Biopharmconsortium Blog

PPAR complex coactivator interaction

This is part 3 of a three-part series on insulin sensitizers for treatment of type 2 diabetes.

In part 1 of the series (posted August 23, 2010), we focused on safety issues with the two marketed thiazolidinedione (TZD) peroxisome proliferator-activated receptor gamma (PPARγ) agonists–rosiglitazone and pioglitazone (Takeda’s Actos). Both of these insulin sensitizing, antidiabetic agents induce weight gain, and carry an increased risk of edema and heart failure. In addition, rosiglitazone carries an increased risk of myocardial infarction. Because of the latter risk, some critics would like to see it removed from the market. However, on July 15, 2010, by a close vote the FDA’s Endocrinologic and Metabolic Drugs Advisory Committee voted to leave rosiglitazone on the market, with some new restrictions.

In part 2 of the series (posted on August 29, 2010, we discussed a breakthrough discovery by Bruce Spiegelman (Dana-Farber Cancer Institute and Harvard Medical School, Boston MA) and his colleagues, published in the 22 July issue of Nature. It was the Spiegelman group that originally identified PPARγ as the master regulator of adipocyte biology and differentiation, which eventual led to the development of the TZD drugs.

In the new research, the Spiegelman group found evidence that the insulin sensitizing and antidiabetic effects of PPARγ agonists may not be due to the agonistic effects of these compounds on PPARγ, but to their ability to inhibit phosphorylation (at Ser 273) of PPARγ by the enzyme cyclin-dependent kinase 5 (CDK5). A weak PPARγ agonist, the benzoyl 2-methyl indole (non-TZD) MRL24, inhibits CDK5 phosphorylation of PPARγ as well as rosiglitazone, and also has very good antidiabetic activity. MRL24 had been discovered by Merck in 2005.

CDK5 phosphorylation of PPARγ does not change the ability of PPARγ to upregulate transcription of genes involved in adipocyte differentiation. However, it inhibits the ability of PPARγ to upregulate transcription of a set of genes, including the gene for the adipokine adiponectin, that induce insulin sensitivity and resistance to obesity. Although both rosiglitazone and MRL24 inhibit CDK5 phosphorylation of PPARγ, treatment with the strong agonist rosiglitazone results in upregulation of both the adipogenic and the pro-insulin resistance sets of genes, while treatment with MRL24 results only in upregulation of the pro-insulin resistance set.

The Spiegelman group’s research indicates that the difference between the action of rosiglitazone and MRL24 is due to the different conformational changes induced in the PPARγ protein molecule by binding of the two compounds. The researchers hypothesized that these two different conformational changes may change the way in which PPARγ interacts with coregulator proteins. Nuclear receptors work together with coregulators (i.e., coactivators and corepressors) to regulate specific sets of genes. Different ligands (natural or synthetic) that modulate nuclear receptor interactions with its coregulators can give different results in terms of which genes are unregulated or downregulated.

Researchers hypothesize that it is the upregulation of the adipogenic gene set that is responsible for the adverse effects of strong agonists of PPARγ–weight gain, edema, and the risk of heart failure. In contrast, the upregulation of adiponectin and the other members of the pro-insulin resistance gene set is thought to be responsible for the desirable, antidiabetic effect of PPARγ agonists. If researchers could develop synthetic PPARγ ligands that would induce PPARγ upregulation of the pro-insulin resistance gene set but not the adiopgenic gene set, these compounds might constitute improved, second-generation insulin sensitizers that would have the desirable effects of the TZDs with fewer adverse effects.

In the 22 July 2010 issue of Nature, there are two essays that discuss using the new breakthrough results of the Spiegelman group to discover and develop such improved insulin sensitizers. These are a News and Views article by metabolic disease researchers Riekelt Houtkooper and Johan Auwerx (Ecole Polytechnique Federale de Lausanne [EPFL] in Switzerland), and a Nature News article by Boston-based journalist Heidi Ledford, Ph.D. of Nature.

In the Houtkooper and Auwerx article, the authors advocate changing screening strategies for PPARγ-modulating drugs, to look for those that inhibit CDK5 phosporylation of PPARγ rather than those that are strong PPARγ agonists. Potential PPARγ-modulating drugs would also induce conformational changes in the PPARγ protein that support its recruitment of coregulators that have favorable effects on metabolism (e.g., induce insulin sensitivity and/or protect against obesity). Specifically, these authors suggest that researchers determine whether CDK5-mediated PPARγ phosphorylation enhances recruitment of coactivators such as TIF2/SRC-2 and RIP140 (which unfavorably affect metabolism), and inhibits recruitment of coactivators such as SRC-1 and PGC-1, which have a favorable effect on metabolism. If this is true, drugs that inhibit CDK5 phosporylation of PPARγ should shift the balance toward recruitment of cofactors that favorably affect metabolism (i.e., promote insulin sensitivity and resistance to obesity).

Houtkooper and Auwerx also advocate biochemical and genetic research on the multiple roles of CDK5 and its upstream regulators in metabolic pathways and disorders, as well as research to identify a PPARγ phosphatase that reverses the effects of CDK5 on PPARγ. These authors do not advocate screening for CDK5 inhibitors. (Several already exist, such as roscovitine, which also inhibits several CDKs that are involved in the cell cycle, and is being developed as the anticancer drug Seliciclib by Cyclacel).  CDK5 inhibitors would interfere with CDK5′s other functions, including in the central nervous system. Instead, drug discovery researchers should focus on discovering compounds that specifically change the conformation of PPARγ so that CDK5 phosphorylation of that molecule is inhibited.

In Heidi Ledford’s News article, she also advocates that pharmaceutical companies screen for inhibition of PPARγ phosphorylation, rather than strong activation of PPARγ. She also reviews the rocky history of the “glitazone” class of drugs (i.e., TZDs and related compounds), many of which have exhibited various safety problems that have kept them from the market or removed them from it. (You can see our take on this history in two 2006 articles on our website [here and here). Some commentators therefore believe that companies are unlikely to perform any new screens to identify PPARγ modulators, given the recent conflicts over Avandia and the checkered history of the glitazones. Other commentators counter that because of the large and growing type 2 diabetes market ($20.2 billion in 2008 and a projected $37.9 billion in 2018), companies will be tempted to try some new drug discovery efforts based on the new Spiegelman research. However, others counter that the new Spiegelman results are only preliminary. In any case, if drug developers would start the new screens now, it would take around 10-15 years for any new drugs to appear on the market.

But some companies may not need to start from scratch, by screening for novel compounds that inhibit CDK5 phosphorylation of PPARγ. These companies already have potential second generation insulin sensitizers in clinical development. In our published 2006 article "Safety Issues Hamper Dual PPAR Agonists Is Partial Antagonism the Solution?", we discussed safety issues both with PPARγ agonists, and with two PPARγ/PPARα dual agonists that were in late-stage development but were discontinued in 2006 because of adverse cardiovascular events. (PPARα is another PPAR nuclear receptor that is involved in control of lipid metabolism, especially of serum triglycerides and high density lipoprotein [HDL].) We proposed that development of what we called “partial agonists” of PPARs might be a solution to these safety problems. “Partial agonists” of PPARγ and of other PPARs are also called “selective PPAR modulators”. In the case of PPARγ, selective modulators are compounds that are less active in activating pathways that result in adipogenesis than strong agonists such as rosiglitzone and pioglitazone, while still upregulating pathways involved in insulin resistance.

In our 2006 article, we especially focused on a compound then called metaglidasen, which was being developed by Metabolex (Hayward, CA) and Johnson & Johnson (JNJ). This non-TZD drug is now called MBX-102/JNJ39659100. Metabolex and JNJ had tested MBX-102 for treatment of type 2 diabetes in eight Phase I and four Phase II clinical studies. However, according to the Metabolex website, the companies have repurposed the drug to treat gout, after discovering that MBX-102 is an effective uricosuric agent (i.e., it increases the excretion of uric acid in the urine).

