Hide and Seek
Led by Dr. Daria Hazuda, researchers tackle HIV latency with HDAC inhibitors
by Chael Needle
With all the talk around viral suppression, it may seem odd at first to hear that researchers are looking for agents that reactivate HIV in the body, but not once you remember that HIV has the ability to go dormant. The virus hides out in stable cellular reservoirs, established early during acute infection. In these resting memory CD4+ T cells, the virus is beyond the radar of the immune system and beyond the reach of antiretrovirals and other biological processes that could eradicate it. This capacity means that antiretrovirals alone will not provide a “cure” for HIV.
HIV latency occurs when the virus infects resting (non-activated) CD4+ T cells. While antiretroviral therapy has been shown to suppress the replication of HIV in T cells, it cannot affect the virus in latently infected cells, whose reservoirs are located throughout the body—the genital tract, bone marrow, lymphoid tissue, the brain, among other sites. Whether the virus infects activated T cells or cells that come to rest, the viral genome—the blueprint for replication—is integrated into the DNA of cells. So, the virus persists in a reversible, low-productive state, and it can “wake up” when the factors that effect productive infection are present.
One approach that researchers have investigated has been to disrupt latency by disrupting the genomic conditions that maintain it in individuals who have been successfully treated with HAART.
First, you need to find the genes that maintain latency. Part of the work of Daria Hazuda, PhD, World Wide Discovery Head of Antiviral and Infectious Disease Research at Merck Research Labs, has been to conduct small interfering RNA screenings to find human genes that maintain HIV latency and to disable them. If the genes are targeted and knocked out, latent HIV is expressed. If HIV is expressed, HIV-induced cell death could occur and, additionally, HIV would be vulnerable to antiretroviral therapy.
Dr. Hazuda’s laboratory has so far found over 400 human genes whose activity is related to the maintenance of HIV latency and, in particular, the screening approach has identified histone deacetylases (HDACs) as viable targets.
Dr. Hazuda, who, previous to our interview, delivered the Bernard Fields Lecture, entitled, “Antiretroviral Drug Discovery: HIV-1 Integrase Inhibitors and Beyond,” at CROI 2013. There, she talked about moving drug discovery efforts in eradication forward, building on the established proof of concept that inducing latent HIV gene expression in suppressed patients with HDAC inhibitors, such as SAHA [suberoylanilide hydroxamic acid, or Vorinostat (brand name Zolinza)], is possible.
Last year at CROI, David Margolis, MD, presented study results of Zolinza, a rare-blood cancer drug from Merck that was found to reactivate latent cells of eight HIV-1-infected patients in vitro. In theory, Zolinza works by inhibiting HDAC, an enzyme that allows HIV to go dormant and escape surveillance. The single-dose study was replicated and extended by Sharon Lewin, MD, who led a multiple-dose study.
The toxic effects of Zolinza and other HDAC inhibitors prevent them from being repurposed as anti-HIV agents, but the HDAC-inhibition template they provide is promising. Notes Dr. Hazuda: “They weren’t designed to be used in a non-oncology setting and what you need in terms of the biochemical profile in a drug that’s going to be used to treat cancer is likely very different than what is necessary to induce HIV gene expression, so that’s why it’s particularly important to go back to the basic biochemistry to really understand that [and] so we can hopefully design molecules that are much better suited for the purpose that we want to use them for.”
Dr. Hazuda is leading research that addresses these questions around the basic biochemistry of expression and eradication. “Even though you can induce expression of HIV in the latent reservoir, it appears that that is not sufficient to eliminate those cells. So one of the things that we have been focusing on is really twofold:
“First, how do we understand how HDAC inhibitors work to increase HIV gene expression in order to increase their activity and dial out some of the toxicity and tolerability issues that all of the HDAC inhibitors that have been developed for oncology have.
“Secondly, just as in the early days of therapy, trying to identify drugs that will work in combination with HDAC inhibitors that will increase the amount of gene expression that we’re getting in cells. We hypothesize that one of the reasons we’re not seeing the elimination of those cells is that we’re not expressing enough HIV gene product. So we know that in the context of HIV replication, the expression of HIV proteins can be toxic to cells, so we think that in the context of latency we’re not expressing enough of these proteins to actually induce the cytopathic effects that normally occur during HIV replication to kill those cells. So that’s one of the ways in which we’re trying to approach the problem and I showed a little bit of data from our high-throughput screening efforts to specifically look for small molecules that will really enhance the immune activity that we’re getting with SAHA.”
