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.

In the December 15, 2009 issue of Neurology, a research report by Stephen Salloway and his colleagues at the Butler Hospital and Brown University (Providence, RI) and an editorial by Dan Kaufer and Sam Gandy (University of North Carolina at Chapel Hill) focus on a Phase II multicenter placebo-controlled clinical trial of Elan/Wyeth’s bapineuzumab (AAB-001) in patients with mild to moderate Alzheimer’s disease (AD). (Wyeth is now part of Pfizer.) (A subscription is required to read the full text of both of these articles.) Bapineuzumab is a monoclonal antibody (MAb) drug that is specific for amyloid-β (Aβ) peptide. The dominant paradigm among AD researchers and drug developers is that the disease is caused by aberrant metabolism of Aβ, resulting in accumulation of neurotoxic Aβ plaques. This paradigm is known as the “amyloid hypothesis”.

The overall result of the study by Salloway et al. was that there was no difference in cognitive function between patients in the drug-treated and the placebo groups. However, the study did not have sufficient statistical power to exclude the possibility that there was such a difference. About 10% of patients treated with the agent also experienced vasogenic edema (VE), which was reversible. (Cerebral VE is the infiltration of intravascular fluid and proteins into brain tissue, as the result of breakdown of the blood-brain barrier.)

Retrospective analysis of the data suggested that bapineuzumab-treated patients who were not carriers of the apolipoprotein E epsilon4 allele (ApoE4) showed improved cognitive function as compared to placebo treatment, and that they had a lower incidence of VE than ApoE4 carriers. The ApoE4 polymorphism is the only known, well-characterized genetic risk factor associated with the development of late-onset AD. Of the three common isoforms of ApoE, ApoE3 is the most common, followed by ApoE4 and ApoE2, respectively. Unlike ApoE4, the ApoE2 allele appears to protect against development of AD. Some researchers estimate that allelic variations in ApoE may account for over 95% of AD cases.

In the study by Salloway et al., nearly two-thirds of the AD patients carried one or more ApoE4 alleles; thus only the remaining one-third of patients appeared to show positive effects of bapineuzumab treatment according to the retrospective analysis. However, the idea that the drug is efficacious in ApoE4 noncarriers is only a hypothesis, which will require prospective clinical trials to confirm. Elan and Pfizer are now conducting large Phase III clinical trials of bapineuzumab, which have prospectively segregated enrollment into ApoE4 carrier and noncarrier groups.

The hypothesized association of ApoE4 noncarrier status of AD patients with bapineuzumab efficacy and safety has been used as a case study in workshops on stratified medicine sponsored by the FDA, MIT, and industry partners in 2009 and 2010. You can read about the October 2009 workshop here. The most recent workshop was held at MIT on January 19, 2010. In these workshops, two case studies were discussed: the use of diagnostic tests for the HER2 receptor in identifying breast cancer patients who are likely to benefit from treatment with trastuzumab (Genentech/Roche’s Herceptin), and the bapineuzumab/ApoE4 case. The HER2/ trastuzumab relationship is well known and well characterized, and is considered to be a paradigm of stratified medicine. This contrasts with the bapineuzumab/ApoE4 association, which remains a hypothesis pending the results of the Phase III prospective clinical studies.

A growing minority of researchers is skeptical that the amyloid hypothesis is sufficient to account for AD pathogenesis in all stages of the disease or in various disease subpopulations, and they are investigating other pathways that may contribute to the disease, either in combination with the amyloid pathway or as alternative mechanisms. We have discussed alternative hypotheses for AD pathogenesis in a 2004 article published in Genetic Engineering News (available on our website), and in book-length reports published by Cambridge Healthtech Institute in 2006 and in 2009.

The search for alternative hypotheses takes on added urgency because of the clinical failure of several AD drugs that had been designed based on the amyloid hypothesis. These include Neurochem’s (now Bellus Health) Alzhemed (3-amino-1-propanesulfonic acid) and Myriad Pharmaceuticals’ Flurizan (tarenflurbil), both of which failed in Phase III clinical trials. Based on the overall results of the Phase II trial of bapineuzumab, most researchers and industry commentators would add bapineuzumab to the list, unless the stratified Phase III trial shows that the drug is significantly efficacious and safe for ApoE4 noncarriers.