MBX-102 is a single optical isomer of halofenate, a compound that had been studied in the 1970′s by Merck as a lipid-lowering agent. Halofenate was serendipitously found to be an insulin sensitizer; Metabolex produced a form of the drug that contained only the active optical isomer, and developed it as MBX-102.

in a 2009 paper, researchers showed that MBX-102 had insulin sensitizing and serum glucose-lowering effects in diabetic rat models (but with much lower potency than rosiglitazone), without the weight gain seen with TZDs. In vitro, MBX-102 did not drive human and mouse adipocyte differentiation, unlike TZDs. Moreover, MBX-102 had a greatly reduced ability to recruit PPARγ coactivators. MBX-102′s ability to recruit coactivators that favorably affect metabolism (SRC-1 and PGC-1) was significantly greater than its ability to recruit coactivators that unfavorably affect metabolism (such as TIF2/SRC2). MBX-102 also potently mediated transrepression of proinflammatory genes in vitro and in vivo. Earlier preclinical studies, discussed in our 2006 article, indicated that MBX-102 produced less cardiac hypertrophy in animals than rosglitazone, and also preserved the function of pancreatic beta cells.

Metabolex had also been developing a second selective PPARγ modulator, MBX-2044, a more potent follow-on compound to MBX-102, which reached Phase II of development. However, due to the company’s limited resources, it cannot conduct Phase III clinical trials without partners, especially the large Phase III trials required for diabetes. Therefore, Metabolex has shelved MBX-2044 for now, and has repurposed MBX-102 for gout, in partnership with JNJ. In a June 23 news release, Metabolex characterized itself as “a research-based company…not a commercial company” However, Metabolex is likely to seek to retain co-promotion rights in any commercialization agreement with pharmaceutical companies.

Meanwhile, another biotech company, InteKrin Therapeutics (Los Altos, CA) is developing another selective non-TZD PPARγ modulator, INT131. InteKrin had licensed the drug (formerly known as AMG131 and originally discovered by Tularik) from Amgen in January 2007. Unlike MBX-102, INT131 was purposefully designed to be a selective PPARγ modulator. INT131 has been in Phase II clinical trials for treatment of type 2 diabetes. in a 2009 paper, Amgen researchers showed that INT131 exhibited a different pattern of PPARγ coregulator recruitment from TZDs. in adipocytes, INT131 only minimally activated genes involved in adipogenesis, and exhibited a greater degrees of activation of genes involved in mediating insulin sensitivity. In a diabetic rat model, INT131 had a similar effect on glucose metabolism to rosiglitazone, with similar potency. But unlike rosigliatazone, INT131 did not induce weight gain, cardiac hypertrophy, or edema, in both rodents and nonhuman primates. In Phase IIa clinical trials (as published by InteKrin in 2010, INT131 showed potent glucose lowering (even at the low, 1 mg dose), without weight gain and fluid retention. High-dose (10 mg) INT131 provided apparently greater benefit to glucose metabolism than maximal-dose TZDs, without weight gain and fluid retention. InteKrin has tested INT131 in Phase IIb clinical trials, in which the drug gave comparable glycemic control to pioglitazone, but without edema and with minimal weight gain. The company has completed an End-of-Phase II meeting with the FDA, and is moving INT131 into Phase III clinical trials.

In order to “close the loop” in comparing studies of such compounds as MBX-102, MBX-2044, and INT131 with the new Spiegelman studies of the mechanism of insulin sensitization by PPARγ modulators, researchers would need to test these compounds for inhibition of phosphorylation of PPARγ by CDK5. Such studies have not yet been done.

While we were completing preparation of this three-part series of articles for our blog, a two-page “News and Analysis” article on selective PPAR (including PPARγ and the two other human PPARs) modulators was published in this month’s (September 2010) issue of Nature Reviews Drug Discovery (NRD). As with our blog posts, this article discussed (in a briefer format) the July 2005 FDA Advisory Committee recommendations on Avandia, the recently published Spiegelman article on the effects of insulin sensitizers on PPARγ phosphorylation by CDK5, and the prospects for selective PPARγ modulators as therapeutics for type 2 diabetes.

The NRD article not only discusses selective PPARγ modulators developed by InteKrin and Metabolex (with the latter company’s PPARγ modulators only being listed in a table), but also selective agonists of other PPARs, including single compounds that address multiple PPARs. Among these compounds is Metabolex’ selective PPARδ agonist MBX-8025 (formerly RWJ-800025, in-licensed from Janssen Pharmaceutica). PPARδ regulates genes involved in multiple aspects of the metabolic syndrome, including lipid storage and transport, fatty acid oxidation, and insulin sensitivity. MBX-8025 is being developed for treatment of mixed dyslipidemia and metabolic syndrome. In late 2008, MBX-8025 completed a Phase II proof-of-concept clinical trial, in which the compound was found to substantially reduce serum triglycerides, increase HDL, lower low-density lipoprotein (LDL), and improve insulin sensitivity in obese patients with metabolic syndrome. The effects of MBX-8025 are complementary to the LDL-lowering effects of statins such as atorvastatin (Pfizer’s Lipitor).

Another compound discussed in the NRD article is Roche’s rationally designed balanced dual PPARα/PPARγ activator aleglitazar, which is in Phase III development for treatment of patients with type 2 diabetes who have experienced a recent acute coronary syndrome.  This compound is designed to combine the improvements in insulin sensitivity associated with activation of PPARγ with the amelioration of dyslipidemia associated with activation of PPARα. Roche believes that aleglitazar will avoid the adverse cardiovascular effects seen with earlier dual PPARα/PPARγ activators, or “glitazars”, as discussed in our 2006 article. A 2009 clinical trial showed that aleglitazar had a positive effect on lipid and glucose metabolism, with no induction of edema or congestive heart failure, and less weight gain than for pioglitazone, over a 16-week treatment period. These short-term results are encouraging, but must be confirmed by the results of ongoing Phase III trials.

Meanwhile, other companies, including Merck, the discoverer of MRL24, have been continuing to develop selective PPARγ modulators for treatment of type 2 diabetes.

In conclusion, the development of selective PPARγ agonists for type 2 diabetes, as we postulated in our 2006 article, is a promising approach to overcoming the issues with current PPARγ agonists, especially rosiglitazone/Avandia. However, it will be important to overcome the cloud that hangs over PPAR modulators, as the result of safety issues with such drug classes as TZDs and glitazars. Encouraging data from ongoing trials of second-generation, selective modulators will hopefully overcome these doubts, enabling companies to develop them and regulators to review them without prejudgment.

It will also be important to “close the loop” with the recent Spiegelman studies, by looking at the effects of these agents on PPARγ phosphorylation by CDK5. Moreover, screening of compounds for their effects on PPARγ phosphorylation may lead to the development of even better agents, especially if agents now in the clinic fail or give less than optimal results in late-stage trials. However, we hope that agents now in development (especially InteKrin’s INT131 and/or the two Metabolex agents if that company can find the resources from partners to complete development for diabetes) will prove to be safe and effective in clinical studies. In the longer term, it will be important to confirm that any new insulin sensitizers work to preserve beta-cell function (which would prevent progression of type 2 diabetes), and to determine if they lower the incidence of cardiovascular complications of diabetes. These are major unmet needs in treatment of type 2 diabetes.

PPARgamma

In part 1 of this three-part series (posted August 23, 2010), we discussed the recent action of the  Endocrinologic and Metabolic Drugs Advisory Committee regarding rosiglitazone (GlaxoSmithKline’s Avandia). The advisory committee, by a close vote, recommended to the FDA that it leave the drug on the market, with new restrictions (e.g., closer supervision and new label warnings).

Avandia and pioglitazone (Takeda’s Actos) are the only marketed members of the thiazolidinedione (TZD) class of peroxisome proliferator-activated receptor gamma (PPARγ) agonists. PPARγ is a nuclear receptor that controls glucose metabolism and adipocyte differentiation. In treatment of type 2 diabetes, TZD modulation of PPARγ results in decreased insulin resistance, thus enabling tissues such as muscle and fat to utilize insulin more efficiently for the uptake of glucose. Agents that work by decreasing insulin resistance are known as “insulin sensitizers”.

