Sci. Aging Knowl. Environ., 9 February 2005
Vol. 2005, Issue 6, p. pe4
[DOI: 10.1126/sageke.2005.6.pe4]

PERSPECTIVES

Drug Discovery in Neurodegenerative Diseases

Hugo Geerts, John Q. Trojanowski, and Virginia M.-Y. Lee

The authors are at In Silico Biosciences (H.G.) and the Center for Neurodegenerative Disease Research, the Institute on Aging, and the Department of Pathology and Laboratory Medicine (H.G., J.Q.T., and V.M.Y.L.), University of Pennsylvania School of Medicine, Philadelphia, PA 19104, USA. E-mail: Hugo-Geerts{at}In-Silico-Biosciences.com (H.G.)

http://sageke.sciencemag.org/cgi/content/full/2005/6/pe4

Key Words: Alzheimer's disease • drug discovery • neurodegeneration • prion • protein misfolding • tauopathies

Introduction

About 120 scientists convened for the Third "Drug Discovery in Neurodegenerative Diseases" symposium at the Cold Spring Harbor Laboratories (CSHL) near New York in early December 2004. The program covered recent advances in drug discovery and design for rare and common neurodegenerative disorders, especially those characterized by protein misfolding. These approaches are classified as "disease-modifying" because protein misfolding directly causes pathology, and therefore therapeutic agents aimed at correcting the defect in protein folding should reduce, stop or reverse ongoing neurodegeneration. The conditions in question include Alzheimer's disease (AD), Parkinson's disease (PD), prion disorders such as Creutzfeld-Jakob Disease (CJD), and Amyotrophic Lateral Sclerosis (ALS), in addition to polyglutamine disorders such as Huntington's disease (HD), Spinal Muscular Atrophy (SMA), and even Dominant Optic Atrophy (DOA). This has been a rewarding series of meetings showcasing rapid and timely advances in the development of therapeutic interventions to treat this group of disorders, and the major focus of this symposium was to provide updates on these advances while elaborating on common therapeutic approaches such as inhibiting abnormal protein fibrillization or engineering neuroprotection. What follows here is a brief summary of some of the highlights of this excellent meeting, which was organized by Sam Gandy (Chair), Virginia M.-Y. Lee, and Marcie MacDonald at the idyllic CSHL site among pristine winter scenery. Notably, in the very near future CSHL intends to post the PowerPoint slide presentations of all consenting speakers on its Web site.

Highlights of the Meeting

The U.S. National Institutes of Health (NIH)�Molecular Libraries�Roadmap Initiative was reviewed by Christopher Austin of NIH (Bethesda, MD, USA), who aptly summarized its focus: performing�high-throughput screening using diverse in vitro assays submitted by the research community to optimize confirmed hits for�in vitro chemical probes, and making all data and�probes available to the entire nonprofit and for-profit research community as well as to the general public.�In so doing, the Molecular Libraries Roadmap hopes to encourage academic labs�to screen�less conventional targets,�such as those relevant to neurodegenerative diseases like SMA�that are less attractive to "big pharma." Specific examples involving therapeutic approaches to SMA, prion disease, and ALS are discussed below.

The approach employed at the Laboratory for Drug Discovery in Neurodegenerative Disease (LDDN) at Harvard Medical School was discussed by Greg Cuny of Harvard University (Cambridge, MA, USA), who pointed out that this was one of the earliest initiatives to pursue this type of research. The objective is to bring a particular compound to the proof-of-principle stage in a suitable animal model. One project addressed the inhibition of neuronal cell death in a stroke model, using a compound that had originally been described as an inhibitor of necrosis. This drug was active in a middle cerebral artery occlusion model when applied 6 hours after occlusion.

