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.