As discussed in our August 23 article, clinical evidence indicates that both Avandia and Actos induce weight gain in type 2 diabetics (who are usually obese to begin with), and carry an increased risk of edema and heart failure. Avandia also carries a significantly increased risk of myocardial infarction (MI). Critics of Avandia who want the drug removed from the market cite the increased risk of MI, and the availability of a safer TZD, Actos.

Despite the major safety issues with TZDs, there is both animal model and human evidence that these agents may work to preserve and/or enhance beta-cell function, and thus to help prevent progression of type 2 diabetes. Moreover, insulin resistance is a major factor in the pathogenesis of the disease. We therefore asked whether it might be possible to discover and develop better, safer insulin sensitizers that would have the desirable properties of the TZDs with fewer adverse effects. In this article, which is part 2 of the series, we discuss a recent breakthrough in the biochemistry of PPARγ that may enable companies to develop better insulin sensitizers. In part 3 of this series, we shall look at how companies might develop such compounds.

It was Bruce Spiegelman (Dana-Farber Cancer Institute and Harvard Medical School, Boston MA) and his colleagues who identified PPARγ as the master regulator of adipocyte biology and differentiation back in 1994. This eventually led to the discovery and development of TZDs such as Avandia and Actos, which are synthetic compounds that are strong agonists of PPARγ. These compounds act as potent insulin sensitizers, and are thus used in the treatment of type 2 diabetes. However, the mechanism of their insulin sensitizing activity is not clear. Administration of  TZDs result in decreased expression in adipose tissue of insulin-resistance inducing hormones such as tumor necrosis factor-alpha (TNF-α), interleukin-1 (IL-1) and resistin. They also induce increased expression of adiponectin, an adipose-derived hormone or adipokine (a member of a class of cytokines that are secreted by adipocytes) that acts as a natural insulin sensitizer.

In a July 2010 research article published in Nature, the Spiegelman group noted that the action of insulin-sensitizing PPARγ agonists does not make biological sense. Obese people and people with insulin resistance (including type 2 diabetics) have no deficiency in PPARγ or PPARγ activity. Therefore, why do synthetic activators of PPARγ have such dramatic insulin sensitizing and antidiabetic activity? Since PPARγ controls adipocyte differentiation, it makes good biological sense that PPARγ agonists can induce weight gain, however. But why do these inducers of weight gain also act as insulin sensitizers, while excessive weight is generally associated with increased insulin resistance? Finally, although the strong PPARγ agonists rosiglitazone and pioglitazone are insulin sensitizing and antidiabetic, some selective PPARγ modulators with poor agonist activity, such as the (non-TZD) benzoyl 2-methyl indole MRL24 (discovered by Merck Research Laboratories) have very good antidiabetic activity.

The Spiegelman group found that the enzyme cyclin-dependent kinase 5 (CDK5) phosphorylates PPARγ at serine 273 (Ser 273). There are no other CDK5 phosphorylation sites on PPARγ. Unlike other members of the CDK family, CDK5 is not a cell-cycle kinase that is regulated by a cyclin, but instead is regulated by p35/25, which are targets of numerous cytokines and other proinflammatory signals. Specifically, cytoplasmic p35, possibly in response to proinflammatory signals, is cleaved to form p25. p25 can then enter the nucleus, where it associates with and activates CDK5. The activated CDK5 can then phosphorylate PPARγ at Ser 273.

Treatment of adipocytes with proinflammatory cytokines or free fatty acids results in enhanced formation of p25, and enhanced phosphorylation of PPARγ at Ser 273. The same results occur in vivo, in mice that are fed a high-fat diet over a prolonged period of time, and thus become obese. It is well known that both free fatty acids and proinflammatory cytokines are elevated in obesity.

The researchers also found that phosphorylation of PPARγ at Ser 273 does not change the ability of PPARγ to upregulate transcription of genes involved in adipocyte differentiation. However, it inhibits the ability of PPARγ to upregulate transcription of certain other genes, including adiponectin. The insulin-sensitizing synthetic compounds rosiglitazone (a strong PPARγ agonist) and MRL24 (a weak PPARγ agonist) both inhibited phosphorylation of PPARγ at Ser 273 in adipocytes. However, treatment of adipocytes with these two compounds gave different results in terms of their effects on PPARγ-regulated genes. Treatment of fat cells with rosiglitazone resulted in upregulation of adiponectin and other genes that are downregulated by Ser 273 phosphorylation. But rosiglitazone treatment also resulted in upregualtion of PPARγ-regulated genes involved in adipogenesis. In contrast, treatment of adipocytes with MRL24 did not upregulate the genes involved in adipogenesis. But it did upregulate the gene set (including adiponectin) that was downregulated by phosphorylation of PPARγ at Ser 273.

Mass spectrometry studies indicated that both rosiglitazone and MRL24 changed the conformation of PPARγ in such a way as to make this protein less favorable for phosphorylation by CDK5. However, rosiglitazone and MRL24 binding results in different conformational changes. The researchers hypothesized that these conformational changes may change the way in which PPARγ interacts with coregulator proteins. Nuclear receptors work together with coregulators to regulate specific sets of genes. Different ligands (natural or synthetic) that modulate nuclear receptor interactions with its coregulators can give different results in terms of which genes are unregulated or downregulated.

The researchers then studied the action of roslglitazone and of MRL24 on phosphorylation of PPARγ at Ser 273 and on modulation of PPARγ-regulated genes in vivo. In mouse models, these two compounds inhibited PPARγ Ser 273 phosphorylation in adipose tissue, and caused similar changes in PPARγ-regulated gene expression as they do in adipose cells in vitro. Moreover, human diabetes patients treated with rosiglitazone usually exhibited decreased phosphorylation of PPARγ at Ser 273 in biopsied subcutaneous fat. However, in some cases, no such decreased phosphorylation was seen. Improvements in insulin sensitivity in these patients correlated with decreased phosphorylation of PPARγ at Ser 273.

On the basis of these results, the researchers concluded that the insulin sensitizing and antidiabetic effects of PPARγ agonists may not be due to the agonistic effects of these compounds on PPARγ, but on their ability to inhibit CDK5 phosphorylation of PPARγ.

In a News and Views article in the same issue of Nature, Riekelt Houtkooper and Johan Auwerx (Ecole Polytechnique Federale de Lausanne [EPFL] in Switzerland) postulate that the new findings of the Spiegelman group may be used to develop PPARγ modulating drugs that lack full agonist activity, but still inhibit CDK5 phosphorylation of PPARγ. Such compounds (of which MRL24 may be a starting point) would not upregulate adipogenic genes, but would upregulate insulin sensitizing genes such as adiponectin. These authors postulate that these novel compounds may thus have strong insulin sensitizing and antidiabetic effects, but would lack such adverse effects as weight gain, edema, and the risk of heart failure.

We shall discuss strategies for developing improved insulin sensitizers in greater depth in Part 3 of this series.

Avandia (rosiglitazone)

On July 15, 2010, the FDA’s Endocrinologic and Metabolic Drugs Advisory Committee voted to leave the diabetes drug rosiglitazone  (GlaxoSmithKline’s  Avandia) on the market, with some new restrictions (e.g., closer supervision and new label warnings). This is the same committee that voted against FDA approval of Vivus’ anti-obesity drug Qnexa on the same day, as discussed in our August 4 blog post. (Some commentators believe that the Qnexa rejection is connected to the decision on Avandia. We shall reserve judgment on that question.)

The FDA usually follows the advice of its advisory panels, but does not always do so.

Of the 33-member panel, 10 voted to keep Avandia on the market under close supervision, seven voted to keep it on the market but with stronger label warnings, and three voted to keep the drug on the market with no new restrictions. Twelve voted to remove Avandia from the market. One member abstained.

One factor in the recommendations of several panelists to add restrictions or to eliminate Avanida from the market altogether is that a competing drug of the same thiazolidinedione (TZD) class, pioglitazone (Takeda’s Actos), appears to have similar efficacy to Avandia, but fewer adverse effects.

The panel members’ decisions were based on their analysis of large amounts (18 presentations worth) of contradictory data.

