Galega officinalis (Goat’s Rue) From JoJan

Metformin (Bristol-Myers Squibb’s Glucophage, generics), an oral biguanide antidiabetic drug, is the most widely prescribed agent for treatment of type 2 diabetes. The drug mainly works by lowering glucose production by the liver, and thus lowering fasting blood glucose.

Although metformin–approved in the United States in 1994, and in Europe prior to that–has been used for many years, its mechanism of action is not well understood. In 2005, signal-transduction pioneer Lewis Cantley (Beth Israel Deaconess Cancer Center/Harvard Medical School, Boston MA), and his colleagues–including Reuben J. Shaw (now at the Howard Hughes Medical Institute, The Salk Institute for Biological Studies, La Jolla, CA)–published a report showing that metformin targets the adenosine monophosphate (AMP)-activated kinase (AMPK) pathway in the liver. We discussed this report and its implications in our 2007 Cambridge Healthtech Institute Insight Pharma Report, Diabetes and Its Complications.

AMPK is found in all eukaryotic organisms, and serves as a sensor of intracellular energy status. In mammals, it also is involved in maintaining whole-body energy balance, and helps regulate food intake and body weight. We  have discussed the potential role of AMPK in regulation of lifespan, and as a target in anti-aging medicine and in metabolic disease, in earlier articles on this blog. (See here and here.)

AMPK is activated by increases in the ratio of AMP to ATP, caused by energy stress. Under conditions of energy stress, AMP levels go up, and AMP binds to a specific site on the AMPK γ subunit. This induces a conformational change that exposes the activation loop of the α subunit. This allows an upstream serine/threonine kinase to phosphorylate this activation loop. In several mammalian cell types, including liver and skeletal muscle, that kinase is LKB1. Drs. Cantley and Shaw in 2005 showed that metformin targets the LKB1-AMPK pathway in the liver, and that metformin requires LKB1 to lower glucose production by the liver. However, neither LKB1 nor AMPK is the direct target of metformin, and as of 2005, that direct target was unknown.

A new genetic study that suggests that ATM kinase may affect the ability of patients to respond to metformin

Now–as of February 2011–comes a Nature Genetics paper that indicates that the serine/threonine kinase ATM (ataxia telangiectasia mutated) acts upstream of AMPK to mediate the therapeutic effects of metformin. ATM is a DNA repair protein that is recruited and activated by double-strand breaks in DNA. It initiates activation of the DNA damage checkpoint, leading to cell cycle arrest, followed by DNA repair or apoptosis. Thus the role of ATM in the AMPK pathway and in the therapeutic effects of metformin is surprising indeed.

In the study reported in the Nature Genetics paper, researchers of The GoDARTS and UKPDS Diabetes Pharmacogenetics Study Group and The Wellcome Trust Case Control Consortium 2 performed a genome-wide association study (GWAS) for glycemic response to metformin in type 2 diabetes patients in the U.K. In a population of nearly 4,000 patients, they identified a single-nucleotide polymorphism (SNP) designated rs11212617, which was associated with treatment success. This SNP occurs in a genetic locus that also contains the gene that encodes ATM. In a rat hepatoma cell line, inhibition of ATM by the specific inhibitor KU-55933 (KuDOS Pharmaceuticals, Cambridge, U.K., which was acquired by AstraZeneca in 2005) attenuated metformin-mediated phosphorylation and activation of AMPK.

The analysis by Morris Birnbaum and Reuben Shaw in the 17 February 2011 issue of Nature

The 17 February 2011 issue of Nature contained a Forum entitled “Genomics: Drugs, diabetes and cancer.” This consisted of two analyses of the implications of the Nature Genetics paper for metformin’s mechanism of action, and for understanding diabetes and the connections of the metformin-activated ATM/AMPK pathway with cancer. The first analysis was by Morris J. Birnbaum, M.D., Ph.D. (University of Pennsylvania Medical School, Philadelphia, PA), who does research on the role of AMPK and insulin in energy metabolism and in diabetes. The second analysis is by Dr. Reuben Shaw, mentioned earlier. Dr. Shaw’s research centers around LKB1 [also known as serine/threonine kinase 11 (STK11)]. LKB1, a serine/threonine kinase, is not only a regulator of hepatic glucose production via AMPK, but is also a tumor suppressor. Germline mutations in LKB1 are associated with the familial cancer Peutz-Jegher syndrome, and somatic mutations in LKB1 are present in various other cancers. In particular, the Lkb1 gene is one of the most frequently muted genes in human lung adenocarcinomas.

