Phenotypic susceptibilities to tenofovir in a large panel of clinically derived human immunodeficiency virus type 1 isolates

Chronic Nucleoside Reverse Transcriptase Inhibitors Disrupt Mitochondrial Homeostasis and Promote Premature Endothelial Senescence

Combination antiretroviral therapy (cART) has improved the life expectancy of HIV patients, thus increasing the number of people living with HIV (PLWH). However, cardiovascular diseases (CVD) are now one of the most prevalent causes of death among PLWH. Nucleoside reverse transcriptase inhibitors (NRTIs) are the backbone of cART, and the emtricitabine (FTC) and tenofovir disoproxil fumarate (TDF) coformulation is commonly used. In prior studies, acute NRTI treatment-induced endothelial dysfunction, increased reactive oxygen species production, and mitophagic activity, suggesting that mitochondrial dysfunction may be critical to NRTI-induced endothelial dysfunction. Mitochondrial dysfunction plays a causal role in endothelial senescence, whereas premature endothelial senescence can promote the development of CVD. We hypothesize that for chronic NRTI treatment, a disruption in mitochondrial homeostasis leads to premature endothelial senescence and predisposes PLWH to CVD. We used human aortic endothelial cells (HAEC) and HIV-1 transgenic (Tg26) mice to test the interrelationship between mitochondrial and vascular dysfunction after chronic NRTI treatment in vitro and in vivo. Mitochondrial DNA copy number was decreased in late-passage HAEC treated with NRTIs, and senescence-associated β-galactosidase accumulation was elevated. In late-passage HAEC, NRTIs decreased the activity of Parkin-mediated mitophagy. In Tg26 mice treated with FTC, plasma nitrite levels were decreased. Endothelium-dependent vasodilation in NRTI-treated Tg26 mice was also reduced. Our work suggests that long-term use of NRTI may disrupt mitochondrial homeostasis, induce premature endothelial senescence, and impair vascular function.

A phase IV randomised, open-label pilot study to evaluate switching from protease-inhibitor based regimen to Bictegravir/Emtricitabine/Tenofovir Alafenamide single tablet regimen in Integrase inhibitor-naïve, virologically suppressed HIV-1 infected adults harbouring drug resistance mutations (PIBIK study): study protocol for a randomised trial

BACKGROUND

Currently recommended boosted protease-inhibitor (bPI) regimens may be associated with increased risk of cardiovascular or chronic kidney diseases; in addition, boosted regimens are particularly associated with drug-drug interactions. Since both cardiovascular and renal disease, and polypharmacy, are common in ageing people with HIV, there is a need for alternative efficacious regimens. bPI-based regimens are often the treatment of choice for individuals with pre-treatment or treatment-acquired resistance but it is plausible that carefully selected HIV-positive individuals with drug resistance, who are virologically suppressed on their current bPI regimen, could maintain virological efficacy when switched to bictegravir, emtricitabine and tenofovir alafenamide (B/F/TAF) fixed dose combination (FDC).

METHODS/DESIGN

A phase IV, investigator-initiated, multicentre, open label pilot, randomised two-arm study to assess the safety and efficacy of switching from bPI regimen to B/F/TAF single tablet regimen in integrase inhibitor-naïve, virologically suppressed adults with HIV-1 infection harbouring drug resistance mutations. Eligible individuals will either continue on their bPI regimen or switch to B/F/TAF FDC. After 24 weeks, all participants in the bPI arm will be switched to B/F/TAF and followed for a further 24 weeks and all participants will be followed for 48 weeks. The primary efficacy endpoint is the proportion of participants with HIV-1 RNA < 50 copies/mL at week 24 using pure virologic response whilst the secondary efficacy endpoint is the proportion of participants with HIV-1 RNA < 50 copies/mL at Week 48. Other secondary outcome measures include between arm comparisons of drug resistance at virological failure, safety and tolerability and patient-reported outcome measures.

DISCUSSION

We aim to provide preliminary evidence of the efficacy of switching to B/F/TAF in patients with virological suppression on a bPI-based regimen who harbour select drug resistance mutations.

TRIAL REGISTRATION

ISRCTN 44453201 , registered 19 June 2019 and EudraCT 2018-004732-30.

Potential of acyclic nucleoside phosphonates in the treatment of DNA virus and retrovirus infections

The acyclic nucleoside phosphonates [HPMPC: cidofovir, Vistide; PMEA: adefovir dipivoxil, Hepsera; and PMPA: tenofovir, Viread] have proven to be effective in vitro (cell culture systems) and in vivo (animal models and clinical studies) against a wide variety of DNA virus and retrovirus infections, for example, cidofovir against herpesvirus [herpes simplex virus type 1 and 2, varicella-zoster virus, cytomegalovirus, Epstein-Barr virus, human herpesvirus type 6, 7 and 8), polyoma-, papilloma-, adeno- and poxvirus (variola virus, cowpox virus, vaccinia virus, molluscum contagiosum virus and orf) infections; adefovir against herpesvirus, hepadnavirus [human hepatitis B virus] and retrovirus [HIV type-1 and 2, simian immunodeficiency virus and feline immunodeficiency virus] infections; and tenofovir against both hepadna- and retrovirus infections. Cidofovir has been officially approved for the treatment of cytomegalovirus retinitis in AIDS patients, tenofovir disoproxil fumarate (Viread) for the treatment of HIV infections (i.e., AIDS) and adefovir dipivoxil for the treatment of chronic hepatitis B.

