Translating Natural Products into Drugs for Alzheimer's and Neurodegenerative Disease

Abstracts

Translating Natural Products into Drugs: Advantages, Disadvantages, and Historical Precedents
David J. Newman, PhD, National Cancer Institute, NIH

Alzheimer's disease is one that has a variety of etiologies with the current "theory" heavily related to the presence of amyloid plaques in the brain of AD patients on autopsy. The current treatments are based on various acetylcholinesterase inhibitors, agents that alter Tau pathologies and agents claimed to be "neuroprotectives", and currently, there is significant interest in the production of ApoE (an apolipoprotein) in patients who are carriers of the gene ApoE4. Investigation of the natural product literature shows a very significant number of pure natural products (as distinct from food supplements, traditional remedies, etc.) that have been used as AChE inhibitors, tau-interference agents, secretase inhibitors and anti-inflammatories (due to the probable relationship between inflammation and neuronal damage). This short presentation will discuss the sources of some of the newer agents (or older agents with potential utility) from plant and marine sources, though there is a strong possibility that microbes are involved in some of the materials to be mentioned. There will also be a discussion of the problems associated with the use of pure natural products; for example source versus synthesis, making certain of supplies when having to source overseas. Examples to be presented will include the problems associated with taxol® (chemical relatives of which may be a potential treatment in due course) and how these can be overcome.
 

Natural Products Discovery — The State of the Art
Grant J. Carr, DPhil, AMRI

Natural product samples have been a rich source of therapeutic agents and starting points for successful lead optimization campaigns. Despite these successes interest in natural products discovery has diminished over the last 25 years to the point that today it is often viewed as the strategy of last resort by commercial enterprises. The drivers of this decline and the strategic and technological advances which are leading a return to prominence are presented.
 

Medicinal Chemistry: The Challenges and Advantages of Natural Compounds
Frank E. Koehn, PhD, Pfizer Global R&D

Natural products derived from plants, microbial fermentation and marine organisms are an unsurpassed source of lead structures for drug discovery. It is now apparent that these molecules offer a means of addressing previously "undruggable" target space. However, natural products are often difficult to advance in a lead development sense because of their complex chemical structures and often- limited availability. Chemical synthesis and now biosynthetic engineering of the producing host organisms offer means to modify natural product leads in ways which are not possible by chemical semisynthesis alone. This talk describes the current role of natural products in lead generation and goes on to describe cases where synthetic and biosynthetic medicinal chemistry have been used to advance natural product drug candidates.
 

The Origins of Galantamine
Bonnie M. Davis, MD, Synaptec

Man's appreciation of the medicinal value of galantamine may extend back to ancient Greece. It has been postulated that the milk-colored flower that Hermes gave to Odysseus to counteract the forgetful and delusional state caused by the potions of the nymph Circe was the snowdrop galanthus nivalis, a source of galantamine. Our current understanding of galantamine began with its isolation and characterization as an acetylcholinesterase inhibitor in the early 1950s. Soon after that, a direct effect on acetylcholine receptors of the skeletal muscles was appreciated. Galantamine's enhancement of the activity of carbachol, a cholinesterase-resistant acetylcholine analog, demonstrated that it was a sensitizer of the muscle receptor, the nicotinic receptor. It was galantamine's nicotinic properties that led it to be identified for use in Alzheimer's disease, based on endocrine changes dependent upon central nicotinic pathways. Galantamine owes its natural selection to the rapid creation of a transient, aversive, memorable, but not lethal experience for an animal who would ingest large amounts in a daffodil bulb. These properties are important to galantamine's clinical activity. Galantamine is well absorbed, crosses the blood–brain barrier, binds competitively to the enzymatic site on acetylcholinesterase, and to a positive allosteric modulatory site on nicotinic receptors, with little nonspecific binding. It has a physiologic half-life, compatible with physiologic cholinergic rhythms, and therefore does not induce substantial counter-regulatory acetylcholinesterase. Galantamine reduces loss of brain volume, cognition and activities of daily living in controlled two year trials. The experience of bringing a natural product to market will be presented.
 

