Keywords
Proximal Tubular function, Fractional excretion of phosphate, tenofovir disoproxil fumarate
Proximal Tubular function, Fractional excretion of phosphate, tenofovir disoproxil fumarate
Combination antiretroviral therapy (cART) has mortality and morbidity benefits in both adolescents and young adults1,2. An ART regimen containing the nucleotide reverse transcriptase inhibitor (NtRTI) tenofovir disoproxil fumarate (TDF) is preferred globally3,4
In Zambia, the use of TDF in children above five years of age, and adolescents was approved in 2010 but guidelines were revised in 2016 to include only adolescents principally due to TDF’s unknown effects on growth, kidney function and bone integrity in children5–9. Despite these concerns, TDF regimens remain the preferred formulation in the Zambian HIV guidelines for adolescents and young adults in line with global guidelines10. However, some adolescents and young adults are maintained on abacavir (ABC) regimens rather than switching to TDF by clinicians. In adults, TDF may increase the risk of proximal tubular dysfunction (PTD) among susceptible individuals4,11–13. Fractional Excretion of Phosphate (FePO4) is the percentage of glomerular filtered phosphate which is excreted into urine expressed as the ratio of the clearance of phosphate to creatinine clearance5,14. Therefore, PTD is associated with elevated urinary excretion of phosphate in the presence of hypophosphatemia11. FePO4 has been used to assess for PTD in people living with HIV (PLHIV) on tenofovir regimens5,15–18.
We investigated the effect of an ART regimen on proximal tubular function (PTF) as measured by FePO4 in adolescents and young adults living with HIV. We hypothesized that a TDF-regimen was associated with a decline in PTF.
We conducted a cross-sectional study involving adolescents and young adults receiving either TDF or ABC-based regimens at the University Teaching Hospitals (UTHs) in Lusaka, Zambia. SmartCare, a national HIV electronic medical database, was used to select eligible participants between May 2018 and March 2019 using a checklist to verify eligibility.
All participants were aged between 10 and 24 years who had received ART for a minimum of 12 months and had a documented normal estimated glomerular filtration rate (eGFR) at initiation of the current ART regimen. Participants with prior diagnosis of diabetes mellitus at the time of ART initiation, current use of prednisone (> 10 mg/day) or dexamethasone (> 1 mg/day) or any other corticosteroid, history of taking vitamin D supplementation within the past six months, or pregnancy at the time of enrolment into the study were excluded. Written informed consent and/or assent for participation in this study were obtained from each participant and/or parent or guardian. The study was approved by the University of Zambia Research Ethics Committee (REF. No. 010-01-18) and the Zambia National Health Research Authority. The study adhered to the Declaration of Helsinki
Individual participant data were collected from Smart Care, interview, and physical examination including current age, sex, current weight, body mass index (BMI), current ART regimen and duration, baseline CD4+ cell count, self-reported 85% ART adherence, previous or current tuberculosis, and baseline creatinine or estimated Glomerular Filtration Rate (eGFR).
Participants were requested to provide two first-morning spot urine specimens for urinary protein and creatinine levels to calculate a mean (2-sample) urine protein to creatinine ratio (UPCR). The second first-morning urine was obtained within 7–14 days of the first specimen. Each spot urine sample was analyzed for markers of tubular function, urinary β2-microglobulin, α1-microglobulin, creatinine, and urine phosphate. Urine β2-microglobulin (E-EL-H2188) and α1-microglobulin (E-EL-H0315) were determined by Enzyme-linked immunosorbent assays (ELISA) with detection range of 31.25—2000 ng/mL and 4.69–300 ng/mL. All reagents, samples and standards were prepared according to the instructions in the manufacturer’s manual (Wuhan Elabscience Biotechnology Co. Ltd, Donghu Hi-Tech Development Area, Wuhan, Hubei, China).
The urinary proteins were expressed as the ratio-to-creatinine (α1-MCR, β2-MCR, and UPCR) to adjust for variations in urine concentration.
Serum or plasma samples were analyzed for CD4+ cell count, HIV viral load, hemoglobin, calcium, potassium, uric acid, urea, phosphate, serum creatinine, and albumin.
1. The eGFR was calculated using the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) for participants 19 years and older19,20. For participants younger than 19 years, the Swartz formula was used21.
2. Reduction of renal function was classified according to the guidelines of the National Kidney Foundation: "mild" 60–89; "moderate" 30–59; "severe" 15–29; "renal failure" < 15 ml/min per 1.73 m221.
3. PTF was measured by FePO4. FePO4 was calculated as: FePO4 (%) = (Urine phosphate*Serum creatinine)/ (Serum phosphate*Urine creatinine) *100. PTD was defined by FePO4 ≥ 20% regardless of serum phosphate and FePO4 ≥ 10–20% when serum phosphate was below 0.81mmol/L17.
