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