TZDs are agonists of peroxisome proliferator-activated receptor gamma (PPARγ), a nuclear receptor that controls glucose metabolism and adipocyte differentiation. In treatment of type 2 diabetes, TZD modulation of PPARγ results in decreased insulin resistance. (Insulin resistance is the inability of tissues such as muscle and fat to utilize insulin efficiently for the uptake of glucose.) Thus treatment with these drugs can result in decreased levels of serum glucose, and amelioration of diabetes. Agents that work by decreasing insulin resistance are known as “insulin sensitizers”.

We have been following safety issues with agonists of PPAR receptors for quite some time. For example, there are two articles (here and here) published in 2006 (and available on our website) that include discussion of PPAR agonist safety.

As discussed in these two articles, Steven Nissen, M.D. (now Chairman of the Robert and Suzanne Tomsich Department of Cardiovascular Medicine, Cleveland Clinic, Cleveland, OH) has been a leading critic of Avandia’s cardiac safety. This began with his 2007 meta-analysis of clinical studies of the drug, published in the New England Journal of Medicine. This meta-analysis indicated that treatment with Avandia was associated with a significant increase in the risk of myocardial infarction (MI), and an increase in the risk of cardiovascular death that had borderline significance. A 2010 meta-analysis by Dr. Nissen and his colleagues (published online ahead of print in the Archives of Internal Medicine on June 28) indicated that treatment with Avandia was associated with a significant increase in the risk of MI, but no increased risk of cardiovascular death or all-cause mortality. The results also suggest an unfavorable benefit to risk ratio for Avandia.

A retrospective study by FDA researchers of Medicare recipients published in the Journal of the American Medical Association in July 2010 indicated that Avandia treatment is associated with an increased risk of the composite of AMI, stroke, heart failure, and all-cause mortality as compared with Actos, in patients 65 years or older.

The consensus of multiple studies is that both Avandia and Actos induce weight gain, and carry an increased risk of edema and heart failure as compared with placebo, but that Avandia has a higher risk of MI than Actos. (Few studies directly compared the two drugs, however. The 2010 FDA study is an exception). However, some studies presented to the FDA Advisory Committee indicated that the risk of cardiovascular events were comparable between Avandia, Actos, and diabetes medications of other classes.

The panel’s decision was a compromise, based not only on the contradictory nature of the evidence, but also on the contention that Avandia may be a better choice than Actos (or other diabetes drugs) for certain groups of patients, but not others. However, the continuing bad publicity about Avandia’s risks have significantly reduced its sales.

The continuing unfavorable safety findings for Avandia, as well as the findings that both Avandia and Actos induce weight gain in type 2 diabetics (who are usually obese to begin with), and carry an increased risk of edema and heart failure, have given new ammunition to critics who believe that physicians treating diabetes should stick to combinations of the older, cheaper drugs–insulin, metformin, and sulfonylureas, and avoid using not only TZDs, but also newer agents. This point of view also suggests that there is no need to discover and develop new antidiabetic agents.

However, the arguments in our 2008 Genetic Engineering News (GEN) article on diabetes (available on our website) still apply. There are still major unmet needs in type 2 diabetes, especially the need to prevent weight gain in diabetes treatment (and even to promote weight loss), and the need to prevent long-term loss of pancreatic beta-cell function. It is the loss of beta-cell function that results in the progression of type 2 diabetes, such that patients who initially succeed in reaching glycemic goals even with multidrug treatment with older antidiabetics eventually experience poor glycemic control on the same regimens.

As we discussed in earlier blog posts, some of the newer antidiabetics, namely the incretin mimetic exenatide (Amylin/Lilly’s Byetta) and liraglutide (Novo Nordisk’s Victoza), may give increased glycemic control while promoting weight loss. There is also evidence from animal studies that these drugs might help to preserve beta-cell function.

Ironically, despite the major safety issues with TZDs, there is both animal model and human evidence that these agents may work to preserve and/or enhance beta-cell function. Moreover, insulin resistance is a major factor in the pathogenesis of type 2 diabetes. Therefore, it would be very advantageous, and perhaps essential, for physicians and patients to have access to safer insulin sensitizers, especially if they work to prevent diabetes progression by preserving and enhancing beta-cell function.

Might it be possible to discover and develop better, safer insulin sensitizers than the TZDs? We shall discuss this question in Part 2 and Part 3 of this series.

Topiramate

Phentermine

2010 was supposed to be the year in which one or more new obesity drugs would be approved by the FDA and reach the market. Three new drugs developed by small California companies–Vivus Pharmaceuticals’ Qnexa, Orexigen Therapeutics’ Contrave, and Arena Therapeutics’ lorcaserin, were up for review by the FDA. This follows a long hiatus, since the FDA has approved no anti-obesity drug since 1999.

On July 15, 2010, the FDA’s Endocrinologic and Metabolic Drugs Advisory Committee voted against FDA approval of Vivus’ Qnexa (phentermine/topiramate), with six votes in favor and 10 against. The FDA usually follows the advice of its advisory panels, but does not always do so.

The advisory committee agreed that the drug caused significant weight loss, with patients who took the highest doses of the drug having lost over 10 percent of their weight in a year. But many panelists questioned the lack of long-term data on efficacy, since the real issue with weight loss regimens (whether diet and exercise alone or with drug treatments) is the inability of patients to keep weight off once it has been lost.

But above all, panelists had concerns about Qnexa’s safety. Major concerns included the potential for fetal exposure during pregnancy and birth defects, depression, cognitive issues, and increases in heart rate. Most panelists who voted no were not strongly against approval, but they had lingering concerns, especially since the drug if approved would be given to large numbers of essentially healthy people over a long period of time–perhaps a lifetime.

Vivus said that it would work with the FDA to address the panelists concerns. For example, the company expects to have longer-term safety data on the drug in the next several months.

Qnexa is a low-dose, controlled release formulation of two FDA-approved drugs: phentermine and topiramate. Qnexa is designed to both suppress appetite (phentermine) and promote satiety (topiramate). Phentermine, an amphetamine, is prescribed as a weight-loss aid that is used short-term. It was the “phen” half of the notorious “Fen-Phen” combination. The “fen” part, fenfluramine (Pondimin) or dexfenfluramine (Redux), were serotonin modulators that caused cardiovascular side-effects. Topiramate is an anticonvulsant. As separate agents, phentermine and topiramate have minimal effects on weight loss. However, according to Vivus’ studies, the two drugs appear to have a synergistic effect, even at low doses, that results in significant weight loss. Vivus’ studies also indicate that the two drugs mitigate each other’s side effects; the low does and controlled release is also designed to reduce side effects.

Side effects of phentermine may include increase in blood pressure and heart palpitations, as well as gastrointestinal side effects. Side effects of topirmate may include cognitive issues, lack of coordination, aggressiveness, changes in ability to taste food and loss of appetite, cardiovascular side effects, and others. The risk of birth defects with ether of these drugs is unknown. However, there is preliminary evidence that topiramate might cause birth defects.

Lorcaserin is up for FDA Advisory Panel review in September 2010 and Contrave is tentatively scheduled for review in December 2010. Lorcaserin is a selective serotonin receptor agonist, which is specific for the 5-HT2C receptor. This contrasts with the nonselective serotonin reuptake inhibitor and serotonin-releasing agents, fenfluramine and dexfenfluramine. Lorcaserin is thus designed to be a more selective agent that works by a similar mechanism to dexfenfluramine or fenfluramine. Since the anorectic effects of fenfluramine/dexfenfluramine is due to their activity on 5-HT2C, but the adverse effects of these agents appears to be due to their activity on 5-HT2B, lorcaserin may be a safer agent that fenfluramine/dexfenfluramine. But like fenfluramine and dexfenfluramine, the efficacy of lorcaserin appears to be minimal.

Contrave, like Qnexa, is a combination of long-acting formulations of two FDA-approved drugs–bupropion and naltrexone. Orexigen designed Contrave to have a dual effect on pathways within the hypothalamus of the brain that control energy balance–increasing anorexia and inhibiting the reward effects of food. The company also believes that Contrave may block the body’s compensation for weight loss–i.e., decreased energy use and increased feeding. (For additional details, see our 2008 book-length obesity report, published by Cambridge Healthtech Institute.)

