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Tenofovir-induced kidney disease: an acquired renal tubular mitochondriopathy

  • Mark A. Perazella
    Correspondence
    Section of Nephrology, Yale University School of Medicine, BB 114, 330 Cedar Street, New Haven, Connecticut 06410, USA
    Affiliations
    Section of Nephrology, Yale University School of Medicine, New Haven, Connecticut, USA
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      Tenofovir, used in combination with other antiretroviral agents, is an effective therapy for HIV infection. Although large clinical studies and post-marketing data support a benign renal profile for tenofovir, numerous cases of kidney injury raise concern for nephrotoxic potential. Early human studies and experimental evidence suggested that tenofovir itself was not associated with mitochondrial toxicity within the kidney. However, recent animal data demonstrate that tenofovir causes mitochondrial DNA depletion and mitochondrial toxicity. Herlitz et al. confirm the nephrotoxicity of tenofovir in humans. They describe its clinical consequences, histopathologic findings, and its mitochondrial toxicity in HIV+ patients.
      Drug research and development facilitates the synthesis and release of novel therapeutics into clinical practice at a remarkable rate. Their availability has undoubtedly revolutionized the treatment of numerous diseases—in particular, devastating processes such as HIV infection. The landscape of this disease has been forever changed by the availability of highly active antiretroviral therapy (HAART) since 1996. The basic premise behind this strategy is to attack the virus at different replication steps using multiple medications. Unfortunately, most success stories have a catch; in this case, the price of success is drug-induced kidney disease, a challenging complication for HIV+ patients and their caregivers.
      • Izzedine H.
      • Harris M.
      • Perazella M.A.
      The nephrotoxic effects of HAART.
      Before the widespread use of HAART, HIV-associated kidney disease was primarily due to direct and/or indirect effects of the virus. However, with successful treatment of HIV infection, drug-induced nephrotoxicity (Table 1) surfaced as an important problem. Acute kidney injury (AKI), various tubulopathies, nephrolithiasis, and chronic kidney disease were noted.
      • Izzedine H.
      • Harris M.
      • Perazella M.A.
      The nephrotoxic effects of HAART.
      HAART causes these renal syndromes via multiple mechanisms, including direct tubular toxicity, allergic reactions, and precipitation of insoluble drug crystals.
      Table 1HAART-associated kidney disease
      Renal syndromeMedication
      Acute kidney injury
       Toxic acute tubular necrosisTenofovir, ritonavir, didanosine
       Acute interstitial nephritisAtazanavir, abacavir, indinavir
       Crystal nephropathyIndinavir, atazanavir
      Tubulopathies
       Fanconi's syndromeTenofovir, didanosine, ritonavir
       Renal tubular acidosisLamivudine, stavudine
       Nephrogenic diabetes insipidusTenofovir, didanosine, indinavir
      NephrolithiasisIndinavir, atazanavir, nelfinavir, amprenavir, saquinavir, efavirenz
      Chronic kidney disease
       Chronic interstitial nephritisIndinavir, tenofovir
       Post-AKI kidney diseaseSeveral HAART drugs
      Abbreviations: AKI, acute kidney injury; HAART, highly active antiretroviral therapy.
      Although many of the early drugs had intolerable adverse effects and difficult dosing schedules, newer agents have overcome many of these issues. One such drug, tenofovir, has gained widespread use on the basis of its efficacy, tolerability, and patient-friendly dosing schedule.
      • Jimenez-Nacher I.
      • Garcia B.
      • Barreiro P.
      • et al.
      Trends in the prescription of antiretroviral drugs and impact on plasma HIV-RNA measurements.
      As it is structurally similar to the acyclic nucleotide analogs adefovir and cidofovir, which are nephrotoxic, concern about adverse renal effects existed for tenofovir as well. These two drugs cause proximal tubulopathies such as AKI as a result of acute tubular necrosis (ATN) and Fanconi's syndrome,
      • Tanji N.
