Genetic Proof of Concept

A consideration when designing drugs that activate hypoxia-inducible factor (HIF) for chronic use in treating anemia is whether activation of HIF can be selective for erythropoiesis and safe over the long term. There are many examples in nature and medicine that support safe and selective chronic HIF activation. For example, exposure to hypoxic environments, such as high altitude, regularly leads to increased red blood cell mass. Athletes have used this natural process for years without apparent ill effects to enhance performance by living at high altitude or using devices that mimic high altitude, such as hypoxic sleeping tents. Studies in healthy volunteers have shown that exposure to intermittent periods of hypoxia over a few weeks stimulates erythropoiesis, and hemoglobin deficiency can be corrected in patients with chronic kidney disease using hypobaric chambers.

Genetic proof of concept for selective and safe chronic activation of HIF is provided by several examples in the medical literature of congenital polycythemia, an inherited disorder in which polycythemia (too many red blood cells), as the name suggests, is the main clinical manifestation. Congenital polycythemia can be caused by enhanced stability and activity of HIF as a result of various mutations in proteins that comprise the body's oxygen-sensing system (e.g., HIF-2α1,2,3 prolyl hydroxylase domain protein 2 [PHD2]4,5,6 von Hippel-Lindau tumor-suppressor protein [VHL]7, 8).

Polycythemia and Thrombosis
Polycythemia results from increased levels of erythropoiesis (production of red blood cells), a process mediated by HIF. Based on studies of Chuvash polycythemia, which is a congenital polycythemia caused by a mutation in the VHL protein and endemic to the people of the Chuvash region of Russia, median survival is approximately 55 years of age.8 Premature mortality in Chuvash patients is related to cerebral vascular events and peripheral thrombosis, the latter being associated with extremely high hemoglobin levels (median hemoglobin = 19.0 g/dL) but not in patients with lower, but still above normal, hemoglobin levels (median hemoglobin = 16.6 g/dL).8 While elevated levels of plasminogen activator inhibitor-1 (PAI-1), a HIF-mediated factor that inhibits the breakdown of blood clots, are observed in Chuvash patients, there is no correlation between these elevations and history of thrombosis or imaging evidence of thrombosis.8  Increased thrombotic risks are also reported for patients carrying mutations in PHD2 and HIF2. Lower blood pressures have been reported in Chuvash polycythemia patients8, which contrasts to the tendency for hypertension observed in anemic patients treated with erythropoietic stimulating agents (ESAs).9, 10

No Increased Risk of Tumorigenesis or Malignancy
Unlike patients with classical von Hippel-Lindau (VHL) syndrome, who have a predisposition to develop a specific subset of malignant tumors, patients with congenital polycythemia associated with chronic HIF activation lack a history, or evidence of, renal-cell carcinoma, pheochromocytomas (adrenal gland tumors), or hemangioblastomas (benign tumors of the central nervous system), all hallmarks of VHL syndrome.4, 8  In one study, the absence of tumors, as confirmed by computed tomography and magnetic resonance imaging scans, was reported despite the finding of increased expression and circulating levels of vascular endothelial growth factor (VEGF).8  A gene mediated by HIF. VEGF is involved in angiogenesis (the formation of new blood vessels) and a target of anti-cancer therapies (e.g., Avastin®). The authors concluded that overexpression of HIF and VEGF is not sufficient for tumorigenesis. The same study found no evidence for the presence of retinal hemagioblastomas, a benign tumor of the eye associated with classical VHL syndrome and increased levels of VEGF.8  Similarly, retinal hemagioblastomas were absent in patients with a PHD2 mutation.4

Pharmacologic Proof of Concept for Selective and Safe HIF Activation in Humans

The pharmaceutical agents cobalt and desferrioxamine (DFO) have long been known to increase erythropoietin production and red blood cell mass. Following the discovery of the HIF pathway, DFO has been demonstrated to increase hemoglobin by non-selective stabilization of HIF via iron chelation, while cobalt displaces iron from the catalytic core of PHDs, rendering them less active and leading to HIF activation. Deferasirox (Exjade®), a second generation iron chelator, which also stabilizes HIF, was approved in 2007. Preclinical studies found that deferasirox was not carcinogenic when administered to rats in a 2-year study.

Selective Erythropoiesis with HIF-PHI
Pharmacological activation of HIF using HIF prolyl hydroxylase inhibitors (HIF-PHI) is more selective than the examples cited above due to mechanism of action, which is not via iron chelation. Intermittent dosing with HIF-PHI and reversible HIF stabilization further increase selectivity, and high plasma protein binding limits tissue distribution.

Learn more about selectivity of HIF-PHI

References

  1. Percy MJ et al., (2008) Novel exon 12 mutations in the HIF2A gene associated with erythrocytosis. Blood 111(11):5400-2.
  2. Percy MJ, et al. (2008) A Gain-of-Function Mutation in the HIF2A Gene in Familial Erythrocytosis. N Engl J Med 2008 Jan 10;358(2):162-8.
  3. Martini M, et al. (2008) A novel heterozygous HIF2AM535I mutation reinforces the role of oxygen sensing pathway disturbances in the pathogenesis of familial erythrocytosis. Haematologica 93(7):1068-71.
  4. Percy MJ, et al. (2006) A family with erythrocytosis establishes a role for prolyl hydroxylase domain protein 2 in oxygen homeostasis. Proc Natl Acad Sci U S A 103(3):654-9.
  5. Percy MJ, et al. (2007) A novel erythrocytosis-associated PHD2 mutation suggests the location of a HIF binding groove. Blood 110(6):2193-6.
  6. Al-Sheikh M, et al. (2007) Disturbance in the HIF-1alpha pathway associated with erythrocytosis: further evidences brought by frameshift and nonsense mutations in the prolyl hydroxylase domain protein 2 (PHD2) gene. Blood Cells Mol Dis 40(2):160-5.
  7. Ang SO, et al. (2002) Disruption of oxygen homeostasis underlies congenital Chuvash polycythemia. Nature Genetics 32(4):614-21.
  8. Gordeuk VR, et al. (2004) Congenital disorder of oxygen sensing: association of the homozygous Chuvash polycythemia VHL mutation with thrombosis and vascular abnormalities but not tumors. Blood 103(10):3924-32.
  9. Aranesp [package insert]. Thousand Oaks, CA: Amgen Inc.; August 2008.
  10. EPOGEN [package insert]. Thousand Oaks, CA: Amgen Inc.; August 2008.
   

 
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