Researchers synthesize potent stimulators of HIV expression in latently infected cells

The eradication of viral reservoirs remains among the most challenging obstacles to curing HIV infection, a problem that researchers have sought to solve by inducing HIV replication in latently infected, resting CD4+ T cells. The expectation is that the cells would then either die as a result of the renewed viral replication or become vulnerable to targeting by antiretroviral drugs or immune responses. 

One drug that researchers hope might stimulate the virus to come out of hiding is known as SAHA. The drug inhibits the enzyme histone deacetylase, which causes histones to wrap around DNA more tightly. By loosening that interaction, SAHA makes DNA more accessible to transcription factors, including those that turn on the expression of latent HIV genes. SAHA is currently being evaluated in human trials, and has been shown to increase intracellular RNA expression (see Stalking HIV’s Sleeper Cells, IAVI Report, Mar.-Apr. 2012). 

But HIV maintains its latency in a variety of ways, so it will likely take a combination of drugs with different mechanisms of action to eradicate all parts of the reservoir, says UCLA researcher Jerome Zack. One possible alternative mechanism operates via the activation of protein kinase C (PKC), an enzyme that in turn activates a transcription factor called NFκB, which induces HIV expression in latently infected cells. “If you hit different mechanisms, you may impact different subsets of [latent] virus” says Zack, adding that SAHA has been shown to synergize with PKC activators in previous studies. 

Now Zack and colleagues report that they have synthesized molecules that resemble the natural PKC activator prostratin, which is extracted from the bark of a tree that grows on the island of Samoa. Two of these new synthetic prostratin analogs, they report, can bind PKC much better than the natural version and are 100-fold more potent in the induction of HIV RNA expression in latently HIV infected CD4+ T cell lines, and in latently infected CD4+ T cells taken from people on highly active antiretroviral therapy (Proc. Natl. Acad. Sci. 2013, doi: 10.1073/pnas.1302634110). 

One of Zack’s collaborators, Paul Wender and his group at Stanford, synthesized the new prostratin analogs from croton oil, which is extracted from seeds and is easier to obtain than natural prostratin. Because the analogues are synthetic, they should be easier to modify if they happen to be toxic, something Zack and colleagues now plan to test in mice. They also plan to test if the prostratin analogs can induce HIV expression in latently HIV infected CD4+ T cells in humanized mice. “If all looked good, one would contemplate doing clinical trials,” Zack says. 

One concern, however, is that the molecules could have some unpredictable side effects, since PKC activates a wide variety of genes. One way to address this is to target the action of the prostratin analogs to HIV infected CD4+ T cells. In 2011, Zack’s group reported that they had placed another PKC activator called bryostatin inside a liposome that carried CD4-specific antibodies on its surface (PLoS One 6, e18270, 2011). As expected, the liposomes only targeted the CD4+ T cells in a mix of CD4+ and CD8+ T cells. A similar strategy might be used to target prostratin analogues to appropriate cells.

Any remaining side effects might be minimized for another reason, Zack says: Any treatment with drugs that eradicate the HIV reservoir would likely be short term. “With a limited number of administrations,” he ventures, “you would hopefully activate the reservoir cells and purge them” before the drug can cause any real collateral damage.