      • Tanji K.
      • Kambham N.
      • et al.
      Adefovir nephrotoxicity: possible role of mitochondrial DNA depletion.
      by disrupting proximal tubular mitochondrial function. A number of mechanisms underlie drug-induced mitochondriopathies, but these drugs act primarily by decreasing mitochondrial DNA (mtDNA) replication by inhibiting mitochondrial DNA polymerase-γ, which is the only enzyme capable of replicating mtDNA.
      • Tanji N.
      • Tanji K.
      • Kambham N.
      • et al.
      Adefovir nephrotoxicity: possible role of mitochondrial DNA depletion.
      As a result, mtDNA and a number of the mtDNA-encoded enzymes involved in electron transport chain function and oxidative phosphorylation are depleted, resulting in disturbed mitochondrial function. This ultimately causes, among other effects, a deficit in adenosine triphosphate production, impaired cell function, and cell injury and/or death.
      Early randomized clinical trials
      • Izzedine H.
      • Isnard-Bagnis C.
      • Hulot J.S.
      • et al.
      Renal safety of tenofovir in HIV treatment-experienced patients.
      and post-marketing data
      • Nelson M.R.
      • Katlama C.
      • Montaner J.S.
      • et al.
      The safety of tenofovir disoproxil fumarate for the treatment of HIV infection in adults: the first 4 years.
      examining tenofovir in relatively healthy HIV+ subjects supported an excellent safety profile, including the absence of significant renal injury. An in vitro experimental study supported this clinical finding.
      • Birkus G.
      • Hitchcock M.J.M.
      • Cihlar T.
      Assessment of mitochondrial toxicity in human cells treated with tenofovir: comparison with other nucleoside reverse transcriptase inhibitors.
      The investigators exposed various cultured human cell lines (liver, muscle, proximal renal tubule) to tenofovir. Minimal mtDNA depletion and insignificant reductions in cellular expression of the mitochondrial protein cytochrome c oxidase were noted with tenofovir. However, with the release of tenofovir into clinical practice and its use in HIV patients with various comorbid conditions, reports of nephrotoxicity began to surface.
      • Rifkin B.
      • Perazella M.A.
      Tenofovir-associated nephrotoxicity: Fanconi syndrome and renal failure.
      These reports described renal consequences such as AKI from toxic ATN, Fanconi's syndrome, and rare cases of nephrogenic diabetes insipidus. Renal histopathology from these cases demonstrated acute tubular injury predominantly in the proximal tubules, as described with adefovir and cidofovir. These observations questioned the renal safety noted in the large clinical trials.
      Further information about nephrotoxic potential was gleaned from a retrospective study that examined mitochondrial ultrastructure and mtDNA levels in kidney tissue obtained from biopsies performed for nephrotoxicity in HIV- and HIV+ patients.
      • Cote H.
      • Magil A.
      • Harris M.
      • et al.
      Exploring mitochondrial nephrotoxicity as a potential mechanism of kidney dysfunction among HIV-infected patients on highly active antiretroviral therapy.
      Investigators found that HIV+ kidneys exposed to tenofovir had altered tubular mitochondrial morphology. However, while HIV+ kidneys had mtDNA levels lower than those in HIV- patients, there was no difference in mtDNA levels in tenofovir-exposed versus unexposed HIV+ kidneys. Furthermore, HIV+/tenofovir+/didanosine+ kidneys had lower mtDNA levels than HIV+/tenofovir+/didanosine- kidneys, which questioned whether mitochondrial toxicity was a significant complication of isolated tenofovir exposure. More recently, two animal studies support the notion that tenofovir causes mtDNA depletion and mitochondrial dysfunction.
      • Kohler J.
      • Hosseini S.
      • Hoying-Brandt A.
      • et al.
      Tenofovir renal toxicity targets mitochondria of renal proximal tubules.
      ,
      • Lebrecht D.
      • Venhoff A.
      • Kirschner J.
