Hi to all!!
Sorry I haven’t posted in a while…I tried to have as computer-free a weekend as I could tolerate…Before I start with today’s post, I realized that I forgot to answer the question of how I heard about Enzo during my last Q&A session. I was informed of Enzo by the poster ‘spanky23’ on this board. He is a good friend, extremely knowledgeable about biological science and medical implications of research, and also recommended affymetrix to me when it was $17 a share (but of course I didn’t listen…), for what that’s worth.
I have noticed some questions recently regarding viral loads and their use in monitoring of AIDS therapeutic agents. I thought I would use this post to try to clear up the issues surrounding viral loads and how they do (and don’t) apply in Enzo’s case.
As I mentioned in the review on HIV biology, there is a continual battle between the HIV-1 virus, which infects and destroys CD4+ T-cells, and the bone marrow stem-cells, which are the predeccesors and producers of these CD4+ T-cells. Since HIV has a very long latent period (5-10 yrs, on average) between infection and the eventual development of AIDS, doctors utilize a number of prognostic tests to learn more about who is winning the battle between the virus and the stem cells.
There are two prognostic tests which are widely used in monitoring the patient with HIV:
1) CD4+ T-cell count. As I mentioned in the review on HIV biology, normal people have CD4+ T-cell levels of 500-1000 cells/ml of blood or greater. During the latent phase of HIV infection (when the patient has the virus but has not yet displayed signs of AIDS), the CD4+ T-cell count is usually between 200-500 cells/ml. Once the CD4+ T-cell count drops below 200 cells/ml, the patient does not have enough of these CD4+ T-cells around to orchestrate an immune response against usually benign bacteria and viruses. This immunodeficiency brought on by the drop in CD4+ T-cells is the disorder known as AIDS. Monitoring of the absolute CD4+ T-cell level gives doctors information about the strength of the patients immune system; monitoring changes in the CD4+ T-cell levels provides information on whether the immune system is becoming stronger or weaker.
2) HIV viral loads. The HIV virus infects CD4+ T-cells (and certain other cells of the immune system), replicates in them (often producing 1000 or more viral particles for each cell infected by a single viral particle), and then leaves the cell and enters the bloodstream, looking for more cells to infect. The HIV viral load is a measure of how much virus is present in the blood, and therefore is a indicator of how much the virus has recently been produced by infected cells. The HIV viral load is usually measured by a technique called reverse transcriptase-polymerase chain reaction (RT-PCR). This is basically a way of converting RNA (the genetic material of HIV is RNA) into DNA and then amplifying the information to measurable levels. One of the key advantages of measuring HIV viral loads compared to CD4+ T-cell levels is that they change much more rapidly and provide more timely feedback as to whether a therapeutic is working or failing…
When a patient initiates traditional anti-retroviral therapy (consisting of reverse-transcriptase inhibitors, nucleoside analogs, and protease inhibitors), doctors can see how well the patient is responding to the treatment by measuring the viral load. If the drugs are effective at preventing HIV replication, the patients viral load will decrease over time (usually several weeks to several months). With the HIV drug ‘cocktails’, the viral load is often reduced to unmeasurable levels, meaning that the virus has essentially disappeared from the blood. This does NOT mean that the virus is gone…only that it has not had much success replicating inside infected cells. Therefore, a drop in the viral load is a strong indication that a traditional anti-HIV pharmacologic has been ffective in inhibiting viral replication. Just as a drop in viral load can be an indication that a therapeutic is working against HIV, a rise in viral load can indicate that a drug is becoming less effective against the virus. For instance, the HIV drug ‘cocktail’ was greeted with enthusiasm by the medical community because the viral load dropped off dramatically in many patients soon after starting the therapy. However, after several years, many patients’ viral loads began to creep back up. This suggests that a mutated strain of the virus (one that is not as inhibited by the drugs) had emerged and had begun replicating in cells and spreading virus into the blood. At this point, the only choice is to try to introduce one or more new drugs to try to stop the virus…but it is almost always a losing battle. As you can see, measurements of the viral load are of great importance in monitoring how well a patient is responding to traditional anti-HIV pharmacologics. But Enzo’s HIV-1 strategy is anything BUT traditional…
The importance of viral load measurements breaks down in Enzo’s
case (especially at this stage) for several reasons…
1) Little is known at this time about the patient population
of the stage I clinical trial. Are the patients on a standard HIV drug
cocktail? Are they people with HIV strains that have mutated to evade the
cocktail? These questions are of great importance because they can influence
(even determine) the viral loads in these patients. For instance, patients
who are responding to the drug cocktail will have undetectable viral loads…regardless
of whether Enzo’s approach is effective or not. Likewise, patients that
are not responding to the drug cocktail will probably have high viral loads…even
if Enzo’s approach IS working (see #2).
2) Enzo’s strategy is not to inhibit HIV replication…it is to
repopulate the immune system with cells that are resistant to the replication
of the virus. The trouble with using viral loads to monitor the efficiency
of Enzo’s approach is that these patients will have two types of CD4+ T-cells…the
unmodified CD4+ T-cells which are SUSCEPTIBLE to viral infection, and the
HGTV43-modified CD4+ T-cells (descendants of the transfected CD34+ stem
cells) which should be RESISTANT to viral replication. As a result, the
virus may be replicating away in the susceptible cells (leading to detectable
viral loads) even if the resistant cells are working perfectly and no viral
replication is occurring within that population of cells. Therefore,
the viral load is NOT a useful measurement of whether Enzo’s strategy
is working. The viral load will only become a useful measurement
when the patient has only one population of CD4+ T-cells (either all sensitive
to HIV infection…as in normal individuals…or all resistant to HIV infection…as
Enzo hopes). As long as two populations of cells exist, it will be difficult
to apply viral load measurements to monitor the treatment’s effectiveness.
With antisense treatment of HIV, Enzo has entered uncharted waters. Traditional indicators of a therapeutics efficacy are no longer valid… I think that the REAL questions that need to be asked to determine whether Enzo is making progress are:
1) Is there a subpopulation of resistant CD4+ T-cells? The press
release last week suggested that a subpopulation exists, but no data was
yet released on whether those cells were resistant to HIV. This question
needs to be answered.
2) What is the proportion of modified CD4+ T-cells to normal
T-cells, and how is that ratio changing? As I mentioned in the posts
commenting on Enzo’s HIV-1 study, Enzo needs to achieve levels of 200+
modified CD4+ T-cells/ml in order to have successfully repopulated the
immune system with resistant cells. This correlates to a ratio of approximately
1 resistant cell in 2-5 cells in normal individuals. It would be useful
to know what this ratio is at the present time. It is important to remember,
though, that it is likely that there would be a change in the ratio (in
favor of the resistant cells) over time as the virus kills off non-resistant
cells.
I would suspect that these are the kinds of questions that Enzo will be focusing on, rather than worrying about viral loads. However, given that Enzo is in stage I trials, I doubt that they are looking too intently at these (efficacy) endpoints. Just my 2 cents…
I hope that this helped reduce some of the confusion surrounding viral loads and Enzo’s strategy…as always, if there are questions/comments/corrections/etc. I’d be happy to try to address them. Thanks to all who made it through…
-icposse2000