The Endocrinologic and Metabolic Drugs Advisory Committee’s recommendation against Qnexa casts a cloud on the upcoming reviews by the same committee of the other two drugs. However, the jury is still out on lorcaserin and Contrave. And approval of Qnexa may (or may not) be reconsidered as Vivus presents additional data.

However, antiobesity drugs that work via the CNS to control appetite by modulating the activity of common neurotransmitter pathways have a generally poor record. First was the fenfluramine/dexfenfluramine/Fen-Phen debacle, in which fenfluramine and dexfenfluramine (Interneuron/Wyeth) were found in the postmarking period to cause heart valve damage, leading to market withdrawal in 1997 and a host of lawsuits. Sanofi Aventis’ rimonabant never reached the U.S. market–in 2007 the FDA rejected the drug due to neurologic and psychological adverse effects. Rimonabant was also suspended from use in Europe in 2008. A related Merck drug, taranabant, was never submitted to the FDA, since it had similar adverse effects to rimonabant. And despite a growing understanding of pathways (involving neurotransmitters and neuropeptides) in the hypothalamus that control appetite, and despite a large number of promising leads that emerged from that research, not one drug derived from this research has yet emerged from early clinical trials.

In many cases, drugs that were designed to address these pathways had unacceptable adverse effects, since the neurotransmitter or neuropeptide receptors that they addressed are also involved in other CNS and/or peripheral tissue pathways that do not control body weight or energy balance. This is also the problem with appetite control drugs that have reached the IND or post-marketing stage. Such drugs as fenfluramine/dexfenfluramine and sibutramine target receptors for such common neurotransmitters as serotonin and noradrenaline, which are involved in many pathways within the CNS and peripheral tissues. Rimonabant is an antagonist of the CB1 cannabinoid receptor, which is widely expressed in the brain and in other tissues and modulates multiple pathways.

Sibutramine (Abbott’s Meridia/ Reductil) is an approved and marketed appetite-control drug that works via the CNS. It is a serotonin–norepinephrine reuptake inhibitor. Sibutramine causes increases in blood pressure and heart rate. Therefore, the drug is contraindicated in patients with uncontrolled blood pressure and certain other conditions.

There is also concern that sibutramine may cause more serious cardiovascular conditions. Early in 2010, the FDA issued a warning that the drug posed an increased risk of heart attack and stroke in patients with a history of cardiovascular disease. This resulted in an additional contraindication on the drug’s label. And a few patients taking sibutramine may experience psychological adverse effects. Because of concerns about sibutramine’s safety, the drug has recently been suspended from use in the U.K. and the E.U. Sibutramine is also under continued review by the FDA.

Sibutramine and the other approved antiobesity drug, orlistat (Roche’s Xenical–also marketed as a low-dose over the counter formulation, GlaxoSmithKline’s alli) have only marginal efficacy. And orlistat, which works not in the CNS, but in the gut to block fat absorption, has unpleasant gastrointestinal adverse effects. Therefore there is a need for safer, more efficacious antiobesity drugs.

Nevertheless, the history of failure of antiobesity drugs, especially appetite-control drugs that work via the CNS and modulate neurotransmitter receptors that are involved in multiple pathways, continues, with the decision of the FDA Advisory Committee on Qnexa being the latest episode.

Perhaps companies will have more success developing antiobesity drugs that primarily address metabolic pathways involved in both obesity and diabetes, rather than being directed at appetite-control pathways in the CNS that involve common neurotransmitters. We discussed this strategy in two earlier articles on this blog, dated October 25, 2009 and January 28, 2010. These articles focused on the incretin mimetics, especially liraglutide (Novo Nordisk’s Victoza). Incretin mimetics [which also include exenatide (Amylin/Lilly’s Byetta)] trigger an increase in insulin secretion by the pancreas, and also reduces gastric emptying. The latter effect slows nutrient release into the bloodstream and appears to increase satiety and thus reduce food intake.

Part of the mechanisms of action of the natural incretin glucagon-like peptide-1 (GLP-1) and of incretin mimetics involves activity in the CNS. However, GLP-1 receptors in the brain appear to be more specific in their activity than receptors for common neurotransmitters like serotonin and norepinephrine. The main adverse effect that has been seen with the incretin mimetics exenatide and liraglutide is a transient nausea. Thus incretin mimetics do not appear to cause the psychological and neurological side effects seen with such drugs as sibutramine, rimonabant, and phentermine, and presumably Qnexa.

Nevertheless, acute pancreatitis has been seen in some patients taking exenatide (which resulted in a warning on the label that patients with a history of pancreatitis should not take the drug, and that the drug should be discontinued if symptoms suggesting pancreatitis should develop). Rodents receiving either exenatide or liraglutide have developed thyroid C-cell focal hyperplasia and C-cell tumors. There is no evidence that humans develop thyroid tumors as the result of taking ether drug, however. Nevertheless, the label for liraglutide carries a “back box” warning highlighting the thyroid tumor results in rodents, and including a contraindicating the use of the drug in patents with a history of medullary thyroid carcinoma.

Companies usually develop dual diabetes/obesity drugs first for diabetes, since the regulatory pathway for that disease is easier than for obesity. This has been the case for both exenatide and liraglutide. However, Novo Nordisk announced on June 22, 2010, that following the FDA approval of liraglutide for treatment of type 2 diabetes, it was restarting Phase III clinical trials of the drug in obesity.

As we noted in our October 25, 2009 article, there are at least several companies with early stage dual diabetes/obesity drugs, which they are developing for diabetes. Early-stage obesity drug development has been mainly on hold, awaiting the regulatory approval of Qnexa, Contrave, and/or lorcaserin. Now the results of the regulatory reviews of these three drugs are starting to come in. If none of the three is approved, than early-stage obesity drug development may remain on hold indefinitely.

While researching the material for the June 11 2010 article on obesity , I ran across the recent work of Katherine M. Flegal and her colleagues at the National Center for Health Statistics of the Centers for Disease Control and Prevention (CDC). Dr. Flegal has long been a leading obesity epidemiologist, and led the research that first identified the increased prevalence of obesity in the United States beginning in the 1980s.

Dr. Flegal’s recent work has been based in large part on the most recent data from the National Health and Nutrition Examination Survey (NHANES), and on long-term data from NHANES between 1960 and the early 2000s. In 2005, she and her colleagues published a report in the Journal of the American Medical Association (JAMA) on mortality as a function of Federally-defined weight class–underweight, normal weight, overweight, and obese. These categories were determined on the basis of the body mass index (BMI), with underweight at <18.5, normal weight at 18.5-24.9,  overweight at 25-29.9, and obesity at >30.  Subsequently, in 2007, Dr. Flegal and her colleagues published a report in the JAMA analyzing excess deaths in the underweight, overweight, and obese classes by cause (e.g., cardiovascular disease, diabetes, cancer, etc.).

Dr. Flegal has published other reports relevant to her analyses in these two papers between 2005 and 2010, and has lectured widely on her findings.  Her research was also discussed in a New York Times article in 2005, a 2007 article in the British newspaper The Independent, and in a January 2010 interview in the Association for Psychological Science Observer.

The surprising conclusion of Dr. Flegal’s research is that people in the overweight class have a lower risk of death than those in either the normal weight or the obese class. According to the 2005 study, obesity is associated with about 112,000 excess deaths per year (with most deaths [about 82,000 deaths] concentrated in the extreme obesity class, BMI >35), and underweight is associated with about 26,000 excess deaths per year, but overweight is associated with preventing 86,000 excess deaths per year. (“Excess deaths” refers to the number of deaths per year as compared to the normal weight class).

According to the 2007 study, underweight was associated with significantly increased mortality due to noncancer, non-cardiovascular disease (CVD) causes, but obesity was associated with associated with significantly increased mortality due to obesity-associated cancers, CVD, diabetes, and kidney disease. Thus excess deaths in the underweight and the obese classes vary by cause. Overweight was not associated with either increased or decreased deaths due to CVD, and overweight was not associated with excess deaths due to obesity-associated cancers. However, overweight was associated with a significantly reduced number of excess deaths due to noncancer, non-CVD causes.

