When an investigational drug causes a 0.4 log drop in viral load, it's evidence of biological activity.
When the first large auto-vaccination study produces the same result, it signals the end of an entire field of HIV research. -- RJ
October saw the first publication of results from the Swiss-Spanish Intermittent Treatment Trial (SSITT), the largest study of structured treatment interruptions in chronic HIV infection conducted to date.
The design of SSITT was based on the "auto-vaccination hypothesis": the idea that short interruptions of HAART might augment the immune response to HIV by exposing the immune system to brief, controlled bursts of viral replication. If the media stories that accompanied the release of the data (published in two papers in J. Virology and Proceedings of the National Academy of Sciences) are to be believed, SSITT was an abject failure: "Drug Holidays Not Beneficial for Fighting HIV," quoth Reuters. A commentary accompanying the PNAS paper, authored by Ume Abbas and John Mellors, was headlined with a lengthy dismissal: "Interruption of antiretroviral therapy to augment immune control of chronic HIV-1 infection: Risk without reward."
The first broad overview of the SSITT results was provided by principal investigator Bernard Hirschel at the February Retrovirus Conference. The trial enrolled 133 chronically infected individuals with CD4 counts >300 and viral loads <50 copies on HAART, with a history of at least six months suppression to less than 500 copies prior to study entry. Participants underwent a two week interruption of treatment followed by eight weeks of re-treatment, and this cycle was repeated four times prior to an open-ended treatment interruption at week 40.
Anyone whose viral load was not re-suppressed to <50 copies/mL during the eight week re-treatment periods was automatically excluded from the protocol. [NB: Many of the details of the Swiss-Spanish treatment interruption study, such as entry criteria, structure of time on and off treatment, as well as rules for expulsion from study and definitions of success and failure, represent "best guess" inputs and in some cases are clearly arbitrary. Future STI and SIT studies will need to experiment with these variables in an attempt to fine-tune these parameters and help to bring the immunity picture into clearer focus.]
At week 52 of the trial, 67 participants remained enrolled. Of these, 23 maintained viral loads below a pre-defined "responder" threshold of 5,000 copies, but seven had viral loads in this range before ever starting HAART. The other 44 had viral loads above this threshold, but did not meet the protocol-mandated criteria for restarting therapy.
At the Retrovirus meeting earlier this year, Dr. Hirschel graphically illustrated his interpretation of the data as regards stimulating HIV-specific immunity. He put up a slide of a gravestone, inscribed "Herein Lies the Auto-Vaccination Hypothesis." The primary basis for this provocative conclusion was the analyses of HIV-specific CD8 T cell responses, which were -- confoundingly -- of a greater magnitude in the non-responders than the responders, at least as measured by the ELISpot assay employed in the study.
The newly published studies represent a more detailed exploration of the data described by Hirschel, focusing on the immunological and virological outcomes. In PNAS, the research team -- led by immunologist Annette Oxenius from the Nuffield Department of Clinical Medicine, John Radcliffe Hospital, Oxford, UK -- reports on the 97 participants in the trial that were recruited in Switzerland. In the Journal of Virology, the same team focuses on a subset of 13 individuals for whom detailed data on CD8 T cell responses and HIV viral load were available. The titles of the papers serve as pithy summaries of the conclusions: "Stimulation of HIV-specific cellular immunity by structured treatment interruption fails to enhance viral control in chronic HIV infection" (PNAS) and "HIV-Specific CD8 T-Cell Responses Do Not Predict Viral Growth and Clearance Rates During Structured Intermittent Antiretroviral Therapy" (J Virology).
Taking the larger PNAS study first, the logical presumption from the title is that the participants' viral load remained unchanged compared to their pre-therapy baseline. Strangely, this does not turn out to be the case: "A comparison of plateau viral [load] to pretreatment viral load showed that plateau viral load was significantly lower than pretreatment viral load (P= 0.005, paired t test); however this difference was small (mean log pretreatment viral load= 4.2826 and mean log plateau viral load 3.896)."
This translates into a change from an average of 19,320 copies pre-HAART to 7,396 copies off therapy at the end of the SSITT protocol -- a decrease in viral load of about 0.4 log. Not spectacular, certainly, but statistically significant nevertheless, and seemingly an indication that the STIs had somehow altered the equilibrium between the host and HIV replication.
