The Biopharmconsortium Blog has been following novel developments in anti-aging medicine and biology for several years. Much of the interest in this field has centered around sirtuins and potential drugs that modulate these protein deacetylase enzymes. Recently–on May 29, 2013–we published our latest blog article on sirtuins.
However, we have long been aware that studies in the biology of aging reveal that lifespan is controlled by sets of complex, interacting pathways. Sirtuins represent only one control point in these pathways, which might not be the most important one.
Now comes a research article in the 9 May 2013 issue of Nature, on the role of inflammatory pathways in the hypothalamus of the brain in the control of systemic aging. This article (Zhang et al.) was authored by Dongsheng Cai, M.D., Ph.D. of the Albert Einstein College of Medicine (Bronx, NY) and his colleagues. The same issue of Nature contains a News and Views mini-review of Zhang et al., authored by Dana Gabuzda, M.D. and Bruce A. Yankner, M.D., Ph.D. (Harvard Medical School).
The role of the neuroendocrine system–and other tissues–in the regulation of lifespan
The News and Views review begins with the statement that classic studies of aging-related pathways in Caenorhabditis elegans by such pioneering researchers as Cynthia Kenyon and Leonard Guarente, as well as later studies in Drosophila suggested that genetic changes that affect the function of nutrient-sensing and environmental-stress-sensing neurons can regulate aging of the entire organism.
In our May 11, 2010 Biopharmconsortium Blog article that reviewed the aging field, we focused on biochemical pathways that may affect lifespan, not the specific tissues in which they act. That article included a link to a 25 March 2010 review in Nature by Dr. Kenyon, entitled “The genetics of ageing”. As we said in our article, that review “discussed the panoply of aging-related pathways in worms, flies, and mice, especially the insulin/insulin-like growth factor-1 (IGF-1) and TOR pathways, as well as several other biomolecules and biological processes”. However, Dr. Kenyon’s article, and several of the references she cites, also deal with the tissues in which these pathways act.
Numerous leading researchers have found evidence that IGF signaling in the C. elegans nervous system regulates longevity. A 2008 article by Laurent Kappeler (INSERM U893, Hopital Saint-Antoine, Paris, France) and his colleagues referenced in the Kenyon review also found evidence that IGF-1 receptors in the brain control lifespan and growth in mice via a neuroendocrine mechanism.
Nevertheless, there is evidence that the insulin pathway in such tissues as the intestine of C. elegans can also regulate lifespan in a non-cell autonomous manner. These studies indicate that changes in insulin pathway gene expression in one tissue–perhaps with certain tissues (the neuroendocrine system, endoderm, adipose tissue) being especially important–results in coordinated changes in insulin pathway activity among all the relevant tissues of the organism. These studies complicate the determination that the neuroendocrine system is the locus of longevity regulation by the insulin pathway and other aging-related pathways.
The potential role of the hypothalamus in the regulation of mammalian aging
Based on the results of studies of neural regulation of lifespan in C. elegans and Drosophila, Zhang et al. asked whether the hypothalamus may have a fundamental role in the process of aging and in regulation of lifespan. The hypothalamus is a region of the brain that is critically involved in regulating such functions as growth, reproduction and metabolism. An important function of the hypothalamus is to link the nervous system to the endocrine system via the pituitary gland, an endocrine gland that is intimately associated with the hypothalamus, and is the master regulator of the endocrine system. According to the Gabuzda and Yankner News and Views article, the mammalian hypothalamus has similar functions to the nutrient-sensing and environmental-stress-sensing neurons of C. elegans and Drosophila that have been implicated in regulation of aging in those organisms.
Zhang et al. studied the increase in NF-κB (nuclear factor kappa-light-chain-enhancer of activated B cells) in the hypothalamus of mice as a function of aging. NF-κB is a transcriptional regulator that is involved in cellular responses to stress, and in particular mediates inflammatory responses; it has also been implicated as a driver of aging-related gene expression in mice. The researchers found that the numbers of microglia (central nervous system cells that functionally resemble macrophages) in the hypothalamus increased as the mice aged. These microglia exhibited inflammatory function, expressing activated NF-κB and overproducing tumor necrosis factor alpha (TNFα). In turn, secretion of TNF-α (an inflammatory cell-signaling molecule) stimulated NF-κB-mediated signaling in hypothalamic neurons.