      • et al.
      Mitochondrial tubulopathy in tenofovir disoproxil fumarate-treated rats.
      A study in HIV+ transgenic mice and their wild-type littermates examined ultrastructure and mtDNA levels with tenofovir, didanosine, and vehicle exposures.
      • Kohler J.
      • Hosseini S.
      • Hoying-Brandt A.
      • et al.
      Tenofovir renal toxicity targets mitochondria of renal proximal tubules.
      Only renal proximal tubules from HIV+ transgenic mice exposed to tenofovir showed ultrastructural mitochondrial abnormalities (irregular shapes with sparse, fragmented cristae) and decreased mtDNA levels, which paralleled the ultrastructural mitochondrial abnormalities. The opposite was seen in liver cells, where didanosine depleted hepatic mtDNA and tenofovir had no effect. Another study explored the potential for mitochondrial toxicity in rats exposed to tenofovir, didanosine, or water.
      • Lebrecht D.
      • Venhoff A.
      • Kirschner J.
      • et al.
      Mitochondrial tubulopathy in tenofovir disoproxil fumarate-treated rats.
      Rats exposed to tenofovir, but not those exposed to didanosine or water, developed proximal tubular dilatation, abnormalities in mitochondrial ultrastructure, depleted mtDNA, and depressed respiratory chain enzyme expression (cytochrome c oxidase and nicotinamide adenyldinucleotide dehydrogenase). In contrast, didanosine induced significant hepatic mtDNA depletion, whereas tenofovir had no liver effects. These studies suggest that tenofovir causes compartmentalized mitochondrial toxicity within renal proximal tubular cells.
      Herlitz and colleagues
      • Herlitz L.C.
      • Mohan S.
      • Stokes M.B.
      • et al.
      Tenofovir nephrotoxicity: acute tubular necrosis with distinctive clinical, pathological, and mitochondrial abnormalities.
      (this issue) now make a strong case for tenofovir as a proximal tubular mitochondrial toxin in humans. The Columbia University group queried their renal biopsy archives in search of a histopathologic correlate to the clinical renal disease—tenofovir-associated nephrotoxicity. According to predefined diagnostic criteria, 13 patients with HIV infection were identified. All patients were on various forms of HAART; and the ten with available data had well-controlled HIV infection by CD4 count and viral load. Duration of tenofovir ranged from 3 weeks to 8 years with a median of 8 months. Biopsy was performed for AKI (n=9) or proteinuria with mild serum creatinine elevation (n=4). Mean serum creatinine at biopsy was 5.7±4.0 mg/dl, mean proteinuria was 1.6±0.3 g/d, and five patients had normoglycemic glycosuria, the latter two findings being suggestive of Fanconi's syndrome. AKI was severe enough to require hemodialysis in five patients. Of 11 cases with mean follow-up of nearly 20 months, kidney function returned to baseline in six cases, while five had partial recovery. No patient remained on dialysis. Thus, although all patients had significant recovery with tenofovir discontinuation, nearly half of the patients were left with some level of CKD.
      Renal histology demonstrated proximal tubular injury and varying degrees of chronic tubulointerstitial scarring. Ten cases had findings consistent with ‘toxic ATN.’ Prominent eosinophilic inclusions within proximal tubular cell cytoplasm, which represented giant, abnormal mitochondria, were noted on light microscopy and constitute an interesting new finding. These inclusions are easily identifiable, as they stain brightly with hematoxylin and eosin stain or fuchsinophilic with trichrome stain. On electron microscopy, mitochondria varied widely in shape and size; some were small and rounded, while others were swollen with irregular contours. Loss and disorientation of cristae were observed in enlarged mitochondria, while the overall number of mitochondria was significantly decreased in some tubular cells.
