From NATAP:TDF, Functional Vitamin D Deficiency?

[StephenCastlecrag]

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?
"The finding of a 26% increase in vitamin D binding protein and a 42% decrease in free 1,25-OH(2)D between the lowest and highest quintiles of plasma tenofovir has not been previously reported.
It has been suggested that TDF-associated renal toxicity causes elevations in parathyroid hormone (17, 48). A lower eGFR is associated with parathyroid hormone elevations in both children and adults with early renal failure (38). While TDF use was associated with a decrease in eGFR and increase in parathyroid hormone, 1,25-OHD was higher in the TDF than in the no-TDF group. In patients with mild renal insufficiency, 1,25-OH(2)D decreases (38). This finding argues against the hypothesis that glomerular damage is causative in TDF-associated increases in parathyroid hormone."

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Overton ET, Chan ES, Brown TT, et al. High-dose vitamin D and calcium attenuates bone loss with ART initiation: results from ACTG A5280. CROI 2014…..reduced total hip bone loss 50%. At week 48 total hip BMD had dropped 3.19% in the placebo group versus 1.46% in the D/calcium group, a significant difference (P < 0.001). Lumbar spine declines were 2.91% with placebo and 1.41% with D/calcium, a difference that stopped short of statistical significance (P = 0.085).
During the question-and-answer session after the presentation attendees wondered whether a lower vitamin D dose may be effective (especially for people with moderate vitamin D deficits), whether calcium might be dropped from the regimen in light of its potentially negative cardiovascular impact [2]…..48 weeks of 4000 IU of vitamin D plus 1000 mg of calcium carbonate daily….reduced total hip bone loss 50%…..Concentrations of 25-OH vitamin D rose significantly in the D/calcium arm, from 28.4 ng/mL at baseline to 56.4 ng/mL at week 48…...http://www.natap.org/2014/CROI/croi_50.htm

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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?
Peter L. Havens,a Jennifer J. Kiser,b Charles B. Stephensen,c Rohan Hazra,d Patricia M. Flynn,e Craig M. Wilson,f Brandy Rutledge,g James Bethel,g Cynthia G. Pan,a Leslie R. Woodhouse,c Marta D. Van Loan,c Nancy Liu,g Jorge Lujan-Zilbermann,h Alyne Baker,i Bill G. Kapogiannis,d Catherine M. Gordon,j Kathleen Mulligan,k the Adolescent Medicine Trials Network for HIV/AIDS Interventions (ATN) 063 Study Team
Children’s Research Institute, Medical College of Wisconsin, Children’s Hospital of Wisconsin, Milwaukee, Wisconsin, USAa; University of Colorado, Skaggs School of Pharmacy and Pharmaceutical Sciences, Department of Pharmaceutical Sciences, Aurora, Colorado, USAb; United States Department of Agriculture–Agricultural Research Service Western Human Nutrition Research Center, Davis, California, USAc; Eunice Kennedy Shriver National Institute of Child Health and Human Development, Bethesda, Maryland, USAd; St. Jude Children’s Research Hospital, Memphis, Tennessee, USAe; University of Alabama at Birmingham, Birmingham, Alabama, USAf; Westat, Rockville, Maryland, USAg; University of South Florida College of Medicine, Tampa, Florida, USAh; Tulane University, New Orleans, Louisiana, USAi; Hasbro Children’s Hospital and Brown University, Providence, Rhode Island, USAj; University of California at San Francisco, San Francisco, California, USAk


http://aac.asm.org/content/57/11/5619.full
ABSTRACT

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.      

[StephenCastlecrag]

DISCUSSION

Central conclusions.We found that a higher plasma tenofovir concentration was associated with higher vitamin D binding protein and lower free 1,25-OH(2)D, offering a possible mechanism to explain the higher parathyroid hormone and higher total 1,25-OH(2)D seen in participants taking TDF (Fig. 3A). The finding of higher intracellular tenofovir diphosphate concentration associated with lower FGF23 levels argues against a primary role of FGF23 in TDF-associated hypophosphatemia and suggests a compensatory drop in FGF23 in response to primary perturbations in other endocrine pathways (Table 1 and Fig. 3B).

