On June 1, 2011, Cambridge Healthtech Institute’s (CHI’s) Insight Pharma Reports announced the publication of our new book-length report, Multitargeted Therapies: Promiscuous Drugs and Combination Therapies.
In the past 20 years or so, pharmaceutical and biotechnology industry R&D has been increasingly aimed at developing drugs to treat complex diseases such as cancer, cardiovascular disease, type 2 diabetes, and Alzheimer’s disease. However, the one drug-one target-one disease paradigm that has become dominant in the post-genomic era has proven to be inadequate to address complex diseases, which have multiple “causes”, and each of which may be more than one disease. This has been a major cause of clinical failure and the low productivity of the pharmaceutical industry.
Moreover, researchers have found that most of the successful, FDA-approved small-molecule drugs that were developed prior to the year 2000 are promiscuous, i.e., they are single drugs that address multiple targets. In addition, the great majority of kinase inhibitors, one of the most successful drug classes of the early 21st century, are also promiscuous.
The study of small-molecule drug promiscuity has spawned the emerging field of network pharmacology, which can be applied both to study drug promiscuity and to rationally design small-molecule multitargeted drugs. (Researchers can discover or design multitargeted kinase inhibitors without the use of network pharmacology, however.)
Meanwhile, the development of targeted drugs such as kinase inhibitors and monoclonal antibodies has resulted in the need to develop multitargeted combination therapies. This has been especially true in cancer, where disease causation may involve multiple signaling pathways. In particular, the development of resistance to targeted antitumor drugs has spawned the need to develop second-generation treatments, many of which are multitargeted combination therapies.
Our report covers both discovery and design of small-molecule promiscuous/multitargeted drugs, and of multitargeted combination therapies.
The design of multitargeted combination therapies is one of the hottest areas of cancer R&D today, especially with respect to developing means to overcome resistance to targeted therapies. This area was the focus of many key presentations at the 2011 American Society of Clinical Oncology (ASCO) Annual Meeting, which was held in Chicago on June 3-7. For example, treatment with vemurafenib (PLX4032) of metastatic melanoma patients whose tumors carry the B-Raf(V600E) mutation has produced spectacular overall response rates and increased survival. However, in nearly all cases, the tumors relapse. The latest results with vemurafenib were discussed at ASCO 2011, as well as strategies to overcome resistance to therapy. Our new report also discusses strategies for overcoming vemurafenib resistance, all of which involve design of multitargeted combination therapies.
Another topic discussed at ASCO 2011 was antitumor strategies based on synthetic lethality. We discussed this strategy in an earlier article on this blog, especially with respect to poly(ADP) ribose polymerase (PARP) inhibitors such as KuDOS/AstraZenaca’s olaparib. At a session at the ASCO meeting entitled “PARP Inhibitors, DNA Repair, and Beyond: Theory Meets Reality in the Clinic”, speakers reviewed current progress in developing PARP inhibitors, of which six are now in clinical development.
This session also included a presentation by Michael B. Kastan, MD, PhD (St. Jude Children’s Research Hospital, Memphis TN) on other ways of using the synthetic lethally strategy, for example by targeting kinases involved in DNA repair pathways such as ATM (Ataxia-Telangiectasia Mutated) or Chk1 checkpoint kinase, or even utilizing features of the tumor microenviroment such as hypoxia. Such strategies might be used to design multitargeted combination therapies that specifically target cancer cells with defects in DNA repair and/or in hypoxic solid tumors, and/or to sensitize cancer cells to radiation.
Our new report includes a chapter on using the synthetic lethality strategy to design combination therapies of a cytotoxic drug with a chemosensitizing agent, and to develop therapies for p53-negative cancers. (The key tumor suppressor p53 is deleted, mutated, or inactivated in the majority of human cancers).
Although design of multitargeted combination therapies, as well as discovery and design of kinase inhibitors, are of key importance for current oncology R&D and are also being applied to other diseases, design of single small-molecule multitargeted drugs via network pharmacology is an early-stage, and perhaps a premature, technology. Nevertheless, given the current pharmaceutical company R&D business model that emphasizes outsourcing early-stage R&D, academic research groups and biotechnology companies that are active in this area may be able to forge partnerships with pharmaceutical companies.
