Definition and manipulation of tumor oxygenation

[Volume-effect in radiation therapy part one: volume-effect and tumour]

Volume is an important parameter of radiation therapy. Local control is inversely related to tumor size and the complication rate increases with the importance of the irradiated volume. Although the effect of irradiated volume has been widely reported since the beginning of radiotherapy, it has been less studied than other radiation parameters such as dose, fractionation, or treatment duration. One of the first organ system in which the adverse effect of increased volume was well defined is the skin. Over the last twenty years, numerous mathematical models have been developed for different organs. In this report we will discuss the relation between irradiated volume and tumor control. In a second article we will study the impact of irradiated volume on radiation adverse effects.

Dynamics of tumor oxygenation, CD31 staining and transforming growth factor-beta levels after treatment with radiation or cyclophosphamide in the rat 13762 mammary carcinoma

PURPOSE

Tumors are dynamic tissues that undergo marked molecular, biochemical, and physiologic changes in response to cytotoxic anticancer therapies. Understanding the changes in tumor oxygenation and transforming growth factor-beta expression may allow improved treatment regimens to be developed.

METHODS AND MATERIALS

The effects of a single dose of radiation therapy (20 Gy) or a single dose of chemotherapy (cyclophosphamide, 250 mg/kg) on several molecular and physiologic parameters of the rat 13762 mammary carcinoma growing subcutaneously in female Fischer 344 rats were explored.

RESULTS

Treatment of the tumor-bearing animals with 20 Gy of radiation killed about two logs (99%) of the 13762 tumor cells, and treatment with cyclophosphamide (250 mg/kg) killed about 1.5 logs (95%) of the 13762 tumor cells. Hypoxia, as determined by a pO2 electrode, initially decreased in the tumors of treated animals until 6 h. posttreatment and then increased, so that 24 h. after administration of the radiation therapy or the chemotherapy the number of intratumoral vessels as determined by CD31 staining increased until about 24 h after cytotoxic therapy. Transforming growth factor-beta1, measured by radioimmunoassay, peaked in the serum between 6 h and 18 h and again between 72 h and 96 h after radiation therapy and peaked in the tumor at 24 h and again at 72 h after radiation therapy. The first serum peak after cyclophosphamide was 3 h after drug injection, with second peaks at 36 h and 48 h after drug administration. In the tumor, transforming growth factor-beta1 peaked between 6 h and 8 h after drug administration and again 36 h and 72 h after drug. Apoptosis was maximal 6 h after 20 Gy and 24 h after cyclophosphamide. Vascular endothelial growth factor was also increased in tumors after cytotoxic therapy.

CONCLUSIONS

These changes in the tumor physiologic status are sufficient to protect the tumor from a second cytotoxic insult administered days afterwards and to result in a restructuring of the tissue.

The effect of tumor size on necrosis and polarographically measured pO2

Tumor necrosis and oxygen status were investigated as a function of tumor size in three syngeneic murine carcinomas, MCa-4, OCa-I, and SCC-VII, in C3Hf/Kam mice. Tumor necrosis was estimated histologically, and tumor oxygenation determined by direct polarographic histography. As tumor volume increased necrosis increased significantly in all three tumor types (p < 0.001). Similarly, as tumor volume increased from 200 to 1400 mm3, hypoxia, defined as the percentage of measured pO2 values < or = 5.0 mm Hg, increased from 55.1% to 95.9%, 70.3% to 81.4%, and 56.8% to 98.5% in MCa-4, OCa-I, and SCC-VII tumors respectively (p < 0.001). Correcting pO2 for necrosis reduced the tumor size dependence of measured tumor hypoxia in all three tumor types but in no case was the reduction significant. The main effect of correction was to shift the fitted curves of percent pO2 values < or = 5.0 mm Hg down toward lower percentages for all tumors. This change was significant for MCa-4 and OCa-1 tumors (p < 0.001), but not for SCC-VII (p = 0.054). Defining the influence of variables such as necrosis that affect polarographic assessment of tumor oxygenation is important to enhance the technique's reliability and prospect as an investigative and predictive tool.