TDF, tenofovir, vitamin D, and parathyroid hormone.The association of TDF use with elevations in parathyroid hormone and total 1,25-OH(2)D has been reported by others (Viread product label [http://www.accessdata.fda.gov/drugsatfda_docs/label/2013/021356s046,022577s003lbl.pdf] and references 20, 21, 23, and 25). The finding of a 26% increase in vitamin D binding protein and a 42% decrease in free 1,25-OH(2)D between the lowest and highest quintiles of plasma tenofovir has not been previously reported. Vitamin D binding protein and albumin are proteins that bind to 1,25-OH(2)D, and when their levels are increased they lead to a decrease in free 1,25-OH(2)D (34). Vitamin D binding protein is synthesized constitutively in hepatocytes in conjunction with albumin and other liver-specific proteins, with some regulation resulting from sex hormones, malnutrition, and liver failure and during pregnancy (35). Vitamin D binding protein binds fatty acids, which may change its affinity for vitamin D metabolites. Vitamin D binding protein has roles in innate immunity (macrophage activation and neutrophil chemotaxis) and response to tissue damage (binding actin released from damaged cells), activities which may affect serum levels (36). Since free 1,25-OH(2)D is an important physiologic determinant of 1,25-OH(2)D signaling, decreased free 1,25-OH(2)D could lead to a secondary increase in parathyroid hormone caused by the deficiency in unbound 1,25-OH(2)D sensed by the parathyroid gland. Lower free 1,25-OH(2)D has been associated with lower calcium absorption, which would also cause an increase in parathyroid hormone (37). Chronic elevation of parathyroid hormone causes low bone density, which is observed in association with TDF use (2–5). Lower eGFR concentrations (also associated with higher tenofovir concentrations) have also been found to be associated with lower free 1,25-OH(2)D concentrations and with higher concentrations of parathyroid hormone (38). Since this is the first time an association of plasma tenofovir concentration with vitamin D binding protein and free 1,25-OH(2)D has been found, it merits confirmation in other cohorts.                

TDF, tenofovir, renal phosphate loss, and FGF23.In the TDF group, we found mild phosphaturia in the absence of significant tubular proteinuria or glycosuria, consistent with other studies showing that TDF-associated increased renal excretion of phosphate occurs in the absence of proteinuria or more generalized renal tubular dysfunction (14, 39). Note that the low TRP was not associated with high parathyroid hormone levels. Even though high parathyroid hormone can directly cause renal tubular phosphate losses (40, 41), the data in this report support the findings of others (25) suggesting that TDF-associated phosphaturia is not due primarily to increased parathyroid hormone.

FGF23 concentrations did not differ between TDF treatment groups, consistent with other studies (18, 42), but the highest quintile of tenofovir diphosphate was associated with lower FGF23 concentration. FGF23, produced in osteocytes and osteoblasts, is increased in response to elevations in dietary and serum phosphorus (43) and in response to increased 1,25-OH(2)D (44) (Table 1). Increased FGF23 directly causes increased renal phosphate excretion in the proximal renal tubule, thereby decreasing serum phosphate (45). Increased FGF23 also causes a decline in 1,25-OH(2)D (44), and that decline would further decrease phosphate absorption in the gastrointestinal (GI) tract and increase renal phosphate excretion. Low FGF23 is found with hypophosphatemia and in individuals with vitamin D deficiency (46).

The finding of low FGF23 in participants with the highest quintile of tenofovir diphosphate could result from a compensatory response to the decreased free 1,25-OH(2)D (similar to the situation in vitamin D deficiency [46]), but the free 1,25-OH(2)D was associated with plasma tenofovir and not with intracellular tenofovir diphosphate. Low FGF23 was most closely associated with the highest quintile of intracellular tenofovir diphosphate, which suggests that tenofovir diphosphate is the primary determinant of TDF-associated hypophosphatemia, and FGF23 decreases in response to that stimulus in order to maintain phosphate homeostasis. Other mechanisms could also be postulated. For example, higher tenofovir diphosphate levels could affect osteocyte function directly and cause low FGF23, a possibility supported by the association of higher tenofovir diphosphate levels with lower bone alkaline phosphatase levels. The possibility of separate endocrine effects of tenofovir (on vitamin D-parathyroid hormone) and tenofovir diphosphate (on phosphate-FGF23) is important to explore given the ongoing development of TAF, which has lower tenofovir and higher tenofovir diphosphate levels than TDF (26).

[StephenCastlecrag]

TDF, tenofovir, and renal function: association with parathyroid hormone?The association of lower eGFR with TDF use (47–49) and higher plasma tenofovir concentrations (50) have been previously described. In this cross-sectional analysis, causality cannot be assessed; it may be that participants with the lowest pretreatment eGFR were the slowest to clear tenofovir.

It has been suggested that TDF-associated renal toxicity causes elevations in parathyroid hormone (17, 48). A lower eGFR is associated with parathyroid hormone elevations in both children and adults with early renal failure (38). While TDF use was associated with a decrease in eGFR and increase in parathyroid hormone, 1,25-OHD was higher in the TDF than in the no-TDF group. In patients with mild renal insufficiency, 1,25-OH(2)D decreases (38). This finding argues against the hypothesis that glomerular damage is causative in TDF-associated increases in parathyroid hormone.

We used analysis of variation in one variable by quintiles of another variable as a way to more fully evaluate changes that might occur in only a small subgroup of persons treated with TDF. This approach seemed justified given the relative infrequency of severe toxicity associated with TDF use. The TDF effect on bone density may be strongest in a subgroup of patients: in treatment studies, 5 of 15 (33%) children (3) and 15 of 143 (10%) (6) adults experienced a ≥6% decline in spine bone density after 48 weeks of TDF treatment. TDF-associated renal failure occurs extremely rarely (47). Quintile analysis offered the best demonstration of the relationship between tenofovir concentrations and vitamin D binding protein and free 1,25-OH(2)D, and it allowed us to identify the relationship between tenofovir diphosphate and FGF23, which has not been evaluated by others.