Dr. Birnbaum’s analysis

Dr. Birnbaum notes that the finding of a role for ATM in metformin responsiveness may be an important clue to the mechanism of action of this drug. However, it may also be a false lead, with ATM having only an indirect effect on metformin’s action. He cites recent evidence that metformin acts independently from LKB1 and AMPK and of transcriptional regulation in general. In these studies, genetic ablation of LKB1 and AMPK was used to show that these mediators are dispensable for metformin’s glucose-lowering activity. Instead, metformin appears to work by inhibiting mitochondrial production of ATP in the liver, thus reducing the level of liver glucose production via gluconeogenesis (which uses ATP). This is in apparent contradiction to the 2005 results of Dr Shaw and his colleagues. Nevertheless, metformin’s inhibition of mitochondrial ATP production increases the ratio of AMP to ATP, and thus activates AMPK. There are also other pathways by which inhibition of mitochondrial ATP production may inhibit gluconeogenesis. Thus the mechanisms by which metformin causes a decrease in glucose production by the liver appear to be very complex, and are not well understood.

Dr. Birnbaum therefore speculates that ATM may affect blood glucose levels via pathways that are parallel to, but not the same as, those modulated by metformin. However, the effects of these other pathways may be synergistic with those modulated by metformin when patients are treated with the drug. Dr. Birnbaum notes that 40 years ago, it was found that patients with ataxia telangiectasia often display a type 2-diabetes-like condition, including insulin resistance. Ataxia telangiectasia is a familial disease caused by germline mutations in ATM. This suggests that  ATM may act to counteract hyperglycemia and insulin resistance.

Dr. Birnbaum concluded that biochemical and cell biology studies should be conducted to determine the nature of the interaction of ATM and the antidiabetic effects of metformin. Key to these endeavors is to determine whether there are any biomolecules other than AMPK that both are influenced by ATM and control metabolism.

Dr. Shaw’s analysis

Dr. Shaw first discusses several animal studies that help elucidate the role in glucose regulation of the biomolecules involved in the putative ATM-LKB1-AMPK pathway. He notes notes that deletion of the Lkb1 gene in mouse liver results in loss of AMPK activity in that organ, and to the development of hyperglycemia and hepatic steatosis–two conditions that are seen in type 2 diabetes. Dr. Shaw also cites the 40-year-old finding about the connection between  ataxia telangiectasia and insulin resistance and diabetes. But as he also mentions the more recent (2006) finding that mice with defective ATM activity show increased insulin resistance and abnormal glucose regulation.

Dr. Shaw then speculates as to how ATM might work to modulate patients’ antidiabetic responses to metformin. He notes that ATM is known to phosphorylate LKB1, which is the key activator of AMPK in the liver. Alternatively, ATM might also regulate AMPK independently of LKB1, and might affect responsiveness of patients to metformin by regulating other relevant targets, independently of AMPK. In this context, ATM is known to phosphorylate other, LKB1 and AMPK-independent components of the insulin signaling pathway.

In the light of these considerations, Dr. Shaw says that it is important to determine whether the rs11212617 genetic variant results in modulation of ATM activity toward AMPK activation or toward other targets relevant to glucose regulation, or indeed whether this SNP affects ATM activity at all.

Dr. Shaw then focuses on the potential relevance of metformin to cancer therapy. Researchers have found, in retrospective studies, that diabetes patients who take metformin have a lower risk of developing cancer than those treated with other antidiabetic medications. Animal studies confirm the anticancer effects of metformin, but–as discussed in a 2010 review by Dr. Michael Pollak (McGill University, Montreal, Quebec, Canada)–they indicate that the anticancer effects of this drug are mechanistically complex. Dr. Shaw asks whether metformin is a general activator of ATM (and/or its targets) in the DNA damage-response pathway, or whether its specific effects on LKB1 and/or AMPK might be responsible for the apparent beneficial effects of metformin on cancer risk.

Dr. Shaw concludes with the statement that future studies of the relationship between metformin action, ATM, LKB1, and AMPK should shed light on the relationship between metformin’s antidiabetic effects and its apparent anticancer effects.

Our conclusions

The finding, based on a genome-wide association study, which suggests that ATM, a kinase best known for its involvement in DNA repair pathways, may also be involved in diabetics’ response to metformin is surprising and intriguing. It may eventually be important in unraveling metformin’s mechanism of action in inhibition of liver gluconeogenesis, and in other antidiabetic activities. This finding indicates a connection between pathways by which metformin exerts its antidiabetic activities, and pathways that are involved in cancer.

Nevertheless, the elucidation of metformin’s mechanism(s) of action in diabetes remains a work in progress. This situation is an example of how science works in the real world (as opposed to textbooks or much of science journalism)–generating more questions than answers.

