanthine. siRNA knockdown of ABCC10 resulted in increased accumulation of TFV in CD4(+) cells (18%; P = .04) and MDMs (25%; P = .04). Two ABCC10 SNPs (rs9349256: odds ratio [OR], 2.3; P = .02; rs2125739, OR, 2.0; P = .05) and their haplotype (OR, 2.1; P = .05) were significantly associated with KTD. rs9349256 was associated with urine phosphorus wasting (P = .02) and β2 microglobulinuria (P = .04).

CONCLUSIONS

TFV is a substrate for ABCC10, and genetic variability within the ABCC10 gene may influence TFV renal tubular transport and contribute to the development of KTD. These results need to be replicated in other cohorts.

Tenofovir nephrotoxicity: 2011 update

Tenofovir is an acyclic nucleotide analogue reverse-transcriptase inhibitor structurally similar to the nephrotoxic drugs adefovir and cidofovir. Tenofovir is widely used to treat HIV infection and approved for treatment of hepatitis B virus. Despite initial cell culture and clinical trials results supporting the renal safety of tenofovir, its clinical use is associated with a low, albeit significant, risk of kidney injury. Proximal tubular cell secretion of tenofovir explains the accumulation of the drug in these mitochondria-rich cells. Tenofovir nephrotoxicity is characterized by proximal tubular cell dysfunction that may be associated with acute kidney injury or chronic kidney disease. Withdrawal of the drug leads to improvement of analytical parameters that may be partial. Understanding the risk factors for nephrotoxicity and regular monitoring of proximal tubular dysfunction and serum creatinine in high-risk patients is required to minimize nephrotoxicity. Newer, structurally similar molecular derivatives that do not accumulate in proximal tubules are under study.

[Treatment of chronic hepatitis B: adherence and safety]

Second generation nucleos (t) idic analogues result in a complete viral suppression after 48 to 96 weeks of therapy in most patients, regardless of the virus (HBV genotype, wild type or pre-C mutant), the underlying liver disease (cirrhosis or not) or the immune status (mono- or HIV/HBV co-infection). This antiviral efficacy may result in HBe or HBs seroconversion. Its clinical impact is important since inactivation of necroinflammation allows, in the absence of liver comorbidities, a stabilisation then a reversal of fibrosis and cirrhosis, and consequently a decrease in the occurrence of carcinomatous or non-carcinomatous complications. The future issues for long-term anti-HBV therapy will be adherence on the one hand and safety on the other hand. Therapeutic failures are mainly related to poor adherence more than to viral resistance. Adherence of patients has to be optimized by therapeutic education and education of physicians. Long-term safety has to be systematically evaluated. More than the neuromuscular or metabolic side effects (lactic acidosis), the renal and bone-related adverse events have to be monitored, followed-up and anticipated by good clinical practices.

Tenofovir-associated kidney toxicity in HIV-infected patients: a review of the evidence

Tenofovir (TDF) is an effective and widely used treatment for both human immunodeficiency virus (HIV) and hepatitis B virus infection. Although studies suggest that TDF has a low overall toxicity profile and only a modest effect on estimated glomerular filtration rate, numerous case reports have since appeared in the literature describing TDF-associated renal tubular dysfunction, and this is now a significant source of HIV-related referrals to nephrologists. The main target of toxicity appears to be the proximal tubule, and in severe cases, patients can develop renal Fanconi syndrome. We review findings from recent studies in this area performed by ourselves and others and discuss our direct experience as practicing nephrologists. In particular, we discuss: (1) the nature and extent of TDF-associated kidney toxicity in the HIV-infected population, (2) potential underlying mechanisms of toxicity in the proximal tubule, (3) risk factors for developing tubular dysfunction, and (4) suggested strategies to monitor patients on TDF therapy.