Novel method to assess antiretroviral target trough concentrations using in vitro susceptibility data

Durable suppression of HIV-1 replication requires the establishment of antiretroviral drug concentrations that exceed the susceptibility of the virus strain(s) infecting the patient. Minimum plasma drug concentrations (C(trough)) are correlated with response, but determination of target C(trough) values is hindered by a paucity of in vivo concentration-response data. In the absence of these data, in vitro susceptibility measurements, adjusted for serum protein binding, can provide estimations of suppressive in vivo drug concentrations. We derived serum protein binding correction factors (PBCF) for protease inhibitors, nonnucleoside reverse transcriptase inhibitors, and an integrase inhibitor by measuring the effect of a range of human serum concentrations on in vitro drug susceptibility measured with the PhenoSense HIV assay. PBCFs corresponding to 100% HS were extrapolated using linear regression and ranged from 1.4 for nevirapine to 77 for nelfinavir. Using the mean 95% inhibitory concentration (IC(95)) for ≥1,200 drug-susceptible viruses, we calculated protein-bound IC(95) (PBIC(95)) values. PBIC(95) values were concordant with the minimum effective C(trough) values that were established in well-designed pharmacodynamic studies (e.g., indinavir, saquinavir, and amprenavir). In other cases, the PBIC(95) values were notably lower (e.g., darunavir, efavirenz, and nevirapine) or higher (nelfinavir and etravirine) than existing target recommendations. The establishment of PBIC(95) values as described here provides a convenient and standardized approach for estimation of the minimum drug exposure that is required to maintain viral suppression and prevent the emergence of drug-resistant variants, particularly when in vivo concentration-response relationships are lacking.

Raltegravir, tenofovir DF, and emtricitabine for postexposure prophylaxis to prevent the sexual transmission of HIV: safety, tolerability, and adherence

Antiretroviral drugs have been recommended for postexposure prophylaxis (PEP) after high-risk sexual exposures for more than a decade. Three drug regimens could offer the highest levels of protection, particularly if the infectious source is taking medication, but drug intolerance has often led to suboptimal adherence. The current study evaluated a novel 3-drug PEP regimen, consisting of raltegravir, tenofovir DF, and emtricitabine. Of 100 participants enrolled in this study at a Boston community health center that has had a comprehensive PEP program for more than a decade, 85 were evaluable at 3 months and none became HIV infected. Fifty seven percent of those enrolled completed the regimen as prescribed, and 27% took their medicine daily, but sometimes missed the second daily dose of Raltegravir. The most common side effects reported included nausea or vomiting (27%), diarrhea (21%), headache (15%), fatigue (14%), abdominal symptoms (including pain, gas, or bloating) (16%), and myalgias or arthralgias (8%), all of which were mild and tended to be self-limited, not resulting in drug discontinuation. The side effects were significantly less common than those reported by historical controls, who used a 3-drug PEP regimen including zidovudine, lamivudine, and a ritonavir-boosted protease inhibitor. Raltegravir, tenofovir DF, and emtricitabine may be useful as a 3-drug regimen for PEP.

The Role of Nucleotide Excision by Reverse Transcriptase in HIV Drug Resistance

Nucleoside reverse transcriptase (RT) inhibitors of HIV block viral replication through the ability of HIV RT to incorporate chain-terminating nucleotide analogs during viral DNA synthesis. Once incorporated, the chain-terminating residue must be removed before DNA synthesis can continue. Removal can be accomplished by the excision activity of HIV RT, which catalyzes the transfer of the 3'-terminal residue on the blocked DNA chain to an acceptor substrate, probably ATP in most infected cells. Mutations of RT that enhance excision activity are the most common cause of resistance to 3'-azido-3'-deoxythymidine (AZT) and exhibit low-level cross-resistance to most other nucleoside RT inhibitors. The resistance to AZT is suppressed by a number of additional mutations in RT, most of which were identified because they conferred resistance to other RT inhibitors. Here we review current understanding of the biochemical mechanisms responsible for increased or decreased excision activity due to these mutations.

Nucleotide HIV reverse transcriptase inhibitors: tenofovir and beyond

PURPOSE OF REVIEW

This review describes the class of nucleotide HIV reverse transcriptase inhibitors and summarises recent findings related to tenofovir and its oral prodrug tenofovir disoproxil fumarate, currently the only nucleotide approved for the treatment of HIV infection. In addition, novel strategies in the design of anti-HIV nucleotides and their prodrugs are discussed.

RECENT FINDINGS

A number of studies have demonstrated a potent and durable clinical efficacy of tenofovir disoproxil fumarate in combination with other antiretrovirals, particularly lamivudine or emtricitabine and efavirenz. The prophylactic antiretroviral effect of tenofovir and tenofovir disoproxil fumarate has been characterized in various animal models and is currently being evaluated in controlled clinical studies. In addition, efficacy of tenofovir disoproxil fumarate against hepatitis B virus has been established and is currently being explored in phase III trials. The identification of GS-7340, an alternative prodrug of tenofovir has raised the possibility of using phosphonoamidates as novel prodrugs allowing for an effective intracellular delivery of nucleotides.

SUMMARY

The concept of nucleotides as a novel class of antiretroviral therapeutics has been successfully validated through tenofovir disoproxil fumarate, a nucleotide prodrug that exhibits potent and durable clinical efficacy and favourable safety profile both in treatment-naïve and experienced HIV-infected patients. Several novel nucleotide reverse transcriptase inhibitors such as GS-9148, PMDTA, and PMEO have recently emerged from continuing preclinical drug discovery efforts.

Virological response to Salvage Therapy in HIV-Infected Persons Carrying the Reverse Transcriptase K65R Mutation

Background

The effect of the HIV reverse transcriptase K65R mutation on virological response to salvage therapy has not been clearly defined.

