Given that HDAC inhibitors can potentially cause toxicity and immune
suppression, Gilead Sciences is looking at a different type of
latency-reversing agent, the toll-like receptor 7 (TLR7) agonist GS-9620.
Toll-like receptors on immune cells play a role in defence against
viruses and other pathogens. TLR7 activation leads to increased antigen
presentation and enhanced activation of natural killer cells, CD8 killer
T-cells and CD4 T-cells.
James Whitney from Harvard Medical School and colleagues evaluated an
analogue of GS-9620 in ten rhesus macaques with SIV, an HIV-like virus that infects
monkeys. The macaques started combination ART about 9 weeks after infection and
achieved stable viral suppression. At 45 weeks post-infection, four of the
monkeys received seven oral doses (0.1-0.3 mg/kg) of the TLR7 agonist every
other week, while the other six received a placebo.
The monkeys showed a consistent increase in CD8 T-cell activation while
taking the TLR7 agonist, as well as a smaller increase in CD4 T-cell
activation, which returned to baseline levels after the last dose.
The first three doses had no effect on plasma viremia, but after the fourth or
fifth dose all treated monkeys had transient increases in plasma viral load (500-1000
copies/ml), which returned to undetectable within 4-7 days after the last dose.
The control animals showed no change in viral load.
Viral DNA levels decreased by 30-90% in peripheral blood mononuclear
cells (PBMCs) and in colon and lymph node biopsy samples from three of the four
treated monkeys; levels again remained unchanged in the control group.
ART was discontinued two weeks after the last TLR7 agonist dose to see
if these transient plasma viral blips and decreases in viral DNA corresponded
to a reduction in the latent viral reservoir. The speed of plasma viral load
rebound was similar in the treated and control monkeys, but those that received
the TLR7 agonist had about a 0.5 log10 lower viral set-point than
the control group.
The researchers concluded that the TLR7 agonist "was safe, induced
transient plasma viremia, reduced viral DNA content in PBMCs, colon and
lymphoid tissues and established lower viral set-point after ART cessation."
"We get
some kick, but also get some kill," Whitney explained at a CROI press
conference. "After repeated dosing we can elicit viraemic blips, which
tells us we're unleashing the latent reservoir." He added that the TLR
agonist was very well tolerated.
In a related
poster presentation, Gilead scientists reported that GS-9620 induced HIV RNA expression in cultures of PBMCs from 18 people
living with HIV on stable suppressive ART who underwent leukapheresis, a
procedure in which blood is removed, white cells are separated out, and the rest
is returned to the donor. In a laboratory study, the collected PBMCs were
exposed to GS-9620 (100 or 1000 nM) or DMSO as a control for four days, along with
antiretrovirals.
GS-9620 activated HIV expression more than DMSO: at either dose GS-9620 induced
at least 2-fold greater HIV activation in cells from 13 of the 18 donors (72%),
with a mean of 9-fold greater and up to 27-fold greater. The drug's variable
effect on cells from different donors may be due to variations in the
combination of 'kick' and 'kill' effects, the researchers suggested.
GS-9620 also induced production of a wide variety of
cytokines (immune cell chemical messengers) including interferons, interleukins,
tumour necrosis factor-alfa, IP-10 (CXCL10) and I-TAC (CXCL11). Interferon
alfa/beta (type I) receptor signalling was required for maximal HIV activation,
and blocking the receptor reduced HIV expression by up to 85%. Viral activation
was also associated with peak production of IP-10 and I-TAC, although the role of these chemokines in HIV activation is not yet known.
The drug's effect on the latent HIV reservoir was assessed by using a
protein kinase C (PKC) agonist to activate CD4 T-cells. GS-9620 led to decreased
HIV expression after activation, but the researchers said more studies are needed
to determine if this indicates reduction of the viral reservoir.
A clinical trial of GS-9620 is now underway in ART-treated people with
HIV. The drug is also being evaluated as a treatment for hepatitis B, and recently
entered a phase 2 trial for that indication.