Tumor radiosensitization through reductions in hemoglobin affinity

Alterations in the oxygen (O2) distribution in a tumor due to changes in the quantity of O2 carried in the blood can affect the response of a tumor to radiation. For example, the blood hemoglobin (Hb) level has been shown to be an important prognostic and therapeutic factor in radiation therapy. Another factor affecting the delivery of O2 to tissues is the Hb affinity for O2. Changes in Hb affinity for O2 result in shifts of the Hb-O2 dissociation curve which increase or decrease tissue oxygenation. The aim of the present studies was to determine whether reductions in Hb affinity prior to irradiation could improve the resultant tumor response. KHT sarcomas were irradiated in female C3H/HeJ mice, possessing either normal or reduced Hb affinities for O2 at the time of treatment. Changes in Hb affinity for O2 were induced by keeping tumor-bearing mice in a 12% O2 environment for various periods of time. Erythrocyte 2,3-diphosphoglycerate (2,3 DPG) was measured as an indicator of Hb affinity for O2. After 36 hr of low O2 exposure, 2,3 DPG levels increased 20-30%. This change in 2,3 DPG reflected a proportional decrease in Hb affinity for O2. Following the exposure of 12% O2, the animals were removed from the low O2 chamber and their tumors locally irradiated while the mice breathed air. After irradiation, tumor cell survival was determined using an in vivo to in vitro excision assay. The results indicate that the fraction of hypoxic cells in tumors of mice whose Hb affinity had been reduced prior to irradiation was approximately 3%. By comparison, the hypoxic fraction in tumors irradiated in mice with normal Hb affinities was approximately 15%. Thus, reductions in Hb affinity prior to irradiation can yield significant radiation sensitization in tumors. These findings form the basis for future investigations of the use of pharmacologic methods for the in vivo alteration of the Hb-O2 dissociation curve to improve tumor oxygenation.