Executive Summary

  • Nrf2 activators bind to Keap1, a protein that coordinates the cellular response to reactive oxygen species (ROS). Binding to Keap1 activates Nrf2, a transcription factor that evolved to orchestrate the resolution of inflammation after the initiating pathogen is cleared or damage is repaired. Nrf2 coordinates this process by normalizing metabolism and increasing mitochondrial energy production (ATP), increasing cellular antioxidant capacity and reducing product of ROS. This activity inhibits inflammatory signaling pathways, such as NF-κB and the inflammasome.
  • Abnormal metabolism, decreased ATP levels, increased ROS production, and chronic inflammatory signaling are common features of a variety of diseases. When left unresolved, these pathologic processes can lead to abnormal cellular proliferation, tissue fibrosis and remodeling, and organ damage. By correcting the underlying molecular aberrations, Nrf2 activators may have the potential to prevent these longer-term consequences and improve disease symptoms.
  • Since mitochondrial dysfunction, oxidative stress, and inflammation are features of many diseases, Nrf2 activators may have many potential clinical applications and have been the subject of more than 200 peer-reviewed scientific papers. The effects of Nrf2 activators are described below in more detail.
Inflammation Resolution:
Inflammation is a protective response of the body to harmful stimuli such as invading pathogens, damaged cells, or irritants. It evolved to neutralize the initial cause of injury, eliminate dead cells, and initiate repair of damaged tissues. A central feature of inflammation is mitochondrial dysfunction and the production of ROS, which promotes activation of inflammatory pathways. In many diseases, inflammation does not resolve normally and is associated with chronic excessive ROS, organ fibrosis, remodeling or other tissue damage, and impaired ATP production1,2,3.

Normalizing Metabolism and Increasing Mitochondrial Energy Production: Mitochondria are often described as the power plants of the cell because they generate energy through the production of ATP, the primary unit of cellular energy. Mitochondrial dysfunction, which is manifested through decreased cellular energy production and increased production of ROS, is a feature of many chronic inflammatory diseases4. Nrf2 activation reduces mitochondrial ROS, promotes the availability of fatty acids and glucose for mitochondrial ATP production, and increases mitochondrial biogenesis5,6,7.

Pharmacology

The foundational biology of Nrf2 underlies our two lead product candidates, bardoxolone methyl and omaveloxolone, and certain of our preclinical programs.

Many diseases have three features in common: inflammation, oxidative stress, and mitochondrial dysfunction. Healthy cells normally sense the presence of pathogenic organisms and other toxic stimuli. When these danger/damage signals are sensed, cells rapidly respond by increasing the production of reactive oxygen species (ROS) and pro-inflammatory cytokines. To facilitate production of these inflammatory mediators, mitochondria adopt an “inflamed” state, wherein normal mitochondrial function is temporarily suppressed. This shift-in-state allows mitochondrial resources to be diverted away from ATP production and toward other pathways that orchestrate normal cellular defense and repair processes. Reorientation of mitochondria from energy production to ROS production allows the affected cells to kill invading pathogens with molecules such as hydrogen peroxide and superoxide. Some of the ROS produced by inflamed mitochondria are released from the mitochondria into the cytoplasm and promote the activity of multiple key pro-inflammatory signaling complexes, including the IκBα/NF-κB complex and the NLRP3 inflammasome. In a normal disease process, after the pathogens have been eliminated the resolution phase of inflammation can begin, and naturally occurring molecules promote the resolution of inflammation, in part, by activating the Keap1/Nrf2 pathway. As a result, pro-inflammatory mediators are reduced, ROS are neutralized, and normal mitochondrial function is restored. However, in many chronic inflammatory and genetic diseases, this resolution process does not occur or is inadequate, leading to mitochondrial dysfunction, oxidative stress, and chronic inflammation, all of which can ultimately lead to tissue damage.

Nrf2 activators mimic the activity of the endogenous molecules that promote the resolution of inflammation and restore homeostasis by binding to Keap1, a protein that coordinates the cellular response to ROS and other stimuli, each of which can cause cellular damage (generally referred to as oxidative stress). Binding to Keap1 activates Nrf2, a transcription factor that increases the levels of antioxidants and transporters, thereby reducing the levels of oxidative stress caused by excess ROS. Nrf2 also restores normal mitochondrial function by increasing the availability of substrates and reducing equivalents that are required to support ATP production. Nrf2 activation inhibits inflammation by reducing ROS levels, restoring normal mitochondrial function, and directly inhibiting pro-inflammatory signaling.

Since mitochondrial dysfunction, oxidative stress, and inflammation are features of many diseases, Nrf2 activators may have many potential clinical applications and have been the subject of more than 200 peer-reviewed scientific papers.

Inflammation Resolution

Reactive oxygen species: ROS are chemically reactive molecules that contain oxygen and have important roles in cell signaling and balancing cellular systems. ROS are formed during mitochondrial ATP production and by a variety of other cellular processes. ROS increase inflammatory signaling, and excessive ROS can cause cellular damage to tissues in critical organs including the muscles, lung, heart, liver, brain, and eyes. Excessive ROS and chronic inflammation have been shown to be the cause of cellular damage in many diseases3. Nrf2 activation increases the cellular content of antioxidant, which makes reducing equivalents available to neutralize ROS8. This suppresses the pro-inflammatory signaling effects of ROS and protects tissues from the damaging effects of excessive ROS.