One of the more provocative NIH Roadmap initiatives is the concerted effort to re-engineer how clinical research is conducted, both within NIH and outside the organization, and a particular clinical trial presented at this meeting was directed at "disease-modifying agents," which attempt to address the neuropathology at the heart of disease. Francine Gervais (Neurochem, Montreal, Canada) gave an update on the clinical status of Alzhemed, a glycosaminoglycan (GAG) mimetic currently under review in two pivotal phase III trials. The compound had its biggest effect in mild cases of AD, both in terms of lowering the concentration of the primary 42-residue form of A{beta} found in amyloid plaques, as reflected in the A{beta}42/40�ratio (of A{beta} peptides derived from the amyloid precursor protein APP; see "Inside Job: Alzheimer's protein kills neurons from within") in cerebrospinal fluid (CSF), and in terms of cognitive stabilization. As an example, 9 of 13 mild AD patients in the open-label extension of the clinical trial (an arm of the trial involving no placebo treatment) did not deteriorate in the cognitive subscale of the Alzheimer's Disease Assessment Scale (ADAS-Cog), measured over a period of 20 months.

Larry Sparks of the Sun Health Research Institute in Sun City, Arizona, USA, presented some results with statin drugs (see "Greasing Aging's Downward Slide" and "Lipid-Control Allies"), based on his long-term research into the relationship between cholesterol and A{beta} metabolism. In a small (63 evaluable subjects) double-blind placebo-controlled 1-year study, the trial HMG-CoA reductase inhibitor atorvastatin was given together with the standard acetylcholinesterase inhibitor used clinically. Atorvastatin-treated patients showed a 4-point superiority over patients who had received the placebo at the end of 1 year in the ADAS-Cog and, unexpectedly, a significant improvement on the Geriatric Depression scale and a trend for benefit on the Neuropsychiatry Index. Interestingly, atorvastatin had previously been documented not to cross the blood-brain barrier.

John Collinge (Medical Research Council, London, UK) reviewed the ongoing preclinical and clinical activities in the area of variant Creutzfeld-Jacob Disease (vCJD), including the design of the recently initiated Prion-1 study using the drug quinacrine. The particular nature of this disease precludes use of the standard double-blind placebo-controlled approach, so the patients were offered three possibilities: (i) open-label nonrandom treatment with quinacrine; (ii) treatment on a "natural history" basis, without quinacrine; or (iii) immediate treatment with quinacrine or quinacrine treatment after a 6-month delay, on a randomized double-blind basis. So far, 16 patients have enrolled in the nonrandom quinacrine treatment arm of the trial, 15 have elected to participate and be followed in the natural history arm of the trial, and no patients opted for the randomized immediate versus delayed quinacrine treatment. Definitive results from this work are still pending.

An innovative approach for supporting the design of clinical trials in central nervous system (CNS) disorders, based on computational neuropharmacology, addressed the possible problem of add-on neuroleptics (antipsychotic drugs) and antidepressant treatment for behavioral problems in the relevant patient populations, and a presentation on this was given by Hugo Geerts, one of the authors of this report. Indeed, G protein-coupled receptors--which are the target of psychoactive drugs, such as the neuroleptic Haldol--are known to interfere with intracellular pathways typically associated with disease-modifying approaches, and vice-versa. Computational neuropharmacology allows a quantitative estimate to be made of the effect of additional medication on disease-modifying therapeutic approaches and, as such, can be helpful in optimizing clinical trial design.

Many neurodegenerative diseases are characterized by the inappropriate aggregation of amyloidic proteins, such as A{beta} peptides derived from APP and paired helical filament (PHF) aggregates of microtubule-associated protein (MAP) tau, which are known respectively as senile plaques and neurofibrillary tangles in the context of AD (see "Detangling Alzheimer's disease" and Berezovska Perspective). Other pathological examples of abnormal protein aggregates include PHF tau in tauopathies and frontotemporal dementias, huntingtin proteins in HD, {alpha}-synuclein in PD, and filamentous prion proteins in CJD. There are common themes in these different protein self-aggregation processes, as indicated by an antibody that recognizes amyloidic oligomers, but not monomers or fibrils, irrespective of their primary sequence (1). This suggests that it may be possible to develop small molecules that behave in a similar or identical fashion--i.e., compounds that can inhibit aggregation of different types of protein and so be of potential use as therapeutic agents (2). Vernon Ingram (Massachusetts Institute of Technology, Cambridge, MA, USA) and James Shorter (Whitehead Institute, Cambridge, MA, USA) discussed kinetic details of the interaction of one such compound, 4,5 dianilinophthalimide (DAPH), with A{beta} or yeast prion protein, respectively.

