October 20, 2000
On October 16, 2000 the FDA's Antiviral Drugs Advisory Committee met to explore issues around what advice the Agency should give to industry on development and approval of immune-based treatments (see announcement in AIDS Treatment News #351, September 22, 2000). It seemed widely agreed that the meeting went well, and was important and successful in that it happened at all. But clearly it did not crack the critical question it had been called to address: How might the FDA encourage or at least not impede research into immune-based treatments for HIV, given current scientific understanding and commercial constraints?
In the end we sensed that while the meeting illuminated many of the issues, it could not approach a solution to the central problem, because within the constraints of prevailing thinking there is in fact no solution. If this is true, then at least in AIDS, the path forward on immune-based therapies -- one of the most critically important frontiers of medicine -- will have to come from unexpected developments, not through public policy.
This article presents our analysis. See below for references to the FDA's Web site for a transcript and other information about this meeting.
Since immune-based therapy will clearly be important not only for HIV and other infectious diseases but for cancer, autoimmune diseases, and probably a considerable number of other conditions as well, it would seem to make sense for a large company to conduct research and development in order to build position and leadership in the larger area, even if there is no visible path of return for the investment. But U.S. business is judged on short-term not long-term results -- and the so-called "innovator" pharmaceutical companies are primarily driven by a marketing not a research culture, so long-term "blue sky" funding by industry is scarce.
In HIV, a new antiretroviral can be initially approved for market based on safety data and improvements in viral load. But there is no similar test for approval of immune therapies. There are dozens of "biological markers" of immune function -- but no accepted "surrogate marker" which can be used for drug approval. While many of the biological markers can have abnormal levels in persons with AIDS, and drugs might be developed to move these values toward normal, there is no proof yet that doing so will necessarily benefit the patient. And without that proof, the FDA is understandably unwilling to approve the drug.
What is a "surrogate" marker? Before viral load and CD4 count were accepted (within limits) for drug approval, any new drug for treating HIV was expected to show benefit through reduction of "clinical endpoints" -- generally interpreted to mean death or a major AIDS-related complication (one serious enough to end the clinical trial for that patient, who was then considered a treatment failure for research purposes). A "surrogate" marker is a blood or other laboratory test which is accepted as being able to substitute for death or other clinical endpoint for the approval of a new drug.
What about using viral load as a surrogate marker for immune-based treatments as well as for antivirals? This is a serious possibility, and was discussed extensively at the meeting. But current policy is that a surrogate marker is specific to a class of drugs -- so viral load is accepted only for approval of new antiretrovirals, but not for immune-based therapies (unless it is separately established as a surrogate for them). And another obstacle is that while one would probably expect a good immune-based therapy to substantially reduce viral load, that is a lot to ask in the early stages of development of these treatments.
Another possibility being discussed is that an immune-based treatment might in theory prove itself by extending the average time before viral rebound during a treatment interruption.
Without an accepted surrogate marker of some sort, immune treatments have been required to show that they can reduce clinical endpoints. (The large trial of Remune, the HIV immunogen developed by the late Dr. Jonas Salk, was an attempt to do just this. The large trials of IL-2 which are now recruiting are also an attempt to prove an immune-based treatment through clinical endpoints -- although it is far from clear that these trials, even if successful, would validate CD4 count, the candidate marker which led to the trials, as a general-purpose surrogate marker for other immune-based therapies in the future, since an increase in the CD4 count can mean many different things).
As these trials illustrate, in the United States today it is almost impossible to meet the clinical-endpoint burden of proof, even if the treatment could in fact greatly reduce clinical endpoints. Due to the success of antiretrovirals there are fewer deaths and AIDS-related infections today -- and of course all participants must be offered the best antiretroviral therapy available. A trial might be put together of patients who have failed all antiretrovirals and decided to stop taking them, so that clinical endpoints are more likely -- but such a trial would include highly diverse, advanced patients who had proved difficult to treat for various reasons, making it hard to prove a new drug in such a trial.
One is that there is more diversity among immune-based treatments than among antiretrovirals. All antiretrovirals share a common target -- HIV. But immune-based treatments can be fundamentally different from each other, and a marker appropriate for one might not be for others.
