BOSTON—Researchers have identified several possible targets for the treatment of Alzheimer's disease. On the forefront of approaches to treat and prevent this disease are inhibitors of amyloid-based ß- and g-secretase enzymes and an immune-based vaccine that has been found to reduce the burden of plaque in transgenic animal models of Alzheimer's disease.

"Despite decades of intensive study, the precise etiology of Alzheimer's disease remains a mystery," said Ivan Lieberburg, MD, PhD, at the 125th Annual Meeting of the American Neurological Association. "Nevertheless, an overwhelming amount of circumstantial evidence points to the role of ß-amyloid as the causal agent." Dr. Lieberburg is Chief Scientific and Medical Officer, Elan Corporation, South San Francisco, California; and Clinical Professor in the Department of Internal Medicine, University of California, San Francisco.

PRODUCTION VERSUS CLEARANCE

"It was always my view that the problem in amyloidosis was really a question of production versus clearance," much like atherosclerosis and hypercholesterolemia, Dr. Lieberburg said. In type 2 hypercholesterolemia, for example, most patients have modestly elevated levels of cholesterol but cannot clear it. Drugs such as the HMG-CoA reductase inhibitors (statins) reduce ongoing production of cholesterol so that clearance mechanisms can operate optimally.

Similarly, therapy being developed for Alzheimer's disease has as its basic premise the question of clearance. "And how do we enhance clearance? We can do it either by decreasing production and allowing endogenous clearance to take place or by enhancing it through some other mechanism." For Alzheimer's disease, this involves three major approaches: inhibiting the 1) ß-secretase and 2) g-secretase enzyme activity involved to allow endogenous clearance and, alternatively, 3) immunization, which enhances ongoing clearance through an immune-mediated approach.

SECRETASE INHIBITION

Most of the work done over the past several years has centered on g-secretase. Last year, two groups independently found evidence that the aspartate residues in the presenilin molecule are the sites responsible for g-secretase activity. Dr. Lieberburg said one problem that has emerged with g-secretase as a target for therapy is that "presenilin is crucial to the cleavage of Notch," which is a signaling protein important to normal development of the embryo and rapidly dividing cells. "If you inhibit Notch cleavage, you actually can accelerate the differentiation scheme in the adult animal and cause a dramatic phenotype," he said.

The g-secretase inhibitors under development are orally bioavailable and extremely potent. "They are central nervous system permeant; they are novel compounds; and they have been tested on animals extensively over the past several years. They are capable of dramatically reducing ß-peptide formation in a chronic fashion," said Dr. Lieberburg. He added that in subchronic studies with these inhibitors, "we've achieved as much as 70% reduction of ß-amyloid in the brains of these animals. Now, how that's going to relate to the human situation remains to be seen. But what's important about this is that all the attendant pathology is also reduced," including gliosis and neuritic pathology.

The challenge is to develop not only "a safe drug that's capable of delivering not only this kind of result" but one "with a reasonable therapeutic index that patients can tolerate," Dr. Lieberburg said. While compounds with a therapeutic index of greater than 100 are generally preferred, g-secretase inhibitors currently under study have indexes that range from 20 to 40. "We understand that this is a very serious disease, but we also understand that we don't want to expose patients to any undue toxicity."

"Part of the reason for the excitement [about] ß-secretase…is because its inhibition may not have many of the attendant therapeutic index issues that are associated with g-secretase." The ß-secretase enzyme makes only one cleavage at a specific site between two amino acids on amyloid precursor protein (APP). It is an aspartyl protease, few of which are found naturally in humans, allowing for specificity as well as selectivity in drug development. He said that ß-secretase would probably require about 25% inhibition to produce a therapeutic benefit, compared with nearly 99% inhibition for HIV protease. Also, unlike g-secretase, ß-secretase is relatively selective to the central nervous system.

INHIBITION…

"The work in this area is not quite as far along as it is with g-secretase," Dr. Lieberburg said. However, "we're not that far away from getting something that could move into the clinic," he added.

Some of the drawbacks to developing inhibitors include the ubiquitous nature of g-secretase, the difficulty of working with proteases as targets, and the inhibitors' difficulty in permeating the blood-brain barrier. He said that simulating amyloid clearance mechanisms is much easier than inhibiting the enzymes that promote the production of amyloid. Dr. Lieberburg and his colleagues thus explored whether "we could enhance clearance by activating the immune system and get amyloid to clear from the brain. And in fact you can, rather remarkably."

…OR IMMUNIZATION?

In a prophylaxis study, transgenic animals immunized with ß-amyloid developed almost no amyloid formation in 12 months; the attendant neuritic dystrophy or astrocytosis typically seen was also not present, he said. Given these results, "we were curious to know what would happen if we treated these animals once they developed disease." Treatment begun at 11 months resulted in a "dramatic reduction in pathology," including correction of loss of synapse formation and reduction in neuritic burden and astrocytosis. In reviewing these results, published in Nature Medicinein August 2000, the investigators concluded that "antibodies against amyloid ß-peptide triggered microglial cells to clear plaques through Fc receptor–mediated phagocytosis and subsequent peptide degradation. These results indicate that antibodies can cross the blood-brain barrier to act directly in the central nervous system and should be considered as a therapeutic approach for the treatment of Alzheimer's disease and other neurologic disorders."

An immunization based on this approach is currently in phase 1 clinical trials. No data are yet available, other than that it is well tolerated, (including a lack of glomerular nephritis or autoimmune encephalomyelitis). Dr. Lieberburg said that other drugs being tested for safety in animal models prior to their entry into clinical trials include "at least six backup adjuvant-antigen combinations," fusion antigens, and monoclonal antibodies.

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