For more information on Multitargeted Therapies: Promiscuous Drugs and Combination Therapies, or to order it, see the Insight Pharma Reports website.
Niacin (nicotinic acid)
In our blog post of May 19, 2011, we discussed the late-stage development of two cholesterol ester transfer protein (CETP) inhibitors, designed to raise serum high-density lipoprotein (HDL), or “good cholesterol”. These agents are Merck’s anacetrapib and Roche’s dalcetrapib. The clinical results with these agents have have reignited enthusiasm for CETP inhibitors in the medical and drug discovery and development community.
Now comes the news that The National Heart Lung and Blood Institute (NHLBI) of the National Institutes of Health (NIH) stopped a large clinical trial of Abbott’s Niaspan, an extended-release formulation of high-dose niacin, because the drug failed to prevent heart attacks and strokes. High-dose niacin is the only drug that is approved for raising HDL. Generic high-dose niacin is usually taken 2-3 times per day, and can cause adverse effects such as skin flushing and itching. Niaspan, as an extended-release formulation of the drug, is taken once a day, and was developed to reduce the extent of these adverse effects. Niaspan is an FDA-approved drug.
Merck has meanwhile been developing its high-dose non-flushing niacin product, Tredaptive/Cordaptive (extended-release niacin/laropiprant). This is a combination product consisting of extended-release high dose niacin plus laropiprant. Laropiprant is designed to block the ability of prostaglandin D2 to cause skin flushing; niacin-induced skin flushing works via the action of prostaglandin D2 in the skin. In 2008, the FDA rejected Merck’s New Drug Application for Tredaptive/Cordaptive, so the drug remains investigational in the US. However, in 2009 Merck launched Tredaptive in international markets including Mexico, the UK and Germany. The drug is approved in over 45 countries. Merck is also conducting a 25,000-person trial of Tredaptive for reducing the rate of cardiovascular events in patients who are at risk for cardiovascular disease (CVD). Merck intends to file for approval of the drug in the US in 2012, based on the results of this trial if it is positive.
According to an NIH press release, the NHLBI trial, known as AIM-HIGH, involved combination therapy with Niaspan and a statin (simvastatin). Participants selected for the trial had been taking a statin and had well-controlled LDL, but were still at risk for cardiovascular events since they had a history of CVD, as well as low serum HDL and high serum triglycerides. In the treatment arm of the study, participants received a combination of a stain and Niaspan, while those in the control arm received a statin plus placebo.
During the 32 months of the study, subjects in the treatment arm exhibited increased HDL and lower triglyceride levels, as compared to participants in the control arm. However, combination Niaspan/statin treatment did not reduce cardiovascular events or strokes as compared to statin treatment plus placebo. The NIH therefore stopped the trial 18 months earlier than planned.
In the AIM-HIGH study, subjects had a lower rate of cardiovascular events and strokes than the trial researchers expected. Of the 1,718 people in the treatment arm, 5.8 people per year had cardiovascular events, as opposed to 5.6 cardiovascular events per year among the 1,696 people in the control arm. There was a small increased rate of strokes in patients taking Niaspan, but researchers cautioned that this may have been due to chance. However, the increased rate of strokes, along with the failure to demonstrate efficacy, contributed to the NHLBI’s decision to end the trial early.
As noted by the AIM-HIGH researchers (and mentioned in the NIH press release), the lack of efficacy of high-dose niacin was unexpected, and in striking contrast to the results of previous trials and of observational studies. For example, a 2010 meta-analysis of clinical trials evaluating niacin, alone or in combination with other lipid-lowering drugs (published between 1966 and mid-2008) found significantly positive effects of niacin in preventing cardiovascular events and in reversing or slowing the progression of atherosclerosis. However, the bulk of the studies analyzed had been performed before statin therapy had become the standard of care. As pointed out in a recent article by clinical outcomes researcher Harlan Krumholz, MD (Yale University School of Medicine), it was important to compare Niasapn with a good treatment–in this case, the standard treatment with a statin–rather than comparing it to a poor treatment or to placebo alone.