Methods

From six Italian clinical centres, all consecutive patients starting salvage antiretroviral therapy after virological failure in the presence of the K65R mutation identified by a genotypic resistance test were selected.

Results

Among 145 subjects included over a 197 person-year follow-up, the estimated probability of virological response (VR, defined as reaching HIV RNA <50 copies/ml after salvage therapy) at 24 and 48 weeks was 36% and 60%, respectively. The strongest independent predictor of VR was the inclusion of a thymidine analogue (TA) in the salvage regimen. The presence of M184V and the introduction of lopinavir/ritonavir as new drug were both marginally associated with better outcome. After 24 weeks of salvage therapy, the median reduction in HIV-1 RNA was -1.36 log10 copies/ml (interquartile range [IQR] 0.10–2.46): at multivariable regression analysis, salvage regimens containing a TA (β=+0.80; P=0.02) and lamivudine (β=+1.21; P=0.02) as new drug had a positive effect on the reduction of HIV-1 RNA.

Conclusions

Development of the K65R mutation does not preclude a high rate of virological response to rescue therapy. Inclusion of a TA in the salvage regimen and the presence of a M184V mutation could have a favourable effect on virological outcome.

Steady-state pharmacokinetics of emtricitabine and tenofovir disoproxil fumarate administered alone and in combination in healthy volunteers

The approved antiretroviral drugs, emtricitabine and tenofovir disoproxil fumarate, were considered good candidates for a fixed-dose combination product that could be administered as a single pill once daily (qd), thereby simplifying existing treatment regimens and promoting patient adherence. As both drugs are extensively renally eliminated, a randomized, 3-way crossover study was conducted in 19 healthy volunteers to formally evaluate the potential pharmacokinetic interaction when the drugs are administered alone and together (ie, 200 mg emtricitabine qd for 7 days, 300 mg tenofovir disoproxil fumarate qd for 7 days, and 200 mg emtricitabine plus 300 mg tenofovir disoproxil fumarate qd for 7 days) with no washout between treatments. Steady-state pharmacokinetic parameters (AUC(tau), C(max), and C(min)) of emtricitabine and tenofovir (as tenofovir disoproxil fumarate) in combination were essentially equivalent versus each drug alone, providing a pharmacokinetic rationale for combining these products in emtricitabine/tenofovir disoproxil fumarate fixed-dose tablets.

Quadruple Nucleoside Therapy with Zidovudine, Lamivudine, Abacavir and Tenofovir in the Treatment of HIV

Highly active antiretroviral therapy (HAART) has significantly reduced morbidity and mortality in HIV-infected patients. However, problems such as short-term or long-term toxicity and the development of drug resistance could necessitate a change in the therapy regimen. Whereas various HAART options with low pill burden and favourable long-term tolerability profiles are available for naive patients, treatment of experienced patients tends to be more complex and remains a challenge. Treatment with class sparing nucleoside-only regimens could be an option in this context, but the combination of zidovudine (AZT), lamivudine (3TC) and abacavir (ABC) has shown to be inferior in terms of virological efficacy compared with the standard regimen. More promising data were obtained when AZT, 3TC and ABC were intensified with tenofovir (TDF), resulting in a quadruple nucleoside therapy. This regimen has demonstrated comparable potency to a standard regimen with AZT, 3TC and efavirenz in treatment-naive patients. Additionally, it has shown to be an efficient treatment option especially in moderately pretreated patients. This is accredited to the potency of the single components and the antagonistic selection pressure of AZT and TDF. The presence of L210W, or at least two of the mutations 41L, 67N, 70R, 215F/Y or 219Q/E, at or before baseline seems to be a predictor of non-response, whereas the presence of M184V does not impede virological response and might even be advantageous. This review summarizes current data on the combined use of AZT, 3TC, ABC and TDF in regard to virological and immunological outcome as well as genotypic predictors of response.

Antiviral Efficacy and Genotypic Resistance Patterns of Combination Therapy with Stavudine/Tenofovir in Highly Active Antiretroviral Therapy Experienced Patients

Objectives

To evaluate antiviral efficacy of stavudine/tenofovir (d4T/TDF) backbone combination in late-line antiretroviral therapy, and to assess clinical and virological determinants of treatment success.

Design

Multicentric retrospective analysis on patients starting d4T/TDF after highly active antiretroviral therapy (HAART) failure.

Methods

The primary endpoint was the change in plasma HIV-1 RNA from the baseline (time of d4T/TDF initiation) to 6 months of therapy; secondary endpoint was the risk of virological failure.

Results

Among 172 patients included, a mean change in HIV-1 RNA of -1.69 (+1.23) and -1.53 (+1.43) log10 cp/ml was observed respectively at weeks 24 and 48 after starting d4T/TDF combination. Any single type-1 thymidine analogue mutation (TAM; M41L, L210W, T215Y) had a negative effort on the change in HIV RNA at 6 months, whereas among type-2 TAMs (D67N, K70R, K219Q), only D67N showed a trend for a negative effect. Presence of M184V mutation was related with a greater reduction in HIV RNA during d4T/TDF exposure. The risk of virological failure at 6 months after d4T/TDF starting was 22%. Type-1 TAMs were associated with a greater risk of failure (adjusted hazard ratio [HR]=1.65; 95% confidence interval [CI] 1.19–2.29). Conversely, M184V showed a protective effect. In 17 genotypic tests available at failure, no K65R mutation was detected, whereas a trend for an increasing prevalence of d4T-associated mutations was found.