Influence of an anti-angiogenic treatment on 9L gliosarcoma: oxygenation and response to cytotoxic therapy

Tissue oxygen tensions were measured in subcutaneously growing rat 9L gliosarcoma under normal air and carbogen breathing conditions prior to and after i.v. administration of a perflubron emulsion. When these animals were treated with the anti-angiogenic agents TNP-470 and minocycline for 5 days prior to oxygen measurement, tumor hypoxia was decreased compared with untreated tumors. Hypoxia, defined as the percent of pO2 readings < or = 5 mm Hg, was decreased from 71% in untreated air-breathing controls to 34% in animals treated with the anti-angiogenic agents, the perflubron emulsion and carbogen breathing. These effects were manifest in the increased response of the tumor to single-dose (10, 20 and 30 Gy) radiation therapy. Twenty-four hours after treatment with BCNU oxygenation of the tumors was not altered; however, 24 hr after administration of adriamycin oxygenation of the tumors was increased such that hypoxia in adriamycin-treated tumors in animals receiving the perflubron emulsion and carbogen was reduced to 21%. Tumor growth delay in the s.c. tumors was increased by the addition of treatment with the anti-angiogenic agents from day 4 through day 18 post-tumor cell implantation along with BCNU or adriamycin on days 7-11. Administration of the perflubron emulsion and carbogen breathing resulted in increased tumor growth delay with the chemotherapeutic agents alone and in combination with the anti-angiogenic agents. Life span in animals bearing intracranially implanted 9L gliosarcoma progressively increased with administration of the anti-angiogenic agents and then the anti-angiogenic agents and perflubron emulsion/carbogen compared to treatment with BCNU or adriamycin.

Potentiation of cytotoxic therapies by TNP-470 and minocycline in mice bearing EMT-6 mammary carcinoma

The ability of the antiangiogenic agents TNP-470 and minocycline, singly or in combination, to potentiate the antitumor effects of several cytotoxic therapies was assessed in the murine EMT-6 mammary carcinoma as well as in two drug resistant sublines of that tumor designated EMT-6/CTX and EMT-6/CDDP. The antiangiogenic agents alone or in combination did not alter the growth of the tumors. However, their administration along with cyclophosphamide, CDDP, or thiotepa substantially increased the tumor growth delay produced by these cytotoxic therapies in tumors responsive to the drugs--the increase was about 2-fold for TNP-470 and minocycline together. In drug resistant tumors, treatment with the antiangiogenic agents did not reverse drug resistance but did increase the effect of the cytotoxic drugs. Treatment with TNP-470/minocycline also increased the oxygenation of each of the three tumors. Thus, TNP-470/minocycline administration increased the efficacy of fractionated radiation therapy, especially when used along with a perflubron emulsion oxygen delivery agent/carbogen. These results indicate that treatment regimens including therapies directed toward the proliferating normal cells within a tumor mass as well as therapies directed toward the malignant cells can produce improved outcomes.

Hypoxia and drug resistance

Biologically and therapeutically important hypoxia occurs in many solid tumor masses. Hypoxia can be a direct cause of therapeutic resistance because some drugs and radiation require oxygen to be maximally cytotoxic. Cellular metabolism is altered under hypoxic conditions. Hypoxia can result in drug resistance indirectly if under this condition cells more effectively detoxify the drug molecules. Finally, there is evidence that hypoxia can enhance genetic instability in tumor cells thus allowing more rapid development of drug resistance cells. The current review describes the effects of hypoxia on tumor response to a variety of anti-cancer agents and also describes progress toward therapeutically useful methods of delivering oxygen to tumors in an effort to overcome therapeutic resistance due to hypoxia. Finally, the use of hypoxic cell selective cytotoxic agents as a means of addressing hypoxic 'drug resistance' is discussed.