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Study Sheds Light on How Tenofovir is Processed in Different Body Tissues

Tenofovir, which is widely used for HIV treatment and prevention, is activated in a tissue-specific manner, being processed by different enzymes in immune cells, the vagina, and the colon, according to a study published in the July 19 advance edition of EBioMedicine. These findings have implications for the effectiveness of tenofovir for pre-exposure prophylaxis (PrEP).

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Tenofovir disoproxil fumarate or TDF (Viread, also in the Truvada, Atripla, Complera, and Stribild coformulations) is among the most widely used antiretroviral drugs for HIV treatment. A new formulation under development, tenofovir alafenamide or TAF, reaches higher levels in HIV-susceptible immune cells when taken at lower doses that are easier on the kidneys and bones.

Oral Truvada is the only agent currently approved for HIV PrEP. Other tenofovir formulations are under study including gels and vaginal rings. The iPrEx trial of mostly gayand bisexual men showed that once-daily Truvada reduced the risk of HIV infection by 44% overall, rising to 92% among participants with blood drug levels indicating consistent use.

Truvada PrEP also performed well in the Partners PrEP and TDF2 studies of heterosexual couples in Africa. But the Fem-PrEP and VOICE trials of young women were unable to show a protective effect of Truvada or tenofovir vaginal gel. This has primarily been attributed to poor adherence, but animal and human studies suggest that there are also physiological reasons why tenofovir may be less effective against vaginal compared to rectal HIV exposure.

Julie Lade and colleagues fromJohns Hopkins University School of Medicine performed a laboratory study to analyze the chemical processing required to convert tenofovir to its pharmacologically active form in the body.

Tenofovir requires 2 phosphorylation steps (addition of phosphate groups), being converted first to tenofovir monophosphate then to the active form tenofovir diphosphate. But the specific kinase enzymes that activate tenofovir in cells and tissues susceptible to HIV infection have not yet been identified, the study authors noted as background.

In this study the researchers examined peripheral blood mononuclear cells (PBMC) and vaginal and colon-rectal tissue samples that were transfected with siRNA targeting nucleotide kinases and incubated with tenofovir in the laboratory. As explained in a Johns Hopkins press release, this had the effect of "knock[ing] out genes for phosphate-adding enzymes one by one."

They also genetically sequenced clinical samples from 142 women who participated in Microbicide Trials Network study MTN-001, which tested oral tenofovir and 1% tenofovir vaginal gel.

Results

  • Adenylate kinase 2 (AK2) performed the first phosphorylation step -- converting tenofovir to tenofovir monophosphate-- in PBMCs, vaginal tissue, and colon tissue.
  • Both pyruvate kinase isoenzymes -- muscle (PKM) and liver/red blood cell (PKLR) versions -- were able to phosphorylate tenofovir monophosphateto tenofovir diphosphatein PBMCs and vaginal tissue.
  • However, the creatine kinase muscle isoenzyme (CKM) performed this conversion in colon tissue.
  • Sequencing of MTN-001 clinical samples detected 71 previously unreported variants in the genes encoding these kinases, several of which could make the enzymes ineffective.
  • 8% of the women had genetic variants that would likely to make them unable to convert tenofovir to its activated form.

"[O]ur results demonstrate that tenofovir is activated in a compartment-specific manner," the authors concluded. "Further, genetic variants have been identified that could negatively impact tenofovir activation, thereby compromising tenofovir efficacy in HIV treatment and prevention."

"Because these enzymes are polymorphic and may be dysfunctional in some individuals, these findings suggest that tenofovir-based HIV PrEP may not be protective for all individuals," they added.

"Tenofovir has been shown in trials to be very effective, so when it doesn’t work, researchers and clinicians tend to assume the individual just wasn’t taking the drug as directed," coauthor Namandje Bumpus explained in the press release. "That is probably true in most cases, but in others, it’s possible that genetic variation is actually at fault."

"If confirmed by further studies, our results suggest that in the future, before prescribing tenofovir to a patient, a doctor could order genetic testing and know in advance if it works, and prescribe a different drug if it won’t," she added.

7/16/15

Reference

JM Lade, EE To, CW Hendrix, and NN Bumpus. Discovery of Genetic Variants of the Kinases That Activate Tenofovir in a Compartment-specific Manner. EBioMedicine. July 10, 2015 (Epub).

Other Source

Johns Hopkins. New Evidence that Genetic Differences May Help Explain Inconsistent Effectiveness of Anti-HIV Drug. Press release. July 9, 2015.

Drug Levels Are Key to Effectiveness of Tenofovir Vaginal Gel for HIV Prevention

Although the VOICE study failed to show an overall protective effect for tenofovir vaginal gel, further analysis suggests that women who had detectable blood drug levels were at lower risk of HIV infection, according to an analysis described in the June 29 advance edition of Journal of Infectious Diseases. Another recent analysis from the CAPRISA 004 trial showed that higher levels of tenofovir in the genital tract were associated with greater reduction in risk.

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CROI 2015: Tenofovir Vaginal Gel Not Effective Overall Against HIV [VIDEO]

The FACTS 001 trial evaluating a vaginal microbicide gel containing tenofovir did not show overall effectiveness for preventing HIV infection among young women in South Africa, although the product did appear to provide some protection for women who were able to use it consistently, according to a presentation at the 2015 Conference on Retroviruses and Opportunistic Infections (CROI) last week in Seattle.

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CROI 2015: Disappointing Result for Tenofovir Gel Microbicide

Among some highly promising results from HIV prevention studies presented at the 2015 Conference on Retroviruses and Opportunistic Infections (CROI) last week in Seattle, there was one disappointment. FACTS 001, a study testing the efficacy against HIV of a vaginal microbicide gel containing tenofovir, produced a null result: there was no difference in the HIV infection rate for young women given the active gel and the rate for those given a placebo gel.

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Study of Truvada PrEP and Tenofovir Vaginal Gel Misses Mark Due to Low Adherence

The final published report in the New England Journal of Medicine from the VOICE trial of HIV prevention for women in 3 African countries mainly reinforces what conference presentations have already shown: this ambitious trial failed to demonstrate the effectiveness of either oral pre-exposure prophylaxis (PrEP) or of a tenofovir-containing vaginal microbicide gel, and the reason for this was that only 25%-30% of women actually used the study product, despite 88% claiming they did so. The gel, however, may have stopped 2 out of 3 infections among women who used it.

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