The cellular basis of radiotherapeutic response

Interfacial properties as predictors of radioresistance in cervical cancer

The prediction of radioresistance of tumours, early in the course of radiotherapy, may help clinicians in deciding the optimal treatment strategy for each case. This study was carried out to investigate an in vitro technique to predict radiosensitivity, after a single radiation dose of 2 Gy in cervical cancer. Langmuir films of tissue homogenates of biopsy samples from 20 cervical cancer patients treated with radiotherapy alone and 15 normal controls were evaluated. The tensiometric profiles before and after giving 2 Gy of radiation, were compared with that of controls and were correlated with the clinical radioresponsiveness evaluated on completion of the radiotherapy course of 70-78 Gy over a period of 50-55 days. The tensiometric profiles measured after a single dose of radiation can be used to fingerprint the clinical radioresponsiveness of the cervical cancer tissues. The hysteresis of the monolayers of completely radioresponsive post-radiotherapy tissue homogenates was 5.8 times greater than that of partially radioresponsive post-radiotherapy tissue homogenates and was statistically significant using Mann-Whitney test (p<0.05). From our results, the following tensiometric criteria for prediction of radioresistance emerge. After first dose of radiation, if the minimum surface tension of tissue homogenate is greater than 50 mN/m and hysteresis area is less than 20 microJ those tissues will be in the partially radioresponsive and for completely radioresponsive tissue homogenates, the minimum surface tension will be less than 47 mN/m and the hysteresis area will be greater than 33 microJ. The cholesterol and phospholipid content of radioresponsive cervical cancerous tissues after radiotherapy was found to be 1.2 and 2.2 times lower than that of the untreated tissues and due to lower lipid content organic phase surface activity of radioresponsive cancerous tissues after radiotherapy was less than that of the untreated tissue organic phase. The radiation induced tensiometric profile changes of radioresponsive cervical cancerous tissues can be correlated to the radiation induced lipid profile changes. This technique, due to its simplicity and high precision, can serve as a predictive tool for radioresponsiveness and is easily translatable to the clinical setting. Randomized large sample trials are necessary to validate this technique further and help in the translation from bench to clinics.

Measuring hypoxia and predicting tumor radioresistance with nuclear medicine assays

Tumor cells at low oxygen tension are relatively radioresistant. The hypoxic fraction of individual tumors before, during and after radiotherapy is likely to have prognostic value but its diagnosis still awaits an accurate and acceptable assay. The recent indications that hypoxia can also induce the expression of specific genes and promote a more aggressive tumor phenotype makes its diagnosis even more important. Over 15 years ago, misonidazole, an azomycin-based hypoxic cell radiosensitizer, was found to link covalently to cellular molecules at rates inversely proportional to intracellular oxygen concentration. The use of bioreducible markers to positively label zones of viable hypoxic cells within solid tumors and to predict for tumor radioresistance was proposed. Several hypoxic markers have now been identified and their selective binding within tumors has been measured by both invasive and non-invasive assays. Research from our laboratory has emphasized both mechanistic and preclinical studies associated with nuclear medicine procedures for measuring tumor hypoxia and predicting tumor radioresistance. This report updates radiation oncologists about the status of nuclear medicine hypoxic marker research and development as of mid-1997. While several potential imaging agents have been identified, their testing and validation in appropriate human tumors will require focused research efforts by individual academic departments and, possibly, by clinical trials performed through cooperative groups. Since the prediction of hypoxia in individual tumors could strongly impact radiotherapy treatment planning, the radiation oncology research community is best positioned to execute the validation studies associated with these markers.

Metabolic binding of misonidazole to mouse tissues. Comparison between labels on the ring and side chain, and the production of tritiated water

The 2-nitroimidazole, misonidazole, is of current interest as an imaging agent for hypoxic regions in tumors and in vascular disease such as stroke. The basis of this technique is the reductive activation and binding of nitroheterocycles which is much more efficient in the absence of oxygen. The appropriate molecular location for an active isotope on the nitroheterocyclic probe depends on the nature of the metabolites retained in tissues after the parent drug has been cleared. Previous studies with tumor cells in vitro indicated that a ring label (2-14C) and a side-chain label (3H) were retained equally efficiently in the acid-insoluble fraction, whereas 1.5 to 3 times more side-chain label was retained in the total pool (acid soluble plus acid insoluble) of metabolites in several normal murine tissues. We show here that the excess side-chain label in six normal tissues, plasma and EMT6 tumors was found entirely in the acid-soluble fraction as a volatile component. This volatile component was tentatively identified as tritiated water. It appeared that, in general, molecular products of misonidazole metabolism were retained in mouse tissues, with the exceptions that a small excess of ring label was found in liver and heart and that tritiated water appeared in the acid-soluble fraction of all tissues. Tritiated water would not be important in imaging studies but could be a factor in studies in which scintillation counting of tritiated nitroheterocyles is used.

Response of human tumor cell lines in vitro to fractionated irradiation

The surviving fraction of human tumor cell lines after 2 Gy (SF2) varies between 0.1 and 0.8. It has been postulated that differences in inherent radiosensitivity of tumor cells are a major determinant of radiation response in vivo. Assays of inherent radiosensitivity based on acute survival are being developed as predictors of tumor response which often assume that the same inherent radiosensitivity persists throughout a fractionated treatment. We have investigated the response of 2 human tumor cell lines (A549 and MCF7) with different inherent radiosensitivities to in vitro fractionated irradiation. A549 cells had an SF2 of 0.62 and a mean inactivation dose (D) of 3.07 Gy whereas MCF7 cells had an SF2 of 0.30 and a D of 1.52 Gy. Split dose repair capacity (at equal survival levels) was less for A549 than for MCF7 cells and recovery kinetics for both cell lines were substantially longer than those of rodent cell lines. Survival after 5 fractions of 2 Gy given 12 hr apart at 37 degrees C was near to that predicted from the acute survival curve, assuming complete repair and no proliferation. Acute survival of A549 cells which survived 5 fractions of 2 Gy given 12 hr apart was similar to the acute survival of unirradiated cells. When A549 cells were incubated at 22 degrees C between 5 fractions of 2 Gy given 12 hr apart, proliferation and split dose repair were substantially inhibited. These studies support the proposals to use in vitro inherent radiosensitivity assays for the prediction of in vivo response of tumors to fractionated treatment.

Covalent binding of a fluorinated 2-nitroimidazole to EMT-6 tumors in Balb/C mice: detection by F-19 nuclear magnetic resonance at 2.35 T

A hexafluorinated 2-nitroimidazole (CCI-103F) has been synthesized and its properties as a hypoxic cell binder investigated. The drug has a plasma half-life of 90 min in Balb/C mice and a tumor-to-plasma ratio of 0.8. Following sustained exposure to the drug and a washout period for the unbound drug, liver and tumor samples were excised. NMR investigation of the excised tissue in a 2.35 T animal sized NMR facility revealed the presence of 19F bound to both tissues. Further improvement in sensitivity is required to make 19F NMR detection of binding in situ feasible.