Pharmacological considerations for tenofovir and emtricitabine to prevent HIV infection

The use of antiretroviral medications in HIV-negative individuals as pre-exposure prophylaxis (PrEP) is a promising approach to prevent HIV infection. Tenofovir disoproxil fumarate (TDF) and emtricitabine exhibit desirable properties for PrEP including: favourable pharmacokinetics that support infrequent dosing; few major drug-drug or drug-food interactions; an excellent clinical safety record; and pre-clinical evidence for efficacy. Several large, randomized, controlled clinical trials are evaluating the safety and efficacy of TDF and emtricitabine for this new indication. A thorough understanding of variability in drug response will help determine future investigations in the field and/or implementation into clinical care. Because tenofovir and emtricitabine are nucleos(t)ide analogues, the HIV prevention and toxicity effects depend on the triphosphate analogue formed intracellularly. This review identifies important cellular pharmacology considerations for tenofovir and emtricitabine, which include drug penetration into relevant tissues and cell types, race/ethnicity/pharmacogenetics, gender, cellular activation state and appropriate episodic or alternative dosing strategies based on pharmacokinetic principles. The current state of knowledge in these areas is summarized and the future utility of intracellular pharmacokinetics/pharmacodynamics for the PrEP field is discussed.

Prospective study of renal function in HIV-infected pediatric patients receiving tenofovir-containing HAART regimens

AIM

to describe the impact of tenofovir disoproxil fumarate (TDF) use on renal function in HIV-infected pediatric patients.

DESIGN

it is a prospective, multicenter study. The setting consisted of five third-level pediatric hospitals in Spain. The study was conducted on patients aged 18 years and younger who had received TDF for at least 6 months. The intervention was based on the study of renal function parameters by urine and serum analyses. The main outcome measures were renal function results following at least 6 months of TDF therapy.

RESULTS

forty patients were included (32 were white and 26 were diagnosed with AIDS). Median (range) duration of TDF treatment was 77 months (16-143). There were no significant changes in the estimated creatinine clearance. Urine osmolality was abnormal in eight of 37 patients, a decrease in tubular phosphate absorption was documented in 28 of 38 patients, and 33 of 37 patients had proteinuria. A statistically significant decrease in serum phosphate and potassium concentrations was observed during treatment (P = 0.005 and P = 0.003, respectively), as well as a significant relationship between final phosphate concentration and tubular phosphate absorption (P = 0.010). A negative correlation was found between phosphate concentration and time on TDF.

CONCLUSIONS

TDF use showed a significant association with renal tubular dysfunction in HIV-infected pediatric patients. Periodic assessment of tubular function may be advisable in the follow-up of this population.

Cidofovir Activity against Poxvirus Infections

Cidofovir [(S)-1-(3-hydroxy-2-phosphonylmethoxypropyl)cytosine, HPMPC] is an acyclic nucleoside analog approved since 1996 for clinical use in the treatment of cytomegalovirus (CMV) retinitis in AIDS patients. Cidofovir (CDV) has broad-spectrum activity against DNA viruses, including herpes-, adeno-, polyoma-, papilloma- and poxviruses. Among poxviruses, cidofovir has shown in vitro activity against orthopox [vaccinia, variola (smallpox), cowpox, monkeypox, camelpox, ectromelia], molluscipox [molluscum contagiosum] and parapox [orf] viruses. The anti-poxvirus activity of cidofovir in vivo has been shown in different models of infection when the compound was administered either intraperitoneal, intranasal (aerosolized) or topically. In humans, cidofovir has been successfully used for the treatment of recalcitrant molluscum contagiosum virus and orf virus in immunocompromised patients. CDV remains a reference compound against poxviruses and holds potential for the therapy and short-term prophylaxis of not only orthopox- but also parapox- and molluscipoxvirus infections.

Long-term therapy with tenofovir is effective for patients co-infected with human immunodeficiency virus and hepatitis B virus

BACKGROUND & AIMS

We investigated the long-term efficacy and renal safety of tenofovir disoproxil fumarate (TDF), administered to patients co-infected with human immunodeficiency virus and hepatitis B virus (HBV) as part of an antiretroviral therapy.

