Proteolytic beta amyloid light chain antibody fragments offer promise as future treatments of Alzheimer’s disease.

The aging population will contribute to an expansion of the already $30 billion neurodegenerative market. The market for Alzheimer's disease therapy is expected to grow from 16 million patients to 21 million by 2010 in the seven major pharmaceutical markets and represents a major component of this market.   Alzheimer's disease drives sales in these regions worth $4.7 billion, a figure set to increase to $6.1 billion by the year 2005 and $ 7.8 billion by the year 2010 (for a full analysis of this field click here or here).

The most widely accepted hypothesis on the etiopathogenesis of Alzheimer's disease proposes that aggregates of the beta amyloid form in the brain. Under normal conditions, the predominant amyloid peptide secreted is beta amyloid(1-40) with about 10-15% being the longer 1-42 form. In Alzheimer's disease there appears to be an increase in the longer more toxic form which is proposed to trigger tau hyperphosphorylation and neural degeneration. Neurotoxicity is thought to be due to altered calcium regulation, mitochondrial damage and/or immune stimulation

Current therapeutic strategies focus on the use of cholinesterase inhibitors to compensate for the loss of cholinergic innervation associated with this disease. Although treatment with acetylcholinesterase inhibitors is largely symptomatic, claims of a neuroprotective effect have been made. Currently the only neuroprotective therapy approved for Alzheimer’s disease is the NMDA receptor antagonist, memantine.

Disease modification and disease prevention remains a major unmet need in the Alzheimer’s disease market.  In the short to medium term opinion leaders are looking to Axonyx's phenserine and Neurochem's Alzhemed in anticipation of successful modification of disease progression. Phenserine, in addition to inhibiting acetylcholinesterase activity slows the build up of beta amyloid.  Alzhemed inhibits beta-amyloid fibrillization, and also binds and reduces soluble beta amyloid. The high level of awareness and positivity amongst the Alzheimer’s disease community bodes well for the launches of these products, which could challenge the market domination of Pfizer/Eisai's Aricept (donepezil) if supported by sufficient marketing strength.

As far as longer-term market entries are concerned considerable work is currently focusing on the development of immunomodulatory strategies for the prevention of Alzheimer’s disease.  Collaboration between Elan and Wyeth has contributed to the development of two immunotherapeutic strategies; the use of beta amyloid based vaccines to stimulate the generation of endogenous antibodies to the protein and; the development of monoclonal antibodies for passive immunization.

AN-1792 is a synthetic form of the beta amyloid peptide that is being developed as a vaccine.  Elan initiated a Phase I study of AN-1792 in 2000, and in 2001 this candidate was advanced to phase II stages of development.  Dosing in the Phase IIa study was however halted after signs and symptoms consistent with encephalitis were reported in four patients. Both trials did however provide data that support the beta amyloid immunotherapy approach.  In particular AN-1792 was found to elicit an anti-beta amyloid antibody response in patients with mild to moderate Alzheimer’s Disease and evidence of beta amyloid plaque clearance was observed in four autopsy cases.  Memory, attention and concentration improved at 12 months in anti-beta amyloid antibody responders, although primary cognitive endpoints did not demonstrate improvement in those treated with AN-1792. Elan and Wyeth continue to evaluate the data from AN-1792, which suggest that the biological effects of the immunotherapy approach may warrant further study.

In 2003, the market for monoclonal antibodies in the treatment of cancer was just short of $2.8 billion and growing at a phenomenal rate. The market is fueled by leading products such as Rituxan and Herceptin.  In 1997, Rituxan became the first monoclonal antibody therapy to be approved in the for the treatment of cancer.  This agent targets CD20 on B-cells and is indicated for B-cell non-Hodgkin's lymphoma (NHL).  Herceptin, approved by the FDA in 1998, targets HER2 and is therefore indicated for the treatment of metastatic breast cancer.  Other products continue to demand more attention each year (for an analysis of therapeutic antibodies click here) and Alzheimer’s represents one emerging indication for antibodies.

In addition to active immunization (ie through the use of vaccines) Elan and Wyeth are currently conducting a Phase I safety study of passive immunotherapy using a humanized monoclonal antibody, AAB-001.  This therapeutic antibody, which binds to and clears beta amyloid peptide, is designed to provide antibodies to beta amyloid directly to the patient. Animal studies have shown that this approach is equally effective in clearing beta amyloid from the brain as traditional active immunization methods.  By providing such a “passive immunization” approach for treatment of Alzheimer’s disease, it is believed that the benefits demonstrated with AN-1792 will be retained, while the safety concerns will be reduced. 

Passive immunization with antibodies such as AAB-001 has been proposed to reduce beta-amyloid load by cross-linking plaques to microglial cells through the Fc receptor and subsequently inducing plaque clearance.  Although antibodies have been classically thought of as molecules that simply bind antigen to elicite an immune response it is now becoming evident that antibody light chain fragments have proteolytic activity.  Researchers at have been exploiting this finding to develop antibody fragments that recognize and crucially proteolytically cleave beta-amyloid. In one study published at the end of last year, Rangan et al reported the identification of proteolytic recombinant antibody light chain fragments able to cleave beta-amyloid.  One fragment, c23.5, showed alpha-secretase-like activity, producing the 1-16 and 17-40 amino acid fragments of beta amyloid. The second light chain antibody fragment, hk14, demonstrated carboxypeptidase-like activity, cleaving sequentially from the carboxyl terminal of beta amyloid.

In a more recent study by the same group, the activity of these proteolytic antibodies was further investigated.

In this month’s edition of the journal, Biochemistry, Liu et al report that cleavage of beta-amyloid by hk14, the light-chain antibody with carboxypeptidase-like activity, alters the aggregation of beta amyloid and neutralizes any cytotoxic effects of the peptide. In particular aggregates consisted of very thin filaments. The ability of hk14 to alter aggregation was proposed to reflect the dependency of this process on the carboxy terminal. Interestingly c23.5, which has alpha-secretase-like activity, substantially increased the aggregation rate of beta amyloid due to the release of the poorly soluble beta-amyloid 17-40 fragment and slightly increased toxicity. 

These important data demonstrate the therapeutic potential of anti-beta amyloid proteolytic antibody light chain fragments. These fragments are able to hone in on beta-amyloid and to reduce its neurotoxicity but since they lack Fc binding heavy chains, the mechanism of this effect differs from other antibodies in development that cause degradation through the recruitment and activation of microglial cells. The lack of dependency on Fc-mediated effector functions, and the reduced size of proteolytic antibody light chain fragments may confer significant advantages.  In particular immunogenic adverse effects are expected to be minimal and high levels of brain penetration should allow ready access to the plaque environment.  Furthermore production costs should be relatively low.  On the other hand the clearance of small antibody fragments can be quite rapid and the half-life of such therapeutics could be problematic.  Such issues will be addressed in the future and the results of these studies are eagerly awaited.

Source: Proteolytic Antibody Light Chains Alter beta-Amyloid Aggregation and Prevent Cytotoxicity. Biochemistry. 2004 Aug 10;43(31):9999-10007.

This article is highlighted in the August 18^th edition of DailyUpdates-CNS Disorder, LeadDiscovery’s unique bulletin of breaking journal articles and press releases for the drug discovery community. 

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