Dr. Hazuda, who deciphered the basic biochemistry of the HIV integrase, and whose lab discovered the mechanisms surrounding integrase inhibition and developed the first-in-class integrase inhibitor, knows well that these are still the “early days” of research.
With enough tools in the toolbox, however, research can move toward in vivo or small pilot clinical trials to test hypotheses about expression and eradication. Says Dr. Hazuda: “Even if enhanced expression by itself doesn’t lead to elimination, enhanced expression certainly will be important when we start to think about combination approaches with respect to immune recognition—the more antigens that we can make these cells express, the more likely they’ll actually reveal themselves to the immune system. So I know there’s a lot of interest by many groups in terms of targeting these cells, and later enhancing the immune function or providing some other ways—antibody-directed approaches—that may be able to eliminate those cells. So, in either case, I think having more expression is going to be an important element of the eradication strategy.”
More is needed to be known about latency, as well. “We can’t think of the latent reservoir as a uniform compartment. I think there are many different kinds of cells, kinds of T cells, in fact, that…are established in the underlying biology. It may be slightly different in each of these compartments. That’s another reason why combination approaches are going to be critical because not only should we be looking at combinations that can increase the amount of expression but the breadth of activity we have across these different biological systems.”
With recent talk of “functional cures,” cures that do not involve the complete eradication of HIV, the problem of latency would seem like an essential piece of the solution. Asked about the extent to which it is possible for someone infected with HIV to go off treatment if virus is cleared to a low enough level, Dr. Hazuda points to the proof of concept established by the VISCONTI cohort, which showed that fourteen individuals treated with antiretrovirals during the earliest stage of infection appear to be controlling the virus years after treatment interruption.
“Are the patients who were treated early and treated for a long enough period of time and, apparently the people who have gone off therapy and successfully remain aviremic, the ones who had the smallest reservoirs?” posits Dr. Hazuda about the relationship about achieving a small enough reservoir and the possibility of going into a state of remission for a period of time without needing treatment.
“It’s almost like thinking about it from an oncological perspective, where you can reduce the burden to a size that becomes manageable. Not that you’re truly cured, but you don’t have any untoward effects of having HIV, at least for long periods of time. So the VISCONTI cohort is the proof of concept that it may be possible to reach some threshold in patients that we’ll make that [sort of “cure”] happen.”
Asked if the long-term goal that’s energizing the research field right now is the possibility of a functional cure, Dr. Hazuda says: “Yes, I think so. I think there have been a lot of things that have come together to make people become far more interested in pursuing these lines of research. One, therapy is highly effective. The second is that it’s going to take a very long time to develop an effective vaccine. And we have some hints that [a functional cure] may be possible—through the Berlin Patient, the VISCONTI cohort, the pediatric case [the baby in Mississippi who appears to have cleared the virus after post-natal antiretroviral therapy].
The much-touted “AIDS-free generation,” where individuals with HIV are on treatment but whose disease state does not progress to AIDS, will be effected through a combination of efforts. Says Dr. Hazuda: “I think if we’re really going to make an impact on the epidemic, globally, it really is going to take multiple approaches, so, yes, we have to make therapy as effective as possible for patients who have HIV and make sure that people who have HIV have access to those therapies. [We have to] destigmatize therapy so people feel that it’s not an issue getting diagnosed and being treated. We have to increase efforts in prevention research and, again, I think we have a wonderful proof of concept that was presented a couple of years ago that antiretrovirals actually do work as chemo-prevention but people have got to take them and that’s very difficult for people who don’t perceive themselves at risk. And [we need to think about] how can we improve the delivery of chemo-prevention so that we can increase adherence and make it much more effective. Then also think about whether or not [patients] may be able to have a functional cure, even if it’s only in a fraction of patients.
“Whenever people talk about a functional cure, I always like to remind people what’s happened in the field of hepatitis C. If you look historically at hepatitis C, with the first interferon-based treatment regimen and the first hint that we would be successful in curing HCV, the success rates were less than ten percent. But it provided a glimpse, it provided the basis for believing that in fact if we could have more effective ways to intervene that we could actually increase [the success rate]. Now with the small pilot studies [for] the direct-acting antivirals, we’re getting eighty to ninety percent. It’s outstanding but it started out with those first small studies with that five to ten percent success rate.”
The same promise may hold true for HDAC inhibitors’ possibility for inducing gene expression, says Dr. Hazuda. Now we need to invest in research to truly “understand why these things work, how they’re working in order to really improve them and make them more effective and less toxic.”
Chael Needle wrote about non-melanoma skin cancer risks in the March issue.
Read the article in the April 2013 digital issue by clicking here.