The results of the AIM-HIGH study may not apply to other patient populations, including higher-risk groups such as patients with acute heart attack or acute coronary syndromes, or in patients who have poorly-controlled LDL despite statin treatment. As a press release from Abbott pointed out, the relevance of the results of AIM-HIGH to patient populations other than the one studied–patients with stable, non-acute, pre-existing cardiovascular disease and very well controlled LDL on simvastatin–is unknown. However, it is not known whether there are any patient populations that might benefit from treatment with Niaspan plus a statin as compared to a statin alone.
The results of AIM-HIGH also do not apply to other drugs that are designed to raise levels of serum HDL. Each drug must be tested in the clinic before drawing conclusions about its efficacy and safety. Various drugs may have different effects on human disease biology. Thus, for example, one should not use the results of the AIM-HIGH trial of Niaspan to conclude that the CETP inhibitors anacetrapib and dalcetrapib, discussed in our previous blog post, are not likely to be efficacious.
The 19 May issue of Nature contains a special Insight section on cardiovascular biology. An article in this section, by leading cardiovascular researcher Peter Libby (Brigham and Women’s Hospital, Boston MA, where he is the Chief of the Division of Cardiovascular Medicine) and his colleagues, is entitled “Progress and challenges in translating the biology of atherosclerosis”. That article refers to HDL as a “frustrating next frontier” (beyond lowering LDL with statins) in cardiovascular drug treatment. HDL biology is complex, with HDL promoting efflux of cholesterol from macrophages in atherosclerotic plaques and exerting anti-inflammatory and other beneficial effects, as also discussed in our previous blog post. HDL particles in blood serum are heterogeneous, with some HDL particles having a greater degree of positive effects on atherosclerotic plaque biology than others. As a result, treatments (e.g., drugs, diet) that raise HDL, as determined by standard clinical assays for serum HDL, may not necessarily result in clinical benefit, because of qualitative changes in populations of HDL particles.
In this connection, the 2010 study by Alan Tall and his colleagues, which we discussed in our May 19, 2011 blog article, provides hope for the efficacy of anacetrapib. These researchers showed that although niacin treatment in humans resulted in a moderate increase in the ability of HDL to promote net cholesterol efflux (measured in in vitro assays), anacetrapib treatment caused a more dramatic increase. This was due not only to a higher level of HDL in anacetrapib-treated subjects, but also to enhanced ability of anacetrapib-induced HDL particles to promote cholesterol efflux, especially at high HDL concentrations. Although this study suggests that anacetrapib treatment induces increases in HDL particles that promote beneficial effects on atherosclerotic plaque biology, we must wait for the results of the REVEAL trial (expected in 2014-2016) to determine the efficacy of this drug. Meanwhile, the clinical trial of Roche’s CETP inhibitor dalcetrapib, known as dal-OUTCOMES, is ongoing, with efficacy results expected in 2012-2013.
Steven Nissen, M.D. (chief of cardiovascular medicine at Cleveland Clinic), a veteran HDL researcher who has often been critical of the pharmaceutical industry, was recently interviewed on public television about the AIM-HIGH trial. He said that ever since the introduction of statins in 1987, we have not had a successful new drug class that provides significant clinical benefits by modulating serum lipids. Even when new types of lipid-modulating drugs have given apparently better biochemical results (e.g., LDL lowering or HDL raising), they have not provided clinical benefit in terms of preventing cardiovascular events.
Nevertheless, despite past disappointments with HDL-raising therapies, and despite the results of AIM-HIGH, Dr. Nissen persists in running clinical studies of novel HDL-raising drugs. He is now working on testing Resverlogix’ (Calgary Alberta, Canada) RVX-208, a small-molecule drug related to resveratrol that induces endogenous production of the protein component of HDL, apolipoprotein A1. And, as discussed in our last blog post, Dr. Nissen is enthusiastic about the prospects of Merck’s anacetrapib, although he states that the FDA will require hard clinical evidence of this drug’s efficacy before approving it.
