Two major analyses of the risk of death or AIDS-related illness in people who started treatment at different CD4 counts have produced conflicting evidence about the benefit of starting treatment substantially earlier than current guidelines recommend, the Sixteenth Conference on Retroviruses and Opportunistic Infections heard on Monday.

A North American cohort study found that people who started treatment with a CD4 count below 500 cells/mm³ had a 60% higher risk of death than people who started treatment above this level, but a collaborative study by European and North American cohorts failed to find any extra benefit of starting above 400 cells/mm³. The second study found that while there was a clear benefit to starting treatment at a CD4 count in the range 350-450 cells/mm³ when compared with a lower count, starting treatment at CD4 count in the range 450-550 cells/mm³ did not provide a further benefit in terms of reducing the risk of AIDS or death.

Treatment guidelines in the developed world all concur that antiretroviral treatment should start around the time the CD4 cell count falls to 350 cells/mm³, although some groups of people, such as those with high viral load or with HIV/hepatitis C co-infection, should consider starting treatment before this point is reached.

However some physicians now believe that treatment should start considerably earlier, when the CD4 count is above 500 cells/mm³, in order to minimise the time during which individuals with HIV are immunosuppressed (the normal CD4 count in healthy adults is in the range of 700-1200 cells/mm³). Immunosuppression might increase the risk of some non-AIDS-defining cancers, which occur more frequently in people with HIV, and might also exacerbate the damage caused by hepatitis C infection, which appears to be more virulent and damaging to the liver in people with lower CD4 cell counts.

At last year’s ICAAC meeting US researchers presented an analysis of US and Canadian patient cohorts which showed a significant advantage to starting treatment before the CD4 count fell below 350 cells/mm³ (a 70% higher risk of death was identified in those who started treatment at a CD4 count below 350 cells/mm³ when compared to above 350 cells/mm³), but that study was not designed to determine whether starting treatment with a CD4 count above 500 cells/mm³ conferred additional benefit.

In order to answer that question Mari Kitahata and colleagues from the North American AIDS Cohort Collaboration looked again at the 9174-patient cohort, and identified 2620 patients who had started treatment at a CD4 count above 500 cells/mm³, and compared their risk of death to those who started treatment later.

Their observational study attempted to mimic the conditions of a randomised trial by only including people who had been diagnosed with HIV at a CD4 count above 500 cells/mm³, in order to take into account the potential biasing effect of missing people who might die with a CD4 count below 500 cells/mm³ before starting treatment. Some previous studies have only looked at people who actually started treatment, and would thus have missed those most likely to die.

They found a 60% higher risk of death for those who deferred treatment (relative hazard 1.60, p<0.001), a highly significant effect, after controlling for potential confounding factors such as age and baseline viral load. They also found that older age was a significant predictor of death, independent of the CD4 count at which treatment was started.

After six years 10% of those who deferred treatment had died, and 15% by eight years, indicating that although the absolute risk of death was small, it was not negligible.

However, this study is still not able to answer the question of whether there is a significant difference between starting treatment in the CD4 count range of 350 to 500 cells/mm³, or starting at a CD4 count above 500 cells/mm³.

A second analysis, using data from 21,247 patients in seven cohorts, yielding 68,256 person-years of follow-up was carried out by the When to Start Consortium, led by Jonathan Sterne at Bristol University in the United Kingdom. That group compared the effects of deferring treatment across a range of CD4 cell bands below 550 cells/mm³, and found that while there was no significant difference in the risk of AIDS or death between those who started in the range 451-550 cells/mm³ and those who started in the range 351 to 450 cells/mm³ (HR 0.99, 95% CI 0.76 – 1.29), nor in the range 401-500 cells/mm³ compared to 301-400 cells/mm³ (HR 1.09, 95% CI 0.85 – 1.38), a significant difference was apparent in the range 351-450 cells/mm³ when compared to 251-350 cells/mm³ (HR 1.28, 95% CI 1.04 – 1.57).