Conclusions

Combining TDF with a thymidine analogue as d4T may be effective as component of antiretroviral rescue regimens in HIV-infected patients with previous exposure to nucleoside analogue reverse transcriptase inhibitor. Previous selection of type-1 TAMs induces a detrimental effect over virological response.

Tenofovir disoproxil fumarate: a review of its use in the management of HIV infection

Tenofovir disoproxil fumarate (tenofovir DF; Viread), an ester prodrug of the nucleotide reverse transcriptase inhibitor (NRTI) tenofovir, is indicated in combination with other antiretroviral agents in the treatment of HIV infection. As a component of an antiretroviral regimen, oral tenofovir DF 300 mg once daily effectively reduces viral load in patients with HIV infection who are treatment-experienced with baseline NRTI resistance mutations or treatment-naive. Tenofovir DF provides a simple and convenient once-daily dosage regimen, and is generally well tolerated and able to produce sustained suppression of viral replication.

Effect of concurrent zidovudine use on the resistance pathway selected by abacavir-containing regimens

OBJECTIVES

Abacavir (ABC) selects for four mutations (K65R, L74V, Y115F and M184V) in HIV-1 reverse transcriptase (RT), both in vitro and during monotherapy in vivo. The aim of this analysis was to compare the selection of these and other nucleoside reverse transcriptase inhibitor (NRTI)-associated mutations by ABC-containing therapies in the presence and absence of concurrent lamivudine (3TC) and/or zidovudine (ZDV) and to assess the effect of these mutations on phenotypic susceptibility to the NRTIs.

DESIGN

This study was a retrospective analysis of the patterns of NRTI-associated mutations selected following virological failure in six multicentre trials conducted during the development of ABC.

METHODS

Virological failure was defined as confirmed vRNA above 400 HIV-1 RNA copies/mL. RT genotype and phenotype were determined using standard methods.

RESULTS

K65R was selected infrequently by ABC-containing regimens in the absence of ZDV (13 of 127 patients), while L74V/I was selected more frequently (51 of 127 patients). Selection of both K65R and L74V/I was significantly reduced by co-administration of ZDV with ABC (one of 86 and two of 86 patients, respectively). Y115F was uncommon in the absence (seven of 127 patients) or presence (four of 86 patients) of ZDV. M184V was the most frequently selected mutation by ABC alone (24 of 70 patients) and by ABC plus 3TC (48 of 70 patients). Thymidine analogue mutations were associated with ZDV use. The K65R mutation conferred the broadest phenotypic cross-resistance of the mutations studied.

CONCLUSIONS

The resistance pathway selected upon virological failure of ABC-containing regimens is significantly altered by concurrent ZDV use, but not by concurrent 3TC use. These data may have important implications for the efficacy of subsequent lines of NRTI therapies.

Effects of the Delta67 complex of mutations in human immunodeficiency virus type 1 reverse transcriptase on nucleoside analog excision

Long-term use of combination therapy against human immunodeficiency virus type (HIV-1) provides strong selective pressure on the virus, and HIV-1 variants that are resistant to multiple inhibitors have been isolated. HIV-1 variants containing amino acid substitutions within the coding region of HIV-1 reverse transcriptase (RT), such as the 3'-azido-3'-deoxythymidine (AZT)-resistant variant AZT-R (M41L/D67N/K70R/T215Y/K219Q) and a variant containing an insertion in the fingers domain (S69SGR70/T215Y), are resistant to the nucleoside RT inhibitor (NRTI) AZT because of an increase in the level of excision of AZT monophosphate (AZTMP) from the primer. While rare, variants have also been isolated which contain deletions in the RT coding region. One such virus, described by Imamichi et al. (J. Virol 74:10958-10964, 2000; J. Virol. 74:1023-1028, 2000; J. Virol. 75:3988-3992, 2001), contains numerous amino acid substitutions and a deletion of codon 67, which we have designated the Delta67 complex of mutations. We have expressed and purified HIV-1 RT containing these mutations. We compared the polymerase and pyrophosphorolysis (excision) activity of an RT with the Delta67 complex of mutations to wild-type RT and the two other AZT-resistant variants described above. All of the AZT-resistant variants we tested excise AZTMP and 9-[2-(R)-(phosphonomethoxy)propyl]adenine (PMPA [tenofovir]) from the end of a primer more efficiently than wild-type RT. Although the variant RTs excised d4TMP less efficiently than AZTMP and PMPA, they were able to excise d4TMP more efficiently than wild-type RT. HIV-1 RT containing the Delta67 complex of mutations was not able to excise as broad a range of NRTIs as the fingers insertion variant SSGR/T215Y, but it was able to polymerize efficiently with low concentrations of deoxynucleoside triphosphates and seems to be able to excise AZTMP and PMPA at lower ATP concentrations than AZT-R or SSGR/T215Y, suggesting that a virus containing the Delta67 complex of mutations would replicate reasonably well in quiescent cells, even in the presence of AZT.