Statins, despite their leading role in cardiovascular therapy, only reduce the risk of heart attack and stroke by 25% to 35%. Thus there is the need for new classes of drugs, and HDL is–frustrating though it be–the next frontier. Thus researchers persist in discovery and development of HDL-raising drugs, and there are promising new candidates on the horizon.
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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.
Atherosclerosis. From Nephron. http://bit.ly/jL6Zos
In the April 29, 2011 issue of Cell, there is a Leading Edge review entitled “Macrophages in the Pathogenesis of Atherosclerosis”, by Kathryn J. Moore (New York University Medical Center, New York, NY ) and Ira Tabas (Columbia University, New York, NY). This 15-page review (including 4 pages of references) covers a big subject–the central role of the macrophage in the pathogenesis of atherosclerosis, and of the resulting acute thrombotic vascular disease, including myocardial infarction, stroke, and sudden cardiac death. The review will be helpful to those who wish to update their knowledge of the mechanistic basis of atherothrombotic disease, or to those who want an introduction to the subject.
Included in the review is a discussion of the role of high-density lipoprotein (HDL), or “good cholesterol” in promoting regression of atherosclerotic plaques. HDL, as well as the protein component of HDL, apolipoprotein A1, are key players in the process of cholesterol efflux, or removal of cholesterol from macrophages in atherosclerotic plaques. HDL may also have other beneficial roles, including prevention of subendothelial apolipoprotein B-lipoprotein (apoB-LP) retention (which starts the atherosclerotic process in the first place), decreasing activation of endothelial cells, and reducing LDL oxidation. (ApoB is the protein component of low-density lipoprotein [LDL], or “bad cholesterol”.) In human populations, low HDL is generally recognized as a major cardiovascular risk factor, and high HDL is recognized as being protective.
In its discussion of therapeutic strategies based on our current picture of the mechanistic basis of atherosclerosis, the authors of the review state that the most effective way to treat the condition would be to decrease subendothelial apoB-LP retention by lowering apoB-LPs in the blood via lifestyle changes and drugs. In order to completely prevent atherosclerosis, serum apoB-LPs (i.e., mainly LDL and VLDL [very low-density lipoprotein]) would need to be lowered below the threshold level required for subendothelial apoB-LP retention in the arteries. However, in Western societies (and in other societies that have been rapidly adopting Western lifestyles), initiation of atherosclerotic lesions occurs in the early teens; thus this preventive approach is not currently feasible.
The leading drugs for lowering serum LDL are the statins, such as atorvastatin (Pfizer’s LIpitor, which is the largest-selling statin; Lipitor will go off-patent in November 2011), pravastatin (Bristol-Myers Squibb’s Pravachol, generics), simvastatin (Merck’s Zocor, generics), and rosuvastatin (AstraZeneca’s Crestor). Statins are generally accepted as being effective in decreasing mortality in patients with cardiovascular disease (CVD). These drugs are also widely prescribed for patients with a high risk of developing CVD; i.e., patients with high LDL, type 2 diabetes, and/or other risk factors. However, some researchers question the value of statins in primary prevention in patients without preexisting CVD but at high risk of developing the disease. For example, a 2010 meta-analysis published in the Archives of Internal Medicine did not find evidence that statin therapy was beneficial in primary prevention of all-cause mortality in patients at high risk of developing CVD. Moreover, although statins are highly effective in decreasing cardiovascular events (up to 60%) and cardiovascular deaths in patients with pre-existing CVD, a large percentage of patients with or at high risk of developing CVD, despite statin treatment, still experience cardiovascular events and cardiovascular death. Therefore, researchers and companies would like to develop other, complementary drugs that work via different mechanisms from the statins.
HDL raising has long been a key target for pharmaceutical and biotechnology companies in their quest to develop CVD drugs that would be complementary to the statins. In the early-to-mid 2000’s, companies had several candidate drugs, of different types, in development. In an article published by Pharmaceutical Executive in 2006, I was quoted as saying that raising HDL was a big field. However, most of the drugs being developed at that time fell by the wayside, mainly due to failure in the clinic.