METHODS

We performed a multicenter, prospective cohort study of 102 patients co-infected with human immunodeficiency virus and HBV who were treated with TDF.

RESULTS

At baseline, 80% of patients had a detectable viral load (HBV DNA >20 IU/mL). Among patients positive for hepatitis B e antigen (HBeAg) (n = 67), 92% had a virologic response (HBV DNA <20 IU/mL) after 5 years of treatment. There was no difference between patients with or without lamivudine resistance at baseline (P = .39). Loss rates of HBeAg and hepatitis B s antigen (HBsAg) were 46% and 12%, respectively. Among HBeAg-negative patients (n = 15), 100% had a virologic response after 4 years of treatment and 2 (13%) lost HBsAg. Twenty subjects (20%, all HBeAg-negative) had undetectable HBV DNA at baseline; during a median follow-up period of 52 months (interquartile range, 41-63 mo), 19 (95%) maintained a virologic response and 2 (10%) lost HBsAg. Overall, one patient acquired a combination of resistance mutations for anti-HBV drugs and experienced a virologic breakthrough. Three (3%) patients discontinued TDF because of increased serum creatinine levels. The estimated decrease in renal function after 5 years of TDF therapy was 9.8 mL/min/1.73 m(2), which was most pronounced shortly after TDF therapy was initiated.

CONCLUSIONS

TDF, administered as part of antiretroviral therapy, is a potent anti-HBV agent with a good resistance profile throughout 5 years of therapy. Only small nonprogressive decreases in renal function were observed.

Nucleoside reverse transcriptase inhibitor toxicity and mitochondrial DNA

IMPORTANCE OF THE FIELD

HIV/AIDS is a worldwide epidemic. While there remains no cure for the HIV-1 infection, nucleoside reverse transcriptase inhibitors (NRTIs) have helped transform the HIV-1 infection from a lethal disease into a chronic illness. Though NRTIs inhibit HIV-1 replication, they exhibit side effects in human tissues that appear to result from NRTI inhibition of human mitochondrial polymerase γ (pol γ).

AREAS COVERED IN THIS REVIEW

this review discusses the current knowledge of NRTI-induced toxicity, specifically the inhibition of pol γ and the mitochondrial toxicity from incorporation of NRTIs into mitochondrial DNA. Details are discussed about general mechanisms of NRTI toxicity and how specific tissue toxicities in mitochondria relate to clinical manifestation.

WHAT THE READER WILL GAIN

a detailed knowledge of the mitochondrial toxicity resulting from NRTI-inclusive therapies and related tissue toxicities are provided. This review presents both the molecular effects of NRTI usage on mitochondrial genetic homeostasis and energy metabolism as well as the clinical manifestations associated with NRTI toxicities.

TAKE HOME MESSAGE

NRTIs remain a critical component of current HIV-1 treatment regimens. Future NRTIs should provide higher specificity for HIV-RT and lower incorporation by pol γ to minimize mitochondrial toxicity. Alternatively, therapeutic interventions to prevent or alleviate mitochondrial toxicity should be addressed.

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.

Renal toxicity associated with tenofovir use

IMPORTANCE OF THE FIELD

Tenofovir (TFV) is a nucleotide analogue widely used for the treatment of HIV infection. Despite its proven efficacy and safety, cases of kidney tubular dysfunction have increasingly been reported and concern exists about the risk of nephrotoxicity associated with the long-term use of TFV.

AREAS COVERED IN THIS REVIEW

Evidences about the renal toxicity associated with TFV use as well as predictors are examined. The most relevant publications assessing TFV safety and those which have reported cases of tubular dysfunction were identified and carefully revised.

WHAT THE READER WILL GAIN

Renal damage of clinical significance caused by TFV is uncommon in the short-mid-term. It occurs more frequently in subjects with underlying kidney conditions. TFV primarily results in kidney tubular dysfunction and less frequently in glomerular abnormalities. Kidney damage may progress over time under long-term TFV exposure but is reversible in most cases on drug discontinuation.

