Novel Radiation Sensitizers Targeting Tissue Hypoxia

Introduction

Classic experiments performed in the early part of this century first established that the absence of oxygen diminished the lethal effects of radiation therapy. In general, under anaerobic conditions, the radiation dose must be increased by a factor of 2.5 to 3 to achieve the same degree of cytotoxicity that occurs under oxygenated conditions. The radiosensitivity of cells increases as the partial pressure of oxygen increases from 0 to 20 to 40 mmHg. Cells at oxygen tensions of 20 to 40 mmHg demonstrate radiosensitivities that are nearly equivalent to those of cells exposed to 100% oxygen. Therefore, increasing oxygen pressure beyond this minimal level is not necessarily beneficial.[1]

Hypoxic conditions are present in tumors and, based on experimental studies, hypoxia appears to be a major cause of treatment failure with radiation therapy and chemotherapy. In animal models, 10% to 20% of tumor cells are generally found to be hypoxic.[2,3] Direct oxygen measurements in human tumors have confirmed tumor hypoxia in glioblastoma multiforme and in carcinomas of the breast, uterine cervix, and head and neck.[2,3] Potential mechanisms of chronic or transient hypoxia include obstruction of blood flow, inadequate or defective (malignant) angiogenesis, and failure of cellular growth control, allowing the cell population to outstrip the capacity of the capillary blood supply. In general, tumor cells are oxygenated up to a distance of about 150 mm from capillaries; beyond this distance, tumor cells become oxygen-depleted and either die or survive in a hypoxic state.[4-8] Since hypoxic cells are substantially more resistant to radiation than are oxygenated cells, even a small hypoxic fraction in a tumor will dominate the overall response to radiation by increasing the probability that some viable tumor cells will survive the treatment. Conversely, few hypoxic cells exist in normal tissues. Therefore, therapies that increase the delivery of oxygen to hypoxic cells are not expected to increase the toxicity of radiation to normal tissues.

Several clinical studies have demonstrated that tumors with low median partial pressures of oxygen have a higher in-field failure rate after radiation therapy. For example, compared with well-oxygenated tumors of similar size and stage, tumors of the uterine cervix have been found to have a higher rate of recurrence if the median partial pressure of oxygen in tissue is < 10 mmHg.[9] A similar phenomenon has been noted in patients with head and neck cancer.[10]

In glioblastoma multiforme, a significant fraction of hypoxic cells may partially account for the poor clinical results with radiation therapy. In vitro, glioblastoma cells are as radiosensitive as cells from tumors that radiation can cure, and yet local recurrence of glioblastoma is universal. Therefore, the clinical radioresistance of glioblastoma in vivo has been postulated to result from factors in the tumor microenvironment, particularly hypoxia.[11] In addition, experiments with rat 9L gliosarcoma have demonstrated that tumor oxygen delivery is directly associated with the efficacy of fractionated radiation.[12]

Intraoperative oxygen measurements in patients with glioblastoma have confirmed significant tumor hypoxia. The median partial pressure of oxygen in the tumor was 7.4 mmHg; 25% of all recorded measurements were < 2.5 mmHg.[13] Although reoxygenation of hypoxic areas can occur during fractionated radiation therapy, this process does not eliminate the problem of hypoxia.[1,14,15] Measures to increase tumor oxygenation are thus worthy of evaluation, particularly in patients with glioblastoma multiforme.

Several therapeutic modalities intended to reduce tumor hypoxia in humans have been evaluated in preclinical and clinical trials, and the results of these investigations suggest that reducing the hypoxia fraction does improve the efficacy of radiation therapy. Hyperbaric oxygen in conjunction with radiation therapy resulted in greater local tumor control and longer survival compared with conventional therapy for squamous cell carcinoma of the head and neck and of the uterine cervix.[16-19] Treatment with a fluorocarbon emulsion and breathing carbogen (95% oxygen/5% carbon dioxide) during radiation therapy was associated with an enhanced response rate to radiation in patients with advanced squamous cell carcinoma of the head and neck.[20] In addition, a study of patients with high-grade brain tumors who received the fluorocarbon emulsion and breathed 100% oxygen during radiation therapy yielded an encouraging proportion of long-term survivors.[21]

Interestingly, these clinical studies have not demonstrated a significant increase in the normal tissue complications of radiation, although high rates of acute mucosal and skin reactions were noted. Each of these measures has suffered from limitations that have hampered its use in larger clinical trials where significant numbers of patients could be enrolled, however. For example, hyperbaric oxygen chambers are too expensive and cumbersome to use with radiation therapy, and effective doses of hyperbaric oxygen are toxic.

Radiation sensitizers mimic the effects of oxygen to increase radiation damage. The most common class of radiation sensitizer that has been evaluated in clinical studies is the nitroimidazoles (eg, misonidazole). However, their major limitation is neurotoxicity, which has prevented the delivery of effective doses with conventional daily fractionated radiation. One randomized trial suggested improved survival when the radiosensitizer nimorazole(Drug information on nimorazole) was used to treat head and neck cancer,[22] but thus far, radiation sensitizers have not led to consistent improvements in the therapeutic index compared with optimal fractionation schedules of radiation used alone.[23]

This report will consider four novel and highly specific therapeutics designed to increase oxygen delivery or sensitize hypoxic tissues during radiation therapy. These modalities, which are all in early clinical investigations, include: (1) tirapazamine, which is a hypoxia-selective cytotoxin that maintains its differential toxicity relative to aerobic cells at higher oxygen concentrations than do other bioreductive agents; (2) gadolinium texaphyrin, an “easy to reduce” metallotexaphyrin that is readily capable of “capturing” hydrated electrons and thus increasing the concentration of hydroxyl radicals available after exposure to a given dose of ionizing radiation; (3) RSR13, an allosteric modifier of human hemoglobin that facilitates oxygen unloading at low partial pressures of oxygen; and (4) bovine hemoglobin modified with polyethylene glycol (PEG) to enhance the oxygen-carrying capacity of human blood.