Predicting immune risk in treatment-naïve HIV patients using a machine learning algorithm: a decision tree algorithm based on micronutrients and inversion of the CD4/CD8 ratio

Introduction

Micronutrients have significant functional implications for the human immune response, and the quality of food is a major factor affecting the severity and mortality caused by HIV in individuals undergoing antiretroviral therapy. A decrease in CD4 lymphocyte count and an increase in CD8 lymphocyte count are the hallmarks of HIV infection, which causes the CD4/CD8 ratio to invert from a normal value of >1.6 to <1.0. In this study, we tried to analyze whether the nutritional status of HIV-positive patients has an impact on the CD4/CD8 ratio inversion by utilizing a machine learning (ML) algorithm.

Methods

In this study, 55 confirmed HIV-positive patients who had not started their anti-retroviral therapy were included after obtaining their informed, written consent. Moreover, 55 age-and sex-matched relatives and caregivers of the patients who tested negative in the screening were enrolled as controls. All individual patient data points were analyzed for model development with an 80-20 train-test split. Four trace elements, zinc (Zn), phosphate (P), magnesium (Mg), and calcium (Ca), were utilized by implementing a random forest classifier. The target of the study was the inverted CD4/CD8 ratio.

Results

The data of 110 participants were included in the analysis. The algorithm thus generated had a sensitivity of 80% and a specificity of 83%, with a likelihood ratio (LR+) of 4.8 and LR-of 0.24. The utilization of the ML algorithm adds to the limited evidence that currently exists regarding the role of micronutrients, especially trace elements, in the causation of immune risk. Our inherent strength lies in the fact that this study is one of the first studies to utilize an ML-based decision tree algorithm to classify immune risk in HIV patients.

Conclusion

Our study uniquely corroborated the nutritional data to the immune risk in treatment-naïve HIV patients through the utilization of a decision tree ML algorithm. This could subsequently be an important classification and prognostic tool in the hands of clinicians.

Vitamin D and Calcium Supplementation Reverses Tenofovir-Caused Bone Mineral Density Loss in People Taking ART or PrEP: A Systematic Review and Meta-Analysis

Background

The decrease of bone mineral density (BMD) after the intake of Tenofovir disoproxil fumarate (TDF)-based drugs in people living with HIV/AIDS (PLWHA) and HIV-negative key populations under pre-exposure prophylaxis (PrEP) regimen raised concerns. Previous findings on the effects of vitamin D (VD) and calcium supplements and the recovery of BMD loss were inconclusive. The optimal doses of VD and calcium and its supplementary duration remained unknown. Therefore, we conducted a systematic review and meta-analysis to synthesize current evidence on VD and calcium supplements to inform clinical practice.

Methods

We searched PubMed, Web of Science, Cochrane library, and EMBASE databases for all placebo-controlled trials and prospective cohort studies published before March 5, 2021 that investigated VD and calcium supplements in participants taking TDF-based drugs. The keywords calcium, vitamin D, Tenofovir, and BMD were used for the searches. The primary outcome was changes of spine and hip BMD. A subgroup analysis was performed to determine the factors that were related to the effects of VD supplements on BMD. Locally weighted regression (loess) was used to determine the relationships of VD supplements, supplementary duration, and changes of BMD. This study was registered at PROSPERO (No. 42021231000).

Findings

Seven eligible studies including 703 participants were included in the analyses. The meta-analysis found that VD and calcium supplementation was related to a significant increase of BMD in the spine and hip [standardized mean difference (SMD) 0.43; 95% CI, 0.25 to 0.61, p = 0.009]. Moreover, positive dose-response relationships were demonstrated between doses of VD and calcium supplements, supplementary duration, and BMD recovery. Patients who took VD with the dose level of 4,000 IU/D obtained the highest BMD improvement (SMD 0.59, 95% CI, 0.43 to 0.74). No side effects were reported on VD and calcium supplementation.

Interpretation

We found the VD and calcium supplementation was associated with increases of BMD in participants taking TDF-based drugs. An optimal supplementary dose of 4,000 IU/D for VD was suggested for clinicians. The findings could be used in clinical practice to improve the BMD outcomes in people who were taking TDF-based drugs.Systematic Review Registration: https://www.crd.york.ac.uk/PROSPERO/.

