Chronic administration of tenofovir to rhesus macaques from infancy through adulthood and pregnancy: summary of pharmacokinetics and biological and virological effects

Abstract

The reverse transcriptase (RT) inhibitor tenofovir (TFV) is highly effective in the simian immunodeficiency virus (SIV) macaque model of human immunodeficiency virus infection. The current report describes extended safety and efficacy data on 32 animals that received prolonged (>or=1- to 13-year) daily subcutaneous TFV regimens. The likelihood of renal toxicity (proximal renal tubular dysfunction [PRTD]) correlated with plasma drug concentrations, which depended on the dosage regimen and age-related changes in drug clearance. Below a threshold area under the concentration-time curve for TFV in plasma of approximately 10 microg x h/ml, an exposure severalfold higher than that observed in humans treated orally with 300 mg TFV disoproxil fumarate (TDF), prolonged TFV administration was not associated with PRTD based on urinalysis, serum chemistry analyses, bone mineral density, and clinical observations. At low-dose maintenance regimens, plasma TFV concentrations and intracellular TFV diphosphate concentrations were similar to or slightly higher than those observed in TDF-treated humans. No new toxicities were identified. The available evidence does not suggest teratogenic effects of prolonged low-dose TFV treatment; by the age of 10 years, one macaque, on TFV treatment since birth, had produced three offspring that were healthy by all criteria up to the age of 5 years. Despite the presence of viral variants with a lysine-to-arginine substitution at codon 65 (K65R) of RT in all 28 SIV-infected animals, 6 animals suppressed viremia to undetectable levels for as long as 12 years of TFV monotherapy. In conclusion, these findings illustrate the safety and sustained benefits of prolonged TFV-containing regimens throughout development from infancy to adulthood, including pregnancy.

FIG. 1.

Changes in TFV clearance: effects of age and dosage regimens. Twenty-four-hour pharmacokinetic studies with subcutaneous administration of TFV were performed. (A and B) Weight- and age-related changes in TFV CL/F, respectively, following a single-dose administration of 10 or 30 mg/kg (subcutaneously) to 23 healthy, uninfected macaques of different ages; 2 of these 23 animals (male 31007 and female 31456) were given two doses of TFV (10 mg/kg,= subcutaneously) 19 months apart. (C) TFV CL/F in animals that received prolonged TFV regimens (once daily subcutaneously; regimens are outlined in Table 1). Pharmacokinetic data were collected either from animals that never showed PRTD (blue) or from animals prior to (red) or after (black) the development of PRTD. Animal 33091 had glucosuria at a single time point when the TFV clearance was 400 ml/h/kg, but this disappeared when clearance increased again after dosage reduction. (D) An overlay of graphs B and C with animals grouped according to the presence of PRTD demonstrates that animals that received chronic TFV treatment and never showed signs of PRTD throughout their observation periods had apparent TFV clearances indistinguishable from those of animals that received a single dose. Based on the available data, the horizontal dotted line indicates the arbitrary cutoff value of reduced TFV clearance (<400 ml/h/kg) that was associated with PRTD in juvenile and adult animals.

FIG. 2.

Correlation between apparent TFV clearance, plasma AUCs, and the development of PRTD. Animals had been on stable subcutaneous TFV dosage regimens for at least 4 months at the time of the pharmacokinetic studies. Symbols indicating whether PRTD (glucosuria and/or hypophosphatemia) occurred refer to the time frame after the pharmacokinetics study was performed. While some animals were previously on higher-dosage regimens (see Table 1), the dosages indicated refer to the time of the pharmacokinetic studies. Because clearance is calculated as dose/AUC, hyperbolas are predicted at a specific dose. The quadrants define where differences and changes in TFV CL/F are physiologic (i.e., due to individual variation and the effect of aging) versus pathological (i.e., nephrotoxicity). The available data suggest that the cutoff values for the different quadrants are approximately AUCs of 20 μg·h/ml and clearance values of 400 ml/h/kg. Quadrant A is the zone where no PRTD was observed; quadrant B is a pre-PRTD stage; quadrants C and D area are associated with clinical PRTD. Glucosuria was detected once for animal 33091 after the pharmacokinetic values were at the intersection of these quadrants; the glucosuria resolved following dosage reduction (which was associated with an increase in TFV clearance [quadrant A]). (Inset) Empirical model for the pathogenesis of PRTD during high-dose TFV regimens. At high TFV exposures (quadrant B), the gradual development of PRTD reduces TFV clearance; this reduced clearance, by further increasing drug exposure (i.e., AUCs), aggravates renal toxicity and drives the animals' pharmacokinetic parameters further and further into quadrant C; only following drastic dosage reductions can values move from quadrant C toward quadrant D.

FIG. 3.

DXA scan evaluation of bones in long-term TFV-treated animals. Details on the TFV dosage regimens are provided in Table 1. DXA scans were performed on the femoral neck, global proximal femur, DR+U, and lumbar vertebrae (L2 to L4). Although all these locations gave similar results, the BMDs of lumbar vertebrae are shown because they had less variability (among both treated and untreated animals) and were therefore more useful for comparison of the TFV-treated animals. The untreated female macaques either were nulliparous or were tested at least 14 months after their most recent infant had been weaned. Except for those animals that had been on prolonged high-dose TFV regimens and had bone mineralization defects before the dosage was reduced (29045, 29046, 29276, and 30577), the TFV-treated animals had BMDs similar to those of age- and sex-matched control animals. M and F indicate male and female, respectively.

FIG. 4.

Effect of long-term TFV treatment on disease progression in SIV-infected macaques. Disease-free survival curves are presented for all studies that allowed evaluation of the effect of prolonged TFV treatment on SIV disease progression. Data from separately performed studies were pooled for the analysis if all input parameters (age of animals, virus isolate, dose and route of virus inoculation) were very similar. On each graph, the survival curves of TFV-treated and untreated animals were compared using the log rank test. Long-term survivors that are discussed in Results are indicated by arrows. (A) Newborn macaques were inoculated orally with a high dose of SIVmac251, and TFV treatment was started on five animals (29003, 29008, 29045, 29055, and 31042) 3 weeks later (59, 64). Three of these TVF-treated animals developed AIDS while on treatment; one animal, 29003, had TFV interrupted at the age of 95 weeks and developed AIDS at 177 weeks; animal 29045 was lost from the study at the age of 7 years due to an unrelated cause (self-injurious behavior). (B) Twelve newborn macaques were inoculated intravenously with a high dose of K65R mutants of SIV, and 3 weeks later, six of them were started on TFV as described previously (60). TFV-treated animal 29276 is currently AIDS free at the age of 12 years. (C) Four-week-old infant macaques were inoculated orally with SIVmac251, and three animals (32990, 33093, and 33109) were started on TFV treatment approximately 11 weeks later, when animals were already immunosuppressed (64). (D) Eleven juvenile macaques (ages, 12 to 17 months) were inoculated orally with SIVmac251, and five animals were started on TFV treatment 2 weeks later, as described previously (66). Two animals (32137 and 32993) developed AIDS, while the other three animals (32186, 33088, and 33091) are currently AIDS free after more than 6 years of SIV infection.