Renal Dysfunction due to Tenofovir-Diphosphate Inhibition of Mitochondrial Complex V (ATP Synthase).

Prescription drugs are a common cause of kidney injury. Druginduced nephrotoxicity, however, is a complex process, and likely involves a combination of factors, including (i) drug characteristics (eg, solubility, structure, and charge); (ii) drug dose and duration of therapy; (iii) inherent drug toxicity; (iv) renal metabolism and excretion of the drug; and (v) patient characteristics that enhance their risk for kidney injury. The mechanisms of drug-induced nephrotoxicity and prevention strategies have been reviewed extensively elsewhere.1,2 Tenofovir disoproxil fumarate (TDF) is a nucleoside reverse transcriptase inhibitor used to treat HIV and HBV infections. TDF therapy, however, has been associated with renal impairment, characterized by a decline in glomerular filtration rate and proximal tubular dysfunction.3 TDF is a prodrug that is rapidly metabolized to the active component tenofovir in plasma. In cells, tenofvoir is metabolized to its active diphosphate form by adenylate monophosphate kinase (tenofovir monophosphate) and 5′-nucleoside diphosphate (tenofovir diphosphate).4 Renal injury is likely related to intracellular tenofovir accumulation in proximal tubule cells. A molecular mechanism of TDF-induced renal toxicity, however, is lacking, but it is thought to be via mitochondrial depletion and structural change, including size and shape changes, and leakage of mitochondrial proteins into the cytosol, with resultant DNA damage, which may even induce apoptosis of the cell. In a recent study, Pearson et al. developed an innovative approach to screen for disease-related functional defects in RPTEC/TERT1 cells, a well-differentiated human-derived cell line that replicates many of the major characteristics of proximal tubular kidney cells in vivo.5 The RPTEC/TERT1 cells were exposed to TDF, and high-throughput imaging was used to generate quantitative readouts of solute transport and mitochondrial morphology, which facilitated development of treatment protocols that reproduced well-described features in patients. By using multiparametric metabolic profiling, including metabolomic screening, oxygen consumption measurements, and RNA-sequencing, the authors determined a molecular fingerprint of TDF toxicity. They found that TDF results in a dose-dependent decrease in mitochondrial ATP synthase, or complex V (EC 3.6.3.14) activity and expression, whereas other mitochondrial functions and pathways were well preserved. Tenofovir disphosphate was found to directly inhibit complex V. Downregulation of complex V expression was also observed in human biopsies. Complex V synthesizes ATP from ADP in the mitochondrial matrix using the energy provided by the proton electrochemical gradient, and mutations in complex V give rise to severe mitochondrial disease phenotypes, ranging from neuropathy, ataxia, and retinitis pigmentosa to maternally inherited Leigh syndrome.6 Of note, in a rat model of TDF nephrotoxicity, the activities of the electron chain complexes I, II, IV, and V were also found to be inhibited.7 Collectively, these studies suggest that modulation of complex V activity and expression by TDF is a plausible mechanism for nephrotoxicity. There are, however, limitations associated with the Pearson et al. study. After oral absorption, TDF is rapidly converted to tenofovir in the plasma, and then intracellularly to the active tenofovir diphosphate. Tenofovir, and not TDF, is the primary metabolite that is excreted renally. In this study, the authors used very high concentrations of TDF (300–500μm). The negative charges of the phosphonate moiety of tenofovir reduce its cellular permeability, while TDF was developed to improve tenofovir permeability. Indeed, in antiviral assays in MT-2 cells,