Vitamin D and Calcium Supplement Attenuate Bone Loss among HIVInfected Patients Receiving Tenofovir Disoproxil Fumarate/Emtricitabine/ Efavirenz: An Open-Label, Randomized Controlled Trial

BACKGROUND

Antiretroviral therapy (ART), especially with tenofovir disoproxil fumarate (TDF), has been associated with accelerated bone turnover and leads to significant bone loss.

OBJECTIVE

We aimed to determine the effect of vitamin D2 and calcium on bone mineral density (BMD) in HIV-infected patients receiving TDF/emtricitabine (FTC)/efavirenz (EFV).

METHODS

A prospective, open-label, randomized controlled study was conducted. Eligible patients were ART naïve HIV individuals who initiated TDF/FTC/EFV. The study group received supplementation with vitamin D2 and calcium carbonate, whereas the control group was administered only ART. The primary outcome was the percentage change in total hip BMD at week 24 compared with baseline.

RESULTS

A total of 18 patients were randomized (9 in each group). The mean (standard deviation; SD) total hip BMD significantly decreased from baseline in both groups, from 0.96 (0.14) g/cm2 to 0.93 (0.13) g/cm2 in the study group (p = 0.006) and from 0.87 (0.11) g/cm2 to 0.84 (0.11) g/cm2 in the control group (p = 0.004). The mean (SD) lumbar spine BMD significantly decreased from baseline in both groups, from 1.00 (0.13) g/cm2 to 0.97 (0.13) g/cm2 (p = 0.004) in the study group and from 0.90 (0.09) g/cm3 to 0.86 (0.08) g/cm2 in the control group (p = 0.006). At week 24, the mean (SD) lumbar spine BMD was significantly greater in the study group than in the control group (p = 0.042). However, there were no significant differences in the percentage change of total hip, lumbar spine, and femoral neck BMD between both groups. No adverse events were reported. In conclusion, as early as 24 weeks after TDF initiation, a significant decline in BMD was detected.

CONCLUSION

Vitamin D2 and calcium supplements should be considered for HIV-infected patients receiving TDF/FTC/EFV in a resource-limited setting where there are limited ART options (Clinicaltrials. gov NCT0287643).

Expression of Inflammatory Markers RANK, MMP-9 and PTHrP in Chronic Apical Periodontitis from People Living with HIV Undergoing Antiretroviral Therapy

To compare the expression of the receptor activator of nuclear factor-kappa B (RANK), matrix metalloproteinase 9 (MMP-9) and parathyroid hormone-related protein (PTHrP) in primary chronic apical periodontitis lesions (CAPLs) between people living with HIV (PLWHIV) undergoing antiretroviral therapy (ART) and HIV- individuals, 32 CAPLs (16 lesions from each group) were submitted to histopathological and immunohistochemical analyses and compared between groups. The majority of the PLWHIV group had undetectable plasma viral loads (n = 13; 81.3%). The means of TCD4+ lymphocytes, exposure to HIV-1 and the time of the use of ART were 542.1 cells/mm³ (SD = 256.4), 6.3 years (SD = 2.9) and 5.0 years (SD = 2.5), respectively. Of all variables studied, only histopathological diagnosis showed a significant difference between groups (LWHIV: granuloma n = 11 (68.0%); cyst n = 5 (31.2%); HIV-: granuloma n = 15 (93.8%); cyst n = 1 (6.2%); p = 0.015). When comparing the expressions of the three inflammatory markers between the groups, no significant differences were seen. There was no difference in the expression of RANK, PTHrP and MMP-9 in primary chronic apical periodontitis lesions between PLWHIV under ART and HIV- individuals.

Pharmacogenetic determinants of kidney-associated urinary and serum abnormalities in antiretroviral-treated HIV-positive patients

Tenofovir disoproxyl fumarate (TDF) has been associated with renal tubular abnormalities, phosphaturia and proteinuria (retinol binding protein, RBP, loss): vitamin D (VD) and PTH affect these markers. Aim was to understand if some single nucleotide polymorphisms (SNPs) were predictors of renal abnormalities in an Italian cohort of HIV-affected patients. DNA was analyzed through real-time PCR, urinary RBP corrected by creatinine (uRBP/Cr). The majority of patients received TDF. Abnormal uRBP/Cr was more frequent in TDF recipients: eGFR <90 mL/min and TDF were predictors in the whole cohort, whereas eGFR <90 mL/min, TDF concentrations and CYP24A1-3999TT in TDF-treated patients. Phosphate levels were higher low VD level patients: age <50 years, CYP27B1 + 2838CC genotype and non-European ancestry were predictors. PTH levels were border-line higher in TDF patients: non-European ancestry, females, TDF, VD levels < 30 ng/mL and SLC28A2-124CT/TT and ABCC2-24CC were predictors. For the first time, SNPs were associated with PTH, phosphate, calcium and tubular dysfunction in HIV-infected patients.