This study also attempted to deal with the problem of deaths that might not be detected by a straightforward comparison of immediate versus deferred treatment, by estimating the rate of events seen at the CD4 counts being studied prior to the adoption of antiretroviral therapy, and then using the observed trends to fill in gaps where follow-up data were missing due to late diagnosis.

It should be noted that the two analyses are not strictly comparable. The North American analysis measured only the risk of death, while the When to Start Consortium measured the risk of death or AIDS. Neither analysis was able to capture the risk of non-AIDS-defining serious illnesses, such as non-AIDS-defining cancers or cardiovascular events, which were detected at a higher frequency in individuals with CD4 counts below 350 cells/mm³ off treatment in the SMART treatment interruption study. Determining the relative risk of these serious illnesses is likely to be a factor in measuring the risk/benefit of earlier treatment, and in particular in determining whether the number needed to treat in order to avoid one event is sufficient to be cost-effective.

The findings sparked a lively debate at the conference about the feasibility of conducting a large 'when to start' trial. In particular, there was concern that the observational analyses might not be able to capture the effect of confounding factors.

As Andrew Zolopa of Stanford University noted, referring to the North American analysis: “Patients who choose to defer [antiretroviral treatment] have different health-seeking behaviours than those who start treatment earlier, who probably wear seat-belts, don’t smoke, etc.”.

However, said Mari Kitahata, the North American cohort analyses had attempted to determine the size of unknown confounding effect that would be necessary in order to affect the result. Any confounding factor would have to reduce the risk of death fourfold to change the result of the analysis, she said – a far greater effect than detected in this study.

Professor Doug Richman of the University of California San Diego questioned whether a 'when to start' trial was worth the expense. “Rather than spend millions on a trial, given that most people aren’t diagnosed until much later, why not use all that money to identify people who have the higher risk?”, he asked.

References
Kitahata M et al. Initiating rather than deferring HAART at a CD4+ count > 500 cells/mm³ is associated with improved survival. Sixteenth Conference on Retroviruses and Opportunistic Infections, Montreal, abstract 71, 2009.

Sterne J for the When To Start Consortium. When should HIV-infected patients initiate ART? Collaborative analysis of HIV cohort studies. Sixteenth Conference on Retroviruses and Opportunistic Infections, Montreal, abstract 72, 2009.

Carl Grunfeld and his colleagues with the Fat Redistribution and Metabolism in HIV Infection (FRAM) study sought to determine whether having HIV is an independent risk factor for atherosclerosis after controlling for other relevant factors. FRAM is an ongoing study of metabolic complications of HIV disease conducted at multiple US medical centres.

One commonly used surrogate marker for asymptomatic or pre-clinical atherosclerosis is intima-media thickness (IMT), a way to assess narrowing of the arteries using ultrasound.

Carotid IMT can be measured in two ways: in the common carotid artery or in the so-called "bulb" region where it branches off into the internal carotid artery supplying the brain. The bulb is an area of blood turbulence, making it more susceptible to atherosclerotic damage. It is more difficult to measure IMT in the internal carotid bulb than in the common carotid, but results may be more accurate.

In this cross-sectional study, investigators compared carotid IMT -- measured in both the common carotid and the internal carotid bulb -- in 433 HIV-positive FRAM participants and 5749 healthy HIV-negative individuals in the Coronary Artery Risk Development In Young Adults (CARDIA) study and the Multi-Ethnic Study of Atherosclerosis (MESA).

FRAM participants were on average younger than those in the general population studies (49 vs 60 years) and were more likely to be men (70% vs 47%). With regard to traditional cardiovascular risk factors, FRAM participants were about twice as likely to be current smokers (36% vs 15%) and had higher mean total cholesterol and triglyceride levels, but were less likely to have diabetes (9% vs 14%).

The investigators found that HIV-positive patients had significantly greater carotid IMT than HIV-negative individuals when measured in the internal carotid bulb (1.17mm vs 1.06), although the difference did not reach statistical significance when looking at the common carotid (0.88mm vs 0.86mm).