A particular focus of pharmaceutical companies has been the development of cholesteryl ester transfer protein (CETP) inhibitors. CETP catalyzes the transfer of cholesteryl esters and triglycerides between LDL/VLDL and HDL, and vice versa. In vivo (in animals and in humans), CETP inhibitor drugs raise HDL and lower LDL.
The leading CETP inhibitor in the early to mid-2000s was Pfizer’s torcetrapib. Pfizer had placed high hopes on torcetrapib, as a potential blockbuster to replace anticipated lost revenues from Lipitor when it went off-patent in 2011. However, in late 2006 Pfizer pulled the drug from Phase 3 trials, after finding that combination therapy with torcetrapib and atorvastatin gave a 50 percent greater mortality rate that atrovastatin alone. This was not only a huge disappointment for Pfizer and its shareholders, but also cast a pall of gloom over the entire HDL-raising drug field, and especially over CETP inhibitors. Researchers speculated that inhibition of CETP might result in producing a form of HDL that is not cardioprotective, and might even be harmful. There were even calls for pushing the HDL field back to the basic research level, with the need to find just how (and what form of) HDL exerted its cardioprotective effects, in people with elevated HDL due to genetics, lifestyle, or treatment with high-dose niacin (the only drug approved to raise HDL).
However, later studies of torceptrapib found that the toxicity of the compound was not due to an untoward effect of CETP inhibition or HDL raising, but was due to off-target effects of the drug. In animals and in humans, torceptrapib raised serum levels of aldosterone, via release of aldosterone from the adrenals. Aldosterone was responsible for the increase of blood pressure seen in animals and in humans treated with torceptrapib, and aldosterone has proatherogenic effects that go beyond its effects on blood pressure. The hypertensive and aldosterone-raising effects of torceptrapib were independent of its CETP inhibitor activity, and other CETP inhibitors (discussed below) do not raise aldosterone levels or blood pressure.
A March 2011 News and Analysis article in Nature Reviews Drug Discovery reviewed the history of the CETP inhibitor field after the demise of torcetrapib. Although the torcetrapib debacle caused several other companies to exit the CETP inhibitor field, Roche and Merck persisted. Roche has been developing the CETP inhibitor dalcetrapib, and Merck’s CETP inhibitor is known as anacetrapib.
As mentioned in the Nature Reviews Drug Discovery mini-review, Dr. Alan Tall (Columbia University), working in collaboration with Merck researchers, showed in 2010 that niacin treatment in humans resulted in a 30% increase in HDL, while anacetrapib treatment resulted in a 100% increase in HDL. Niacin treatment in humans resulted in a moderate increase in the ability of HDL to promote net cholesterol efflux (measured in in vitro assays) while anacetrapib treatment caused a more dramatic increase. This was due not only to a higher level of HDL in anacetrapib-treated subjects, but also to enhanced ability of anacetrapib-induced HDL particles to promote cholesterol efflux, especially at high HDL concentrations. HDL from both niacin-treated and anacetrapib-treated subjects also exhibited anti-inflammatory activity. This study should help lay to rest the idea that pharmacological inhibition of CETP might result in abnormal pro-atherogenic HDL, as theorized by some researchers after the clinical failure of torceptrapib.
Currently, Roche’s dalcetrapib is in a 15,600-patient Phase 3 clinical trial known as dal-OUTCOMES; this trial was initiated in 2008, and efficacy results are expected in 2012-2013. As of the time of the Nature Reviews Drug Discovery article, Merck planned to initiate its 30,000-patient REVEAL trial of anacetrapib in April 2011. Efficacy results of REVEAL are anticipated in 2014-2016.
In December 2010, the results of Merck’s moderate-sized (1623 patients with or at high risk for CVD, who were already being treated with a statin) Phase 3 DEFINE trial of anacetrapib were published in the New England Journal of Medicine. The DEFINE trial was designed as a safety study. In this 76-week study, anacetrapib showed no significant differences from placebo in terms of safety, as measured by a pre-specified cardiovascular endpoint (defined as cardiovascular death, myocardial infarction, unstable angina or stroke). These cardiovascular events occurred in 16 anacetrapib-treated patients (2.0 percent) compared with 21 placebo-treated patients (2.6 percent). There were also no significant differences in blood pressure, serum electrolytes, or aldosterone levels between anacetrapib-treated and placebo-treated patients.