Vitamin D3 Supplementation Increases Spine Bone Mineral Density in Adolescents and Young Adults With Human Immunodeficiency Virus Infection Being Treated With Tenofovir Disoproxil Fumarate: A Randomized, Placebo-Controlled Trial

Background

Tenofovir disoproxil fumarate (TDF) decreases bone mineral density (BMD). We hypothesized that vitamin D3 (VITD3) would increase BMD in youth receiving TDF.

Methods

This was a randomized, double-blind, placebo-controlled trial of directly observed VITD3 vs placebo every 4 weeks for 48 weeks in youth aged 16-24 years with HIV, RNA load <200 copies/mL, taking TDF-containing combination antiretroviral therapy (TDF-cART) for ≥180 days. Participants (N = 214) received a daily multivitamin containing VITD3 400 IU and calcium 162 mg, plus monthly randomized VITD3 50000 IU (n = 109) or placebo (n = 105). Outcome was change from baseline to week 48 in lumbar spine BMD (LSBMD). Data presented are median (Q1, Q3).

Results

Participants were aged 22.0 (21.0, 23.0) years, 84% were male, and 74% were black/African American. At baseline, 62% had 25-hydroxy vitamin D (25-OHD) <20 ng/mL. Multivitamin adherence was 49% (29%, 69%), and VITD3/placebo adherence 100% (100%, 100%). Vitamin D intake was 2020 (1914, 2168) and 284 (179, 394) IU/day, and serum 25-OHD concentration was 36.9 (30.5, 42.4) and 20.6 (14.4, 25.8) ng/mL at 48 weeks in VITD3 and placebo groups, respectively (P < .001). From baseline to week 48, LSBMD increased by 1.15% (-0.75% to 2.74%) in the VITD3 group (n = 99; P < .001) and 0.09% (-1.49% to 2.61%) in the placebo group (n = 89; P = .25), without between-group difference (P = .12). VITD3 group changes occurred with baseline 25-OHD <20 ng/mL (1.17% [-.82% to 2.90%]; P = .004) and ≥20 ng/mL (0.93% [-.26% to 2.15%]; P = .033).

Conclusions

For youth taking TDF-cART, LSBMD increased through 48 weeks with VITD3 plus multivitamin, but not with placebo plus multivitamin, independent of baseline vitamin D status.

Clinical Trials Registration

NCT01751646.

Effect of Vitamin D Supplementation on Bone Turnover Markers During HIV Pre-Exposure Prophylaxis Using Tenofovir Disoproxil Fumarate-Emtricitabine in Men Who Have Sex with Men

Pre-exposure prophylaxis (PrEP) with tenofovir disoproxil fumarate-emtricitabine (TDF-FTC) reduces bone mineral density in HIV-uninfected men who have sex with men (MSM). We hypothesized that PrEP with TDF-FTC would increase bone turnover markers (BTMs) at week 24 and that vitamin D supplementation from weeks 24 to 48 would blunt this increase. Participants were from a cohort of 398 MSM and transgender women who received daily TDF-FTC for PrEP. At week 24, a prospective intervention group initiated vitamin D3 4,000 IU daily. Concurrent controls were selected from the cohort who took ≤400 IU/day of vitamin D3 matched by age, race, and body mass index. The primary endpoint was the change in procollagen-I N-terminal propeptide (P1NP) from weeks 24 to 48. Paired t-tests were used to compare changes in BTMs between intervention and controls. Among 48 intervention-control pairs, median age was 33 years. At baseline, 68.9% of the intervention group and 77.3% of controls were vitamin D sufficient (≥20 ng/mL, p = .94). P1NP, C-telopeptide, parathyroid hormone (PTH), and 25-OH vitamin D3 did not increase significantly at week 24. P1NP fell by a mean ± SD of -27.6 ± 49.9 pg/mL from weeks 24 to 48 with vitamin D and -2.5 ± 40.2 pg/mL in controls (p = .01). There were no significant between-group differences in the weeks 24-48 change in C-telopeptide, PTH, or 25-OH vitamin D3. Vitamin D3 supplementation with 4,000 IU/day resulted in a significant reduction in the BTM P1NP compared with controls, suggesting that this intervention has potential to improve bone health during PrEP.