Although it is unclear in the PNAS paper whether the reinitiation of therapy was accounted for in this viral load analysis, the J. Virology substudy echoes the finding (an average 0.4 log drop in viral load, in this case from a mean of 4.6 to 4.2 log), and clearly states that the data from individuals that restarted HAART was based on two or three samples taken just prior to beginning therapy. It is therefore a little surprising when the researchers go on to say: "This result is comparable to studies of single treatment interruptions where the ensuing plasma viral load was similar to pretreatment viral load."
Perhaps it is the failure to identify any immunological correlates that has caused the researchers to shy away from making too much of the observed change in viral load setpoint. Critical questions, however, are left unanswered -- particularly in terms of whether the responses measured by ELISpot truly reflect functional HIV-specific immunity. A number of recent studies have suggested that this may not be the case, demonstrating differences between ELISpot results and those obtained using techniques that measure HIV-specific T cell proliferation (proliferation is a distinct property possessed by some, but not all, antigen-specific T cells).
A team of researchers led by Mark Connors at the NIH has recently shown clear differences in the proliferative responses of HIV-specific CD8 T cells in long-term non-progressors (LTNPs) compared to both untreated and HAART-treated individuals with progressive disease. No such differences were observed when the HIV-specific CD8 T cell response was assessed based on interferon-gamma production. British immunologist Francis Gotch has reported a similar disconnect between proliferative capacity and cytokine production when HIV-specific CD4 T cells from LTNPs and progressors are compared.
The overarching theme that emerges from these data is one that has been sounded since the very earliest days of HIV research: the immunological correlates of control of HIV replication are opaque, and need to be clarified. Recent basic immunology research in animal models has shed new light on the development and maintenance of virus-specific T cell and antibody responses, but efforts to translate these findings to the setting of human HIV infection are in their infancy. Given this fact, it seems important to question whether the "small" reduction in viral load setpoint seen in SSITT is really a basis for ending all research evaluating the immunological and virological effects of STIs in chronic infection, as Abbas and Mellors argue in their commentary.
When monotherapy with nucleoside analogue antiretrovirals was shown to produce a small and transient drop in viral load, these drugs were not dismissed, but rather improved by their eventual inclusion in HAART regimens which produced more lasting effects. Similarly, it can be argued that SSITT represents a first attempt at enhancing control of HIV replication through the use of STIs, and the results -- by showing that viral load setpoint can indeed by reduced -- set a standard for future STI studies in chronic infection to improve upon. Neither the SSITT authors nor Abbas and Mellors, however, appears willing to countenance such a possibility.
In part, the negative interpretation of the SSITT results appears driven by concerns regarding the risks associated with the use of STIs in chronic infection. Abbas and Mellors cite several potential risks, including symptomatic acute retroviral syndrome, re-seeding of viral reservoirs, CD4 T cell decline, increased risk of transmission, poor adherence to continuous therapy and the potential risk of drug resistance (especially NNRTI and 3TC) due to declining drug levels during the few days immediately after stopping HAART.
Clearly, the failure of the SSITT protocol to produce any obvious and dramatic immunological or virological benefit threatens to cast a pall over future STI research, at least in terms of auto-vaccination. Efforts to secure funding and Institutional Review Board (IRB) approval for such studies may well be affected, undermining investigators -- such as immunologist Luis Montaner from the Wistar Institute in Philadelphia -- attempting to pursue answers to some of the outstanding questions described in this article.
The political aspect of STI research may also be playing a role in the interpretation of SSITT. As Cal Cohen from the Community Research Initiative of New England has observed, there is no incentive for pharmaceutical companies (or, for that matter, the many researchers they fund) to support strategies that aim to allow people with HIV to safely go without drug treatment for extended periods. To ensure that future STI research is guided by science -- not prejudice or politics -- the treatment activist community will need to closely monitor developments in this field.
Richard Jefferys' complete analysis, "SSITT Downer?", is available here.
|Pre-Treatment Viral Setpoint||Viral Setpoint Post-Rx+STIs|
|Pt. 1||561,831 cps/mL||76,805 cps/mL|
|Source: Journal of Virology, 10/2002|
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