Zhang et al. showed–via using genetic models such as specific gene knockouts–that activation of the NF-κB pathway in hypothalamic neurons accelerated the aging process and shortened lifespan. Conversely, inhibiting the NF-κB pathway resulting in delayed aging and increased lifespan. They further showed that activation of neural NF-κB signaling resulted in declines in gonadotropin-releasing hormone (GnRH) levels. Since GnRH stimulates adult neurogenesis in the hypothalamus and hippocampus, decline in GnRH secretion suppressed neurogenesis. Conversely, hypothalamic administration of GnRH reversed aging-associated declines in neurogenesis.
The researchers also treated old mice with GnRH peripherally (i.e., via subcutaneous injection, rather than via hypothalamic administration). Such treatment with GnRH resulted in amelioration of aging-related changes in muscle, skin, and the brain. Notably, GnRH treatment resulted in amelioration of aging-related cognitive decline. The researchers hypothesize that peripherally-administered GnRH (a neurohormone that is secreted by specific neurons in the hypothalamus) exerts its anti-aging effects via its action on one or more of the GnRH-responsive brain regions that lack a blood–brain barrier, such as the median eminence, subfornical organ and area postrema.
As pointed out by Gabuzda and Yankner in their News and Views article, a 2011 study showed that dendrites of hypothalamic GnRH-producing neurons extend through the blood–brain barrier. These dendrites are able to sense inflammatory and metabolic signals in the blood. Gabuzda and Yankner hypothesize that inflammatory signals in the periphery (which are known to be associated with aging, and such aging-related conditions as insulin resistance, obesity, and cardiovascular disease) may feed back via these dendrites to downregulate GnRH production in the hypothalamus. Such a feedback loop might be analogous to the coordination between peripheral and neural tissues of aging-related pathways seen in C. elegans by Dr. Kenyon and other researchers.
Despite these hypotheses (as pointed out by Zhang et al.), the mechanisms by which GnRH (especially peripherally-administered GnRH) exerts its anti-aging effects are not well-understood, and need further investigation. The researchers conclude, however, that the hypothalamus can integrate NF-κB-directed immunity and the GnRH-driven neuroendocrine system to program development of aging.
With respect to anti-aging therapy, the study of Zhang et al. suggests two potential therapeutic strategies–inhibition of inflammatory microglia in the hypothalamus, and restoration of levels of GnRH. Given the difficulties of specific targeting of microglia in the hypothalamus (across the blood-brain barrier), the second of these alternatives seems to be the more feasible of these strategies.
The GnRH receptor agonist leuprolide as a potential therapy for Alzheimer’s disease
GnRH has a short half-life in the human body, and thus cannot be used as a medication unless it is delivered via infusion pumps. However, the GnRH receptor agonist leuprolide acetate (AbbVie’s Lupron, Sanofi’s Eligard) has been approved by the FDA since 1985. It is available as a slow-release implant (AbbVie’s Lupron Depot) or a formulation delivered via subcutaneous/intramuscular injection. Leuprolide is approved for treatment of prostate cancer, endometriosis, fibroids, and several other conditions. Although leuprolide is the largest-selling GnRH agonist, there are other approved nanopeptide GnRH agonists, such as goserelin (AstraZeneca’s Zoladex) and histrelin (Endo’s Supprelin and Vantas).
As discussed in a 2007 review by Wilson et al., leuprolide acetate has also been under investigation as a therapeutic for Alzheimer’s disease (AD). Studies in mice indicated that leuprolide modulated such markers of AD as amyloid-β (Aβ) and tau phosphorylation, and prevented AD-related cognitive decline. For example, in a 2006 study in a classic mouse model of AD (Tg2576 amyloid precursor protein transgenic mice carrying the Swedish mutation) by Casadesus et al., the researchers demonstrated that leuprolide acetate halted Aβ deposition and improved cognitive performance.
Although Casadesus et al. attributed the efficacy of leuprolide to its suppression of the production of luteinizing hormone, Wilson et al. speculated that it might be possible that leuprolide works directly via GnRH receptors in the brain. Those receptors had only been identified in 2006 by the first author of the review, Andrea Wilson, and her colleagues at the University of Wisconsin. The direct action of leuprolide on GnRH receptors in the brain to ameliorate aging-related cognitive decline–and perhaps AD itself–is consistent with the 2013 findings of Zhang et al.
