Once-monthly oral pill as PrEP against HIV: islatravir study to start in 2021

Simon Collins, HIV i-Base

On 16 November 2020, Merck (MSD outside the US) announced details of a new phase 3 study for oral PrEP with the investigation HIV drug islatravir, that if successful, would involve taking only one pill a month. [1]

The proposed islatravir study, named IMPOWER22, will randomise approximately 4,500 cisgender women and adolescent girls aged 16 to 45 to either once-monthly islatravir or daily TDF/FTC plus matched placebo that will involve all participant taking daily pills throughout the study.

The study is being funded by the Bill & Melinda Gates Foundation for sites in countries in southern Africa, with Merck supplying the study drug and covering costs for US trial sites.

Other studies are planned for gay men and transgender women.


Although these are exciting plans, the timing of the announcement suggests marketing rather than any urgency of medical news. This islatravir study is not due to start until early 2021 and details have not yet been posted to the trial registry

However, on 9 November 2020, the HPTN 084 study reported early news that cabotegravir LA injections for PrEP were effective at preventing HIV transmission and that the study could end its blinded phase earlier than expected and offer injections to all trial participants. [2]

Islatravir is still in research studies for use as a treatment for HIV and these are exciting plans for PrEP.

The monthly pill also has the potential for a single pill to be used as post-exposure prophylaxis (PEP) similar to a morning after pill, based on early results in animal studies. [3]

If these studies are effective, and access to islatravir is easy and affordable – both significant challenges – then the impact on achieving the 2030 target for eliminating HIV transmission globally will be very exciting.


  1. MSD press release. MSD advances phase 3 trial to evaluate investigational islatravir as once-monthly oral PrEP for women at high risk for acquiring HIV-1. (16 November 2020).
  2. Two-monthly cabotegravir injections prevent HIV infection in African women: HPTN 084 study recommends early unblinding. HTB (11 November 2020).
  3. Monthly islatravir for PEP and PrEP: 12 pills a year could cover unmet need for HIV prevention in billions of people globally. HTB (12 March 2020).

This report was first posted on 30 November 2020.


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