Tenofovir Disoproxil Fumarate Is Associated with a Set-Point Variation in the Calcium-Parathyroid Hormone-Vitamin D Axis: Results from a German Cohort

Background

Higher levels of parathyroid hormone have been associated with the use of tenofovir disoproxil fumarate (TDF) in people with and without HIV infection. Yet, alterations in calcium levels have never been elucidated in detail.

Objective

To compare the association of parathyroid hormone with serum calcium levels and other markers of calcium and bone metabolism in people living with HIV on TDF- and non-TDF-containing antiretroviral therapy.

Patients and Methods

A retrospective single center cohort study in Munich, Germany. Median and interquartile ranges and absolute and relative frequencies were used to describe continuous and categorical variables, respectively. The Mann–Whitney U test and chi²-test were used for comparisons. Multivariate median regression was performed in a stepwise backward approach.

Results

1,002 patients were included (786 (78.4%) male; median age 48 (40–55) years). 564 patients (56.3%) had a TDF-containing ART regimen. PTH concentrations were 46.9 (33.0–64.7) pg/mL and 35.2 (26.4–55.4) pg/mL (P=0.001), 43.3 (30.8–59.8) pg/mL and 31.8 (22.3–49.6) pg/mL (P < 0.001), 46.1 (29.5–65.4) pg/mL and 33.4 (22.6–50.1) pg/mL (P < 0.001), and 37.8 (25.3–57.9) pg/mL and 33.8 (20.1–45.3) pg/mL (P=0.012) within the first, second, third, and fourth quartile of corrected calcium levels for patients with and without TDF-containing ART, respectively. In multivariate median regression, PTH concentration was significantly associated with Ca_corr. (−32.2 (−49.8 to −14.8); P < 0.001), female sex (5.2 (1.2–9.2); P=0.010), 25(OH)D (−0.4 (−0.5 to −0.3); P < 0.001), and TDF-use (9.2 (6.0–12.5); P < 0.001).

Discussion

Higher levels of PTH seem to be needed to maintain normal calcium levels in PLWH on TDF-containing ART compared to non-TDF-containing ART. Optimal concentrations for 25-hydroxy vitamin D and calcium might therefore be different in people using TDF than expected from general populations but also people living with HIV with non-TDF-containing antiretroviral therapy. This might require different supplementation strategies but warrants further investigation.

Zoledronic acid is superior to tenofovir disoproxil fumarate-switching for low bone mineral density in adults with HIV

OBJECTIVE

To compare the effects of switching tenofovir disoproxil fumarate (TDF) or treatment with an intravenous bisphosphonate on bone mineral density (BMD) in HIV-positive adults with low bone mass.

DESIGN

Two-year, randomized, open-label study at 10 sites in Australia and Spain.

PARTICIPANTS

Of 112 adults on TDF-based antiretroviral therapy (ART) screened, 87 with low BMD (T-score < -1.0 at hip or spine by dual-energy X-ray absorptiometry) and undetectable plasma HIV viral load were randomized to either switch TDF to another active antiretroviral drug or to continue TDF-based ART and receive intravenous zoledronic acid (ZOL) 5 mg annually for 2 years.

PRIMARY OUTCOME MEASURE

Change in lumbar spine BMD at 24 months by intention-to-treat analysis. Secondary outcomes included changes in femoral neck and total hip BMD, fractures, safety, and virological failure.

RESULTS

Forty-four participants were randomized to TDF switch and 43 to ZOL, mean age 50 years (SD 11), 96% men, mean TDF duration 5.9 years (SD 3.1), and mean spine and hip T-scores -1.6 and -1.3, respectively. At 24 months, mean spine BMD increased by 7.4% (SD 4.3%) with ZOL vs. 2.9% (SD 4.5%) with TDF-switch (mean difference 4.4%, 95% CI 2.6-6.3; P < 0.001). Mean total hip BMD increased by 4.6 (SD 2.6%) and 2.6% (SD 4%), respectively (mean difference 1.9%, 95% CI 0.5-3.4; P = 0.009). There was one fracture in the ZOL group vs. seven fractures in four TDF-switch participants. Virological failure occurred in one TDF-switch participant. Other safety endpoints were similar.