Relative replication fitness of multi-nucleoside analogue-resistant HIV-1 strains bearing a dipeptide insertion in the fingers subdomain of the reverse transcriptase and mutations at codons 67 and 215

A two-serine insertion at position 69 (i69SS) of human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT) appears to be critical to enhance multi-nucleoside RT inhibitor resistance (MNR) in the sequence context of multiple zidovudine (AZT) resistance mutations (i.e., M41L, L210W, T215Y). In this study, we measured the replication capacity relative to the wild-type (WT) HIV-1 of a series of recombinant viruses carrying the i69SS in the background of a clinical isolate with MNR in which we introduced mutations D67N, Y215T, Y215S, or Y215N. In vitro measurements included replication kinetics and growth competition assays at different multiplicities of infection (MOI). While the addition of D67N had a minor effect on replication capacity, the reversion of Tyr-215 to Thr, Ser, or Asn was sufficient to increase the virus ability to replicate in a drug-free environment. The same genotypic changes at position 215 rendered the MNR virus susceptible to AZT and stavudine. Interestingly, the presence of the insertion together with mutation T215Y in an otherwise WT sequence background was not sufficient to confer high-level resistance to AZT, although its replication capacity was clearly impaired. Therefore, the RT residue 215 plays a critical role in both replication capacity and drug resistance of multidrug-resistant viruses containing the i69SS.

Tenofovir resistance and resensitization

Human immunodeficiency viruses in 321 samples from tenofovir-naïve patients were retrospectively evaluated for resistance to this nucleotide analogue. All virus strains with insertions between amino acids 67 and 70 of the reverse transcriptase (n = 6) were highly resistant. Virus strains with the Q151M mutation were divided into susceptible (n = 12) and highly resistant (n = 8) viruses. This difference was due to the absence or presence of the K65R mutation, which was confirmed by site-directed mutagenesis. Viral clones with various combinations of the mutations M41L, K70R, L210W, and T215F or T215Y were analyzed for cross-resistance induced by thymidine analogue mutations (TAMs). The levels of increased resistance induced by single, double, and triple mutations at the indicated positions could be ranked as follows: for mutants with single mutations, mutations at positions 41 > 215 > 70; for mutants with double mutations, mutations at positions 41 and 215 > 70 and 215 = 210 and 215 > 41 and 70; for mutants with triple mutations, mutations at positions 41, 210, and 215 > 41, 70, and 215. Viral clones with M184V or M184I exhibited slightly increased susceptibilities to tenofovir (0.7-fold). Almost all clones with TAM-induced resistance were resensitized when M184V was present (P < 0.001). Among the viruses in the clinical samples, the rate of tenofovir resistance significantly increased with the number of TAMs both in the samples with 184M and in those with 184V (P = 0.005 and P = 0.003, respectively). A resensitizing effect of M184V was confirmed for all samples exhibiting at least one TAM (P = 0.03). However, accumulation of at least two TAMs resulted in more than 2.0-fold reduced susceptibility to tenofovir, irrespective of the presence of M184V. Decision tree building, a classical machine learning technique, was used to generate models for the interpretation of mutations with respect to tenofovir resistance. The application of previously proposed cutoffs for a reduced response to therapy and treatment failure demonstrated the central roles of positions 215 and 65 for 1.5- and 4.0-fold reduced susceptibilities, respectively. Thus, clinically relevant resistance may be conferred by the accumulation of TAMs, and the resensitizing effect of M184V should be considered only minor.

Biological effects of short-term or prolonged administration of 9-[2-(phosphonomethoxy)propyl]adenine (tenofovir) to newborn and infant rhesus macaques

The reverse transcriptase inhibitor 9-[2-(phosphonomethoxy)propyl]adenine (PMPA; tenofovir) was previously found to offer strong prophylactic and therapeutic benefits in an infant macaque model of pediatric human immunodeficiency virus (HIV) infection. We now summarize the toxicity and safety of PMPA in these studies. When a range of PMPA doses (4 to 30 mg/kg of body weight administered subcutaneously once daily) was administered to 39 infant macaques for a short period of time (range, 1 day to 12 weeks), no adverse effects on their health or growth were observed; this included a subset of 12 animals which were monitored for more than 2 years. In contrast, daily administration of a high dose of PMPA (30 mg/kg subcutaneously) for prolonged periods of time (>8 to 21 months) to 13 animals resulted in a Fanconi-like syndrome (proximal renal tubular disorder) with glucosuria, aminoaciduria, hypophosphatemia, growth restriction, bone pathology (osteomalacia), and reduced clearance of PMPA. The adverse effects were reversible or were alleviated following either complete withdrawal of PMPA treatment or reduction of the daily regimen from 30 mg/kg to 2.5 to 10 mg/kg subcutaneously. Finally, to evaluate the safety of a prolonged low-dose treatment regimen, two newborn macaques were started on a 10-mg/kg/day subcutaneous regimen; these animals are healthy and have normal bone density and growth after 5 years of daily treatment. In conclusion, our findings suggest that chronic daily administration of a high dose of PMPA results in adverse effects on kidney and bone, while short-term administration of relatively high doses and prolonged low-dose administration are safe.

Structures of HIV-1 RT-DNA complexes before and after incorporation of the anti-AIDS drug tenofovir

Tenofovir, also known as PMPA, R-9-(2-(phosphonomethoxypropyl)adenine, is a nucleotide reverse transcriptase (RT) inhibitor. We have determined the crystal structures of two related complexes of HIV-1 RT with template primer and tenofovir: (i) a ternary complex at a resolution of 3.0 A of RT crosslinked to a dideoxy-terminated DNA with tenofovir-diphosphate bound as the incoming substrate; and (ii) a RT-DNA complex at a resolution of 3.1 A with tenofovir at the 3' primer terminus. The tenofovir nucleotide in the tenofovir-terminated structure seems to adopt multiple conformations. Some nucleoside reverse transcriptase inhibitors, including 3TC and AZT, have elements ('handles') that project beyond the corresponding elements on normal dNTPs (the 'substrate envelope'). HIV-1 RT resistance mechanisms to AZT and 3TC take advantage of these handles; tenofovir's structure lacks handles that could protrude through the substrate envelope to cause resistance.

