Pharmaceutical giant Merck has announced successful clinical trial results for its new medication, islatravir. The drug is being investigated as both a treatment for HIV and as a form of PrEP.

Merck announced the results of an ongoing Phase 2a clinical trial at the virtual 11th International AIDS Society Conference on HIV Science (IAS 2021) this week.

The trial is studying islatravir as a once-a-month tablet for PrEP.

Subjects in the study were given either 60mg of islatravir, 120mg of islatravir, or a placebo. The study group was at low risk of HIV, and the primary aim was to determine tolerance and the level of islatravir that remained in the patients' bloodstream and for how long.

The participants took the drug once a month for six months. After this period, follow-ups were done to monitor how quickly the drug left their system.

The study found that eight weeks after taking their last tablet, those who took either 60mg or 120mg of islatravir still had enough of the medication in their bloodstream to prevent HIV infection. This fell, however, at the 12-week follow-up.

"The 24-week analysis of investigational, once-monthly oral islatravir not only builds upon the PK data we have already seen but also provides encouraging support for the safety and tolerability profile of this HIV-1 PrEP regimen," said Dr. Joan Butterton, vice president, global clinical development, infectious diseases, Merck Research Laboratories, in a statement.

Merck is currently recruiting Gay men and Trans women for a similar Phase 3 trial to see if the results are the same for people at high risk of HIV. That study will begin later this year.

Matthew Hodson, chief executive of UK-based HIV information organization Aidsmap, said he welcomed the development of longer-acting PrEP.

"There are now a number of drugs that can be used as either HIV treatment or for PrEP that have a long life in the body. Monthly pill dosing is currently in trial, and dosing by injection or implant could soon be as infrequent as twice a year."

Hodson said that PrEP is "extremely, almost totally, effective," but that adherence to daily drug regimes was a problem. Longer-lasting medications promised to overcome this issue, "as well as being more discreet for people who may be stigmatized for their PrEP use, or for their HIV treatment," Hodson said.

As a treatment for HIV, islatravir is being jointly studied as a combination treatment with Gilead's doravirine, as a potential once-a-month HIV treatment pill. Again, trials to date have shown promising results. If successful, it could be another useful medication regimen for those showing resistance to other drugs.

In July, Gilead announced it had applied for FDA approval for a once-a-month injectable HIV treatment known as lenacapavir. While it will be marketed initially as an HIV treatment, that drug also has potential as a PrEP medication.