Data are presented as means and standard deviations for continuous variables if normally distributed, and median and inter quartile range (IQR) if skewed. Unpaired t-test or Mann-Whitney U test were used to compare continuous variables between the groups if the data were normally distributed or skewed respectively. Chi-square or Fisher’s exact test was used for categorical variables depending on the data distribution.
To predict risk factors for declining PTF as measured by FePO4, simple and multivariable linear regressions were used to assess the effect of the covariates on FePO4, with 95% confidence intervals (95% CI).
To explain confounding, we developed a model (Model 1) to predict PTF as measured by FePO4 that included clinically important or significant covariates at univariate analysis. Independent covariates that were significant at the 5% level were included in a multivariable linear model to adjust for the effects of these factors on PTF. STATA software, version 14.0 (Stata Corp 4905 Lakeway Dr, College Station, TX 77845) was used for statistical analyses.
The study population included 180 persons aged 10 to 24 years on ART for at least 12 months22. The median age was 15 years (IQR 19–11), and 55 percent (99 of 179) of participants were female. The median duration on ART was 36 months (IQR 54-26). The proportion of PTD as defined by FePO4 was six percent (11 of 179) as reflected in Table 1. PTD was more common among participants in the TDF group than the ABC group though not statistically significant (10% vs 2%, p=0.058). The median FePO4 (PTF) was higher in the TDF group than the ABC group (5.18% vs. 2.97%, P=<0.001).
N(%) Number (Percent); ART, Antiretroviral Therapy; ABC, abacavir; TDF, tenofovir ; CD4 cell, Cluster of Differentiation 4 cell; m2, Square meters; S- Phosphate, serum Phosphate; S-Urea, serum urea; S-Creatinine, serum Creatinine; eGFR, estimated Glomerular Filtration Rate; Urine β2-MCR, Urine β2-microglobulin Creatinine Ratio; Urine α1-MCR, Urine α1-microglobulin Creatinine Ratio; FePO4, Fractional Excretion of Protein; TmP/GFR, Maximum Tubular Re-absorption of Phosphate per Glomerular Filtration Rate; UPCR, Urine Protein Creatinine Ratio. P values for the unpaired t-test / Mann-Witney U test and chi-square/ Fishers’ exact test comparisons between groups are in the right most column
The mean serum phosphate was significantly lower in the TDF group (1.2 ± 0.26 mmol/L) than the ABC group (1.4 ± 0.24mmol/L; P<0.001). Likewise, the median urine phosphate was significantly higher in the TDF group (7.04 mmol/L; IQR12.66-3.87) than the ABC group (8.96 mmol/L; IQR15.74-4.94, p=0.050). Four (2.2%) participants (three in the TDF group) had eGFR < 90 mL/min/1.73m2 and no participant had an eGFR < 60 mL/min/1.73m2. Only one of the four participants with eGFR < 90 mL/min/1.73 m2 had FePO4 ≥20%.
Table 2 shows demographic and laboratory characteristics participants as stratified by PTF as defined by FePO4.
N (%) Number (Percent); ART, Antiretroviral Therapy; ABC, abacavir; TDF, tenofovir Diproxil Fumarate; CD4 cell, Cluster of Differentiation 4 cell; m2, square meters; S-Phosphate, serum Phosphate; S-Urea, serum urea; S-Creatinine, serum Creatinine; eGFR, estimated Glomerular Filtration Rate; Urine β2-MCR, Urine β2-microglobulin Creatinine Ratio; CRP, C-Reactive Protein; FePO4, Fractional Excretion of Phosphate; UPCR, Urine Protein Creatinine Ratio. P values for the unpaired t-test / Mann Whitney U test and chi-square/ fishers’ exact test comparisons between groups are in the right most column
The following factors were significantly different between participants with normal PTF (FePO4) and elevated FePO4 or PTD; median age (p<0.002), median BSA (p<0.045), mean serum urea (p<0.038), median serum creatinine (r=0.32, p<0.001), median urine-protein excretion (p<0.001), median UPCR (p<0.001), dipstick proteinuria (p=0.003), median urine α1-MCR (p=0.030),Statistical analysis revealed no significant differences between the two groups with respect to eGFR, urine β2-MCR, ART regimen, ART duration, serum uric acid, current CD4+ cell count, viral load and gender.
Three of the four study participants with hypophosphatemia (serum phosphate<0.81mmol/L), had PTD. All the four participants with hypophosphatemia were on a TDF regimen.
The following covariates were moderately positively correlated with PTF as measured by FePO4; age (r=0.33, p<0.0001), and BSA (r=0.32, P<0.0001) as shown in Figure 1. Other factors with weakly positive correlations with FePO4were fasting blood glucose (FBG) (r=0.16, p= 0.026), serum uric acid r=0.23, p=0.002), serum urea (r= 0.15, p=0.034), urine protein excretion (r=0.21 p=0.0043). The rest of the factors were not significantly correlated.