Molecular mechanisms of tenofovir resistance conferred by human immunodeficiency virus type 1 reverse transcriptase containing a diserine insertion after residue 69 and multiple thymidine analog-associated mutations

Two amino acids inserted between residues 69 and 70 of human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT) are rare mutations that may develop in viruses containing multiple thymidine analog (zidovudine [AZT], stavudine)-associated mutations and that confer high-level resistance to all currently approved chain-terminating nucleoside and nucleotide RT inhibitors (NRTIs). The two known mechanisms of resistance to NRTIs are decreased incorporation and increased excision. The mechanism used by RT insertion mutants has not been described for tenofovir (TFV), a recently approved agent in this class. A patient-derived HIV-1 strain (strain FS-SSS) that contained an insertion mutation in a background of additional resistance mutations M41L, L74V, L210W, and T215Y was obtained. A second virus (strain FS) was derived from FS-SSS. In strain FS the insertion and T69S were reverted but the other resistance mutations were retained. The FS virus showed strong resistance to AZT but low-level changes in susceptibilities to other NRTIs and TFV. The FS-SSS virus showed reduced susceptibilities to all NRTIs including TFV. Steady-state kinetics demonstrated that the relative binding or incorporation of TFV was slightly decreased for FS-SSS RT compared to those for wild-type RT. However, significant ATP-mediated excision of TFV was detected for both mutant RT enzymes and followed the order FS-SSS RT > FS RT > wild-type RT. The presence of physiological concentrations of the +1 nucleotide inhibited TFV excision by the wild-type RT and slightly inhibited excision by the FS RT, whereas the level of excision by the FS-SSS RT remained high. Computer modeling suggests that the increased mobility of the beta3-beta4 loop may contribute to the high-level and broad NRTI resistance caused by the T69 insertion mutation.

Molecular determinants of multi-nucleoside analogue resistance in HIV-1 reverse transcriptases containing a dipeptide insertion in the fingers subdomain: effect of mutations D67N and T215Y on removal of thymidine nucleotide analogues from blocked DNA primers

Human immunodeficiency virus type 1 isolates having dipeptide insertions in the fingers subdomain of the reverse transcriptase (RT) show high level resistance to 3 '-azido-3 '-deoxythymidine (AZT) and other nucleoside analogues. Insertions are usually associated with thymidine analogue resistance mutations, such as T215Y. The resistance phenotype correlates with increased ATP-dependent phosphorolytic activity, which facilitates removal of thymidine analogues from inhibitor-terminated primers. In this report, we show that substituting Thr, Ser, or Asn for Tyr-215 in a multidrug-resistant RT, bearing a Ser-Ser insertion between codons 69 and 70, leads to AZT and stavudine resensitization through the loss of the ATP-mediated removal activity. The mutation D67N, which is rarely found in insertion-containing strains, had no effect on excision and a minor influence on resistance. Substituting Tyr-215 had a larger effect than deleting the dipeptide insertion. The presence of both the insertion and mutation T215Y in the wild-type BH10 RT conferred significant ATP-mediated removal activity and moderate resistance to AZT. However, resistance levels and unblocking activities were lower than those observed with the multidrug-resistant enzyme. Removal reactions can be inhibited by the next complementary dNTP. Both Tyr-215 and the dipeptide insertion affect RT-DNA.DNA-dNTP ternary complex formation, an effect that was not detected in the presence of foscarnet. Based on crystal structures of binary and ternary complexes of HIV-1 RT, we propose that Tyr-215 exerts its action by facilitating a proper orientation of the pyrophosphate donor molecule, whereas the effects on dNTP binding are indirect and could be related to significant conformational changes occurring during polymerization.

Natural variation of drug susceptibility in wild-type human immunodeficiency virus type 1

Wild-type viruses from the ViroLogic phenotype-genotype database were evaluated to determine the upper confidence limit of the drug susceptibility distributions, or "biological cutoffs," for the PhenoSense HIV phenotypic drug susceptibility assay. Definition of the natural variation in drug susceptibility in wild-type human immunodeficiency virus (HIV) type 1 isolates is necessary to determine the prevalence of innate drug resistance and to assess the capability of the PhenoSense assay to reliably measure subtle reductions in drug susceptibility. The biological cutoffs for each drug, defined by the 99th percentile of the fold change in the 50% inhibitory concentration distributions or the mean fold change plus 2 standard deviations, were lower than those previously reported for other phenotypic assays and lower than the clinically relevant cutoffs previously defined for the PhenoSense assay. The 99th percentile fold change values ranged from 1.2 (tenofovir) to 1.8 (zidovudine) for nucleoside reverse transcriptase RT inhibitors (RTIs), from 3.0 (efavirenz) to 6.2 (delavirdine) for nonnucleoside RTIs, and from 1.6 (lopinavir) to 3.6 (nelfinavir) for protease inhibitors. To evaluate the potential role of intrinsic assay variability in the observed variations in the drug susceptibilities of wild-type isolates, 10 reference viruses with different drug susceptibility patterns were tested 8 to 30 times each. The median coefficients of variation in fold change for the reference viruses ranged from 12 to 18% for all drugs except zidovudine (32%), strongly suggesting that the observed differences in wild-type virus susceptibility to the different drugs is related to intrinsic virus variability rather than assay variability. The low biological cutoffs and assay variability suggest that the PhenoSense HIV assay may assist in defining clinically relevant susceptibility cutoffs for resistance to antiretroviral drugs.