HSP90 Inhibitors for Neurodegeneration — Geldanamycin as a Starting Point
Gabriela Chiosis, PhD, Memorial Sloan-Kettering Cancer Center

Hsp90 is a molecular chaperone with important roles in regulating pathogenic transformation. While the HSP90 protein was first discovered in the 1980s, it has drawn little interest as a potential target. After all it is abundantly and ubiquitously expressed in most if not all human cells, knockdown of even 50% has little phenotypic outcome whereas knock-out of the HSP90ß paralog is embryonically lethal. Such findings match poorly with the belief that a good therapeutic target has to be crucial to the disease phenotype and be of low expression in vital organs and tissues. Interest on the target potential of HSP90 grew however after the serendipitous discovery of a natural product, geldanamycin. While surprising in light of the available genetic data, low, non-toxic concentrations of this natural product led to a reversal of the disease phenotype and to neuroprotection in several models of neurodegenerative diseases, indicating that for HSP90, pharmacologic and genetic modulation have a distinct outcome. Indeed, much of our understanding on the distinct biology of HSP90 in neurodegeneration and other diseases comes from pharmacologic studies with geldanamycin and other much improved small molecule HSP90 inhibitors. These efforts elucidated a role for HSP90 in stabilizing mutated or abnormally-modified disease-causing proteins, and shed light on mechanisms behind the selective targeting by small molecule HSP90 inhibitors of "pathogenic" HSP90 species. The talk will give an overview of such concepts and present studies towards the potential translation of HSP90 inhibitors for the treatment of neurodegenerative diseases.
 

Targeting Tau with Novel Marine-Sourced Natural Products and their Derivatives
Chad Dickey, University of South Florida

Natural products chemistry has produced several effective Alzheimer's disease (AD) therapy leads, including the amyloid aggregation inhibitor, curcumin, isolated from turmeric; _ENREF_22 the microtubule stabilizer, paclitaxel from the Pacific yew tree; and the Streptomyces-derived Hsp90 inhibitor, geldanaymcin. In fact, ~60% of all therapeutics currently in use for human disease were derived originally from natural products. While most target-based drug discovery efforts in AD have centered on modulation of the amyloid β peptide, these have met with limited success in the clinic. Therefore, recent efforts have focused on targeting the microtubule-associated protein tau, which more closely correlates with the neuronal loss in AD. By analyzing extracts from natural sources with a modern cell-based screening platform that measures tau levels, we found that extract from Myrica cerifera (Bayberry/southern wax myrtle) potently reduced both endogenous and over-expressed tau protein levels in cells and murine brain. The bayberry flavonoids myricetin and myricitrin were confirmed to contribute to this potency but a diarylheptanoid, myricanol, was the most effective anti-tau component in the extract. (+)-S-myricanol isolated from M. cerifera was significantly more potent than commercially available (±)-myricanol. Myricanol may represent a novel scaffold for drug development efforts targeting tau turnover in AD. Through medicinal chemistry efforts, we have now developed a more stable and reliable derivative of (+)-S-myricanol that does not rely on chirality for its activity. Taking these efforts a step further, we have now turned our discovery platform towards a more novel collection of natural product extracts derived from cold water marine organisms recovered from the Southern/Antarctic Ocean. Initial work has identified that some of these extracts and purified compounds have very potent anti-tau efficacy. Efforts are underway to identify the active components of these extracts and understand their mechanism of action.
 

Epothilone D: History and Future Directions in Alzheimer's
Kurt R. Brunden, PhD, University of Pennsylvania

Neurons within the brains of those with Alzheimer's disease (AD) and other related neurodegenerative disorders contain inclusions comprised of hyperphosphorylated tau protein. Tau is normally enriched in axons, where it binds and stabilizes microtubules (MTs). In disease, tau hyperphosphorylation can promote its disengagement from MTs and subsequent aggregation, which likely leads to reduced MT stability that could negatively affect neuron function. Accordingly, a possible therapeutic strategy for AD and related "tauopathies" is the utilization of MT-stabilizing drugs, such as those currently used for the treatment of cancer. However, most existing MT-stabilizing agents are not suitable for brain disorders because of poor blood-brain barrier (BBB) permeability. We have identified epothilone D (EpoD), a compound previously tested in oncology clinical trials, as a brain-penetrant MT-stabilizing drug candidate with unique pharmacokinetic properties. This led to the testing of EpoD in an established tau transgenic mouse model with AD-like pathology, using both preventative and interventional dosing schemes. EpoD at doses that were a fraction of those used in human cancer clinical trials resulted in a normalization of axonal MT density and axonal transport in the tau transgenic mice. Moreover, EpoD reduced the extent of tau pathology in aged tau transgenic mice, with an improvement of cognitive performance. Importantly, no adverse side effects were observed in the EpoD-treated mice. These results suggest that EpoD might be a potential treatment for tauopathies, and this drug candidate is now undergoing clinical testing in AD patients.
 