After adjusting for age, sex, and race/ethnicity, the difference in IMT was considerably larger in the internal carotid bulb that in the common carotid (0.19mm vs 0.043mm, respectively).

After adjusting further for traditional cardiovascular risk factors including smoking, diabetes, high blood pressure and blood lipid levels, the effect of HIV infection on carotid IMT was somewhat weaker, but still apparent. Again, the effect was more obvious in the internal carotid bulb than in the common carotid (difference of 0.15mm vs 0.033, respectively).

Furthermore, the effect of HIV on atherosclerosis appeared to be somewhat stronger amongst women than men, especially when measuring IMT in the internal carotid bulb.

The researchers estimated that even after adjusting for other risk factors, HIV infection independently increased the extent of atherosclerosis by about the same amount as the strongest traditional risk factors such smoking, diabetes and male sex.

Dr Grunfeld explained that the difference in IMT measured in the common carotid versus the internal carotid bulb might help explain conflicting results from previous studies, some of which showed that HIV and its treatment increased the risk of atherosclerosis and some of which did not see this effect. Although more difficult, carotid IMT measurement in the internal bulb is more likely to reveal an effect, and he recommended using both methods if possible.

Discussing the findings, Dr Grunfeld noted that cardiovascular risk in people with HIV is probably a multi-factorial phenomenon involving both host and viral factors. But the effect of HIV is "very large" -- much greater than the small effect associated with use of specific antiretroviral drugs -- and on the question of whether HIV itself is associated with atherosclerosis, he said, "I believe we've cleared that up."

Reference
Grunfeld C et al. HIV infection is an independent risk factor for atherosclerosis similar in magnitude to traditional cardiovascular disease risk factors. Sixteenth Conference on Retroviruses and Opportunistic Infections, Montreal, abstract 146, 2009.

There's nothing like hearing the results of studies directly from those who actually conducted the research. In this interview, you'll meet one of these impressive HIV researchers and read his explanation of a study he presented at CROI 2009. After his explanation, he then answers several questions from the audience.

Robert Siliciano: What I'm going to talk about today is a new way of looking at how well HAART [highly active antiretroviral therapy] works.¹ As you know, in patients who are doing well on treatment, the viral load becomes undetectable. It's below the limit of detection of clinical assays. But with special assays developed by John Coffin and others, you can still see a small amount of free virus in the plasma. This is called residual viremia.

Robert Siliciano, M.D., Ph.D.

There's been a big argument in the field about whether this represents simply the release of virus from a stable reservoir, such as the one that our lab has been studying for a long time; or, alternatively, that it represents continuous replication of the virus that's occurring, because the drugs haven't really completely controlled replication. The latter scenario is clearly a disturbing one, because it raises a possibility of the evolution of resistance.

I'm going to argue that the residual viremia is due to release of the virus from stable reservoirs. We have directly analyzed the residual viremia. We don't see evidence for it evolving. What we do see, though, is that it seems to be coming from at least two different sources: this latent reservoir in T cells that we described a long time ago, and another -- as yet unidentified -- reservoir. And that is a disturbing result with regard to eradication.

But I think the optimistic thing is that we don't see evidence for ongoing replication. Really, the definitive experiment that people in the field have been proposing for a long time is to intensify therapy -- take a patient who is doing well on treatment and add a fourth drug, and see whether this trace amount of viremia goes down any more.

We have done this in collaboration with John Coffin and others, and it turns out that it doesn't. What this means is that we have already reached the theoretical limit of antiretroviral therapy. We will never reduce viremia any further using antiretroviral drugs, because this little bit of free virus that's in the plasma is not coming from new cycles of replication. It's actually coming from cells that were infected prior to therapy. And so, if we want to get rid of the virus, we have to deal with these stable reservoirs.

The optimistic conclusion is that the drugs that we have now actually, in an adherent patient, completely stop the virus from replicating. A lot of people still have trouble with this idea, and most of the work I'm going to talk about today is actually a pharmacologic explanation of this idea. We think there is a missing dimension in the analysis of how well antiretroviral drugs work. It has to do with the steepness of the dose-response curve.

