Depletion of latent HIV-1 reservoir with valproic acid: interesting data but not a cure

13 October 2005. Related: Treatment access, Cure-related research, Basic science and immunology.

Gareth Hardy, HIV i-Base

In the 13 August edition of the Lancet, Ginger Lehrman of University of Texas South Western Medical Centre, USA, reported a small study investigating the effects of the drug, valproic acid (an anticonvulsant) on depletion HIV-1 reservoirs. This proof of concept study was conduced in four patients who had been treated with fully virally suppressive HAART (<50 copies/mL) for at least two years and who then were initiated on an intensified HAART regimen which included their original regimens with the addition of 90 mg T-20 (enfuvirtide) twice-daily. Following a further 4-6 weeks of stable T-20, patients were initiated on three months of 500-700 mg twice-daily oral valproic acid.

The authors explained that valproic acid has been previously described to inhibit the enzyme Histone deacetylase 1 (HDAC1). Histone deacetylase 1 mediates chromatin remodeling, viral gene expression and virion production, since histone deaceteylation is important in maintaining latency of HIV provirus and repression of HIV-1 gene expression. Valproic acid has, therefore been demonstrated in vitro to induce the expression of latent HIV from resting CD4 T cells in patients on HAART maintaining undetectable viraemia. Such induction of latent expression by valproic acid is as efficient as that induced by mitogen, but in contrast does not give rise to cellular activation. Approaches designed to achieve activation of latent integrated virus without cellular activation are likely to be considerably more successful than previous strategies which have involved global T cell activation in the patient, presenting both an abundance of new host cells for induced virus to replicate in and significant, potentially life-threatening toxicity.

Leukopheresis was conducted on each patient at baseline (before T-20) and following 16-18 weeks treatment with T-20-intensified HAART and valproic acid. 200-1200 million resting CD4+ T cells were obtained from the resting leukocytes and were purified for recovery and quantification of replication-competent virus in CD8+ T cell-depleted PBMC outgrowth assays. Real-time PCR for integrated HIV DNA was also carried out on purified resting CD4+ T cells. The number of infected units of resting CD4+ T cells per billion (IUPB) was estimated using a maximum likelihood method. In addition, plasma viral RNA was quantified by a real-time RT-PCR assay capable of quantifying and detecting HIV-1 RNA down to 1 copy/mL and immunophenotypic analysis was carried out for cell surface activation and differentiation markers by flow cytometry.

The authors report that the treatment regimen was well tolerated and all patients adhered well to therapy. Patient 1 had a single episode of transient viraemia (71 copies/mL), which was self limiting and associated with an upper respiratory tract infection, and which resolved without intervention. No significant changes were noted in the proportion or level of expression of cell surface activation markers of naive and memory CD4+ or CD8+ T cells. Three of the four patients were culture negative for viral outgrowth culture at baseline and remained negative throughout the study period. The fourth patient was culture positive at baseline and at initiation of valproic acid therapy (after 6 weeks HAART intensification). Following 11 weeks of valproic acid, this patient became culture negative. One of the patients who maintained culture negativity throughout had detectable levels of viral RNA at various time points during the study, though always below 10 copies/mL. Thus replication competent viral DNA was not always demonstrable despite the occasional presence of plasma viral RNA and the persistent presence of integrated viral DNA.

Most interestingly, there was a substantial decline of integrated viral DNA during the study. This decline in resting CD4+ T cell infection was much greater than predicted by the half-life estimates previously reported (44.2 months on normal HAART, 10.3 months on some intensified regimens). The minimum decline seen in IUPB was 29%, with a decline of more than 50% in the other three patients. Declines were: patient 2 – >84%, patient 3 – 68% and patient 4 – 72% (patient 2’s result was estimated on a single macroculture as outgrown virus could not be recovered at week 18 – end of valproic acid therapy). Based on these figures the authors suggest that valproic acid decreases the half-life of HIV latently-infected resting CD4+ T cells to 2-3 months. This is however based on a single time point determination of IUPBs following 3 months treatment, although multiple determinations were made prior to valproate therapy, which confirmed the stability of the latent reservoir on normal HAART regimens. Taken together with the substantial decline in the frequency of replication-competent HIV recovered from peripheral resting CD4+ T cells in these patients, the authors suggest that this data argues in favour of the use of HDAC inhibitors in long-term HAART treated patients with stable undetectable viral RNA levels. They go on to suggest that the eradication of established HIV infection might be achieved by such an approach, with the concomitant intensification of HAART.

Comment

It is unclear whether the mechanism for maintaining HIV latency targeted by valproic acid is responsible for maintaining all latent HIV in the body. In addition, although in theory T-20 should not be able to impact the resting CD4+ T cell reservoir, as there is no data on this, the impact of valproic acid vs additional of T-20 in this study is not known. At least one previous study has reported that intensification of anti-HIV therapy decreases the half-life latently infected CD4+ T cells [2]. However, as two patients had residual viremia after addition of T-20, it is unclear whether this intensification is either necessary or particularly beneficial.

In addition, the assays used to quantify the resting CD4 T cell reservoir are complex, cumbersome and the intra-person variability from day to day is uncertain. Even defining and isolating resting CD4 T cells is complicated and the samples aren’t 100% pure. Absolute CD4 counts vary on a daily basis in patients on stable therapy and this may impact on the latent pool of infected cells.

This is interesting work, but it presented tentative data on a potential mechanism in four patients, none of whom show any evidence that eradication is possible, and with no control group. The possibility of a cure should never be forgotten or abandoned, but the way the Lancet presented this data, using a quote on their front cover to suggest that a cure is suddenly more likely, and generating subsequent ‘cure’ reports in the lay press, was just exploitative.

References:

1. Lehrman G, Hogue IB, Margolis DM et al. Depletion of latent HIV-1 infection in vivo: a proof-of-concept study. Lancet 2005, 366:549-555.
2. Ramratnam B, Ribeiro R, Markowitz M et al. Intensification of antiretroviral therapy accelerates the decay of the HIV-1 latent reservoir and decreases, but does not eliminate, ongoing virus replication. J Acquir Immune Defic Syndr 2004, 35:33-37.