At CROI last year, Gregory Laird presented results from a laboratory study that tested the activity of several candidate HIV latency-reversing agents (LRAs) on latently infected CD4 T cells isolated from HIV-positive individuals on ART. The results were disappointing: only one LRA showed a very modest effect on HIV gene expression and none induced significant production of HIV virions (the study was later published in Nature Medicine). Laird concluded his CROI talk by offering a glimpse at more hopeful preliminary data suggesting that combinations of LRAs could perform better, and these experiments have since been completed and were published recently in the Journal of Clinical Investigation (the paper is open access).
The nub of the results is that pairings of LRAs from different classes can show synergistic or additive effects, depending on the combination. In particular, HDAC inhibitors—several of which are undergoing evaluation in clinical trials—showed evidence of synergy with the PKC agonists bryostatin-1 and prostratin. The researchers used the extent of latency reversal achieved with maximal CD4 T cell activation as the benchmark for judging the activity of LRA combinations and, as an example, bryostatin-1 and the HDAC inhibitor panobinostat prompted an induction of HIV gene expression that averaged around 50% of that obtained with T cell activation. Although there are concerns about the potential toxicity of bryostatin-1, it is being evaluated in a small trial in Spain and other PKC agonists—most notably an extract from a tropical shrub named Ingenol B—are being considered for clinical testing. Laird’s study found that low doses of bryostatin-1 worked synergistically with the HDAC inhibitor romidepsin, suggesting a means to try and minimize the risk of adverse effects.
The paper also presents data derived from mathematical models indicating that, in clinical trials, effective LRAs or LRA combinations should produce bursts of HIV production of sufficient magnitude to be detected with standard viral load assays. The authors note that so far evidence of this effect has only been reported for the HDAC inhibitor romidepsin (see the AIDS 2014 conference presentation by Dr. Ole Schmeltz Søgaard).
The senior author of the study is latency expert Robert Siliciano from Johns Hopkins University. In a separate open access review just published in Trends in Microbiology, Siliciano and colleagues describe the current tools available to measure the latent HIV reservoir in vivo. The pros and cons of each are delineated, and the need for improved technologies highlighted in the concluding remarks: “Without a high-throughput, sensitive, and well-validated assay, it will remain difficult for researchers to identify novel LRAs, move forward with clinical trials, and develop strategies to eradicate HIV-1.”
J Clin Invest. doi:10.1172/JCI80142.
Gregory M. Laird1, C. Korin Bullen1, Daniel I.S. Rosenbloom2, Alyssa R. Martin3,Alison L. Hill4, Christine M. Durand1, Janet D. Siliciano1 and Robert F. Siliciano1,5
Authorship note: Gregory M. Laird and C. Korin Bullen contributed equally to this work.
Published March 30, 2015
Submitted: November 21, 2014; Accepted: February 6, 2015.
Reversal of HIV-1 latency by small molecules is a potential cure strategy. This approach will likely require effective drug combinations to achieve high levels of latency reversal. Using resting CD4+ T cells (rCD4s) from infected individuals, we developed an experimental and theoretical framework to identify effective latency-reversing agent (LRA) combinations. Utilizing ex vivo assays for intracellular HIV-1 mRNA and virion production, we compared 2-drug combinations of leading candidate LRAs and identified multiple combinations that effectively reverse latency. We showed that protein kinase C agonists in combination with bromodomain inhibitor JQ1 or histone deacetylase inhibitors robustly induce HIV-1 transcription and virus production when directly compared with maximum reactivation by T cell activation. Using the Bliss independence model to quantitate combined drug effects, we demonstrated that these combinations synergize to induce HIV-1 transcription. This robust latency reversal occurred without release of proinflammatory cytokines by rCD4s. To extend the clinical utility of our findings, we applied a mathematical model that estimates in vivo changes in plasma HIV-1 RNA from ex vivo measurements of virus production. Our study reconciles diverse findings from previous studies, establishes a quantitative experimental approach to evaluate combinatorial LRA efficacy, and presents a model to predict in vivo responses to LRAs.
Trends Microbiol. 2015 Apr;23(4):192-203. doi: 10.1016/j.tim.2015.01.013. Epub 2015 Mar 5.
Bruner KM, Hosmane NN, Siliciano RF.
The latent reservoir (LR) of HIV-1 in resting memory CD4+ T cells serves as a major barrier to curing HIV-1 infection. While many PCR- and culture-based assays have been used to measure the size of the LR, correlation between results of different assays is poor and recent studies indicate that no available assay provides an accurate measurement of reservoir size. The discrepancies between assays are a hurdle to clinical trials that aim to measure the efficacy of HIV-1 eradication strategies. Here we describe the advantages and disadvantages of various approaches to measuring the LR.