Radiation Oncology/Radiobiology/Hypoxia

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 ( 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