New treatment a cure? Moment of truth draws nigh
No signs of virus in blood or organs after two years
As the time rapidly approaches when a group of research patients reach the point where HIV theoretically should be completely eradicated from their bodies, biopsies of their lymph nodes, semen, and spinal fluid show these compartments are clear of infectious virus even after more than two years of combination therapy. The findings give researchers more hope that once treatment is stopped, the patients could continue to remain virus-free, or to put it another way cured of their infection.
"The ultimate test is to stop therapy and see if the virus will or will not come back, and that point is rapidly approaching for a number of our patients," said David Ho, MD, director of the Aaron Diamond AIDS Institute in New York, where the patients are being studied. "While some may view this with great trepidation, the patients are very understanding and cooperative, and these regimens are hard to tolerate, so it would be a joy if they could stop. We don’t know how this is going to go, but the results could be quite clear after stopping medication."
Speaking at a recent forum sponsored by the Whitehead Institute for Biomedical Research in Boston, Ho gave an update on 20 patients who began triple-drug therapy shortly after they became infected and have been followed for nearly two and a half years. Because antiretroviral therapy short-circuits the infection of new cells only, and not those already infected, the therapy must continue until all infected cells have died a natural death; otherwise, any remaining cells could theoretically resurrect the infection.
Based on their studies of the half-life of the different classes of cells that become infected with HIV, Ho and other researchers have estimated it would take the body at least two and a half years to clear out infectious HIV from all the compartments known to harbor the virus. Earlier this year, it was reported that none of the patients were willing to go off treatment, and Ho was criticized for taking the next step without knowing more about the status of the virus in bodily compartments that are harder for antiretroviral drugs to penetrate.
The new findings suggest that the virus, indeed, is clear in all 20 patients, he said. Although proviral DNA is found in these patients, it is believed that these particles are mutations or fragments that cannot replicate or cause infection. Nonetheless, there could be other compartments that harbor the virus or virus particles that are undetectable through current assays.
"We don’t really know if we will ever get rid of the virus," Ho said on a more sober note. "It is pretty clear by doing these aggressive early interventions that we have eliminated 99.99% of the virus, but that last bit we think is a real problem."
How big a problem? To answer that question, the institute has been performing ongoing follow-up studies since two years ago when it first established the groundbreaking findings that the virus was multiplying during the early stage of infection at a much faster rate than previously thought as much as 10 billion copies per day. The fact that this intense viral activity takes place not only in plasma but in lymphatic tissue as well has led to a new emphasis on treating HIV infection early in the course of disease, before it wreaks havoc on the immune and lymphoid systems.
These studies also have tried to determine why treatment results in an initial dramatic drop in virus, which then levels out after several months and reaches a "set point," a steady-state level at which the amount of new virus produced roughly equals the amount killed by the body. The virus often remains at this set point for months and even years.
Ho’s research has rapidly filled in the holes in this picture of the viral dynamic, showing that this second phase of infection has two features:
• The slower clearance of virus suggests that virus in these bodily compartments dies off more slowly.
• Even though combination therapy can reduce virus to undetectable levels in a matter of weeks, during this second phase there are still lurking latently infected CD4 cells and macrophages that die off much more slowly, taking several days or weeks compared to the six-hour half-life estimate for plasma virus. (See table above.)
While these latently infected cells and macro phages make up only an estimated 10% of the proportion of infected cells, their eradication time is estimated at 400 to 600 days. Moreover, the other 90% of infected cells are made up of defective or mutated cells whose half-life is estimated at around 100 days and could take seven to 10 years to eradicate from the body, said John Coffin, PhD, professor of molecular biology at Tufts University Medical School in Boston.
"While these DNA copies are presumed to be defective and individually incapable of restarting infection, it is at least a theoretical possibility that rare fusion of such cells could give rise to replicating virus at some later time," he reported at the 7th Annual Clinical Care Options Symposium in early June in Potomac, MD. There may exist an undetected class of infected cells that would take even longer to eradicate, he added.
With slower decay profiles, said Ho, these cellular compartments could serve to rekindle the infection if the drug regimen were withdrawn too soon.
Because two and a half years is only an estimate based on numerous variables, a patient would be reluctant to go off medication a regimen endured faithfully and yet with numerous side effects only to have the virus fire back up. Should that happen, Ho said, the patient would be put back on the same regimen and should respond, because theoretically no new and therefore possibly mutant virus was being produced during treatment.
Virus crops back up after months of treatment
Indeed, the one study patient who had to discontinue treatment for a time saw his virus level rise to detectable limits, but the level fell once treatment was restarted, he added. This patient’s experience is enlightening because it showed that even with months of treatment powerful enough to suppress the virus to undetectable levels, "the virus was still around," Ho points out.
These newly discovered viral dynamics of HIV form the underpinnings of new recommendations that monotherapy should not be used anymore "and that we should go ahead and use the big guns that we have in combination. It is no wonder that monotherapy doesn’t work," he explains. "It is easy for HIV to quickly mutate and become resistant to many of the therapies we have. And in fact it suggests that the only way we could do this [attain undetectable viral levels] is to apply so much pressure that the virus has to mutate at many positions at the same time."
Further research is trying to establish the possibility that plasma virus has a half-life shorter than six hours, which would have treatment implications in that the whole timetable for complete eradication could be shortened.
Two studies attempting to nail down the exact time suggest that the half-life might be minutes rather than hours. In one study, a monkey was injected with simian immunodeficiency virus. The virus particles were "radio-labeled" and tracked while the virus was constantly infused. The time it took for the virus to reach a steady state or equilibrium was 10 minutes. That is the same half-life found in similar studies with other viruses, such as polio and vaccinia, Ho said.
If the same dynamic is going on in humans infected with HIV, each day the virus in the extracellular fluid pool turns over 100 times all the more reason to quench this raging fire as soon as possible. Unfortunately, the fever and aches that are the only symptoms of the onset of HIV infection don’t show up in many infected patients or are mistaken for the flu. The relatively asymptomatic nature of early HIV infection, coupled with reluctance of many people at high risk for HIV to be tested, makes it unlikely that the majority of HIV-positive patients will be treated during primary infection, when combination therapy is considered most beneficial, he added.
(Editor’s note: Copies of talks presented at the 7th Annual Clinical Care Options for HIV Symposium are available on the Internet at http://www.healthcg. com.)