Inhibition of inflammatory signaling: Mitochondrial ROS, NF-κB, and the NLRP3 inflammasome are important activators and regulators of the inflammatory response. Nrf2 activators, through reduction of ROS and inhibition of NF-κB and the inflammasome, suppress production of TNFα, IL-6, IL-1, and other inflammatory cytokines, or cellular messengers. The suppression of these inflammatory cytokines inhibits their downstream pro-inflammatory signaling pathways8,9,10.

Reduction of enzymes associated with fibrosis and tissue remodeling: Tissue remodeling and fibrosis can be caused by chronic inflammation due to deposition of collagen and other factors. In a variety of models and settings, suppression of ROS and inhibition of NF-κB has been observed to reduce the expression of enzymes associated with tissue remodeling that are implicated in the progression of PH, certain types of cancer, arthritis, and many other diseases1,8.

Inhibition of cellular proliferative pathways: Inhibition of NF-kB and the NLRP3 inflammasome by Nrf2 activators inhibits a number of signaling pathways that promote hyperproliferation (harmful excessive growth) of cells. The antiproliferative effects of Nrf2 activators have been demonstrated in a variety of models of cancer and inflammatory or fibrotic diseases11,12,13.

Normalizing Metabolism and Increasing Mitochondrial Energy Production

ATP production: ROS are produced in the mitochondria as a byproduct of ATP production. Nrf2 activation improves mitochondrial efficiency by making antioxidant enzymes available to reduce or neutralize ROS2. The management of these reducing equivalents is a constant and critical balancing act within the mitochondria. Disease processes that increase ROS deplete reducing equivalents available for ATP production4. Accordingly, the induction of antioxidant proteins through Nrf2 activation augments mitochondrial ATP production5,14.

Efficient consumption of fats and sugars: Nrf2 activation promotes the transport of fatty acids to the mitochondria where they are converted into reducing equivalents used to produce ATP4. Nrf2 also promotes the transport of glucose from the bloodstream into the cells where it is converted into reducing equivalents used to produce ATP. Nrf2 activators have been shown to promote glucose uptake and oxygen consumption in animal models of diet-induced obesity and diabetes15,16.

Mitochondrial biogenesis: PGC1α is a protein that increases the number of mitochondria in a cell. Activation of Nrf2 has been shown to increase PGC1alpha expression in skeletal muscle, which may increase ATP production7,17.

References

  1. Harijith et al. Reactive oxygen species at the crossroads of inflammasome and inflammation. Front Physiol. 2014;5:352.
  2. Brϋne et al. Redox control of inflammation in macrophages. Antioxid Redox Signal. 2013 Aug 20;19(6):595-637.
  3. Liby and Sporn. Synthetic oleanane triterpenoids: multifunctional drugs with a broad range of applications for prevention and treatment of chronic disease. Pharmacol Rev. 2012;64(4):972-1003.
  4. Greco et al. Neuroprotection through stimulation of mitochondrial antioxidant protein expression. J Alzheimers Dis. 2010;20 Suppl 2:S427-37.
  5. Holmstrom et al. Nrf2 impacts cellular bioenergetics by controlling substrate availability for mitochondrial respiration. Biol Open. 2013 Jun 20;2(8):761-70.
  6. Uruno et al. The Keap1-Nrf2 system prevents onset of diabetes mellitus. Mol Cell Biol. 2013 Aug;33(15):2996-3010.
  7. Wenz et al. Activation of PPAR/PGC-1α pathway prevents a bioenergetics deficit and effectively improves a mitochondrial myopathy phenotype. Cell Metab. 2008 Sep;8(3):249-56.
  8. Whitman et al. Nrf2 modulates contractile and metabolic properties of skeletal muscle in streptozotocin-induced diabetic atrophy. Exp Cell Res. 2013 Oct 15;391(17):2673-83.
  9. Rossi et al. Anti-inflammatory cyclopentenone prostaglandins are direct inhiibtors of IκB kinase. Nature. 2000 Jan 6;403(6765):103-8.
  10. Honda et al. Novel synthetic oleanane triterpenoids: A series of highly active inhibitors of nitric oxide production in mouse macrophages. Bioorg Med Chem Lett. 1999 Dec 20;9(24):3429-34.
  11. Deeb et al. Oleanane triterpenoid CDDO-Me inhibits growth and induces apoptosis in prostate cancer cells by independently targeting pro-survival Akt and mTOR. Prostate. 2009 Jun 1;69(8):851-60.
  12. Hyer et al. Apoptotic activity and mechanism of 2-cyano-3,12-dioxoolean-1,9-dien-28-oic-acid and related synthetic triterpenoids in prostate cancer. Cancer Res. 2008 Apr 15;68(8):2927-33.
  13. Yore et al. The synthetic triterpenoid 1-[2-cyano-3,12-dioxooleana-1,9(11)-dien-28-oyl]imidazole blocks nuclear factor-κB activation through direct inhibition of IκB kinase β. Mol Cancer Ther. 2006 Dec;5(12):3232-9.
  14. Cassarett and Doull. Toxicology. 8th Ed. Chapter 3.
  15. Kensler et al. Cell survival responses to environmental stresses via the Keap1-Nrf2-ARE pathway. Annu Rev Pharmacol Toxicol. 2007;47:89-116.
  16. Saha et al. The Triterpenoid 2-Cyano-3,12-dioxooleana-1,9-dien-28-oic-acid Methyl Ester Has Potent Anti-diabetic Effects in Diet-induced Diabetic Mice and Leprdb/db Mice. J Biol Chem. 2010; 285(52): 40581-40592.
  17. Shin et al. Role of Nrf2 in prevention of high-fat diet-induced obesity by synthetic triterpenoid CDDO-Imidazolide. Eur J Pharmacol. 2009; 620(1-3): 138-144.