Age by cART (Panel 1.1); Serum Urea by cART (Panel1.2); Haycock BSA by ART (Panel 1.3); Urine β2-MCR by cART Regimen (Panel 1.4).
The crude effects of the covariates for prediction of FePO4 (PTF) are depicted in Table 3. A TDF regimen, dipstick proteinuria, age, BSA, serum urea, urine protein excretion, urine β2-MCR, and CRP were all associated with worsening FePO4 or PTF.
FePO4, Fractional Excretion of Phosphate; Urine β2-MCR, Urine β2-microglobulin Creatinine Ratio; CRP, C-Reactive Protein; Urine α1-MCR, Urine α1-microglobulin Creatinine Ratio; UPCR, Urine Protein Creatinine Ratio; ART=Antiretroviral Therapy; TDF, tenofovir Diproxil Fumarate; BSA, body surface are; S-phosphate, Serum phosphate; U-phosphate 2, Urine phosphate; S-Uric acid, Serum Uric acid; eGFR, estimated Glomerular Filtration Rate; U-Dipstick; Urine Dipstick; U-Protein Excretion, Urine Protein Excretion, S-Creatinine, Serum Creatinine; FBS, Fasting Blood Sugar; TmPGFR, Maximal Tubular reabsorption of phosphate per glomerular filtration rate; β-coef, Beta-coefficient; T, T-Statistic; 95% CI, 95% confidence Interval. P-value ≤ 0.050 was considered significant
The adjusted effects of the significant or clinically important covariates associated with FePO4 at univariate analysis are shown in Table 4. After adjusting for sex, age, body, BSA, ART regimen, serum uric acid, urine β2-MCR, CRP and UPCR, a TDF regimen was not associated with worsening FePO4 or PTF. Age (coef 0.637, 95% CI 0.612-1.113) and urine-β2_MCR (coef 0.010, 95% CI 0.006-0.014) were associated with worsening FePO4 or PTF. The other covariates independently associated with worsening FePO4 or PTF, were UPCR (coef 0.013, 95% CI 0.001-0.025, and CRP (coef 0.155, 95% CI 0.033-0.277). Figure 2 shows the prediction of FePO4 with Urine-β2 MCR and age. ART regimen showed a big difference in the crude analysis (Coef 3.72, 95% CI 1.45-6.02, p=0.002) but little difference in the adjusted analysis (Coef -2.3, 95% CI -6.93-2.07, p=0.287). CRP, Haycock, and urea also showed big changes between crude and adjusted analysis. When ART regimen was only adjusted for age, CRP, urea and haycock, ART regimen was only confounded by age.
FePO4, Fractional Excretion of Phosphate; Urine β2-MCR, Urine β2-microglobulin Creatinine Ratio; CRP, C-Reactive Protein; UPCR, Urine Protein Creatinine Ratio; ART=Antiretroviral Therapy; TDF, tenofovir Diproxil Fumarate; BSA, body surface area; S-phosphate 1, Serumphosphatespline 1; S-phosphate 2, Serumphosphatespline 2; S-Uric acid, Serum Uric acid; bsa, body surface area;β-coef, Beta-coefficient; Std. Err, Standard Error; 95% CI, 95% confidence Interval. P-value ≤ 0.050 was considered significant
In this cross-sectional study among black African adolescents and young adults living with HIV on two different ART regimens, the prevalence of PTD as measured by FePO4 was 6%. We found a higher prevalence of isolated PTD in the TDF group (10%) than the ABC group (2%), but this was non-significant after adjusting for age.
The clinical implication of subclinical PTD without progressive reduction in eGFR remains obscure5,19,20. In fact, only four participants had mild reduction of glomerular function (eGFR<90mls/mL/1.73m2), and three of these were on TDF with normal PTF implying that reduced eGFR was probably independent of PTD.
A tenofovir regimen was not associated with a worsening PTF after adjusting for confounding. This was unexpected finding as TDF exposure has been implicated with urine phosphate wasting13,23. Although, some studies show no association between TDF and urine phosphate wasting5,12,24,25, our study was compromised by the substantial age difference between the groups and further analysis showed that age confounded the effect of ART regimen on PTF. Therefore, it is likely that a randomized controlled trial is required to answer this question definitively. Our study suggests that a position of equipoise is present - we have no evidence that TDF is associated with worsened tubular dysfunction - and therefore a clinical trial is justified.
Of concern was the lower serum phosphate and higher urine phosphate in the TDF group than the ABC group respectively. In fact, all the four patients that had low serum phosphate were on TDF and three had PTD implying that TDF may be contributing significantly to urinary phosphate losses resulting in hypophosphatemia with possible long-term implications for bone health.
