Tenofovir (TFV) is a nucleotide analogue active against HIV and hepatitis B virus. Although TFV rarely affects the glomerular function, abnormalities in the kidney tubular function appear to be quite common. The relationship between TFV exposure and kidney tubular dysfunction (KTD) was examined prospectively in 92 HIV-infected individuals. Median TFV plasma trough concentration was higher in patients with KTD than in the rest (182 vs. 106 ng/ml; P = 0.001). This dose-dependent effect further supports an involvement of TFV in KTD.
aDepartment of Infectious Diseases, Hospital Carlos III, Madrid, Spain
bService of Infectious Diseases, University of Torino, Torino, Italy
cDepartment of Nephrology, Hospital Infanta Leonor, Madrid, Spain.
Received 19 July, 2009
Revised 12 August, 2009
Accepted 18 August, 2009
Tenofovir (TFV) is a nucleotide reverse transcriptase inhibitor widely used in the treatment of HIV infection. Although TFV is relatively well tolerated for the kidney with a very low rate of renal insufficiency , cases of tubular dysfunction, including the development of Fanconi's syndrome, have been reported [2–8]; and concern exists about its long-term use. TFV undergoes renal clearance by a combination of glomerular filtration and active tubular secretion. A mechanism of toxicity related with the route of elimination of TFV has been suggested. Polymorphisms at genes coding for transporter proteins involved on TFV elimination (ABCC2 or ABCC4 genes) have been associated with tubular renal damage. Examples of these polymorphisms are the haplotype ‘CATC’ at ABCC2 and the genotype CC at -24 ABCC2 [9,10]. These studies suggest that alterations on TFV elimination could be the cause of renal toxicity, and that overexposure to TFV could cause kidney tubular cell damage. Herein, we analyse the association between TFV concentration and kidney tubular dysfunction (KTD) in HIV-infected patients under TFV therapy.
Patients included in this study belonged to a cohort of HIV-infected individuals followed for longer than 6 months at our institution . The study protocol was approved by the hospital ethical committee. A total of 284 patients underwent study of KTD in 24-h urine. Patients were divided into three groups: patients under highly active antiretroviral therapy (HAART) containing TFV, n = 154, patients under HAART without TFV, n = 49; and antiretroviral-naive patients, n = 81. The percentage of patients with KTD was higher in the group receiving TFV than in the rest (22% vs. 6% vs. 12%, P < 0.05). For the present substudy, only patients on TFV treatment were selected. KTD was determined on the basis of the following abnormalities : nondiabetic glucosuria, altered resorption of phosphorus, hyperaminoaciduria, β2-microglobulinuria, and abnormal uric acid excretion. KTD was defined when at least two of these abnormalities were present, with at least one being a Fanconi's syndrome criteria (glucosuria in nondiabetics, hyperaminoaciduria or hyperphosphaturia). Mid-dose TFV concentration (C12) was measured using a validated HPLC-mass method . Time sample was between 10 and 14 h. Statistical analyses were performed using SPSS package, version 11.0 (SPSS Inc., Chicago, IL, USA).
Plasma samples and informed consent were obtained from 92 patients. Patients were divided into two groups: Eighteen (19%) fulfilling criteria for KTD and 74 (81%) with normal tubular function. The median exposure to TFV was 33 months (interquartile range = 10–46). Creatinine clearance (24 h) was normal and comparable between groups. Moreover, both groups were well matched in sex, age, weight, hepatitis C virus coinfection, months under TFV, exposure to nephrotoxic drugs, concomitant protease inhibitors use, and hypertension. Diabetes was more frequent in cases than controls (44% vs. 22%; P = 0.05).
Median TFV plasma levels were higher in patients with KTD than in those with normal tubular function [182 (105–220) vs. 106 (75–148) ng/ml, respectively; P = 0.001] (Fig. 1). The best TFV plasma concentration threshold to discriminate KTD was 160 ng/ml (AUROC 0.75 [95% confidence interval (CI) = 0.63–0.87], P = 0.001) (61% sensitivity and 80% specificity). Multivariate analysis including all relevant variables showed that only TFV plasma levels more than 160 ng/ml were associated with KTD [odds ratio (OR) = 4.8 (95% CI = 1.5–16), P = 0.008]. Interestingly, the multivariate analysis showed that female sex [OR = 71 (95% CI = 33–111), P < 0.01] and the ratio body weight/plasma creatinine [OR = −1.19 (95% CI = −2.2- −0.56), P < 0.01] were independently associated with higher TFV plasma levels.
These results argue in favour of a dose-dependent effect of TFV on KTD. This observation is in agreement with prior findings in which TFV concentration exceeded the one found in pharmacokinetic studies in patients with KTD [6,13,14]. The mechanism of kidney tubular damage by TFV is probably related with its renal clearance, supporting that higher TFV plasma levels may lead directly to a greater accumulation of TFV in the renal tubular cells and, consequently, to kidney toxicity .
The ratio body weight/serum creatinine (BW/SCr) was associated with TFV plasma levels in our study. This finding agrees with a previous pharmacokinetic study in which a strong relationship was found between TFV clearance and BW/SCr, being TFV exposure diminished as the BW/SCr ratio increased . Being female was also strongly associated with increased TFV exposure. Although sex differences in TFV plasma levels have not been reported so far, for other antiretrovirals such as zidovudine and lamivudine, differences in intracellular concentration of the triphosphate forms have been reported according to sex .
KTD associated with TFV use appears to be multifactorial. Increased TFV exposure plays an important role, along with other factors, such as polymorphisms at renal transporter proteins, age, body weight, etc. [9,10]. The ultimate mechanism of TFV kidney toxicity is still unclear because tenofovir has shown low mitochondrial toxicity in vitro, due to its low affinity for the polymerase gamma.
Several limitations of our study must be acknowledged. First, a limited number of patients were examined and we could have missed individuals with more serious renal toxicity manifested at early time points. Second, we only had one TFV measurement per patient and a relatively high intraindividual variation on TFV clearance has been described . Lastly, the cross-sectional nature of our study warrants that results should be confirmed in larger, prospective studies.
In summary, these results suggest an association between TFV plasma levels and TFV-associated renal toxicity, primarily recognizable as kidney tubular dysfunction. This concentration-dependent effect is important because TFV may be used in conditions other than HIV (e.g. chronic hepatitis B) in which dose reduction might be an option. If tubular damage persists for long periods in the absence of renal insufficiency, laboratory and clinical manifestations other than those associated with kidney failure may develop (premature osteoporosis due to bone mineral loss) under long-term TFV therapy. Further studies examining the role of TFV plasma measurements to detect and prevent tubulopathy are warranted.
Role of each author: S.R.N., P.L. and V.S. designed the study. S.R.N., A.D.A., M.S., S.B. and G.D.P. participated in the quantitative measurement of tenofovir plasma concentrations. P.L., P.B., E.V. and V.S. participated in the collection of clinical information. S.R.N. and V.S. wrote the manuscript.
There are no conflicts of interests.
Financial support: This work was supported in part by grants from Ministerio de Educación y Ciencia (SAF-2007-63329), Fondo de Investigaciones Sanitarias (FIS-CP07/00016), Fundación para la Investigación y Educación en SIDA (FIES), Red de Investigación en SIDA (RIS, project ISCIII-RETIC RD06/006) and the European NEAT Project.
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