Hypoxia

Overview

• Oxygen "fixes" (makes permanent) damage caused by free radicals (ion pair -> free radical -> DNA damage -> O2 fixation by making permanent DNA-peroxide bond)
  • Must be present during or immediately (~5 msec) after RT
• Oxygen enhancement ratio (OER) ~2.5x
  • Decreases with increasing LET: photons 2.5, neutrons 1.5, alpha 1.0
  • Similarly, decreases with increasing RBE
  • For photons, increases with dose and dose rate. If dose/fx <2 Gy OER ~1.5x, if dose/fx >2 Gy OER ~3x
• Measurements of oxygenation
  • Evaluated directly by polarographic Eppendorf oxygen probes
  • Exogenous: Nitroimidazoles (reduced and irreversibly bound under low O2 tension), EF5, carbon black
  • Endogenous compounds: carbonic anhydrase (CA9), HIF, GLUT1 (need biopsy)
  • Noninvasive imaging: PET F-18-miso, PET Cu-64-Cu-ATSM, SPECT I-123-azomycin arabinoside
• Hypoxia markers:
• O2 concentration
  • Air 155 mmHg, 100% oxygen 760 mmHg
  • O2 tension in tissues varies between 1 - 100 mmHg, venous blood ~30 mmHg. Many tissues normally borderline hypoxic
  • Cell survival very sensitive to low level O2. At 0.2% O2 (1 mm Hg), survival curve noticeably different
  • At 0.5% O2 (3 mm Hg), survival halfway to aerated
  • Most rapid change between 0 and 30 mmHg
  • Virtually no change in survival curve from venous to arterial to 100% oxygen
• O2 diffusion distance in metabolic active tissue 100-200 µm (Tomlinson-Gray hypothesis, PMID: 13106296)
• Hypoxia varies
  • Spatially: within tumor
  • Temporary: chronic (diffusion-mediated due to distance from blood vessels) vs acute (perfusion-mediated due to transient fluctuations in blood flow due to malformed vascular supply)
  • From patient to patient
• Hypoxic fraction
  • Can estimate by extrapolating back from shallow portion of the biphasic survival curve (steep portion is for oxygenated and shallow portion is for hypoxic cells)
  • Ranges from 0-50%, on average ~15%
• Reoxygenation
  • RT preferentially kills oxygenated cells. Hypoxic cells survive, but typically become re-oxygenated within 24 hours, just in time for the next fraction
  • Therefore, if re-oxygenation is achieved, hypoxic cells do not have a significant effect on outcome of fractionated RT
  • Lack of reoxygenation is potentially a concern for single fraction SRS/SBRT treatment approaches
  • The extent and rapidity of re-oxygenation varies dramatically from tumor to tumor, and depend on proportion of chronic vs acute hypoxia present
• Hypoxic conditions may play a role in malignant progression, by decreasing apoptosis, increasing genomic instability and gene amplification, and by promoting angiogenesis
• Radiosensitization of hypoxic cells
  • Improved oxygen delivery: Hyperbaric oxygen, perfluorocarbons, carbagen
  • Tobacco cessation
  • Drugs: nitroimidazoles (misonidazole showed limited effect, nimorazole significant improvement in a Danish H&N trial) for chronic hypoxia, nicotinamide for acute hypoxia
  • Concurrent chemo: Mitomycin C, Tirapazamine
• Hypoxia imaging (PMID: 28540739): the principal noninvasive approaches to imaging tumor hypoxia currently include magnetic resonance and radionuclides (PET and single-photon emission computed tomography), but other techniques, such as optical imaging or electron spin resonance, are under investigation
• Angiogenesis (see more below):
  • Pro-angiogenesis: HIF-1α, VEGF, PDGF, FGF
  • Anti-angiogenesis:
    • Natural: VHL, TSP-1, Angiostatin, Endostatin, Heparin
    • Drugs: bevacizumab, sunitinib, sorafenib, thalidomide

Hypoxia Inducible Factor (HIF)

• HIF proteins are a constitutively expressed family, including HIF-1α, HIF-1β, and HIF-2α
• EGLN2 enzyme acts as a cellular oxygen sensor. It is a prolyl hydroxylase (PHD)
  • Under normoxic conditions, it hydroxylates HIF-1α using available oxygen
  • Hydroxylated HIF-1α is recognized by Von Hippel-Lindau (VHL) protein, and marked for degradation by ubiquination
  • Under hypoxic conditions EGLN2 does not have access to oxygen, and thus does not hydroxylate HIF-1α
• HIF-1α subsequently binds to HIF-1β, and the complex acts as a transcription factor on DNA hypoxia-responsive elements (HREs). Targets include
  • VEGF: promotes angiogenesis
  • GLUT-1: promotes glycolysis and oxygen-independent ATP production
  • Stimulation of erythropoiesis
  • Increase in apoptosis by interaction with Bcl-2 protein family and p53
• However, HIF-1α levels are also influenced by Ras and PI3K pathways, so that HIF-1α activity may not directly correlate with hypoxia

Review

• Duke 2005 PMID 16098463 -- "Pleiotropic effects of HIF-1 blockade on tumor radiosensitivity." (Moeller BJ, Cancer Cell. 2005 Aug;8(2):99-110.)
  • Radiation increases HIF-1 activity in tumors, both sensitizing and protective:
    • Radiosensitization: promotes ATP metabolism, proliferation, p53 activation
    • Radioresistance: stimulates endothelial cell survival
  • Net effect of HIF-1 blockade highly dependent on treatment sequencing, with "radiation first" being more effective due to preventing development of radioresistance effect
  • Comment from MSKCC PMID 16098459

Vascular Endothelial Growth Factor (VEGF)

• Family of proteins resulting from alternate splicing of mRNA from a single VEGF gene
• Bind to tyrosine kinase receptors (VEGF-R) on cell surface, causing them to dimerize
• VEGF-R2 appears to modulate most known cellular responses
  • Angiogenesis (endothelial cell migration, mitosis, creation of blood vessel lumen, fenestrations, etc)
  • Chemotaxis for macrophages and granulocytes
  • Vasodilation through NO release
• VEGF-R3 appears to mediate lymphangiogenesis
• Anti-VEGF therapies
  • Monoclonal antibodies: bevacizumab for cancer indications, ranibizumab for macular degeneration
  • Small molecule TKIs: sunitinib, sorafenib

Lymphangiogenesis

• 2007 PMID 17878481 -- "Role of lymphangiogenesis in cancer." (Sundar SS, J Clin Oncol. 2007 Sep 20;25(27):4298-307.)
  • Review