Anacetrapib treatment also decreased LDL by 40 percent (from 81 to 45 mg/dl vs. 82 to 77 mg/dl for placebo) and increased HDL by 138 percent (from 40 to 101 mg/dl vs. 40 to 46 mg/dl for placebo). Anacetrapib also had other favorable effects on lipid levels (e.g., 36.4% reduction in lipoprotein(a), and 6.8% reduction in triglycerides, beyond the changes seen with placebo treatment).
Although the DEFINE study was too small to provide definitive results regarding the safety of anacetrapib, it gave a 94% predictive probability that treatment with anacetrapib is not associated with the rate of cardiovascular events seen with torcetrapib. The trial also indicated that anacetrapib treatment does not result in the effects (especially raising of serum aldosterone levels) thought to be responsible for torcetrapib’s toxicity. Moreover, anacetrapib treatment resulted in a dramatic increase in HDL levels (beyond that seen with torcetrapib) in the DEFINE study, and the 2010 study by Dr. Tall and his colleagues indicates that anacetrapib-induced HDL is highly effective in promoting cholesterol efflux.
The results with anacetrapib have reignited enthusiasm for CETP inhibitors in the medical community. Even the often-critical Dr. Steven Nissen (Cleveland Clinic) expressed enthusiasm for anacetrapib. However, despite these promising results, the efficacy of CETP inhibitors, in terms of significantly reducing the rate of cardiovascular events, has not yet been demonstrated. Only large, adequately-powered Phase 3 clinical trials, such as dal-OUTCOMES for Roche’s dalcetrapib and REVEAL for Merck’s anacetrapib, can definitively establish both the efficacy and the safety of these drugs.
The development of CETP inhibitors represents a situation in which the leading drug in the class failed because of off-target effects. However, these off-target effects were not class effects, and targeting CETP in order to raise HDL now seems like a good idea after all. Pfizer ignored warning signs (especially the modest elevation in blood pressure induced by torcetrapib, which did not appear to be very significant) in pursuit of its commercial goals, while Roche and especially Merck pursued a more moderate and science-based approach to development of CETP inhibitors. Other companies stopped development of their CETP inhibitors, thus losing their opportunities in this field. Meanwhile, various companies and academic group have been developing other approaches to HDL raising, such as apolipoprotein A1 mimetics, which are in early stages of development.
Despite its early setbacks, HDL-raising drugs may turn out to be a big field after all.
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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.
I will lead a workshop entitled “Developing Improved Animal Models in Oncology and CNS Diseases to Increase Drug Discovery and Development Capabilities” at the World Drug Targets Summit in Cambridge MA in July 2011.
Workshops will be held on July 19, and the main conference on July 20-21. I am planning to attend the entire conference.
Our workshop will be a discussion of 2-3 case studies involving development of novel animal models in oncology and CNS diseases, aimed at more closely modeling human disease than current models. Drug discovery and development in these therapeutic areas has been severely hampered by animal models that are poorly predictive of efficacy. This is a major cause of clinical attrition in these areas.
We shall discuss the implications of these case studies for developing novel therapeutic strategies, target identification and validation, drug discovery, preclinical studies, and reducing clinical attrition. We shall also discuss hurdles to industry adoption of novel animal models developed in academic laboratories.
The main conference will focus on ways of building successful target strategies to minimize drug attrition in the clinic, and specifically how to identify and validate targets that can lead to commercially differentiated products. Speakers will include target discovery and validation leaders from such companies as Pfizer, Merck, NeurAxon, Gilead Sciences, Boehringer Ingelheim, Merrimack Pharmaceuticals, Bayer Schering Pharma AG, FORMA Therapeutics, Roche, Novartis, Tempero Pharmaceuticals, UCB Pharma, Infinity Pharmaceuticals, and from such academic institutions as Harvard Medical School.
The conference agenda and brochure, as well as online registration, are available on the conference website.
Galega officinalis (Goat’s Rue) From JoJan http://bit.ly/l5Ybco
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.
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