GS-9620, an investigational toll-like receptor or TLR7 agonist, led to immune activation in a study of macaque monkeys infected with an HIV-like virus, and two of the animals treated with multiple doses have maintained viral suppression for at least three months after stopping antiretroviral treatment, according to research presented at the Conference on Retroviruses and Opportunistic Infections (CROI 2016) last week in Boston.
Researchers working on a functional cure for HIV have studied various 'kick and kill' strategies aimed at reactivating latent virus and then attacking it. Histone deacetylase or HDAC inhibitors have been most extensively studied as latency-reversing agents, but they may have non-selective activity that causes toxicity.
TLR7 agonists are an alternate approach. Toll-like receptors on immune cells are part of the innate or immediate immune response, but they promote adaptive immunity, or recognition of and response to specific viruses and other pathogens. TLR7 activation leads to increased antigen presentation and enhanced activity of natural killer cells, antibody-producing B-cells, and CD4 and CD8 T-cells.
James Whitney from Beth Israel Deaconess Medical Center presented findings from a study evaluating whether Gilead Sciences' GS-9620, a selective orally administered TLR7 agonist, would influence HIV RNA levels, perturb the viral reservoir and limit viral rebound after interrupting antiretroviral therapy (ART). Previous research showed that GS-9620 induced HIV expression in laboratory cultures of peripheral blood mononuclear cells (PBMCs) from people with HIV.
At last year's CROI, Whitney reported that rhesus macaques infected with a simian relative of HIV (SIVmac251) and treated with seven doses of GS-986 (a TLR7 agonist similar to GS-9620) showed increased CD4 and CD8 cell activation. While the first three doses had no effect on plasma SIV levels, subsequent doses led to transient viral load increases. However, viral rebound after stopping ART was no different in monkeys treated with TLR7 or placebo, and the study drug led to interferon-alfa production, which causes side-effects.
At this year's meeting, Whitney presented data from a longer-term follow-up study in which SIV-infected rhesus monkeys were treated with GS-986 or GS-9620 at lower doses that hopefully would not trigger interferon-alfa production and would be clinically relevant for treatment of people with HIV.
The 11 monkeys started ART using tenofovir, emtricitabine and dolutegravir at 65 days post-infection and achieved viral suppression (<50 copies/ml). On day 467 (about 5.5 months) after infection, three groups started GS-986 at 0.1 mg/kg or GS-9620 at 0.05 mg/kg or placebo every other week for 10 doses, then took a three-month break, then restarted the same treatment for nine more doses. The fourth group received GS-9620 at a dose of 0.15 mg/kg every other week for 10 doses, then took a seven-month break with no resumption. At that point all the monkeys were taken off ART.
The first two doses of the TLR7 agonists had no notable effect on plasma virus levels. But the third through tenth doses led to transient viral blips while still on ART. Whitney said the lower doses affected the frequency of blips but the magnitude was similar. After the three-month pause, however, further doses did not produce additional changes in viral load.
Monkeys treated with the TLR7 agonists showed increased activation of lymphocyte subsets including CD4 and CD8 cells. Plasma interferon-alfa was intermittently detected in the GS-986 group, but mostly unchanged in the GS-9620 and placebo groups. Treated macaques had elevated levels of various cytokines including interleukin-1 RA, I-TAC and MCP-1, as well as increased activity of interferon-stimulated genes. However, these showed no direct correlation with virus blips.
The treated macaques showed a more pronounced decline in viral DNA levels in memory CD4 T-cells in peripheral blood, lymph nodes and colon biopsies. When stimulated with a mitogen (an agent that activates T-cells) seven of the nine treated monkeys showed reductions in inducible virus. The remaining two had undetectable virus production in both PBMCs and lymph nodes before stopping ART. Animals in the placebo group showed no change.
ART was interrupted two weeks after the last TLR7 dose and most of the macaques experienced rapid viral load rebound. However, two monkeys – one that received 0.1 mg/kg GS-986 and one that got GS-9620 0.15 mg/kg – continued to have undetectable plasma viraemia through 3-4 months after antiretroviral discontinuation. The same two animals also had no mitogen-inducible virus in PBMCs or lymph node cultures.
Repeated low doses of GS-986 or GS-9620 led to "induction of transient plasma viremia," with no blips after doses 11-19, and two of nine monkeys treated with the TLR7 agonists have remained aviraemic for at least three months after stopping ART, the researchers concluded. There was little to no change in interferon-alfa and multiple doses were well-tolerated.
A phase 1b clinical trial of GS-9620 in people with HIV on ART is now underway. The drug has also shown antiviral activity against hepatitis B virus and is being evaluated for hepatitis B treatment in a phase 2 trial.
"Our ultimate goal with TLR7 agonist therapy is to stimulate the body to drive latent HIV out of viral reservoirs in infected cells and to enhance virus-specific immune responses in HIV-infected individuals," Whitney stated in a Gilead press release. "This study demonstrates the approach has promise, and that lower, longer-term TLR7 agonist dosing may be a potentially useful approach to inducing long-term HIV-remission."
Whitney JB et al. Repeated TLR7 agonist treatment of SIV+ monkeys on ART can lead to viral remission. Conference on Retroviruses and Opportunistic Infections (CROI), Boston, abstract 95LB, 2016.