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Tenofovir alafenamide PrEP protects monkeys from infection, but rectal and vaginal levels in humans may be too low
Liz Highleyman, 2016-02-25 12:00:00
A proof-of-concept study
showed that the new tenofovir alafenamide (TAF) plus emtricitabine (FTC, Emtriva) protected macaques from
infection with an HIV-like virus, with a degree of protection similar to that
previously seen with the older tenofovir disoproxil fumarate (TDF), researchers
reported at the Conference on Retroviruses and Opportunistic Infections (CROI
2016) this week in Boston. But another study looking at TAF concentrations in rectal
and genital tissue samples from women found lower than expected levels, which
could mean it won't be as effective as TDF/emtricitabine (Truvada) for pre-exposure prophylaxis (PrEP).
Gilead Sciences’ Truvada
is currently the only agent approved for HIV prevention in the US, France and a
few other countries. Studies have shown that it dramatically reduces the risk
of HIV infection when used consistently, with around 90% or better efficacy among
gay men. TDF is generally considered safe and well-tolerated, but it can cause
modest bone loss and kidney problems in susceptible individuals.
TAF is a new tenofovir pro-drug that delivers the
active agent, tenofovir diphosphate, more efficiently to cells. TAF produces high
intracellular drug levels with a 10-fold lower dose than TDF (25mg vs 300mg),
which means about 90% lower drug concentrations in the blood plasma and less
exposure for the kidneys, bones and other organs and tissues.
Gilead has requested
approval of a dual coformulation of TAF/emtricitabine, which could be a successor to Truvada.
Several studies – including one
presented this week at CROI – found that antiretroviral regimens containing
maintain viral suppression as well as those containing TDF/emtricitabine when used for HIV
treatment, but with less detrimental effects on kidney function and bone
Kidney and bone side-effects
among HIV-negative people using TDF/emtricitabine for PrEP are not fully understood. Most clinical trials – which excluded
people with pre-existing kidney problems – did not see significant kidney
toxicity or bone loss, but these have been reported in PrEP demonstration
projects and ‘real world’
Some researchers and
advocates have suggested that TAF could potentially be a safer alternative to
TDF for PrEP, but it is not yet known whether it will be equally effective,
given that it is distributed differently in the body.