A drug like metformin, with its complex and still poorly understood mechanism of action, could not have been discovered by modern, post-genomics drug discovery strategies. Metformin was discovered via research on natural products derived from the plant Galega officinalis (known as the French lilac, goat’s rue, and by various other names), which had been known by herbalists for centuries. It is fortunate that researchers were able to study the effects of extracts of this plant, and ultimately to develop metformin, well in advance of the modern era of drug discovery. Diabetics and their physicians now have access to metformin as an inexpensive generic drug.

The continued study of the antidiabetic mechanism(s) of action of metformin may yield additional insights into control of gluconeogenesis and other metabolic pathways. Some of the findings of these studies might be relevant to drug discovery and development, for example the development and use of AMPK activators in metabolic disease and in anti-aging medicine.

Continued study of the mechanism(s) of action of metformin may also be relevant to developing new therapies for cancer. As suggested by Dr. Pollak, although metformin is off-patent and is thus not an attractive agent for development as an oncology drug by pharmaceutical or biotechnology companies, other biguanides or related compounds might be better anticancer compounds, and would be patentable. In addition to identifying such compounds, it will be important to determine and define which groups of cancer patients could best benefit from them (perhaps via biomarkers). It will then be important to conduct personalized medicine hypothesis-testing clinical trials (as discussed in an earlier blog post) designed to obtain proof-of-concept that such compounds can indeed benefit specific groups of patients.


As the producers of this blog, and as consultants to the biotechnology and pharmaceutical industry, Haberman Associates would like to hear from you. If you are in a biotech or pharmaceutical company, and would like a 15-20-minute, no-obligation telephone discussion of issues raised by this or other blog articles, or of other issues that are important to  your company, please click here. We also welcome your comments on this or any other article on this blog.


In statements to Fierce Biotech and to the Myeloma Beacon, GlaxoSmihtKline (GSK) said that it has stopped all development of its proprietary resveratrol formulation SRT501. Thanks also to the “In the Pipeline” blog for the information on the Myeloma Beacon statement.

As you all may recall, GSK acquired the sirtuin-pathway specialty company Sirtris (Cambridge, MA) for $720 million in June 2008. This gave GSK ownership of Sirtris’ sirtuin modulator drugs, including SRT501. GSK also appointed Christoph Westphal, then CEO of Sirtris, as the Senior Vice President of GSK’s Centre of Excellence in External Drug Discovery (CEEDD), and Michelle Dipp, then vice president of business development at Sirtris, as Vice President and the head of the US CEEDD at GSK.

According to the Fierce Biotech article, the precipitating factor in GSK’s decision to halt development of SRT501 was the result of a Phase 2a study of the drug in advanced multiple myeloma. The company suspended the study after several patients developed kidney failure. GSK said that in its analysis, the company concluded that SRT501 “may only offer minimal efficacy while having a potential to indirectly exacerbate a renal complication common in this patient population.” It then said that the company has “no further plans to develop SRT501.”

Instead, GSK intends to focus on development of Sirtris’ non-resveratrol synthetic selective sirtuin 1 (SIRT1) activators, which in addition to their greater potency, have more favorably drug-like properties. In its statement to the Myeloma Beacon, GSK in particular mentioned SRT2104 and SRT2379 as the focus of its continuing activity. According to the Sirtris website, SRT2104 is in Phase 2 studies in metabolic and cardiovascular disease, and SRT2379 is in Phase 1 studies in healthy volunteers. Neither compound is currently being tested in cancer.

We discussed Sirtris’ SIRT1 activators in the context of the anti-aging biology field, in a February 10, 2010 blog post. In summary, the mechanism of action of reseveratrol and of Sirtris/GSK’s sirtuin activators is unclear. They apparently activate multiple targets, and they may not be direct SIRT1 activators at all. Nevertheless, Sirtris’ studies of these compounds in mice indicate that they have efficacy in treatment of metabolic diseases. The Phase 2 clinical trials in humans are still ongoing.

To complicate matters further, a study published in the journal Diabetes in March 2010 by NIH researcher Jay H. Chung and his colleagues indicates that resveratrol works indirectly, via the energy sensor AMP-activated protein kinase (AMPK), to activate sirtuins. Since activation of AMPK increases fatty acid oxidation and upregulates mitochondrial biogenesis, the effect of resveratrol on AMPK may be more important than its more indirect activation of sirtuins, at least in the case of metabolic diseases.

Thus Sirtris/GSK’s “sirtuin activators” are under a cloud.