Development of a leuprolide-based Alzheimer’s disease treatment by Voyager Pharmaceuticals
As discussed in the review of Wilson et al., a Phase 2 clinical trial in women with mild-to-moderate AD receiving acetylcholinesterase inhibitors and implanted subcutaneously with leuprolide acetate showed a stabilization in cognitive decline at 48 weeks. A subsequent study in men (clinical trial number NCT00076440) was documented on ClinicalTrials.gov between 2004 and 2007. However, no results of this trial were ever posted.
The clinical development of a formulation of leuprolide (known as Memryte) as a treatment for AD had been carried our by a small Durham, NC biotech company called Voyager Pharmaceutical Corporation. Memryte was a biodegradable implant filled with leuprolide acetate, designed to treat mild to moderate AD.
After struggling to develop their treatment for nearly a decade, Voyager ran out of money, and stopped its R&D operations in 2007. In 2009, Voyager acquired a new set of investors, and changed its name to Curaxis. In 2010, Curaxis did a reverse stock merger, which enabled it to become a publicly traded company. Also in 2010, Curaxis attracted $25 million from a Connecticut investment firm. Nevertheless, Curaxis noted in its SEC filing that it would need “at least $48 million through 2014” to complete development of Memryte and file a New Drug Application (NDA) with the FDA.
However, Curaxis failed to find the needed funding and/or a partner to complete its development plans. In July 2012, the company filed for bankruptcy.
The failure of Voyager/Curaxis as a company does not necessarily mean that leuprolide may not be a viable treatment for AD. However, as we noted in earlier Biopharmconsortium Blog articles, development of an AD therapy is an enormously long, expensive, and risky proposition, which is beyond the capacity of a small biotech (such as Voyager/Curaxis) unless it attracts a Big Pharma partner. Moreover, treating AD once it reaches the “mild to moderate” stage is unlikely to work.
As we discussed in our April 5, 2013 article, the FDA has been working with industry and academia to develop guidelines for clinical trials of agents to treat the very earliest stages of AD, before the development of extensive irreversible brain damage. If another company endeavors to develop a formulation of leuprolide (or another GnRH pathway activator) for AD, it would probably do best to aim to treat very early-stage disease using the new proposed FDA guidelines.
Why should drug discovery and development researchers and executives be interested in anti-aging research? No pharmaceutical company will be able to run a clinical trial with longevity as an endpoint. However, the hope is that an “anti-aging” drug approved for treatment of one disease of aging will have pleiotropic effects on multiple diseases of aging, and will ultimately be found to increase lifespan or “healthspan” (the length of a person’s life in which he/she is generally healthy and not debilitated by chronic diseases). Numerous pharmaceutical and biotechnology companies have been working on discovery and development of treatments for major aging-related diseases, such as type 2 diabetes and AD. It would be truly spectacular if a new drug for (for example) type 2 diabetes would also be effective against AD.
Moreover, their are also major aging-related conditions, such as sarcopenia (aging-related loss of muscle mass, quality, and strength) that are not normally targets for drug development, but are major causes of disability and death. It would also be amazing if an anti-aging drug aimed at (for example) AD would be effective against sarcopenia as well. Note that Zhang et al. showed that GnRH treatment ameliorated aging-related changes in muscle in their mouse models. (Update: The statement that sarcopenia is “not normally a target for drug development” is no longer true. See our September 4, 2013 article on this blog, “Novartis’ Breakthrough Therapy For A Rare Muscle-Wasting Disease”.)
The study of Zhang et al. constitutes a new approach to anti-aging biology and target and drug discovery. However, aging biology is complex and not well-understood. Researchers will need to study many aspects of aging-related pathways for anyone to be able to discover and develop successful anti-aging drugs. This includes, of course, the sirtuin field, as well as the role of mitochondria in aging related metabolic imbalance, as exemplified by a recent paper in Cell. As discussed in our other Biopharmconsortium Blog articles on anti-aging biology and medicine, there are many other avenues for investigation as well.
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