Dr. Flegal and her colleagues found that the association of mortality with BMI appears to be much weaker in the most recent surveys as compared to earlier ones. In the most recent data, the association of overweight and mild obesity with risk of death appears to be weak and not statistically significant. This suggests that the association between mortality and weight has been decreasing with time, perhaps due to improvements in medical treatments and in public health. The results of other researchers confirm this hypothesis, and indicate that better management of the risk of death from CVD (e..g, the use of preventive measures such as blood pressure medications and statins, as well as better management of heart attacks via such procedures as angioplasty and stent placement) is responsible for the decreasing risk of obesity-associated death.

The other interesting issue is the risk of weight-related mortality and age. The association  of mortality with weight decreases in older people, especially for those over 70, with the overweight group again having a lower risk of death than the underweight and the obese. Since most people die when they are over 70, this may account, at least in part, for the reduced risk of death in the overweight group in Dr. Flegal’s studies.

Dr. Flegal’s analysis of population weight data over time leads her to dispute the term “obesity epidemic” that most researchers and commentators in the field (including me) have been using. The prevalence of obesity had been stable between 1960 and 1980, but then increased markedly between 1980 and 2000. This increase is what has been referred to as an “epidemic”, since it was expected to continue. However, the increase in prevalence of obesity appears to have diminished since 2000. Moreover, limited data going back to the Civil War suggests that weight in the American population has been increasing since that time, and increasing at a slower rate in recent decades than in the latter half of the 19th century. Dr. Flegal therefore sees obesity as endemic, rather than epidemic.

As might be expected, Dr. Flegal’s conclusions have generated a lot of controversy. Many researchers do not believe the findings, in some cases on the basis of their own earlier studies. Others are simply reluctant to go against the received wisdom that excess weight is a major health hazard, and perhaps the biggest public health problem facing the United States and many other countries. Many researchers note that Dr. Flegal’s studies measure only mortality, not the incidence of such diseases as diabetes and CVD. This is a valid criticism, calling for more epidemiological research. However, many epidemiologists note that Dr. Flegal’s methodology and data are solid, and that she and her colleagues are well respected in their field.

Dr. Flegal’s studies indicate that the designation of obesity and overweight as America’s biggest health problem, and the main cause cause of the rise in health care costs–as discussed in our June 11 2010 blog post–may be overblown. The emphasis for most overweight and moderately obese people may need to be exercise and diets that promote health, whether patients lose weight or not. Given the difficult that most overweight or obese people have in losing weight and keeping it off over the long term, this may be a more realistic approach.

What is a “diet that promotes health”? What is considered “healthy” changes over time, as the result of new research as well as other factors. It is not the purpose of this blog to prescribe or discuss different diets. There are many, many blogs–not to mention books, television programs and other media–that do that. A good place to start, however, might be the work of Walter Willett. (Dr. Willett has disputed the findings of Dr. Flegal’s research, which indicates the level of complexity and disagreement in the obesity field.) Diet and exercise issues should of course be discussed with one’s doctor, but informed patients will usually get better results than uninformed ones.

For those of us in the pharmaceutical and biotechnology industry, the controversies about weight and diet affect the enterprise of drug discovery and development, especially in the metabolic disease and CVD fields. Drugs for such conditions as diabetes and dyslipidemia, as well as antiobesity drugs, are indicated as “adjuncts to diet” or “adjuncts to diet and exercise”. Clinical studies do indicate that these drugs work best when combined with diet and exercise. However, which diet and exercise regimens might be best for various groups of patients, and which diet and exercise regimens might best potentate the efficacy of a drug for various groups of patients, is called into question by the results of Dr. Flegal’s research and the debate over them in the obesity research community. And if weight-associated mortally has been decreasing with time due to improvements in medical treatments, we need to keep up the good work, and develop improved treatments and preventives for such diseases as diabetes and its complications, and for CVD.  (It is the complications of diabetes that are responsible for the greatest level of mortality and disability due to diabetes, as well as the bulk of diabetes-related health care costs.) These new drugs should address unmet medical needs in the metabolic disease and CVD fields.

Health care policy makers should also stop blaming the overweight and the obese and their “lack of personal responsibility” for the woes of the health care system. As we discussed in our June 11 article, this is true whether Dr. Flegal’s conclusions are valid or not.

What causes obesity? To many people, the answer is obvious. Obesity is caused by eating too much food and/or not getting enough exercise. Obese and overweight people lack “personal responsibility” or have become addicted to food the same way that one becomes addicted to smoking. The alarming worldwide rise in obesity over the past several decades is due to an increasing lack of personal responsibility, perhaps as the result of the lure of bad eating habits and lack of exercise caused by increasing affluence.

This “common-sense view” of obesity is gaining increased currency, as the result of rising health care costs and health insurance premiums, and the drive to rein in these costs. Even some leaders of health insurance and health care providers blame the lack of personal responsibility of the obese for rising health care costs, and advocate using education, exhortation, and “economic incentives” (i.e., penalizing the obese, perhaps by raising their insurance rates) to combat obesity.

However, genetic and physiological research shows that obesity is a disease, not just the result of bad habits. This research has shown that weight is as heritable as height, and has uncovered a set of complex pathways that control energy balance. According to this “enlightened, science-based view”, the worldwide epidemic of obesity is mainly the result of the interaction between a set of social and economic factors (e.g., increased consumption of meals away from home, decreased prices for unhealthy versus healthy food, and decreased requirements for physical activity at work and for transportation) and genetic factors that make some people more susceptible to obesity than others. In the industrialized world, between 60%–70% of the variation in obesity-related phenotypes such as body mass index (BMI) and hip circumference appears to be heritable. People who undertake even the best systematic weight-loss programs are fighting a set of complex physiological pathways that have evolved to combat starvation. These pathways are only partially understood. Most people who manage to lose a significant amount of weight usually regain it over the medium to long term.

The “enlightened, science-based view” is discussed, for example, in our 2008 book-length report on obesity, and in our October 25, 2009 article on this blog. It is also the view of pharmaceutical and biotechnology companies that are developing antiobesity drugs, and the view of basic researchers who are endeavoring to understand pathways that control energy balance and may render individuals subject to obesity and its comorbidities.

Recent genetic studies provide an increasing amount of evidence that favors the  “enlightened, science-based view”. For example, researchers have recently identified associations between common variants in the fat mass and obesity-associated (FTO) gene and increases in BMI and waist circumference in several human populations. The FTO gene codes for a 2-oxoglutarate-dependent nucleic acid demethylase. It is expressed in the hypothalamus, a region of the brain that is involved in regulation of feeding and energy metabolism. Hypothalamic FTO appears to be involved in the regulation of energy intake, but not feeding reward. However, the mechanism of action of FTO in control of energy balance is not understood.

The findings on FTO adds to the large amount of evidence for the genetic determination of obesity, and thus for the “enlightened, science-based view” of this condition. Many academic and corporate researchers, including most of the recognized leaders in obesity research, believe that continued basic and translational research on the genetic and molecular basis of obesity will lead to new therapeutic strategies to control this disease.

Now comes a new research report that might result in a “game-changing view” of obesity, published in the 9 April issue of Science. Andrew Gewirtz (Emory University, Atlanta, GA) and his colleagues studied T5KO mice, which are genetically deficient in TLR5, a Toll-like receptor (TLR) that recognizes bacterial flagella as a ligand and is expressed on the surface of both intestinal epithelial cells and cells that mediate innate immunity. TLRs are receptors that recognize conserved molecules derived from bacteria or viruses, and activate immune responses, thus serving as a first line of defense against infection. Researchers hypothesize that TLR5 in gut mucosa may have a role in maintaining a harmonious relationship between the host and the complex population of intestinal microbes.