However, as we discussed in our blog posts of November 8, 2009 and February 10, 2010, basic research on anti-aging biology has yielded ample material for drug discovery which may eventually lead to novel treatments for metabolic diseases, and perhaps for other conditions such as various cancers. For example, several companies are developing AMPK activator drugs. Thus there are other avenues for harnessing basic research on anti-aging pathways to discover and develop novel drugs for multiple conditions, even if the Sirtris compounds prove to be a dead end.


As the producers of this blog, and as consultants to the biotechnology and pharmaceutical industry, Haberman Associates would like to hear from you. If you are in a biotech or pharmaceutical company, and would like a 15-20-minute, no-obligation telephone discussion of issues raised by this or other blog articles, or of other issues that are important to  your company, please click here. We also welcome your comments on this or any other article on this blog.

1. Continuing Controversy

In our blog post on 10 February 2010, we discussed the controversy over Sirtris/GlaxoSmithKline’s reseveratrol formulation, and its second-generation sirtuin-1 (SIRT1) activators. Researchers at Amgen and Pfizer found that the apparent in vitro activation of SIRT1 by these compounds was an artifact of the experimental method used by Sirtris researchers. The Amgen group found that the fluorescent SIRT1 peptide substrate used in the Sirtris assay is a substrate for SIRT1, but in the absence of the covalently linked fluorophore, the peptide is not a SIRT1 substrate. Although resveratrol appears to be an activator of SIRT1 if the artificial fluorophore-conjugated substrate is used, resveratrol does not activate SIRT1 in vitro as determined by assays using two other non-fluorescently-labeled substrates.

Last month, I attended two meetings at which this controversy was discussed. One was the Bio-IT World Conference & Expo in Boston. At that conference, Christoph Westphal (then CEO of Sirtris) gave a keynote address. In that presentation, Mr. Wesphal stuck with the story that Sirtis’ compounds and its assays are valid. The day after his presentation, Mr. Westphal resigned as Sirtris’ CEO, and now is the head of GSK’s SR One venture fund. He and other Sirtris and Vertex founders also started the Longwood Founders Fund in February of this year.

At the other meeting (which was Harvard-related), one of the most respected leaders of the longevity-related pathway field (whose name I am withholding) stated that the whole resveratrol/sirtuin-activator story is nonsense. He did, however, concur with our views on anti-aging pathways as expressed in our November 8, 2009 article on this blog. We do not go as far as calling the resveratrol story nonsense, but remain unconvinced of the mechanistic basis for resveratrol action pending further evidence.

Meanwhile, Derek Lowe’s “In the Pipeline” blog has a discussion of Mr. Wesphal’s talk at the Bio-IT conference.

In its 25 March 2010 issue, Nature also has a News Feature centered upon the controversy. This article (written by Cambridge MA-based Nature reporter Heidi Ledford) basically says that the controversy remains unsettled, but that several laboratories are working to resolve the assay issue. These include corporate researchers at Sirtris, Leonard Guarente of MIT (another leader in the longevity-related pathway field, who is co-chair of Sirtris’ scientific advisory board), and Anthony Sauve of Weill Cornell Medical School (also a member of Sirtris’ scientific advisory board).

2. Opportunity

There was a review of longevity-related pathways in the 16 April 2010 issue of Science. It covers all the bases of anti-aging research in yeast, worms, flies, and mammals, with an emphasis on the TOR and insulin-like growth factor-1 (IGF-1) pathways. Sirtuins and resveratrol rate a minimal mention in the review.

Cynthia Kenyon, another leader in the longevity pathway field, published a review on the genetics of aging in a special Nature Insight section on aging in the 25 March 2010 issue. In this review, Dr. Kenyon discussed the panoply of aging-related pathways in worms, flies, and mice, especially the insulin/IGF-1 and TOR pathways, as well as several other biomolecules and biological processes. Dr. Kenyon discusses sirtuins, but notes the unknowns in aging-related mechanisms involving sirtuins, especially in mammals. She also notes the difficulties in interpreting results with resveratrol. In addition to the issue with the assays involving the fluorescent substrate, she notes that although (in studies conducted by Sirtris researchers and their academic colleagues) resveratrol has been found to extend the lifespan of mice fed a high-fat diet, it did not extend the lifespan of mice fed a normal diet. Dr. Kenyon also cited the results of studies with resveratrol in yeast, worms, and flies that are not consistent with the hypothesis that resveratrol extends lifespan by acting as a sirtuin activator.

The bottom line of the discussion in the two reviews in Science and Nature is 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. Thus no one company “owns” the anti-aging field in terms of drug discovery and development, and there is a lot of opportunity out there. Even Mr. Westphal stated as much in his Bio-IT World presentation.