Fingolimod Therapy for Multiple Sclerosis: A Fungal Derivative Mimicking a Natural Lipid
Jerold Chun, MD, PhD, The Scripps Research Institute

Fingolimod (also known as FTY720 and Gilenya (Novartis)) was FDA approved in 2010 as the first oral treatment for relapsing forms of Multiple Sclerosis (MS). MS is an autoimmune disorder that affects the central nervous system (CNS) and can result in neurodegeneration. Fingolimod arose from studies of fungal metabolic derivatives. It showed initial bioactivities affecting the immune system through an unknown mechanism of action. Subsequent work demonstrated that fingolimod was a chemical analog of a naturally occurring lipid known as sphingosine: upon in vivo phosphorylation, fingolimod-phosphate is produced which mimics sphingosine 1-phosphate (S1P), a member of the lysophospholipid group of signaling lipids. Lysophospholipids produce biological effects through a family of 7-transmembrane, G protein-coupled receptors that have broad tissue expression including the immune system and the brain. Fingolimod thus accesses S1P lysophospholipid signaling to alter lymphocyte trafficking, which has been thought to produce its efficacy signal in MS. However, recent work has identified CNS activities that may also contribute to efficacy, which may have particular relevance to reducing neurodegenerative sequelae. The interface between natural product derivatives and endogenous lipid signals may provide new therapeutic avenues for the treatment of MS and perhaps other neurodegenerative conditions.
 

Rapamycin as a Potential Therapeutic for Alzheimer's Disease
Salvatore Oddo, PhD, University of Texas Health Science Center, San Antonio

Accumulation of amyloid-β (Aβ) and tau is an invariant feature of Alzheimer disease (AD). However, the molecular mechanisms linking Aβ and tau accumulation to cognitive decline remain to be elucidated. Here we show that the buildup of Aβ increases the mammalian target of rapamycin (mTOR) signaling, while decreasing mTOR signaling reduces Aβ levels, thereby highlighting an interrelation between mTOR signaling and Aβ. The mTOR pathway plays a central role in controlling protein homeostasis and hence neuronal functions; indeed mTOR signaling regulates different forms of learning and memory. Using rapamycin, a product of the bacterium Streptomyces hygroscopicus and known mTOR inhibitor, we show that restoring mTOR signaling rescues cognitive deficits and ameliorates Aβ and tau pathology in two independent animal models. Mechanistically, the changes in Aβ and tau appear mediated by a rapamycin-mediated increase in autophagy induction. The results presented here provide a molecular basis for the Aβ-induced cognitive deficits and, moreover, show that rapamycin, an FDA approved drug, improves learning and memory and reduces Aβ and tau pathology.
 

Biologically Active Grape-Derived Polyphenols for Targeting Tau Oligomerization and Beta-Amyloidosis
Giulio Maria Pasinetti, MD, PhD, Mount Sinai School of Medicine

Potential development of polyphenols for the prevention or treatment of neurological disorders is largely hindered by their complexity and limited knowledge about bioavailability, metabolism, and bioactivity, especially in the brain. We recently found that polyphenol metabolites characterized in the brain are able to improve cognitive function. We report for the first time that a biosynthetic epicatechin metabolite, 3'-O-methyl-epicatechin-5-O-β-glucuronide (3'-O-Me-EC-Gluc), one of the proanthocyanidin (PAC) metabolites identified in the brain following monomeric treatment, promotes basal synaptic transmission and long-term potentiation at physiologically relevant concentrations in hippocampus slices. PAC has beneficial effects on learning and memory in AD dementia, possibly through mechanisms associated with the prevention of oligomerization of Aβ and abnormal phosphorylation of tau. Our studies suggest select brain-targeted PAC metabolites benefit cognition by improving synaptic plasticity in the brain, and provide impetus to develop 3'-O-Me-EC-Gluc and other brain-targeted PAC metabolites to promote learning and memory in AD and other forms of dementia. Our studies have a major impact by providing vital knowledge on the protective roles of polyphenols in AD.