A "Trojan horse" strategy has been designed that uses a chemical trick to increase the molecular weight of small-molecule inhibitors of protein-protein interactions. This approach makes use of�bifunctional compounds consisting of a targeting moiety, which binds specifically to {beta}-amyloid, and a recruitment moiety, which binds to the endogenous chaperone FKBP (FK506 binding protein, which binds the immunosuppressant drug FK506). The resulting drug-FKBP complex substantially increases the steric bulk of the small molecule (3), thus improving the potency of small-molecule inhibitors of amyloid aggregation to yield inhibitors of aggregation effective at nanomolar concentrations, as summarized by Isabella Graef of Stanford Medical School (Stanford, CA, USA).

One novel functional approach to drug discovery for neurodegenerative disease was illustrated by Rebecca Pruss (Trophos Inc., Marseille, France), in which functional assays based on primary cultures of specific brain neurons (such as trophic factor-deprived motoneurons relevant to the pathology of ALS and SMA�or genetically modified striatal medium spiny neurons for HD) have been applied to high-throughput imaging screening. Once a hit (a small molecule with pharmacological activity in the screen) has been identified, lead optimization is performed, which involves making small chemical modifications to render a compound more potent; the target compound is then investigated for toxicity and pharmacokinetics (studies probing how fast a drug is taken up, metabolized by, and cleared from the body). This approach has identified compounds that are active in animal models for the relevant neurodegenerative disease(s), and one compound is now�in Phase I clinical trials.

Automated screening of�Drosophila models for specific human diseases can greatly accelerate the process of discovering new therapeutic lead compounds because of the relative simplicity of the work needed to develop transgenic files, the short life span of Drosophila, and the advantage of whole-organism phenotypic screening (see "Flies Like Us"). One example of a drug discovery process involving inhibitors of histone deacetylase protein(s) being carried out in a model of HD was given by Christopher Cummings (EnVivo Pharmaceuticals, Boston, MA, USA). D. Brunner (Columbia University, New York, NY, USA) described another novel approach to increasing the throughput of in vivo pharmacological testing [behavioral studies such as prepulse inhibition (PPI, in which a quiet tone may prevent startling by a subsequent louder tone), motor assessment, rotarod (in which experimental animals must walk to avoid falling from a rotating rod), and object recognition] based both on advances in automation and robotics and�on the�use of bioinformatics to increase statistical power. Clever optimization analysis is also used to reduce substantially the number of expensive animals (such as the R6/2 transgenic mouse model for HD) needed, by designing successive phases�of screening and prioritization and preserving the necessary statistical power to detect beneficial effects.

Todd Golde (Mayo Clinic, Jacksonville, FL, USA) reviewed the therapeutic role of nonsteroidal anti-inflammatory drugs (NSAIDs) in AD (see "Knife Jugglers"). Some NSAIDs, such as sulindac sulphide, ibuprofen, flurbiprofen, indomethacin (4), and meclofen, are interesting mechanistically, as they appear to shift the production of A{beta}40 and A{beta}42 to shorter forms such as A{beta}37 and A{beta}38, thereby resulting in a lower overall concentration of A{beta}. However, other related compounds, such as celecoxib, appear to increase the abundance of A{beta}42, which is obviously problematic for long-term therapy of AD.

Eckhard Mandelkow discussed the extensive work of the Max-Planck Institute Laboratory that he and Eva Mandelkow direct in Hamburg, Germany, on tau protein as a therapeutic target in the battle against AD. Using tools ranging from electron microscopy to molecular biology and transgenic animals, they have identified key parts of the tau protein necessary for PHF formation. A number of small compounds were found that were able both to prevent PHF formation and to disassemble preformed PHF in an in vitro cell culture system. It is now planned to test these compounds in a transgenic mouse expressing an inducible form of {Delta}K280 tau (a model with neuropathological features strongly resembling tangle pathology in AD).