      The comprehensive clinical–histopathologic correlation described in this paper addresses a number of issues related to tenofovir nephrotoxicity. First, it confirms that tenofovir causes ‘toxic ATN’ of varied severity and Fanconi's syndrome in exposed HIV+ patients. Second, patients can develop the clinical renal syndrome at any time point in tenofovir therapy, both early and late. Third, although tenofovir discontinuation is associated with significant renal recovery, many patients may suffer from CKD. Finally, histopathology confirms that tenofovir is a mitochondrial toxin in renal tubular cells and has a distinctive finding on light microscopy. However, the study does not answer one important question: Why does tenofovir cause renal injury in only a small subset of HIV+ patients? To try to answer this question, host characteristics and drug pharmacokinetics must be considered.
      Host factors that potentially enhance tenofovir nephrotoxicity include mtDNA depletion, a complication of HIV infection itself, which likely primes patients for more severe mitochondrial dysfunction with exposure to a drug that targets this organelle. In addition, underlying kidney disease with low glomerular filtration rate and genetic defects in certain renal drug excretion pathways also enhance risk for tenofovir-induced nephrotoxicity.
      • Perazella M.A.
      Renal vulnerability to drug toxicity.
      Tenofovir is eliminated by a combination of glomerular filtration and proximal tubular secretion, which in part explains the compartmental toxicity of tenofovir.
      • Perazella M.A.
      Renal vulnerability to drug toxicity.
      Tenofovir is transported via organic anion transporter-1 (OAT-1) from the basolateral circulation into proximal tubular cells (Figure 1), where it is eventually translocated into the urine through apical efflux transporters such as multidrug resistance protein-2 (MRP-2) and MRP-4. Renal impairment with reduced glomerular filtration rate will increase the amount of tenofovir that is secreted, increasing trafficking through the proximal tubular cells. Increased activity of OAT-1 may increase the amount of tenofovir entering proximal tubular cells, although this remains unproven. Impaired MRP-driven efflux activity can reduce tenofovir secretion and increase intracellular concentrations. A single-nucleotide polymorphism in the MRP-2 efflux transporter gene (ABCC2) has been documented in HIV+ patients who developed tenofovir-induced nephrotoxicity, supporting this hypothesis.
      • Izzedine H.
      • Hulot J.S.
      • Villard E.
      • et al.
      Association between ABCC2 gene haplotypes and tenofovir-induced proximal tubulopathy.
      Similarly, endogenous anions and other drugs may compete with tenofovir for these efflux transport pathways.
      Figure thumbnail gr1
      Figure 1Proximal tubular cell transport pathway for tenofovir. (a) The organic anion drug tenofovir (TF) is delivered to the basolateral membrane of proximal tubular cells and is transported into the cell by organic anion transporter-1 (OAT-1). Once within the cell, TF is transported via carrier proteins and is subsequently secreted into the urinary space by the apical efflux transporters multidrug resistance protein-2 (MRP-2) and MRP-4. (b) Disturbances in the secretory pathway of TF (increased OAT-1 activity or decreased MRP efflux transport activity) may lead to increased TF concentrations within the cell, which can cause mitochondrial DNA depletion and dysfunction. This can ultimately cause a proximal tubulopathy characterized by acute kidney injury and Fanconi's syndrome. Abbreviations: NaDC, sodium–dicarboxylate symporter; OCT, organic cation transporter; Pgp, P-glycoprotein.
      Ultimately, these excretory pathway defects can lead to increased tenofovir trafficking through and/or increased concentrations within proximal tubular cells, enhancing risk for mtDNA depletion and mitochondrial dysfunction. In a subgroup of patients, these energy-deficient tubular cells may manifest as the clinical renal syndromes previously described. The actual development and onset time of tenofovir-induced nephrotoxicity will likely depend on the number and severity of patient risk factors present. Genetic risk factor testing (for the single-nucleotide polymorphism in ABCC2) to identify high-risk patients and targeted interventions, such as probenecid to reduce OAT-1 transport of tenofovir into tubular cells, may allow HIV+ patients to garner benefit from effective therapies such as tenofovir.

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