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A therapeutic HIVACAT T-cell immunogen (HTI) vaccine regimen was safe, and about 40% of recipients were able to stay off antiretroviral therapy (ART) for several months, a researcher said.

In a first-in-human phase I/II randomized trial that began with 45 participants, eight of 20 participants receiving the vaccine successfully went 22 weeks without ART, compared with one who received placebo, reported Beatriz Mothe, MD, of IrsiCaixa Institute for AIDS Research in Badalona, Spain.

Moreover, five vaccine participants and one placebo participant maintained viral loads of less than 2,000 copies/mL during the analytical treatment interruption period of up to 24 weeks, though all participants experienced some level of viral rebound.

These promising results, presented as a late-breaking study at the virtual Conference on Retroviruses and Opportunistic Infections (CROI), caught the attention of the International AIDS Society. IAS President Adeeba Kamarulzaman, MD, characterized the trial as "an exciting step forward to HIV cure research" in a statement.

"The level of viral control off ART was not as strong as what our long-term goal is in HIV cure studies," said Kamarulzaman, of the University of Malaya in Kuala Lumpur, Malaysia. "However, the beneficial effect of immunization on controlling viral load was clear and represents the first proof of concept in People Living with HIV that stimulation of HIV-specific T-cells can contribute to cure strategies."

The goal of any cure strategy is to achieve a viral load of less than 200 copies/mL in the absence of ART, she added.

Mothe emphasized that these results are a long ways away from changing current clinical practice of HIV management, but "our encouraging data strongly support the use of HTI vaccines as a T-cell backbone for future combination cure strategies."

To date, therapeutic vaccines have shown limited ability to contain viral replication once ART is interrupted, but HTI vaccines are designed to induce specific immune responses associated with better viral control, Mothe said.

was a single-center randomized trial comprised of patients with early treated HIV infection, meaning they had to start early with a three-drug therapy within 6 months of documented HIV acquisition and had to be virologically suppressed for at least a year.

Participants were randomized 2:1 to receive a prime-boost vaccine regimen: three doses of DNA vaccine followed by two doses of viral vector vaccine, and then three more doses of viral vector vaccine, or matching placebo before entering the analytical treatment interruption period of 24 weeks.

There were 45 participants in the first phase, and 42 agreed to continue to the second phase. All but one entered the analytical treatment interruption period. Participants were nearly all men who have sex with men (MSM), with an average age of 34-37.

Notably, a minority of participants had potentially beneficial human leukocyte antigen (HLA) class I alleles, which may have contributed to favorable virologic responses. Mothe and colleagues excluded them when assessing participants' ability to maintain analytical treatment interruption, leaving 32 participants without any potential beneficial HLA class I alleles.

Mothe's group also noted that the magnitude of HTI-specific responses at the time of analytical treatment interruption positively correlated with time off ART in the vaccinated group.

Next steps include testing the vaccine in a broader population, potentially including the vaccine as part of a combination approach alongside a latency-reversing agent or an agent to boost immune response and help reduce the viral reservoir.

"That does not mean therapeutic vaccines could not work among people who haven't started ART," Mothe said at a press conference.

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