Interestingly, Dr Kenyon notes that different closely related animals can have large differences in lifespan. For example, rats live for three years, but squirrels for 25. She speculates that differences in longevity might be easily evolvable, and mechanisms by which lifespan changes during evolution (perhaps involving mutations in regulatory genes or that affect rates of respiration) might constitute novel intervention points.

3. Good News

Now for some good news about aging. In an article in the 25 March 2010 Nature Insight section by James W Vaupel (Max Planck Institute for Demographic Research, Rostock, Germany, University of Southern Denmark, Odense, and Duke University), the author presents evidence that human senescence (i.e., deterioration with age)—at least in advanced countries—has been postponed by a decade. This process, first noted in 1994, is continuing. The factors that are making this possible are prosperity (which promotes good health) and medicine (including medical and surgical interventions to prevent or treat disability, and public health efforts). These two factors enable people to reach old age in better health, as well as promoting better health in older people.

This ongoing postponement of senescence and mortality provides a foundation for ongoing anti-aging research and eventual treatments based on that research. (One must remember, however, that regulatory agencies as well as the practical considerations of drug development will not permit researchers and companies to utilize mortality as an endpoint in clinical trials. Companies must therefore develop putative “anti-aging drugs” for specific diseases associated with aging, such as diabetes, cancer, various cardiovascular indications, and dementia.) The postponement of senescence also has profound implications for how one lives one’s life, as well as for social policy and the practice of medicine.

On November 8, 2009, we posted an article entitled “Anti-aging biology: new basic research, drug development, and organizational strategy” on this blog. This article focused on new findings in anti-aging biology, their applications to drug discovery and development, and on how this field has affected the organizational strategy of GlaxoSmithKline (GSK).

GSK acquired Sirtris for $720 million in 2008. Later that year, GSK appointed Christoph Westphal, the CEO and co-founder of Sirtris, as the Senior Vice President of GSK’s Centre of Excellence in External Drug Discovery (CEEDD). The CEEDD works to develop external alliances with biotech companies, with the goal of acquiring promising new drug candidates for GSK’s pipeline. Michelle Dipp, who was the vice president of business development at Sirtris at the time of GSK’s appointment of Dr. Wesphal, became Vice President and the head of the US CEEDD at GSK. Thus GSK has been using its relationship with Sirtris to restructure its organizational strategy, attempting to become more “biotech-like” in order to improve its R&D performance.

Now we learn that several research groups and companies have been questioning whether resveratrol (a natural product derived from red wine which has been the basis of Sirtris’ sirtuin-activator platform), as well as Sirtris’ second-generation compounds, may not modulate the sirtuin SIRT1 at all. Thanks to Derek Lowe’s “In the Pipeline” blog for the information. This issue was also covered in a second post on the same blog. It was also covered by articles in the 15 January 2010 issue of New Scientist and in the January 26, 2010 issue of Forbes. Nature also covered this story in an online news article.

In a report published in December 2009, researchers at Amgen found evidence that the apparent in vitro activation of SIRT1 was an artifact of the experimental method used by Sirtris researchers. The Amgen group found that the fluorescent SIRT1 peptide substrate used in the Sirtris assay is a substrate for SIRT1, but in the absence of the covalently linked fluorophore, the peptide is not a SIRT1 substrate. Although resveratrol appears to be an activator of SIRT1 if the artificial fluorophore-conjugted substrate is used, resveratrol does not activate SIRT1 in vitro as determined by assays using two other non-fluorescently-labeled substrates.

More recently, researchers at Pfizer published a study of SIRT1 activation by resveratrol and three of Sirtris’ second-generation sirtuin activators (which the Pfizer researchers synthesized themselves, using published protocols). These researchers also found that although these compounds activated SIRT1 when a fluorophore-bearing peptide substrate was used, they were not SIRT1 activators in in vitro assays using native peptide or protein substrates. The Pfizer researchers also found that the Sirtris compounds interact directly with the fluorophore-conjugated peptide, but not with native peptide substrates.

Moreover, the Pfizer researchers were not able to replicate Sirtris’ in vivo studies of its compounds. Specifically, when the Pfizer researchers tested SRT1720 in a mouse model of obese diabetes, a 30 mg/kg dose of the compound failed to improve blood glucose levels, and the treated mice showed increased food intake and weight gain. A 100 mg/kg dose of SRT1720 was toxic, and resulted in the death of 3 out of 8 mice tested.

The Pfizer researchers also found that the Sirtris compounds interacted with an even greater number of cellular targets (including an assortment of receptors, enzymes, transporters, and ion channels) than resveratrol. For example, SRT1720 showed over 50% inhibition of 38 out of 100 targets tested, while resveratrol only inhibited 7 targets. Only one target, norepinephrine transporter, was inhibited by greater than 50% by all three Sirtris compounds and by resveratrol. Thus the Sirtris compounds have a different target selectivity profile than resveratrol, and all of these compounds exhibit promiscuous targeting.