Nonetheless, available evidence indicates that hypophosphatemia associated with TDF maybe transient and normalizes during ART21. Pontrelli et al. among 49 adolescents showed normalization of serum phosphate levels without interruption of therapy at 2 years of follow-up after an initial significant decline in the groups receiving TDF. However, the authors in their conclusion still thought that the long-term clinical impact of a TDF regimen would be answered by a prospective study with a longer follow up period21.
There was no significant difference between the two groups with respect to eGFR. This finding is consistent with other studies in adolescents and young adults PLHIV that did not find significant changes in eGFR between participants on a TDF versus non-TDF regimens5,11,13,21,24. Of note, our study population was younger without co-morbidities like diabetes mellitus and hypertension which are risk factors for TDF-induced renal dysfunction in adults12 which partly explains our findings.
Urine β2-microglobulin creatinine ratio (β2-MCR) has been reported in some studies to be a marker of PTD in PLHIV6,23,26. We also report an independently significant association between urine β2-MCR and worsening PTF or FePO4. Unlike our study, there are also reports of no association of urine β2-MCR with PTF27.
The reasons for the lack of association between Urine β2-MCR and tubular dysfunction are unclear in these studies, except that urinary pH may have played a role. Studies that did not control urine pH by treating the urine samples with acid or alkali at the time of urine collection reported negative results27.
Likewise, protein excretion is enhanced in PLHIV with subclinical PTD without glomerular function impairment3.
In univariable analysis, dipstick proteinuria, protein excretion and UPCR were all associated with worsening PTF or FePO4. UPCR, the only urine protein covariate considered in multivariable analysis, was independently associated with worsening PTF orFePO4. Our findings are comparable with other studies that considered proteinuria to be a marker of PTD3. Therefore, proteinuria may be a cheaper tool to use in resource-limited settings to monitor for early PTD. CRP was independently associated with elevated FePO4, indicating the possible role of inflammation in PTD12,28.
Our study’s strengths included a relatively larger sample size than most pediatric studies, and a comparison of participants on two different ART regimens to control for confounding. The median duration of 36 months on ART allowed us to assess the medium to long-term renal impact of TDF exposure. Furthermore, because our participants were drawn from patients who receive ART in the general HIV care programs, even though at a tertiary care level, our findings may be generalized to the general population of adolescents and young adults with HIV.
There were also limitations that affected our study findings. As this was a cross sectional study, we could not compare our findings with the pre-ART PTF because in our setting, FePO4 measurements are not done routinely. As earlier stated, the big difference in age between the two groups confounded the effects of ART on PTF. Therefore, more research is needed to investigate if TDF is associated with PTD in adolescents with similar age structure.
The majority (>90%) of patients in the TDF group were previously switched from ABC, so we don’t know the impact of this on our findings. None of our participants reported using nephrotoxic medications like non-steroidal anti-inflammatory drugs, but we did not collect information on use of such medications, so there is possibility of some residual confounding.
Even though subclinical PTD was more common among participants in the TDF group than the ABC group, a TDF regimen was not associated with worsening PTF as measured by FePO4. Age was a confounder on the effect of ART on PTF and therefore, further research is required. CKD was very rare with only four participants with mildly reduced eGFR. Of concern, though not significant, were the four participants on TDF with hypophosphatemia
Figshare: Effect of anti-retroviral regimen on proximal tubular function in Zambian adolescents and young adults living with HIV, https://doi.org/10.6084/m9.figshare.22044047.v122.
This project contains the following underlying data:
Data are available under the terms of the Creative Commons Zero "No rights reserved" data waiver (CC0 1.0 Public domain dedication).
We acknowledge the hospital team that helped with data collection especially Sr Beauty Kanfwa.
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Is the work clearly and accurately presented and does it cite the current literature?
Yes
Is the study design appropriate and is the work technically sound?
Yes
Are sufficient details of methods and analysis provided to allow replication by others?
Yes
If applicable, is the statistical analysis and its interpretation appropriate?
Partly
Are all the source data underlying the results available to ensure full reproducibility?
Yes
Are the conclusions drawn adequately supported by the results?
Yes
Competing Interests: No competing interests were disclosed.
Reviewer Expertise: HIV
Is the work clearly and accurately presented and does it cite the current literature?
Partly
Is the study design appropriate and is the work technically sound?
Partly
Are sufficient details of methods and analysis provided to allow replication by others?
Yes
If applicable, is the statistical analysis and its interpretation appropriate?
Partly
Are all the source data underlying the results available to ensure full reproducibility?
Yes
Are the conclusions drawn adequately supported by the results?
Yes
Competing Interests: No competing interests were disclosed.
Reviewer Expertise: - HIV pharmacotherapy, Clinical Research
Alongside their report, reviewers assign a status to the article:
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Version 1 20 Mar 23 |
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