CONCLUSION

ZOL is more effective than switching TDF at increasing BMD in HIV-positive adults with low bone mass.

Bone Loss in HIV Infection

Human immunodeficiency virus (HIV) infection is an established risk factor for low bone mineral density (BMD) and subsequent fracture, and treatment with combination antiretroviral therapy (cART) leads to additional BMD loss, particularly in the first 1-2 years of therapy. The prevalence of low BMD and fragility fracture is expected to increase as the HIV-infected population ages with successful treatment with cART. Mechanisms of bone loss in the setting of HIV infection are likely multifactorial, and include viral, host, and immune effects, as well as direct and indirect effects of cART, particularly tenofovir disoproxil fumarate (TDF) and the protease inhibitors (PIs). Emerging data indicate that BMD loss following cART initiation can be mitigated by prophylaxis with either long-acting bisphosphonates or vitamin D and calcium supplementation. In addition, newer antiretrovirals, particularly the integrase strand transfer inhibitors and tenofovir alafenamide (TAF), are associated with less intense bone loss than PIs and TDF. However, further studies are needed to establish optimal bone sparing cART regimens, appropriate screening intervals, and preventive measures to address the rising prevalence of fragility bone disease in the HIV population.

Decline in Bone Mass With Tenofovir Disoproxil Fumarate/Emtricitabine Is Associated With Hormonal Changes in the Absence of Renal Impairment When Used by HIV-Uninfected Adolescent Boys and Young Men for HIV Preexposure Prophylaxis

BACKGROUND

We aimed to define the relative importance of renal and endocrine changes in tenofovir disoproxil fumarate (TDF)-related bone toxicity.

METHODS

In a study of daily TDF/emtricitabine (FTC) preexposure prophylaxis (PrEP) in human immunodeficiency virus (HIV)-uninfected young men who have sex with men, we measured changes from baseline in blood and urine markers of the parathyroid hormone (PTH)-vitamin D-fibroblast growth factor 23 (FGF23) axis, creatinine, and renal tubular reabsorption of phosphate (TRP). We explored the relationship of those variables to changes in bone mineral density (BMD). Tenofovir-diphosphate (TFV-DP) in red blood cells was used to categorize participants into high and low drug exposure groups.

RESULTS

There were 101 participants, median age 20 years (range 15 to 22). Compared with low drug exposure, high-exposure participants showed increase from baseline in PTH and decline in FGF23 by study week 4, with no differences in creatinine, phosphate, or TRP. At 48 weeks, the median (interquartile range) percent decline in total hip BMD was greater in those with high- compared to low- exposure (-1.59 [2.77] vs +1.54 [3.34] %, respectively; P = .001); in high-exposure participants, this correlated with week 4 TFV-DP (inversely; r = -0.60, P = .002) and FGF23 (directly; r = 0.42; P = .039) but not other variables.

CONCLUSIONS

These findings support the short-term renal safety of TDF/FTC PrEP in HIV-seronegative young men and suggest that endocrine disruption (PTH-FGF23) is a primary contributor to TDF-associated BMD decline in this age group.

CLINICAL TRIALS REGISTRATION

NCT01769469.

Validation Protocol of Vitamin D Supplementation in Patients with HIV-Infection

Hypovitaminosis D and secondary hyperparathyroidism are frequent among HIV-infected patients. As there are no data about the best supplementation therapy both in treatment and in maintenance, we conducted an observational study of 300 HIV-infected patients for whom vitamin D and parathormone (PTH) had been measured in order to validate a protocol of vitamin D supplementation in patients with HIV-infection. Patients with vitamin D deficiency (defined as 25(OH)D < 10 ng/mL), insufficiency (defined as 25(OH)D < 20 ng/mL), or hyperparathyroidism (PTH > 65 pg/mL) were supplemented with cholecalciferol 16.000IU (0.266 mg) weekly (if deficiency) or fortnightly (if insufficiency or high PTH levels). Rates of normalization of 25(OH)D (levels above 20 ng/mL) and PTH levels (<65 pg/mL) were analyzed. Multivariate analysis of factors related to normalization was carried out. With a median follow-up of 2 years, 82.1% of patients with deficiency and 83.9% of cases with insufficiency reached levels above 20 ng/mL. However, only 67.2% of individuals with hyperparathyroidism at baseline reached target levels (<65 pg/mL). Independent factors for not achieving PTH objective were tenofovir (TDF) and protease inhibitors use. In HIV-infected patients with hypovitaminosis, the protocol of cholecalciferol supplementation normalized vitamin D levels regardless of antiretroviral regimen in a high proportion of patients but it was less effective to correct hyperparathyroidism.