Since ApoE4 carrier status is such a prominent risk factor for developing late-onset AD, might ApoE4 itself be a target for drug discovery in AD? Drs. Kaufer and Gandy suggest that such an approach might be fruitful, whatever the outcome of the Phase III trial of bapineuzumab. Several academic laboratories have been investigating mechanisms by which ApoE4 may be involved in the pathobiology of AD. You may read two recent papers on this subject here and here. ApoE4 may contribute to AD pathogenesis via multiple mechanisms, including by causing synaptic deficits and mitochondrial dysfunction in neurons, and by inducing endoplasmic reticulum stress leading to astrocyte dysfunction.

Given the prominence of ApoE4 expression as a risk factor for AD, the study of the mechanistic basis of ApoE4’s role in AD pathobiology needs greater attention. Hopefully, this research will lead to the development of novel therapeutic strategies for AD.

In the December 10 2009 issue of Nature, researchers at Agios Pharmaceuticals (Cambridge, MA) and their academic collaborators published an article implicating mutations in a metabolic enzyme, cytosolic isocitrate dehydrogenase (IDH1) as a causative factor in a major subset of human brain cancers.

The mutated forms of IDH1 are found in around 80% of human grade II-III gliomas and secondary glioblastomas. The mutations occur in arginine 132, which is usually mutated to histidine. (In other less common mutations, arginine 132 is mutated to serine, cysteine, glycine, or leucine.) Typically, only one allele of IDH1 is mutated. These mutations appear to occur early in the process of tumorigenesis, and often appear to be the first mutation that occurs. The mutant forms of IDH1 are also found in a subset of acute myelogenous leukemia (AML).

The wild-type form of IDH1 catalyzes the NADP+-dependent oxidative decarboxylation of isocitrate to α-ketoglutarate. However, the researchers found that the mutant forms of IDH1 no longer catalyzes this reaction, but instead catalyzes the NADPH-dependent reduction of α-ketoglutarate to R(-)-2-hydroxyglutarate (2HG). This is the result of changes in the active site of the enzyme, as demonstrated by structural studies carried out by the researchers. Tumors that harbor the mutant form of IDH1 have elevated levels of 2HG. The researchers therefore hypothesize that these elevated levels of 2HG are a causative factor in tumorigenesis and/or tumor progression in human gliomas.

This hypothesis is supported by the effects of the familial metabolic disorder 2-hydroxyglutaric aciduria. This disease is caused by a deficiency of 2-hydroxyglutarate dehydrogenase, an enzyme that converts 2HG to α-ketoglutarate. Patients with this metabolic disease have elevated levels of 2HG in bodily fluids and in the brain, and an increased risk of developing brain tumors.

The mechanism by which 2HG might contribute to tumorigenesis is unknown. The authors advance several hypotheses, including increasing reactive oxygen species (ROS) levels, serving as an NMDA (N- methyl-D-aspartate) receptor agonist, and competitive inhibition of enzymes that use glutamate and/or α-ketoglutarate resulting in the induction of hypoxia-inducible factor-1α, a transcription factor that facilitates tumor growth under conditions of hypoxia.

According to the authors, these results suggest that in patients with low-grade gliomas containing mutant forms of IDH1, therapeutic inhibition of 2HG production may slow or halt progression of these tumors to lethal secondary glioblastomas. 2HG levels may also be used as a prognostic test for IDH1 mutations, since patients with these mutations tend to live longer than patients with gliomas that have other mutations.

The company that led this research, Agios Pharmaceuticals, is developing a pipeline of oncology drugs based on targeting metabolic pathways in cancer cells. Interestingly, Agios means “holy” in Greek.

