A direct in vitro assay for studying radiosensitization in mammalian cells

Double-stranded DNA in methanol-ethanol-buffer solvent system

DNA in a solvent system consisting of roughly equal volumes of methanol and ethanol and 5% buffer has a conservative circular dichroism (CD) spectrum of very low intensity above 220 nm and an increase of epsilon258 comparable to that of denatured DNA (about 40%). A direct comparison of this spectrum with the CD of single-stranded DNA reveals many differences, indicating DNA in this solvent system has a conformation different from that of denatured DNA. When the alcohols are removed, the B form conformation and normal epsilon258 are restored in native DNA, while single-stranded DNA remains denatured. A double-stranded structure of DNA in the methanol-ethanol-buffer solvent system is confirmed by the neutral cesium chloride density gradient centrifugation of DNA in which one chain is labeled with [14C]thymidine and the other [3H]5-bromodeoxyuridine. The doubly labeled DNA exposed to the alcohol solvent system has a centrifugal pattern identical with that of control DNA; the two radioactivities cosediment and form a superimposing band, distinctly different from that of single-stranded DNA; 3H-labeled (thymidine) chains sediment further than 14C-labeled chains (5-bromo-deoxyuridine). Denatured DNA exhibits varying CD spectra depending on solvents. It is suggested that single-stranded DNA in different solvent systems assumes different modes of base stacking.

Radiosensitization of a human cell line lacking repair replication

A human cell line has been isolated that is deficient in X-ray-induced repair replication. The slope of the radiation survival curve is greater in the presence of the anthracycline antibiotic, adriamycin. The results suggest that radiosensitization by DNA-binding drugs is more probably related to DNA strand breakage phenomena than to the polymerase step of excision-repair.

Combination Adriamycin and radiation therapy in gynecologic cancers

Anthracyclic antibiotics, of which Adriamycin is representative, have the ability to bind to cellular DNA and thereby interfere with the X-ray repair process. When radiation survival curves of tissue cultures were studied, increased cell-killing was noted in those cultures with Adriamycin over those without the drug. The mechanism by which this occurs may be related to a reduced rate of DNA strand break rejoining, as demonstrated by use of alkaline sucrose gradient techniques. A preliminary clinical Phase I study, in which patients with advanced gynecologic malignancy were treated by simultaneous Adriamycin and X radiation, suggests that combined therapy is well-tolerated, and that such combinations may prove useful in selected patients.