John Trojanowski (University of Pennsylvania, Philadelphia, PA, USA) reviewed new features on the landscape of tauopathies--3 microtubule-binding repeat (3R) versus 4 microtubule-binding repeat (4R) isoforms of tau, and the neuronal versus glial localization of tau pathology. It has been shown that altered ratios of 3R to 4R tau isoforms cause a neurodegenerative tauopathy and that sequestration of tau into tangles and related inclusions leads to microtubule destabilization, a loss of axonal transport function, and neuron degeneration (5). By offsetting the loss of tau using microtubule-stabilizing drugs such as taxol, it may be possible to treat AD and related tauopathies, which could open up a variety of new therapeutic approaches using U.S. Food and Drug Administration (FDA)-approved drugs such as taxol and related microtubule-stabilizing compounds currently used to treat cancer. A new formulation of taxol has been shown to be effective in a transgenic animal model of tauopathy (6).

Jeff Rothstein (Johns Hopkins University, Baltimore, MD, USA) described examples of NIH-supported efforts to identify therapeutically effective drugs for neurodegeneratative diseases, employing an FDA-approved compound library provided to investigators by NIH. Glutamate has been proposed to be an important excitotoxin in diseases such as ALS, and the glutamate antagonist riluzole has been shown to slow disease progression in clinical trials. Although new compounds were screened by using an assay for neuroprotection, further studies illustrated that the beta-lactam antibiotic ceftriaxone increased glutamate transporter protein expression and function, both in vitro and in vivo. Ceftriaxone was also active in three other in vitro screening assays based on mutant SOD1 toxicity and glutamate toxicity [a small number of ALS patients have a mutation in superoxide dismutase 1 (SOD1)]. Furthermore, the drug-induced overexpression of the transporter was neuroprotective in vitro, in models of ischemic injury and glutamate-mediated motor neuron degeneration. In the G93A SOD1 transgenic animal model of ALS, the drug delayed loss of grip strength, slowed loss of motor neurons, and increased survival (7). Rothstein noted that this compound is scheduled to enter clinical trials in 2005.

Patients suffering from SMA harbor genetic abnormalities that lead to a missing or dysfunctional SMN1 gene, culminating in the SMN protein being absent or reduced in abundance. A private patient and disease advocacy group known as Families of Spinal Muscular Atrophy has taken the initiative to fund a drug discovery program through deCODE Chemistry, Inc., that is aimed at increasing the amount of SMN protein derived from the remaining functional SMN2 gene. As described by Mark Gurney (deCODE Chemistry, Inc., Columbus, OH, USA), this new program has already yielded a compound with a quinazoline scaffold that may prove to be a useful lead compound for developing new therapies for SMA. The compound has an interesting pharmacokinetic/metabolic profile (i.e., the drug is stable in vivo and no inactive metabolites are formed) and will now be tested in appropriate animal models.

Concluding Remarks

This conference addressed a number of key scientific and pharmaceutical issues related to therapeutic interventions for neurodegenerative disease, and it was clear to many of us who had attended earlier CSHL meetings on this topic that tremendous progress and momentum are building in this research arena. However, the conference also took into thoughtful consideration the contextual and ethical aspects of developing new and successful drugs for these debilitating and often relentlessly progressive and ultimately lethal disorders. Scientific progress has benefited much from the ability to carry out genetic manipulation in various model species, including mouse, C. elegans, zebrafish, and Drosophila, providing a very positive impact on the development of functional assays needed for the drug discovery process. The new NIH Molecular Libraries Roadmap initiative has also provided an incentive and a support framework for many academic centers to capitalize on their biological expertise in the discovery of new drugs, and it is to be hoped that NIH will continue to support this project. Meanwhile, the pioneering work from the UK prion group has provided a scientifically compelling strategy and a suitable ethical approach for the design of clinical trials to find new and better drugs for these devastating incurable diseases.


February 9, 2005
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Citation: H. Geerts, J. Q. Trojanowski, V. M.-Y. Lee, Drug Discovery in Neurodegenerative Diseases. Sci. Aging Knowl. Environ. 2005 (6), pe4 (2005).








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