Clinical monitoring and correlates of nephropathy in SIV-infected macaques during high-dose antiretroviral therapy

The nucleotide reverse transcriptase inhibitor (NRTI) PMPA or tenofovir has become one of the most commonly used antiretroviral drugs due to its favorable efficacy and safety profile, based on data collected over more than 9 years for HIV-infected adults. The acyclic nucleoside phosphonate PMPA is renally excreted by a combination of glomerular filtration and active tubular secretion [1]. The effective uptake of acyclic nucleoside phosphonates by organic anion transporters in proximal tubules leads to accumulation in tubular cells and dose-limiting toxicity in animals [2]. Renal toxicity is usually manifested as renal insufficiency and proximal renal tubular dysfunction (PRTD). FTC or emtricitabine is the (-) enantiomer of a thio analog of cytidine, which differs from other cytidine analogs in that it has a fluorine in the 5-position. It is another nucleoside analog HIV-1 reverse transcriptase inhibitor and also mainly eliminated by the kidney. ZERIT^® is the brand name for d4T or stavudine, a synthetic thymidine nucleoside analogue. D4T is phosphorylated by cellular kinases to the active metabolite d4T triphosphate, which inhibits the activity of HIV-1 reverse transcriptase (RT) by competing with the natural substrate thymidine triphosphate and by causing DNA chain termination following its incorporation into viral DNA. D4T triphosphate inhibits cellular DNA polymerases β and γ and markedly reduces the synthesis of mitochondrial DNA. Urinary excretion is the major route of d4T elimination.

Although systemic ART is clearly of benefit, a variety of antiretroviral drugs including protease inhibitors and NRTIs, have been linked to nephrotoxicity [3]. SIV-infected macaques also develop renal disease that mimics the scope and etiology of that observed in HIV-infected people. Rhesus macaques develop opportunistic infections such as SV40, cytomegalovirus and adenovirus infection that may produce renal pathology and resemble the disease processes recognized in HIV-infected patients. Furthermore, a segmental glomerulosclerosis has been described in SIV-infected animals that have progressed to AIDS, which is similar morphologically to the HIV-nephropathy observed in human patients [4‐6]. Finally macaques may also develop renal dysfunction subsequent to antiretroviral therapy with PMPA [7‐11]. PMPA is the biologically active metabolite of the prescription drug Viread^®. It is commonly used in SIV pathogenesis studies because it can be administered by the parenteral route and is highly effective at reducing viral loads. Previous work of others has revealed that long-term administration of PMPA at 30 mg/kg resulted in a Fanconi-like syndrome with glucosuria, aminoaciduria, hypophosphatemia, growth restriction and bone pathology [2]. In this report, the serum biochemical correlates of renal morphologic alterations in SIV-infected macaques that received PMPA, d4T and FTC combination therapy are described and guidelines to prevent and identify serious renal sequellae in future experiments are proposed.

The results presented indicate that ART nephropathy was common and persistent in SIV-infected rhesus macaques receiving combination PMPA-containing ART. Animals developed morphologic and biochemical evidence of disease despite normal renal function at the onset of the study. This is in contrast to human patients, in whom disease is said to be infrequent and associated with pre-existing renal pathology. These differences may be related to differences in species, drug or drug dose, and route. The dose of the prodrug tenofovir disoproxyl fumarate (TDF; Viread^®) in human patients is substantially lower on a per kilogram basis than the equivalent PMPA dose used in rhesus macaques, which in combination with species differences in drug pharmacokinetics results in lower plasma drug levels in humans [13, 14]. In addition, while Viread^® is given orally in human patients, PMPA is administered as a single subcutaneous injection once daily in macaques, which may further increase peak plasma drug values. In addition, the FTC dose that was used in these studies (50 mg/kg) is also higher than the equivalent regimen in humans. Others demonstrated that a dose of 20 mg/kg of FTC is equivalent to the human dose [15]. This higher dose of FTC may also have contributed to toxicity. However, for d4T, the dose administered for NHP in this study (2.4 mg/kg/day) was slightly lower than the dose recommended for humans, based on a body surface conversion model [16]. Although morphologic evidence of ART nephropathy was frequent, associated acute renal failure was less common (7.1%) and overt chronic renal failure demonstrated by azotemia following completion of the ART regimen was not observed over the time period monitored. Changes in BUN over time suggest that if a longer follow-up period is allowed, a subset of animals might develop chronic renal failure. In the present study, the occurrence of ART nephropathy after termination of treatment did not appear to effect clinical outcome or survival. Nonetheless, subclinical renal dysfunction could impact experimental outcome through alteration of the pharmacokinetics of co-administered drugs, changes in calcium/phosphorus balance or initiation of other sequellae of chronic renal disease. It was also found that animals may have substantial morphologic alterations at the light microscopic level and normal BUN and creatinine levels in serum, reinforcing the relative insensitivity of these tests in diagnosing renal pathology. For these reasons, investigators should be made aware of ART nephropathy, and a better understanding of risk factors associated with its development should be sought.

The toxic effects of PMPA on bone and kidney in SIV-infected macaques have previously been described in detail [2, 17]. These findings included growth restriction and biochemical and morphologic features of renal tubular dysfunction, which were frequently observed in animals receiving PMPA at 30 mg/kg for periods exceeding 8 months [2]. The results presented here differ in that short term (30 days) administration of PMPA at this dose was associated with acute renal failure in a small subset of animals and was more frequently associated with morphologic and biochemical evidence of renal dysfunction for up to 300 days following cessation of treatment. The reason for this difference is unknown, and since our cohort received combination therapy, it is possible that co-administration of the other NRTIs, d4T and FTC, may have potentiated the nephrotoxic effects of PMPA [18‐21]. This cohort was also substantially older than the neonatal and juvenile animals previously examined. Van Rompay reported that PMPA renal clearance was lower in adult as compared to juvenile animals, and thus age differences may play a role in determining disease susceptibility and course [2]. Lower PMPA clearance may allow for higher plasma concentrations to be achieved and promote drug accumulation within the PCT epithelium, thereby exacerbating renal toxicity. Once renal damage occurs, PMPA clearance may decrease leading to a further reduction in tubular function.

It was found that individual serum chemistry values were of limited use in predicting acute renal or persistent renal pathology. The identified changes were consistent with the proposed pathogenesis of ART nephropathy and changes previously observed in humans and macaques.

Several recommendations are proposed to reduce the impact of ART nephropathy on future studies: