DNA interstrand cross-links induced by the cyclopropylpyrroloindole antitumor agent bizelesin are reversible upon exposure to alkali.

Bizelesin, a cyclopropylpyrroloindole (CPI) antitumor agent, has been shown to alkylate and cross-link DNA within A/T-rich tracts. Previous studies have shown that covalent reaction of the CPI adozelesin with DNA was reversible [Warpehoski, M. A., Harper, D. E., Mitchell, M. A., & Monroe, T. J. (1992) Biochemistry 31, 2502-2508]. That is, the monofunctional adduct could be lost from DNA, thus restoring the fidelity of DNA. In this study, we demonstrate that covalent DNA adducts induced by bizelesin at the adenine N3 position undergo two subsequent competing reactions: one which causes DNA strand cleavage, via depurination, and one which proceeds through loss of the DNA adduct (adduct reversal with restoration of DNA integrity). Our results were obtained by studying the chemical stability of synthetic DNA oligonucleotides which contained either a distinct DNA monofunctional adduct or DNA interstrand cross-links. Quantification of adduct reversal was performed on the basis that drug-modified DNA, upon exposure to heat followed by hot piperidine treatment, was resistant to strand cleavage at the site of alkylation. The rate of adduct reversal was found to increase with increasing temperature and was found to be maximum at 70-80 degrees C. The rate of adduct reversal was also found to increase with increasing pH and ionic strength. In contrast, the rate of depurination and subsequent DNA strand cleavage decreased as pH and ionic strength were increased. Adduct reversal was favored in DNA containing interstrand cross-links, whereas rapid depurination occurred preferentially within monofunctionally alkylated DNA.