TAKE HOME MESSAGE

Severe renal damage associated with TFV use is uncommon and of multifactorial origin. However, mild tubular dysfunction is recognized in a substantial proportion of TFV-treated individuals and tends to increase with cumulative exposure.

Recent developments in HIV-related kidney disease

Although kidney disease has been a recognized complication of HIV infection since the beginning of the HIV epidemic, its epidemiology, underlying causes and treatment have evolved in developed countries where HAART has been widely available. HIV-associated nephropathy and HIV immune complex-mediated kidney disease were the prominent renal diagnoses in the earlier period of the HIV epidemic. While HIV immune complex-mediated kidney disease remains a common finding among HIV-infected individuals with kidney disease, the incidence of HIV-associated nephropathy has been diminishing in developed countries. The role of the metabolic effects of long-term HAART exposure and nephrotoxicity of certain antiretroviral medications on the development and progression of chronic kidney disease is now of increasing concern. The long-term clinical implications of acute kidney injury among HIV-infected persons are increasingly recognized. Kidney disease in HIV-infected persons continues to be a major risk factor for morbidity and mortality in this patient population; therefore, early recognition and treatment of kidney disease are imperative in lessening the impact of kidney disease on the health of HIV-infected individuals. This review focuses on recent developments and ongoing challenges in the understanding, diagnosis and management of HIV-related kidney disease.

Organic anion transporters: discovery, pharmacology, regulation and roles in pathophysiology

Our understanding of the mechanisms behind inter- and intra-patient variability in drug response is inadequate. Advances in the cytochrome P450 drug metabolizing enzyme field have been remarkable, but those in the drug transporter field have trailed behind. Currently, however, interest in carrier-mediated disposition of pharmacotherapeutics is on a substantial uprise. This is exemplified by the 2006 FDA guidance statement directed to the pharmaceutical industry. The guidance recommended that industry ascertain whether novel drug entities interact with transporters. This suggestion likely stems from the observation that several novel cloned transporters contribute significantly to the disposition of various approved drugs. Many drugs bear anionic functional groups, and thus interact with organic anion transporters (OATs). Collectively, these transporters are nearly ubiquitously expressed in barrier epithelia. Moreover, several reports indicate that OATs are subject to diverse forms of regulation, much like drug metabolizing enzymes and receptors. Thus, critical to furthering our understanding of patient- and condition-specific responses to pharmacotherapy is the complete characterization of OAT interactions with drugs and regulatory factors. This review provides the reader with a comprehensive account of the function and substrate profile of cloned OATs. In addition, a major focus of this review is on the regulation of OATs including the impact of transcriptional and epigenetic factors, phosphorylation, hormones and gender.

Antiretroviral medications: adverse effects on the kidney

The widespread introduction of highly active antiretroviral therapy (HAART) in the mid-1990s dramatically altered the course of human immunodeficiency virus (HIV) infection, with improvements in survival and reductions in the incidence of AIDS-defining illnesses. Although antiretroviral therapy has been shown to reduce the incidence of both AIDS-defining and non-AIDS conditions, long-term exposure to HAART may also be associated with significant toxicity. This article reviews the potential nephrotoxicity of specific antiretroviral agents and the impact of antiretroviral therapy on related metabolic disorders. The antiretroviral agents most strongly associated with direct nephrotoxicity include the nucleotide reverse transcriptase inhibitor, tenofovir, and the protease inhibitor indinavir, although other agents have been implicated less frequently. Tenofovir and related nucleotide analogs have primarily been associated with proximal tubular dysfunction and acute kidney injury, whereas indinavir is known to cause nephrolithiasis, obstructive nephropathy, and interstitial nephritis. Kidney damage related to antiretroviral therapy is typically reversible with early recognition and timely discontinuation of the offending agent, and nephrologists should be familiar with the potential toxicity of these agents to avoid delays in diagnosis.