Highly uneven distribution of tenofovir-selected simian immunodeficiency virus in different anatomical sites of rhesus macaques

Antiviral tenofovir monotherapy was used to determine whether drug-selected simian immunodeficiency virus (SIV) variants replaced their wild-type progenitors at the same rate in different tissues of six rhesus macaques. The relative frequencies of drug-resistant and wild-type genotypes were measured longitudinally in blood and in 23 lymphoid and nonlymphoid tissues collected at necropsy. The mutant/wild-type genotype ratio was measured using a heteroduplex tracking assay targeting tenofovir-selected SIV reverse transcriptase codons. After the initiation of tenofovir treatment in animals with high steady-state viremia levels, resistant genotypes emerged in the plasma within 1 to 8 weeks and in five of six cases reached frequencies of nearly 100% within 4 to 25 weeks. The appearance of tenofovir-resistant genotypes in peripheral blood mononuclear cell (PBMC) DNA was generally delayed by 1 to 2 weeks and in one case was completely absent. Necropsies performed 8 to 55 weeks after the initiation of tenofovir treatment showed the frequency of resistant SIV genotypes to be generally higher in tissue RNA than DNA fractions. The frequency of drug-resistant genotypes varied widely between anatomical sites, including different lymph nodes of the same animal. Except for the epidydimis, the tissues with the lowest rates of proviral replacement by tenofovir-resistant genotypes differed between animals. The highly uneven distribution of tenofovir-resistant genotypes in different tissues seen shortly after the initiation of tenofovir monotherapy may reflect differences in local antiviral drug selection pressures and/or the stochastic effect of small effective populations of drug-resistant variants randomly seeding different anatomical sites early in therapy.

Clinical potential of the acyclic nucleoside phosphonates cidofovir, adefovir, and tenofovir in treatment of DNA virus and retrovirus infections

The acyclic nucleoside phosphonates HPMPC (cidofovir), PMEA (adefovir), and PMPA (tenofovir) have proved to be effective in vitro (cell culture systems) and in vivo (animal models and clinical studies) against a wide variety of DNA virus and retrovirus infections: cidofovir against herpesvirus (herpes simplex virus types 1 and 2 varicella-zoster virus, cytomegalovirus [CMV], Epstein-Barr virus, and human herpesviruses 6, 7, and 8), polyomavirus, papillomavirus, adenovirus, and poxvirus (variola virus, cowpox virus, vaccinia virus, molluscum contagiosum virus, and orf virus) infections; adefovir against herpesvirus, hepadnavirus (human hepatitis B virus), and retrovirus (human immunodeficiency virus types 1 [HIV-1] and 2 [HIV-2], simian immunodeficiency virus, and feline immunodeficiency virus) infections; and tenofovir against both hepadnavirus and retrovirus infections. Cidofovir (Vistide) has been officially approved for the treatment of CMV retinitis in AIDS patients, tenofovir disoproxil fumarate (Viread) has been approved for the treatment of HIV infections (i.e., AIDS), and adefovir dipivoxil (Hepsera) has been approved for the treatment of chronic hepatitis B. Nephrotoxicity is the dose-limiting side effect for cidofovir (Vistide) when used intravenously (5 mg/kg); no toxic side effects have been described for adefovir dipivoxil and tenofovir disoproxil fumarate, at the approved doses (Hepsera at 10 mg orally daily and Viread at 300 mg orally daily).

HIV type 1 genotypic resistance in a clinical database correlates with antiretroviral utilization

We established a database of HIV-1 reverse transcriptase (RT) and protease (PR) sequences and mutations to monitor the prevalence of antiretroviral drug resistance and mutational patterns in clinical samples submitted for testing to a major U.S. reference laboratory. At the end of 1998, 80% of the clinical samples tested harbored HIV strains with genotypically predicted resistance to at least one antiretroviral (ARV) drug. By the third quarter of 2002, the frequency of genotypically predicted resistance declined to 65% of samples tested. The prevalence of both PR and nucleoside RT inhibitor resistance declined over this period, while an increase in resistance to nonnucleoside RT inhibitors was found. These genotypic results strongly correlated with a nationwide decrease in the prescription of PR and nucleoside RT inhibitors, and an increase in the prescription of nonnucleoside RT inhibitors over the time period. The increased number of strains that were genotypically sensitive to all classes of ARV probably indicates an increase in genotypic assay use in ARV-naive individuals, however, the trends and correlations in this data set were similar when evaluated after removal of genotypically sensitive strains. Continued monitoring of ARV resistance prevalence, patterns, and utilization trends in clinical databases provides insight into the evolving relationship between clinical practice and ARV resistance.

Tenofovir disoproxil fumarate

OBJECTIVE

To review the pharmacology, virology, pharmacokinetics, efficacy, safety, resistance profile, and clinical use of tenofovir disoproxil fumarate.

DATA SOURCES

A MEDLINE search was performed (1966-August 2002) using the following terms: tenofovir, tenofovir disoproxil fumarate, PMPA (9-(R)-[2-(phosphonomethoxy)propyl]adenine), and Viread. Abstracts from HIV-related meetings were reviewed. DATA EXTRACTION AND STUDY SELECTION: Publications and meeting abstracts regarding tenofovir were reviewed. The most recent and pertinent items were included.

DATA SYNTHESIS

Tenofovir disoproxil fumarate is a nucleotide prodrug that is diphosphorylated to its active moiety, tenofovir diphosphate. In this form, tenofovir acts as a reverse transcriptase inhibitor to inhibit HIV-1 replication. In clinical trials, tenofovir was effective at suppressing HIV-1 RNA and boosting CD4+ cell counts. Tenofovir has a long intracellular half-life, which permits once-daily dosing. Since tenofovir does not interact with the cytochrome P450 pathway, it exhibits minimal drug interactions, with the exception of didanosine. Compared with other reverse transcriptase inhibitors, tenofovir may have advantages in terms of toxicity and medication adherence profiles. Ongoing studies are also analyzing tenofovir's activity against hepatitis B virus.