The researchers found that as compared to wild-type mice, T5KO mice showed increased fat mass and body weights 20% higher than wild-type mice, and features of the metabolic syndrome (insulin resistance, elevated serum cholesterol and triglycerides, and elevated blood pressure).  The adipose tissue of TK5KO mice exhibited higher production of the proinflammatory cytokines interferon-γ and interleukin-1β.  T5KO mice were also hyperphagic, eating 10% more than wild type mice. When the researchers restricted the food fed to T5KO mice to the amount eaten by wild type mice, they no longer exhibited increased fat mass or body weight, or abnormalities in blood glucose and lipids. However, they still were insulin resistant.

When both wild type and T5KO mice were fed a high-fat diet, both populations showed increases in fat mass and body weight, as well as elevated levels of blood lipids. However, unlike wild type mice, T5KO mice fed a high-fat diet has blood glucose levels of greater than 120 milligrams per deciliter, and thus were diabetic. The T5KO mice also showed inflammatory infiltrates in their pancreatic islets, and hepatic steatosis. Thus a high-fat diet exacerbated the metabolic syndrome shown by T5KO mice.

The researchers asked whether other mediators of the immune system were involved in the induction of metabolic syndrome shown by T5KO mice. Deletion of the Toll-like receptors TLR2 and/or TLR4 in T5KO mice had no effect on their metabolic syndrome.  Deletion of RAG1 (which is necessary for development of the T and B cells of the adaptive immune system) also had no effect. However, deletion of the intracellular protein MyD88 in T5KO mice resulted in normalization of the metabolic syndrome. Since MyD88 is necessary for signaling by all TLRs except for TLR3, and for signaling by receptors for interleukin-1β and interleukin-18, this suggests that another TLR and/or signaling by one or both of these two cytokines might be necessary, together with TLR5 knockout, for induction of the metabolic syndrome.

Since TLR5 is expressed in the gut mucosa and recognizes bacterial flagellin, the researchers tested the hypothesis that the metabolic syndrome seen in T5KO mice might be due to alterations in the population of gut microbes as the result of the loss of TLR5 function. When the researchers treated newly weaned T5KO mice with broad-spectrum antibiotics, the number of gut bacteria was reduced by 90%. This treatment eliminated the metabolic syndrome, hyperphagia, and obesity of the T5KO mice.  Conversely, when the researchers transplanted the gut microbiota of T5KO mice to the guts of wild type germ-free mice, the recipient mice exhibited hyperphagia, obesity, metabolic syndrome, and elevated levels of proinflammatory cytokines in their adipose tissue. Analysis of the gut microbiota of T5KO and wild type mice showed that the species composition of the gut bacteria of T5KO mice was significantly different from that found in wild type mice.

These results suggest that obesity and metabolic syndrome may be caused at least in part by genetically determined differences in interactions between the innate immune system of the gut mucosa and the intestinal flora. Obesity-prone individuals may develop a gut microbe population that interacts with the immune system in such a way as to promote obesity. Interactions between gut microbes and innate immunity that promote obesity might result in changes in proinflammatory cytokines and in adipokines in adipose tissue (and perhaps also in muscle and liver) that not only cause increased inflammation and metabolic syndrome, but also disrupt signals within the brain that promote appetite control and energy balance. They further suggest that treatments that target intestinal microbes may be effective therapies for obesity and its sequelae.

These conclusions are based on a mouse model, which may or may not have much to do with the pathogenesis of human obesity. However, there is evidence that the the composition of gut bacteria differs between obese and nonobese humans in similar ways to differences in gut flora between obese and nonobese mice. Colonization of germ-free mice with the gut microbiota of obese mice results in significantly greater increase in body fat than colonization with the gut microbiota of lean mice. Researchers obtained evidence that gut microbes from obese mice have an increased ability to harvest energy from food than do the gut bacteria of lean mice. They therefore hypothesize that this extra energy harvest may help promote obesity, in both mice and humans. But it is also possible that obesity-associated gut microbe populations might promote systemic low-grade inflammation that contributes to the pathogenesis of metabolic syndrome and obesity.

In addition to the research report itself, two commentaries on the report were published in April 2010–one by Darleen A Sandoval and Randy J Seeley (University of Cincinnati in Ohio) and the other by Ping Li and Gökhan Hotamisligil (Harvard School of Public Health, Boston MA). Drs. Sandoval and Seeley conclude that the new findings may allow researchers to develop means of preventing obesity by manipulating gut microbe-immune system interactions by such means as drugs, diet, or probiotics.  Drs.  Li and Hotamisligil take a more nuanced view. (Dr. Hotamisligil is a leader in the study of pathways involved in control of energy metabolism, and their relationship to inflammation and metabolic diseases.) They state that the results seen in T5KO mice were relatively mild, and thus probably cannot account for the full spectrum of metabolic dysfunction seen in obesity. They essentially see the gut microbiota/innate immunity interaction as one factor in the complex networks that determine obesity and metabolic syndrome. They call for more research into the gut microbe/immune system relationship, and believe that such research will lead to a better understanding of metabolic syndrome.

What if the gut microbiota/immune system interaction is a major factor in obesity, at least in a subpopulation of obese subjects? That would resemble the situation with peptic ulcers. Peptic ulcers were once considered a disease of “lifestyle”, due to the “type A personality”, “a stressful lifestyle”, and/or eating spicy foods. However, eventually Drs.  Barry J. Marshall and Robin Warren of Australia discovered that a high percentage of ulcers were caused by chronic inflammation due to infection with Helicobacter pylori. It is now accepted that this bacterium is responsible for 60% of gastric ulcers and up to 90% of duodenal ulcers. Treatment involves administration of combinations of antibiotics together with a proton pump inhibitor and sometimes a bismuth compound.

The majority of scientists and physicians resisted the idea that peptic ulcers were caused by a microbial infection for a long time. Dr. Marshall even had to do and publish a self-experiment, drinking a culture of bacteria from a patient, before the scientific community would accept his findings. Finally, In 2005, Drs. Marshall and Warren received the Nobel Prize in Physiology or Medicine for their [re]discovery of H. pylori and its role in gastritis and peptic ulcers.

The role of gut microbes in obesity and metabolic syndrome may not be simple as the role of H. pylori in gastric ulcers. Nevertheless, this hypothesis deserves intensive investigation, and it may lead to a game-changing view of metabolic disease and eventually important new treatments. In any event, it would be wise for the scientific, medical, and policy communities to take the advice of Dr. Jeffrey Friedman (Rockefeller University, New York, NY), who is arguably the founder of the “enlightened, science-based view” of obesity and metabolic syndrome, with his breakthrough discovery of the hormone leptin in 1994:  “A war on obesity, not the obese.”

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.

On October 25, 2009, we posted an article on this blog that focused on liraglutide (Novo Nordisk’s Victoza) as a potential treatment for obesity. As we stated in the article, at that time liraglutide had recently been approved in Europe for treatment of type 2 diabetes. The drug was also awaiting FDA approval for that indication.

On January 26, 2010, after a 21-month review, the FDA approved liraglutide for treatment of type 2 diabetes. This followed the approval of the drug in Japan a week earlier.

The approval process for liraglutide in the United States had not been straightforward. In April 2009, the FDA’s Endocrinologic and Metabolic Drugs Advisory Committee voted 6-6 (with one abstention) on approval versus disapproval of liraglutide, because of the finding of thyroid C-cell tumors in studies of the drug in rodents. There is no evidence, however, that liraglutide has ever caused thyroid tumors (or other types of cancer) in humans.

As a result, the drug’s label carries a black box warning of the risk for thyroid cancer, and requires a risk-mitigation strategy. However, as we discussed in our article, liraglutide has an advantage over most antidiabetic drugs in that it induces weight loss. It also has a low risk of triggering hypoglycemia, which is a problem with several antidiabetic drugs.

As we also discussed in our article, liraglutide belongs to a class of agents known as incretin mimetics. The first incretin mimetic to reach the market was exenatide (Amylin/Lilly’s Byetta). Exenatide, which is approved for type 2 diabetes, also induces weight loss. Physicians therefore sometimes prescribe exenatide off-label for treatment of obesity. However, exenatide has a relatively short half-life, and must be self-injected twice a day. In contrast, liraglutide has a longer half-life than exenatide, and is self-injected only once a day. Amylin and Lilly are developing a longer-acting, once-weekly formulation of exenatide (known as Exenatide Once Weekly) for treatment of type 2 diabetes. The new formulation is being developed in collaboration with Alkermes, which developed the long-acting drug-delivery technology. Amylin, Lilly, and Alkermes are awaiting FDA approval of the NDA for Exenatide Once Weekly.