Another serious problem is that a surrogate marker, as understood today, can only be established in a highly successful trial of a drug of the same class. So a surrogate marker is never available for the first drug of a new kind -- only for later ones. Since there is no immune-based HIV treatment today that has been proven to reduce death or other clinical endpoints, there is by definition no surrogate marker for immune-based HIV treatments. And if there is no viable opportunity to approve an immune-based drug, there will not be a marker. (Even worse, it turns out to be much more difficult for data to establish that a marker works, than to establish that a drug works -- making it even less likely that surrogate markers can be validated for approval of immune-based drugs.)
The bottom line is that today there is not even a substantial beginning of a professional consensus on how an immune-based treatment for HIV could be approved. Therefore, research and funding in the area are neglected -- completing the vicious circle, as little progress is made toward developing scientific consensus for the future. So the FDA called a meeting about this problem, and on October 16 the FDA's own experts and outside experts talked for a day, putting lots of information and issues on the table but with no real movement toward resolution. If the process continues as it is going now, the best prediction is that in a few years there will be a little more consensus, but no approved HIV immune-based drugs or markers. Then the FDA will call another meeting, and the experts will gather to talk again.
To see how a different outcome is possible, consider the history of viral load, the clearly successful surrogate marker for antiretrovirals. Today viral load is well established technically as a surrogate marker for antiretroviral treatments -- an outcome that would not have been possible without the great success of modern HAART treatments, which provided the successful trials through which this surrogate marker could be validated. But historically, viral load was used in drug approval ahead of full, rigorous statistical proof that it was a surrogate marker, because the mechanism made sense, and the alternatives were so grim. Today there is more skepticism about new treatments (mainly due to the side effects of antiretrovirals), so progress in immune-based markers might not follow the same path.
An engineering analogy might help to illustrate the choice before us. Assume that we have a digital video as a computer file, which must be transmitted online to anyone who requests a copy, so that they can view the video on their computer. Digital video files are likely to be very large and therefore difficult for some computer users to receive -- but they can be mathematically compressed to allow faster transmission. And here we have the choice. If the file must be received perfectly -- guaranteed not even one pixel changed -- then it cannot be compressed very much. But if we allow the possibility of minor errors that will probably never be noticed, the file can be made many times smaller.
The analogy with surrogate markers is that if we demand perfect proof, we will probably block drug development and prevent progress in immune-based therapies. But if we settle for less than perfection, and use our best judgment while accepting a small chance of being wrong, we could move much more rapidly in drug development, fueling basic research in immunology as well. Our concern is that this decision may not be made rationally, because many people like the idea of being right, and are attracted to the case for taking the time to do it right -- no matter what research opportunities will be sacrificed for the assurance of certainty.
Here are some specific suggestions for unblocking research in this field:
Even if the FDA can make a better medical decision than the "market" of HIV-treating physicians -- which could be debated -- that is not the right comparison. The issue is whether to let the medical community make that decision several years down the road, vs. not having the choice because the research and development were never done. If the FDA opens the door now, doctors will make their decisions in several years, with the benefit of the additional knowledge.
Today this change is unlikely, because the FDA's institutional dynamic -- like that of almost any active organization -- is to expand its role, not contract it. But we are facing an emergency here. Professional consensus could make a difference, and we need more discussion of this issue.
But in immune-based treatments for HIV, other kinds of proof of clinical benefit could lead to much faster results. For example, the treatment could be tested in patients who need continuous therapy to control thrush or other AIDS-related infection, to see if HIV treatment reduced the need for the other drugs, or otherwise benefit patients in measurable ways. Such trials could be much smaller than clinical-endpoint trials, and would produce results much faster, because they do not need to wait for thankfully rare events in order to get the data that counts.
What might most speed development of immune-based treatments and markers would be examples of clear success in the area. This week's announcement of a successful vaccine test in animals (see "Vaccine Advance: Monkeys Still Infected, but Protected from Illness," in this issue) is one. The research establishment must be able to move quickly to take advantage of unexpected success.
ISSN # 1052-4207
Copyright 2000 by John S. James. Permission granted for noncommercial reproduction, provided that our address and phone number are included if more than short quotations are used.
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