Major strengths of the study include the lack of comorbidities in the young population studied compared to studies in older populations where a drug effect may be confounded by aging-related diseases. Measurement of plasma tenofovir and intracellular tenofovir diphosphate concentrations allows a more nuanced understanding of exposure-effect relationships compared to studies that only allow analysis by TDF use. However, we measured tenofovir and tenofovir diphosphate concentrations at a single time point after an unobserved dose, a limitation that might have kept us from identifying other pharmacodynamic associations.

A cross-sectional analysis can only be seen as hypothesis generating. The findings of this study need to be confirmed with data obtained in longitudinal studies, including measurements of bone density in individuals initiating therapy with TDF. These post hoc analyses were performed to better understand the relationship between TDF use and increased parathyroid hormone. Further studies are needed to confirm these findings.

In summary, these data suggest that tenofovir-associated increased vitamin D binding protein is associated with decreased free 1,25-OH(2)D and functional vitamin D deficiency, which in turn leads to increased parathyroid hormone secretion. The data further suggest that tenofovir diphosphate induces a hypophosphatemic stress related more closely to intracellular tenofovir diphosphate concentrations than plasma tenofovir concentrations. In response, FGF23 production decreases to compensate and maintain phosphate homeostasis. It is also possible that tenofovir diphosphate directly inhibits FGF23 production within osteocytes. Further study is needed to test these hypotheses and to clarify whether the apparent plasma tenofovir-vitamin D-parathyroid hormone relationship is truly distinct from the intracellular tenofovir diphosphate relationship with FGF23.      

            

[StephenCastlecrag]

INTRODUCTION

The prodrug tenofovir disoproxil fumarate (TDF) is commonly used in combination antiretroviral therapy (cART) for persons infected with HIV, and, when coformulated with emtricitabine, it is also used for preexposure prophylaxis in HIV-seronegative adults (1). Following absorption, TDF is converted to free tenofovir and then phosphorylated intracellularly by host enzymes to tenofovir diphosphate. TDF is highly effective, but its use is associated with bone, endocrine, and renal toxicity.

The mechanism(s) by which TDF causes these organ-specific toxicities is unclear. There is a complex interplay between bone, endocrine, and renal physiology (Table 1). TDF use is associated with decreased bone mineral density (2–6), possibly caused by TDF-induced phosphaturia (7–14) with accompanying hypophosphatemia and osteomalacia (15–17). The phosphaturic hormone fibroblast growth factor 23 (FGF23) is not altered by TDF use (18, 19). TDF-associated bone changes may occur through a TDF effect on parathyroid hormone (16, 20), as parathyroid hormone secretion increases soon after initiating TDF (21). Vitamin D deficiency (22) may exacerbate this increase in parathyroid hormone (20, 23), but TDF-associated increased parathyroid hormone is found even in persons with sufficient vitamin D (24). Short-term vitamin D supplementation reduces TDF-associated increased parathyroid hormone but does not ameliorate TDF-associated phosphate loss (24, 25). The magnitude of phosphaturia is generally small and only rarely clinically important, while significant declines in bone density may occur in 10 (6) to 28% (Viread product label, available at http://www.accessdata.fda.gov/drugsatfda_docs/label/2013/021356s046,022577s003lbl.pdf) of TDF-treated adults.                  

TDF may induce a state of functional vitamin D deficiency (24), or a parathyroid hormone-like factor may cause these TDF-associated metabolic changes (18). The active form of vitamin D, 1,25 dihydroxy vitamin D [1,25-OH(2)D], is increased by TDF administration (Viread product label [http://www.accessdata.fda.gov/drugsatfda_docs/label/2013/021356s046,022577s003lbl.pdf]), in contrast to the trend expected with vitamin D deficiency but consistent with the changes expected with increased parathyroid hormone secretion (Table 1). The association between TDF use and vitamin D binding protein (VDBP), which could alter free 1,25-OH(2)D and affect circulating parathyroid hormone concentration, total 1,25-OH(2)D, and bone density, has not been assessed.

Different TDF-associated toxicities (bone, phosphate, and renal) may be associated with different pharmacokinetic properties of TDF, either plasma tenofovir or intracellular tenofovir diphosphate. This distinction may be important given the development of tenofovir alafenamide fumarate (TAF), a tenofovir prodrug with low concentrations of plasma tenofovir but high concentrations of intracellular tenofovir diphosphate (26). A study of TAF use showed equivalent virologic suppression and less bone toxicity compared to TDF, suggesting that bone toxicity is more closely related to tenofovir, while virologic efficacy is most closely related to intracellular tenofovir diphosphate concentrations (27).

This report uses baseline data from a trial of vitamin D supplementation in HIV-infected youth treated with cART containing or not containing TDF (24) to explore the relationship of TDF use with vitamin D-calcium balance and phosphate metabolism and to evaluate the relationship of plasma tenofovir and intracellular tenofovir diphosphate concentrations with those metabolic and endocrinologic variables.

(Presented in part at the 18th Conference on Retroviruses and Opportunistic Infections [CROI], Boston, MA, February 2011.)