CONCLUSIONS

Tenofovir has been shown to be active against HIV-1 in combination with other antiretrovirals. The drug's benefit as a single-agent intensifier of highly active antiretroviral therapy in treatment-experienced patients has been established, and preliminary data for treatment-naïve patients are encouraging.

Tenofovir: a nucleotide analog for the management of human immunodeficiency virus infection

Tenofovir disoproxil fumarate, an acyclic nucleotide analog of adenosine monophosphate, is the most recent addition to the antiretroviral arsenal. After conversion to tenofovir by diester hydrolysis, subsequent phosphorylation by cellular enzymes to form the active tenofovir diphosphate is necessary for antiretroviral activity. Preliminary data suggest that tenofovir is as safe and efficacious as stavudine when given in combination with lamivudine and efavirenz for the treatment of antiretroviral-naïve patients. In antiretroviral-experienced patients, the addition of tenofovir to stable background antiretroviral therapy resulted in approximately a 0.6 log10 copies/ml reduction in viral load relative to placebo. Extended follow-up suggests that such virologic gains may be durable. In vitro, recombinant human immunodeficiency virus (HIV) expressing the K65R mutation showed a 3-4-fold increase in the 50% inhibitory concentrations of tenofovir when compared with wild type. In vivo, this mutation thus far appears to occur infrequently and is associated with variable virologic responses. Response rates to tenofovir vary with the number and pattern of thymidine analog mutations present before starting treatment with this agent. Tenofovir appears to be a well-tolerated agent in patients who are heavily pretreated and who have advanced disease. The main adverse effects appear to be gastrointestinal in nature and include nausea, vomiting, and diarrhea. In animals, osteomalacia and nephrotoxicity have occurred with tenofovir at exposures much higher than those observed in humans. Although no patient had to discontinue therapy as a result of elevated creatinine levels or hypophosphatemia through 58 weeks of treatment, the toxicities associated with long-term tenofovir therapy in humans are unknown. Concomitant administration of tenofovir and didanosine increases the area under the concentration-time curve of the latter by 44-60%; monitoring for signs and symptoms of didanosine toxicity is recommended. The approved dosage of tenofovir is 300 mg (one tablet) once/day with meals. Given the ease of administration and relative safety from the perspectives of adverse effects and drug interactions, tenofovir has the potential to assume a large role in the treatment of patients with HIV infection.

Tenofovir disoproxil fumarate: a nucleotide reverse transcriptase inhibitor for the treatment of HIV infection

BACKGROUND

Tenofovir disoproxil fumarate (DF) is the first nucleotide reverse transcriptase inhibitor approved for use in combination with other antiretroviral agents in the treatment of HIV-1 infection in the United States. Unlike the nucleoside reverse transcriptase inhibitors, which must undergo 3 intracellular phosphorylation steps for activation. nucleotide analogues such as tenofovir require only 2 such steps. This reduction in the phosphorylation requirement has the potential to produce more rapid and complete conversion of the drug to its pharmacologically active metabolite.

OBJECTIVE

This article describes the pharmacologic properties and potential clinical usefulness of tenofovir DF.

METHODS

Relevant information was identified through searches of MEDLINE (1996-April 2002), Iowa Drug Information Service (1996-April 2002), and International Pharmaceutical Abstracts (1970-April 2002), as well as from meeting abstracts of major HIV/AIDS conferences (1996-2002), using the search terms tenofovir tenofovir disoproxil fumarate, PMPA, bis(POC)PMPA, GS-4331-05, acyclic nucleoside phosphonate, and nucleotide reverse transcriptase inhibitor. Additional information was obtained from material submitted to the US Food and Drug Administration by the manufacturer of tenofovir DF in support of its New Drug Application.

RESULTS

In vitro, tenofovir DF has exhibited anti-HIV activity in various HIV-infected cell lines and has produced a synergistic or additive effect against HIV when combined with other antiretroviral agents. In adult humans, tenofovir has a volume of distribution of 0.813 L/kg, is minimally bound to plasma protein (7.2%), has a plasma elimination half-life of 12.0 to 14.4 hours, and is mainly excreted unchanged in urine (70%-80%). Dose adjustment based on sex or body weight does not appear to be necessary, although dose reduction may be necessary in the elderly; there are currently no data on tenofovir DF in renal or hepatic insufficiency. The results of clinical trials suggest the efficacy of tenofovir DF in reducing plasma levels of HIV-1 RNA when used as an add-on to a stable antiretroviral regimen. The most commonly (>3%) reported adverse events in clinical trials have included nausea, diarrhea, asthenia, headache, vomiting, flatulence, abdominal pain, and anorexia. The most commonly (>2%) reported laboratory abnormalities (grade III or IV) included increases in creatine kinase, triglycerides, amylase, aspartate aminotransferase, and alanine aminotransferase, as well as hyperglycemia and glucosuria. Serious adverse events leading to discontinuation of tenofovir DF were infrequent (5%), occurring with an incidence similar to that with placebo (8%). The recommended dosage of tenofovir DF in adults is 300 mg/d PO; pharmacokinetic and efficacy studies in children are ongoing.

CONCLUSION

Although additional studies are needed, tenofovir DF appears to be a promising agent for the treatment of HIV infection.