If you take that into account, what you find is that the drugs are actually way more effective than we thought they were. Just to give you an example: Some of the best protease inhibitors can produce a 10-log reduction in a single round of infection. That's a 10 billion-fold inhibition. This was not apparent before, because the assays weren't available to see this. But we think that by analyzing the inhibitory potential of antiretroviral drugs in the correct way, we'll be able to do a better job of choosing drug regimens, particularly in the case of patients who are failing therapy. By really knowing how much replication is occurring in the patient, and how much each drug actually inhibits replication or contributes to the inhibition of replication, we'll have a more rational approach to choosing drug therapy.

Reporter #1: Do you think, at the end of the day, that we should continue down the road towards the kind of eradication that you've described? Or whether we should really be promoting research in the basic sciences that look at ways in which we can physically get our bodies to coexist with the virus?

Robert Siliciano: I think we should be pursuing strategies for eradication. The first step in eradication is stopping the virus from replicating, and I would say that that step has been accomplished, as a result of the efforts of everybody who participated in the development of HAART. What remains, and which actually may prove much harder, is to find all of the stable reservoirs -- we know there are at least two -- and find ways to eliminate them. There is a lot of momentum now to try and do that.

Reporter #2: What are the implications of this for when to start therapy? Do reservoirs continue to be seeded over the course of active infection, and would it be better to start immediately?

Robert Siliciano: That's a good question. When we looked in patients who were treated during acute infection -- during symptomatic primary HIV infection -- and then kept on HAART for a couple of years, the reservoirs were already established. It's possible that by catching people very, very quickly, we may reduce the size to some extent. And it's possible that delayed treatment may increase the size slightly. But I don't think that's really going to give us a way to attack the problem.

Reporter #3: Do you know what percentage [of residual viremia] comes from the known reservoirs and what percentage comes from the unknown reservoirs?

Robert Siliciano: In about half of the patients that we have analyzed, more than half of the residual viremia is coming from this alternative source. But in other patients, we don't see this. So we still have a lot of work to do. The problem is that it's extremely difficult to do these studies, because the number of viruses that are present in the blood of somebody who's doing well on treatment, at the average level, is about one virus particle per mL of plasma. So it's extraordinarily hard to collect these viruses and analyze them. The best I can say right now is that in about half of the patients, the residual viremia is dominated by this alternative source.

Reporter #4: I'm wondering if you have any clues about what the mystery reservoir might be.

Robert Siliciano: Our current hypothesis is that it represents infection of a progenitor cell in the monocyte/macrophage lineage, and then that cell proliferates after infection. When it does, it copies the viral genome, without any mutation, into multiple progeny cells that go off and produce the same identical virus. The reason I say this is because the unique characteristic of this predominant plasma population is that it's very oligoclonal, which is very unusual in HIV infection. HIV tends to diversify very dramatically, so finding an oligoclonal population is very unusual. One way to explain it is that it's being generated by cell division, which copies an integrated viral genome without error. But we don't actually know the cell source. We can't find any of the cells in the blood.

Reporter #2: Why is it that the virus doesn't continue to seed reservoir sites with ongoing replication?

Robert Siliciano: In patients on HAART? If there is ongoing replication in a patient who's not on treatment or who is failing therapy, there will be new sequences being deposited in these reservoirs. But the size of a latent reservoir in resting T cells is very small. It's one in a million cells. And it's not an efficient process by which these reservoirs are generated. When you have suppression of replication to below 50 copies/mL, there are just no new infection events, and the reservoir is not turning over anymore. You are just left with what you had when you started HAART.

There had been the idea that the reservoirs were being continuously replenished. We've directly looked at that, and we don't see that.

This transcript has been lightly edited for clarity.

Reference

1. Siliciano R. New approaches for understanding and evaluating the efficacy of ARVs. In: Program and abstracts of the 16th Conference on Retroviruses and Opportunistic Infections; February 8-11, 2009; Montréal, Canada. Abstract 16.