Exenatide’s label carries no warning with respect to thyroid cancer. However, it does carry a warning concerning the risk of drug-associated pancreatitis. Moreover, the FDA Advisory Committee raised concerns that the risk of thyroid C-cell tumors may be a class effect of incretin mimetics. The FDA has mandated that Amylin conduct postmarketing studies to deal with this concern; depending on the results of the studies, a warning of a risk for thyroid cancer may (or may not) appear on the labels of Byetta and Exenatide Once Weekly.

Despite these safety concerns, the stocks of not only Novo Nordisk, but also Amylin and Alkermes, rose on the news that the FDA had approved Victoza. Stock analysts predicted that the approval of Victoza implied that the FDA was likely to approve Exenatide Once Weekly later in 2010.

Our October 2009 blog post discussed exenatide and liraglutide in the context of the obesity drug market, and specifically drugs that treat both type 2 diabetes and obesity. Neither exenatide not liraglutide is approved for treatment of obesity in any jurisdiction, however. As we discussed in our original article, Novo Nordisk has been developing liraglutide for obesity, but Amylin is developing other, earlier-stage drugs for that indication despite the weight loss benefits seen with exenatide. Novo Nordisk had been waiting for FDA approval of liraglutide for treatment of type 2 diabetes before proceeding with further development of the drug for obesity. Now that the company has obtained that approval, we expect that development of liraglutide for obesity will be restarted.

The field of obesity drugs has been a very difficult one for the pharmaceutical industry. Attempts to develop these drugs have been plagued by major safety failures, notably the notorious “Fen-Phen” case that led to market withdrawal and numerous lawsuits. More recently, rimonabant (Sanofi-Aventis’ Acomplia) failed to gain FDA approval due to psychiatric adverse effects, and the company also later withdrew the drug from the market in Europe. Currently marketed drugs have marginal efficacy and troublesome side effects. The complex physiology of weight control, and our inadequate knowledge of pathways that control energy balance, make development of effective agents difficult.

Moreover, there is a lingering perception that obesity is merely a “lifestyle issue” and a failure of “personal responsibility”. This is despite the consistent finding that weight is as heritable as height, and that there are physiological factors that militate against long-term, medically significant weight loss by overweight or obese individuals. These research results indicate that safe and efficacious obesity drugs will be necessary, in addition to diet and exercise, to ward off obesity and its comorbidities in the rapidly growing, worldwide overweight population.

Currently, late-stage drugs developed by three small California companies, Vivus Pharmaceuticals, Orexigen Therapeutics, and Arena Pharmacuticals, are approaching NDA submission. This follows a long hiatus, since the FDA has approved no anti-obesity drug since 1999. The companies hope that the drugs will reach the market in late 2010 or early 2011. All three drugs work in the brain to suppress appetite, as does the currently marketed prescription drug sibutramine (Abbott’s Meridia/ Reductil). The other current agent, orlistat, is available in prescription form as Roche’s Xenical, and in a low-dose over-the-counter form, GlaxoSmithKline’s alli. Orlistat works in the gut to reduce absorption of fats.

Now comes a report in the 23 October 2009 issue of the Lancet, comparing the effects of liraglutide (Novo Nordisk’s Victoza) and orlistat on weight loss in a 20-week double-blind, placebo-controlled Phase II trial in 564 obese healthy volunteers on a hypocaloric diet and increased physical activity. (A subscription is required to see the complete article). The researchers found that in the 20-week period, subjects on liraglutide lost a significant 4.8-7.2 kilograms (10.6-15.8 pounds), depending on the dose, as compared to 4.1 kilograms (9.0 pounds) on orlistat and 2.8 kilograms (6.2 pounds) on placebo. 76% of subjects on the 3.0-milligram/day dose of liraglutide lost over 5% of their body weight, as compared to 30% of subject on placebo. All doses of liraglutide reduced blood pressure, and the 1.8 mg through 3.0 mg doses reduced the prevalence of prediabetes (e.g., fasting plasma glucose above normal, but below that which is classified as diabetes) by between 84-96%. The most common side effects of liraglutide were nausea and vomiting, which usually occurred during the first month of treatment. However, these effects were mainly transient and rarely led to subjects discontinuing treatment. No serious adverse effects were seen.

In an open-label extension of the trial, subjects on liraglutide maintained their weight loss, according to Novo Nordisk. Additional questions need to be addressed, including whether subjects on liraglutide maintain their weight loss after they stop taking the drug.

Unlike the two currently marketed obesity drugs, liraglutide is administered via subcutaneous self-injection. Liraglutide was approved in Europe earlier this year, and is currently marketed in Europe for treatment of type 2 diabetes. However, it is awaiting FDA approval for that indication. It is not yet approved for treatment of obesity in any jurisdiction.

Liraglutide is a member of a class of drugs called incretin mimetics. An incretin is a gastrointestinal hormone that triggers an increase in insulin secretion by the pancreas, and also reduces gastric emptying. The latter effect slows nutrient release into the bloodstream and appears to increase satiety and thus reduce food intake. The major physiological incretin is glucagon-like peptide 1 (GLP-1), and incretin mimetic drugs are peptides with homology to GLP-1 that have a longer half-life in the bloodstream than does GLP-1.

The first incretin mimetic to reach the market is exenatide (Amylin/Lilly’s Byetta), which is based on a Gila monster lizard salivary peptide and was approved for treatment of type 2 diabetes in 2005. Physicians sometimes prescribe exenatide off-label for treatment of obesity. Exenatide has a relatively short half-life, and must be self-injected twice a day. Amylin and Lilly are therefore developing a longer-acting, once-weekly formulation for treatment of type 2 diabetes. Researchers working with Amylin and Lilly also reported positive results of a clinical trial of exenatide in treatment of nondiabetics for obesity at a scientific meeting earlier this year. Amylin is also developing two earlier-stage biologics, pramlintide/metreleptin and davalintide, for treatment of obesity. Neither is an incretin mimetic.

Liraglutide is a GLP-1 analogue designed to bind to human serum albumin in the bloodstream, and thus has a longer half-life than exenatide, and is self-injected only once a day. Liraglutide is thus more convenient for patients to use than exenatide. The results of a study published in the Lancet earlier this year indicate that liraglutide is more effective than exenatide in long-term reduction in blood glucose (measured as hemoglobin A1c) in patients with type 2 diabetes.

The development of liraglutide for obesity represents part of a larger trend—the development of drugs that treat both type 2 diabetes and obesity. In the case of development of obesity drugs, the regulatory pathway for diabetes is easier than for obesity. Companies therefore tend to develop dual diabetes/obesity drugs first for diabetes. As the drugs prove themselves in the clinic, with respect to safety, antidiabetic efficacy, and effects on weight loss, companies may also develop them for obesity. This is the case with liraglutide.

In the case of treatment of type 2 diabetes, reducing weight in obese diabetics undergoing drug treatment is a major unmet need. Antidiabetics that also induce weight loss are therefore of special value. We discussed this issue in our 2008 article, “Addressing unmet type 2 diabetes needs”.

There are at least several companies with early stage dual diabetes/obesity drugs. These companies generally prefer to develop these drugs for diabetes. Early stage obesity drug development is mainly on hold, awaiting the regulatory approval of the three late-stage drugs now nearing NDA submission.

Novo Nordisk is also waiting to hear from the FDA regarding regulatory approval of liraglutide for treatment of type 2 diabetes before proceeding with further development of the drug for obesity.

We have produced two additional resources for understanding drug development in type 2 diabetes and obesity. These are, Diabetes and Its Complications: Strategies to Advance Therapy and Optimize R&D and Obesity Drug Pipeline Report Overview, both published by Cambridge Healthtech Institute.