Sensitive detection of DNA modifications induced by cisplatin and carboplatin in vitro and in vivo using a monoclonal antibody.

An assay that is based upon a monoclonal antibody (ICR4) is described that enables the quantitation of cisplatin-induced adducts on DNA down to 3 nmol Pt/g DNA (i.e., 1 Pt adduct/10(6) bases), the level necessary to produce toxic effects in cells in vitro and in vivo, using just a few micrograms of DNA. Detection is possible below this level (although probably not necessary for in vivo studies) but the cross-reactivity of unmodified DNA sequences complicates absolute quantitation of adducts. Therefore, it will be possible to investigate the distribution of clinically useful platinum drugs in patients undergoing chemotherapy. Rats of strain F344 appeared to be the best, among several tested, for the production of antibodies to modified DNA, and they were used for the production of hybridomas. Fifteen hybridomas which secreted antibodies that bound to DNA that was highly modified with cisplatin but not to normal DNA were obtained. One (ICR4) was chosen for further characterization because of its relatively strong binding to DNA modified to a moderate level with cisplatin. The characterization included the development of a sensitive competitive enzyme-linked immunoabsorbent assay and the use of DNA that had been reacted with cisplatin both in vitro and in vivo. The levels of platination of both types of DNA samples were determined by atomic absorbance spectroscopy. For DNA that had been exposed to cisplatin in vitro, 50% inhibition of antibody binding was caused by about 15 fmol of total DNA-bound Pt/assay well. At moderate levels of platination, heating of the DNA solution at 100 degrees C for 5 min increased its immunoreactivity such that 50% inhibition was caused by 2.5 fmol Pt adducts/well. Pt adducts on DNA extracted from cells that had been treated with cisplatin were less immunoreactive than DNA treated with cisplatin in vitro, but after heating the immunoreactivity increased such that 50% inhibition in the assay was caused by 2 fmol Pt adduct/well. This sensitivity was invariant over a wide range of levels of platinum adduct frequency. DNA adducts formed by the second generation anticancer drug carboplatin were recognized similarly to the adducts formed by cisplatin, but those formed by the clinically inactive trans-diamminedichloroplatinum(II) or chloro(diethylenetriamine)-platinum(II)-chloride were not significantly immunoreactive. Control DNA cross-reacted in the competitive assay but the immunoreactivity per mol base was 10(7) times lower than the immunoreactivity of cisplatin adducts.