Sirtris and GSK dispute the findings of the Amgen and Pfizer researchers. One issue raised by Sirtris is that the Sirtris compounds synthesized by Pfizer may have contained impurities, resulting in the toxicity and lack of specificity of the compounds in vivo. Researchers associated with Sirtris and GSK also contend that although the Sirtris compounds only work with fluorophore-conjugated peptides in vitro, they appear to increase the activity of SIRT1 in cells. However, other researchers assert that since resveratrol interacts with many targets in cells, the results of the cellular assays are difficult to interpret. In the Forbes article, GSK’s CEO Andrew Witty is quoted as calling the dispute over the activity of the Sirtris compounds “a bit of a storm in a teacup”. He says that the compounds that Pfizer tested in mice are not the same ones that Sirtris and GSK are currently testing in clinical trials for treatment of diabetes and cancer. (Sirtris’ compounds in clinical trials, discussed in the next paragraph, are in fact different from the ones tested by the Pfizer researchers.)

Currently, Sirtris is testing its proprietary formulation of resveratrol, SRT501, in a Phase IIa clinical trial in cancer. The company reports that SRT501 lowered blood glucose and improved insulin sensitivity in patients with type 2 diabetes in a Phase IIa trial. Sirtris is also testing a second-generation SIRT1 activator, SRT2104, in Phase IIa trials in patients with metabolic, inflammatory and cardiovascular diseases. SRT2104 was found to be safe and well tolerated in Phase I trials in healthy volunteers. Sirtris is also testing another second-generation SIRT1 activator, SRT2379, In Phase I trials. SRT2379 is structurally distinct from resveratrol and from SRT2104.

As we discussed in our original blog post, Elixir Pharmaceuticals is also developing various sirtuin inhibitors and activators for metabolic and neurodegenerative diseases and for cancer. One of Elixir’s products, the SIRT1 inhibitor EX-527, was in-licensed by Siena Biotech (Siena, Italy) in 2009, and was entered into Phase I clinical trials in January 2010. Siena Biotech is developing this compound for treatment of Huntington’s disease.

Despite the dispute over whether Sirtris’ compounds are real SIRT1 activators, the numerous studies on the biology of sirtuins are still valid. Companies with assays that use native peptide substrates and are amenable to high-throughput screening could use these assays to discover novel sirtuin activators. For example, Amgen published a report in 2008 describing such assays. The ability of companies such as Amgen and Pfizer to commercialize such novel sirtuin activators would depend on whether they could overcome the intellectual property position of Sirtris (and Elixir). Since Amgen and Pfizer are working in this area, this indicates that they believe that they can do so.

The efficacy of high doses of resveratrol in improving metabolic parameters of mice fed a high-calorie diet is also not invalidated by the Amgen and Pfizer studies. However these studies cast doubt on the mechanisms by which resveratrol exerts these effects. The apparent efficacy of SRT501 in improving metabolic parameters in patients with type 2 diabetes in a Sirtris Phase IIa trial is consistent with the mouse studies.

Finally, as we discussed in our November 8, 2009 blog post, longevity is controlled by numerous interacting pathways, which may provide at least several targets for drug discovery. Researchers are hard at work to gain additional understanding of these pathways, and some companies are working to discover and develop compounds that modulate these targets. For example, several companies are developing AMPK activators, as discussed in our original blog post. And numerous research groups are reportedly attempting to find drugs that act similarly to rapamycin in increasing lifespan in mice (the main focus of our November blog post), without rapamycin’s immunosuppressive effects.

In the 2 October issue of Science (the “Ardipithecus ramidus issue”), there was a Perspective (authored by Matt Kaeberlein and Pankaj Kapahi) and a Report (authored by Colin Selman and his colleagues) on recent findings in anti-aging biology.

Since the late 1980s, researchers have found that caloric restriction (CR) (reduction in caloric intake while maintaining essential nutrients) slows aging in a variety of organisms—yeasts, nematodes, fruit flies, mice, and most recently rhesus macaques. In the recently published 20-year study in rhesus macaques, CR not only increased lifespan, but also delayed the onset of a suite of aging-related disease conditions—diabetes, cancer, cardiovascular disease, and brain atrophy. This parallels the studies with other organisms.