Vitamin D3 supplementation increases fibroblast growth factor-23 in HIV-infected youths treated with tenofovir disoproxil fumarate

BACKGROUND

Tenofovir (TDF) is associated with phosphaturia and elevated 1,25 dihydroxy vitamin D (1,25-OH(2)D). Fibroblast growth factor 23 (FGF23) causes phosphaturia and increases in response to elevated 1,25-OH(2)D. Vitamin D-binding protein (VDBP) binds to 1,25-OH(2)D, decreasing its biological activity, and is elevated in individuals with higher plasma tenofovir concentrations. We compared FGF23 and VDBP before and after vitamin D3 (VITD) supplementation in youths treated with combination antiretroviral therapy (cART) containing or not containing TDF.

METHODS

A randomized controlled trial in HIV-positive youths aged 18-25 years enrolled participants based on cART treatment with TDF (TDF; n=118) or without TDF (no-TDF; n=85), and randomized within those groups to VITD (50,000 IU every 4 weeks) or placebo (PL). We measured FGF23 and VDBP and calculated free 1,25-OH(2)D at baseline and week 12, and compared changes by TDF treatment and VITD randomized group.

RESULTS

At baseline, serum FGF23 concentration showed a quadratic relationship with 1,25-OH(2)D most pronounced in the TDF group. At week 12, total and free 1,25-OH(2)D increased in the VITD but not PL groups, independent of TDF use. FGF23 increased in the TDF group receiving VITD, but there was no FGF23 change in the no-TDF group receiving VITD or the PL groups. The adjusted mean change in FGF23 from baseline to week 12 was 7.7 pg/ml in the TDF/VITD group, compared with -1.7 (no-TDF/VITD, P=0.010), -1.3 (TDF/PL, P=0.006) and 1.1 (no-TDF/PL, P=0.035).

CONCLUSIONS

These results suggest that TDF-containing cART may alter the FGF23 response to vitamin D supplementation in HIV-infected youths. Clinical trials number: NCT00490412.

Association of higher plasma vitamin D binding protein and lower free calcitriol levels with tenofovir disoproxil fumarate use and plasma and intracellular tenofovir pharmacokinetics: cause of a functional vitamin D deficiency?

Tenofovir disoproxil fumarate (TDF) causes bone, endocrine, and renal changes by an unknown mechanism(s). Data are limited on tenofovir pharmacokinetics and these effects. Using baseline data from a multicenter study of HIV-infected youth on stable treatment with regimens containing TDF (n = 118) or lacking TDF (n = 85), we measured cross-sectional associations of TDF use with markers of renal function, vitamin D-calcium-parathyroid hormone balance, phosphate metabolism (tubular reabsorption of phosphate and fibroblast growth factor 23 [FGF23]), and bone turnover. Pharmacokinetic-pharmacodynamic associations with plasma tenofovir and intracellular tenofovir diphosphate concentrations were explored among those receiving TDF. The mean age was 20.9 (standard deviation [SD], 2.0) years; 63% were male; and 52% were African American. Compared to the no-TDF group, the TDF group showed lower mean estimated glomerular filtration rates and tubular reabsorption of phosphate, as well as higher parathyroid hormone and 1,25-dihydroxy vitamin D [1,25-OH(2)D] levels. The highest quintile of plasma tenofovir concentrations was associated with higher vitamin D binding protein, lower free 1,25-OH(2)D, higher 25-OH vitamin D, and higher serum calcium. The highest quintile of intracellular tenofovir diphosphate concentration was associated with lower FGF23. Higher plasma tenofovir concentrations were associated with higher vitamin D binding protein and lower free 1,25-OH(2)D, suggesting a functional vitamin D deficiency explaining TDF-associated increased parathyroid hormone. The finding of lower FGF23 accompanying higher intracellular tenofovir diphosphate suggests that different mechanisms mediate TDF-associated changes in phosphate handling. Separate pharmacokinetic properties may be associated with distinct TDF toxicities: tenofovir with parathyroid hormone and altered calcium balance and tenofovir diphosphate with hypophosphatemia and FGF23 regulation. (The clinical trial registration number for this study is NCT00490412 and is available online at http://clinicaltrials.gov/ct2/show/NCT00490412.).