Way back in 1924, Otto Warburg demonstrated a difference between cancer cells and normal adult cells in glucose metabolism. In the presence of oxygen, most normal adult cells metabolize glucose to pyruvate via the process of glycolysis, generating two molecules of ATP (the energy currency of the cell) per glucose molecule. In the mitochondria, they then utilize oxygen to catabolize pyruvate to CO2 and water, in the process generating 36 molecules of ATP per glucose molecule. Cancer cells, however, predominantly carry out aerobic glycolysis, in which they carry out glycolytic conversion of glucose to pyruvate, followed by reduction of pyruvate to lactate. Despite the presence of oxygen, cancer cells generate the bulk of their ATP from glycolysis, not mitochondrial oxidative phosphorylation, in the process consuming large amounts of glucose. The reliance of cancer cells on aerobic glycolysis for their metabolism is known as the “Warburg effect”.

Agios’ platform is based in part on the work of signal-transduction pioneer Lewis Cantley (Beth Israel Deaconess Cancer center/Harvard Medical School, Boston MA). It is Dr. Cantley’s work on the connection between growth factor-mediated signal transduction and aerobic glycolysis that is the basis for Agios’ platform. In particular, Dr. Cantley and his colleagues found that pyruvate kinase M2 (PKM2) is a link between signal transduction and aerobic glycolysis. PKM2 binds to tyrosine-phosphorylated signaling proteins, which results in the diversion of glycolytic metabolites from energy production via mitochondria oxidative phosphorylation to anabolic processes required for rapid proliferation of cancer cells.

Agios closed a $33 million Series A financing in July 2008, co-led by Third Rock Ventures, Flagship Ventures and ARCH Venture Partners. In June 2009, Fierce Biotech named Agios to the 2009 FierceBiotech “Fierce 15” list. On December 21, 2009, Agios received funding from the nonprofit organization Accelerate Brain Cancer Cure (ABC2), to supplement Agios’s research on the development of IDH1-based therapeutics and diagnostics. Agios expects to have a lead compound in the clinic some time in 2010.

The Agios website calls cancer metabolism “one of the most exciting new areas of cancer research”. But the study of cancer metabolism, and especially the Warburg effect, is not new—the Warburg effect is a classic observation going back 85 years. Moreover, biotechnologists working in such areas as production of recombinant proteins in CHO cells have been familiar with aerobic glycolysis, which is carried out by most mammalian cell lines in culture, for decades. Nevertheless, cancer metabolism has been well out of the mainstream of cancer drug discovery. It was Dr. Cantley’s work, which links the classic Warburg effect to the mainstream area of signal transduction and protein kinases, which has made Agios’ platform possible.

Similarly, it was Julian Adams’ work on the biology of the proteasome in the 1990s, through a series of biotechnology company mergers that eventually led him to Millennium Pharmaceuticals (now Millennium: The Takeda Oncology Company), which resulted in Millennium’s proteasome inhibitor Velcade (bortezomib). Velcade, the only proteasome inhibitor on the market, is now approved by the FDA for the treatment of multiple myeloma and mantle cell lymphoma. Prior to Dr. Adams’ work, proteasome biology and protein degradation were out of the mainstream of cancer drug discovery. Now Joseph Bolen, the chief scientific officer of Millennium, sees “protein homeostasis” as one of the most exciting areas of cancer research.

Finally, although the development of protein kinase inhibitors to target signaling pathways in cancer is now well within the mainstream of oncology drug discovery, prior to the discovery and development of imatinib (Novartis’ Gleevec/Glivec) (approved by the FDA in 2001), specific targeting of protein kinases was though to be unlikely, since all of these enzymes have a high degree of similarly in their ATP binding sites. Thus the field of protein kinase inhibitors did not enter the mainstream until the late 1990s-early 2000s.

The take-home lesson is that drug developers may find fertile areas for innovation in seemingly obscure or out-of-the mainstream areas of biology (or of chemistry, as we have discussed in previous blog posts). Some of these areas may be technologically premature, and not quite ready for exploitation by drug developers. However, as demonstrated by our blog post on monoclonal antibodies, even some technologically premature areas may yield to innovators who are willing and able to develop enabling technologies to move these areas up the development curve.