Researchers who have been studying the CR model have been attempting to elucidate the mechanisms by which CR works to slow the aging process and to retard aging-related disease. They hope to find targets for drugs to mimic the effects of CR in humans, since long-term CR is not practical for most people. Over the years, researchers have discovered several pathways by which CR appears to exert its effects. The Report describes new research results on one such pathway, the mammalian target of rapamycin (mTOR) pathway. The Perspective reviews this research in the context of related recent studies.

In a report published in Nature earlier this year (16 July 2009), researchers found that rapamycin administered in food increased the median and maximal lifespan of genetically heterogeneous laboratory mice, whether it was fed to middle-aged (600 days old) or young adult (270 days old) mice. Rapamycin feeding beginning at 600 days of age led to an increase in lifespan of 14% for females and 9% for males, on the basis of age at 90% mortality.

Rapamycin targets mTOR (mammalian target of rapamycin), a kinase that regulates signaling pathways that affect many cellular processes. mTOR forms two protein complexes that are active in intracellular signaling—mTORC1 and mTORC2. It is mTORC1 that is most sensitive to rapamycin. mTORC1 works to coordinate cellular growth and survival responses induced by changes in the availability of nutrients, and also responses to cellular stresses (e.g., hypoxia, DNA damage and osmotic stress). Genetic inhibition of TORC1 in yeast and invertebrates has been found to extend their lifespan. In particular, in the nematode Caenorhabditis elegans, TORC1 interacts with the insulin pathway (via raptor, a component of TORC1) to control lifespan. The role of the insulin pathway in the enhancement of lifespan by CR in C. elegans has been known for many years. The role of mTORC1 at the junction of nutrient and stress sensing pathways, together with these results in invertebrates and now mice, has led researchers to hypothesize that the mTORC1 pathway may be involved in CR-mediated enhancement of lifespan, and that drugs that modulate this pathway may substitute for CR in lifespan extension.

In other studies, inhibition of the mTOR pathway in mice was found to retard development of such aging-related conditions as cancer, metabolic disease, and cardiovascular disease. This effect has also been seen in studies of CR in mice and in nonhuman primates, as stated above.

Rapamycin is an immunosuppressant that is marketed as Wyeth’s (now Pfizer’s, since the October 2009 merger) Rapimmune, to prevent organ transplant rejection. More recently, a derivative of rapamycin, temsirolimus (Wyeth/Pfizer’s Toricel) has been approved for treatment of renal cell carcinoma. The authors of the Nature paper therefore hypothesized that rapamycin may have extended lifespan in the mice either by working via CR-related pathways that control lifespan, by postponing death from cancer, or both.

The finding that oral rapamycin can retard aging in mice, even when fed to 600-day-old mice (the equivalent of 60 years old in humans) raises hope for the development of anti-aging drugs for human use. However, rapamycin itself cannot be used for this purpose because of its immunosuppressant effects. (In the mouse rapamycin feeding studies, the mice were kept under specific pathogen-free conditions.) If researchers were to attempt to modulate the mTORC1 pathway to extend lifespan, they would therefore need to discover other drugs that modulate that pathway without rapamycin’s side effects. Learning more about specific pathway components that may be targeted to increase lifespan may help researchers discover such drugs.

In the new Selman et al. report, researchers endeavored to learn more about how the mTORC1 pathway might extend lifespan in mice. They constructed knockout mice that lacked S6 protein kinase 1 (S6K1). S6K1 is a downstream target of mTORC1, which upregulates mRNA translation and protein synthesis in response to mTORC1 signaling. The researchers found that deletion of the gene for S6K1 resulted in a 19% increase in median lifespan in female mice (as compared to wild-type females), and also increased maximum lifespan. S6K1 deletion had no effect on the lifespan of male mice. This was in contrast to the study with rapamycin feeding, which showed lifespan extension in both sexes, even though the effect in female mice was greater. However, the results of the two studies are not strictly comparable, since mice of different genetic background were used in the two studies.

Female S6K1 knockout mice also showed improvement in several biomarkers of aging (e.g., motor and neurological function, level of physical activity, insulin sensitivity, glucose tolerance, fat mass, immunological parameters). Hepatic gene expression in 600-day-old female S6K1 knockout mice resembled that of wild type mice subjected to CR. Female S6K1 knockout mice showed increased hepatic, muscle, and adipose tissue expression (as compared to wild-type mice) of genes associated with other pathways associated with longevity, including genes for sirtuin-1 (SIRT1) and adenosine monophosphate-activated protein kinase (AMPK).

Selman et al. went on to obtain evidence that the effect of S6K1 knockout on lifespan in female mice is due to activation of AMPK. The gene expression profile of muscle tissue of long-lived female S6K1 knockout mice resembled the profile of wild-type mice treated with the AMPK activator aminoimidazole carboxamide ribonucleotide (AICAR). Hepatocytes from S6K1 knockout mice also showed enhanced AICAR activation of AMPK as compared to hepatocytes from wild type mice. A parallel study in C. elegans showed that deletion of the aak-2 gene, which encodes a subunit of AMPK, suppresses lifespan extension in mutants that lack rsks-1, the nematode homolog of S6K1. These results suggest that S6K1 knockout may exert its pro-longevity effects via activation of AMPK.

AMPK is found in all eukaryotic organisms, and serves as a sensor of intracellular energy status. In mammals, it also is involved in maintaining whole-body energy balance, and helps regulate food intake and body weight. AMPK has been implicated in metabolic response to CR in eukaryotic organisms from yeasts to humans, and it mediates the effects on lifespan of at least one type of CR regimen in C. elegans. Thus the hypothesis that lifespan extension via the mTORC1-S6K1 pathway works via AMPK activation is an attractive one.

However, it is not known how deletion of S6K1 (or its inhibition via mTORC1 in rapamycin-treated mice) might activate AMPK. Moreover, as pointed out by Kaeberlein and Kapahi, there are other downstream targets of S6K1 that might play a role in anti-aging effects of SK61 deletion or inhibition. Among these is hypoxia-inducible factor-1α (HIF-1α). Moreover, there are other biomolecules and pathways that have been implicated in the effects of CR on retarding aging. These especially include the sirtuins, an evolutionarily conserved family of nicotinamide adenine dinucleotide (NAD+)-dependent protein deacetylases.

As shown by the Perspective and Report in the 2 October issue of Science, anti-aging research is an exciting area of basic biological research, and researchers still have much to learn about pathways that mediate the effects of CR on longevity. However, this field is already being applied to drug discovery and development. A basic issue in applying anti-aging research to the development of drugs is that one clearly cannot use increased lifespan as an endpoint in clinical trials. Companies must test putative anti-aging drugs against one or more diseases of aging. The hope is that any “anti-aging” drugs 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).

The two principal types of “anti-aging” drugs currently in company pipelines are sirtuin modulators and AMPK activators. Sirtris Pharmaceuticals (Cambridge, MA, a wholly-owned subsidiary of GlaxoSmithKline [GSK]) is developing the SIRT1 activators SRT501 (a proprietary formulation of the natural product resveratrol) and SRT2104 (a novel synthetic small-molecule SIRT1 activator that is structurally unrelated to resveratrol and is up to 1000-fold more potent). SRT501 is in Phase II clinical trials in type 2 diabetes. SRT2104 has been tested in Phase I trials in healthy volunteers, and was found to be safe and well tolerated. Elixir Pharmaceuticals (Cambridge, MA) is developing a preclinical-stage SIRT1 inhibitor for treatment of Huntington’s disease and certain cancers, and a preclinical-stage SIRT1 activator for treatment of type 2 diabetes and obesity. Elixir also has a research-stage SIRT2 inhibitor under development for treatment of type 2 diabetes and obesity.

Companies developing AMPK activators include a collaboration between Metabasis Therapeutics (La Jolla, CA; about to be acquired by Ligand Pharmaceuticals, San Diego, CA) and Merck–preclinical oral AMPK activators, for treatment of type 2 diabetes and hyperlipidemia), Mercury Therapeutics (Woburn, MA)–research and preclinical-stage oral AMPK activators for treatment of type 2 diabetes, and Betagenon (Umea, Sweden)–the preclinical-stage oral AMPK activator BG8702, for treatment of type 2 diabetes.

The relationship between sirtuin-modulator developer Sirtris and GSK represents a prime example of the attempt of large pharmaceutical companies to become more “biotech-like” in order to improve their R&D performance. We discussed this strategy in our recent report, Approaches to Reducing Phase II Attrition. GSK acquired Sirtris for $720 million in June 2008. In December 2008, GSK announced that it had appointed Christoph Westphal, the CEO and co-founder of Sirtris, as the Senior Vice President of GSK’s Centre of Excellence in External Drug Discovery (CEEDD). The CEEDD works to develop external alliances with biotech companies, with the goal of acquiring promising new drug candidates for GSK’s pipeline. Michelle Dipp, who was the vice president of business development at Sirtris at the time of GSK’s appointment of Dr. Wesphal, is now Vice President and the head of the US CEEDD at GSK. Dr. Westphal, who is also a former venture capitalist, remains as CEO of Sirtris, and is based at Sirtris’ Cambridge location.

Thus anti-aging research, despite the fact that it is mainly in the basic research stage, is not only beginning to produce drug candidates, but has also been having an impact on the organizational strategy of one of the major pharmaceutical companies, GSK.