Received for publication January 24, 2008.
Revised April 14, 2008.
Accepted for publication April 15, 2008.

Formaldehyde-activated pixantrone is a monofunctional DNA alkylator that binds selectively to CpG and CpA doublets

Abstract

The topoisomerase II poison mitoxantrone is important in the clinical management of human malignancies. Recently, it was established that pixantrone, a novel aza-anthracenedione developed to improve the therapeutic profile of mitoxantrone, can efficiently alkylate DNA following formaldehyde activation. In vitro transcriptional analysis has now established that formaldehyde-activated pixantrone generates covalent adducts selectively at discrete CpG or CpA dinucleotides, suggesting that the activated complex binds to guanine or cytosine (or both) bases. The stability of pixantrone adduct-induced transcriptional blockages varied considerably, reflecting a mixture of distinct pixantrone adduct types that may include relatively labile monoadducts and more stable interstrand crosslinks. The dimethyl N-substituted analogue of pixantrone BBR 2378 could not form adducts, suggesting that pixantrone alkylates DNA through the primary amino functions located in each side-chain of the drug. Pixantrone generated DNA adducts only when guanine was present in substrates and exhibited a lack of adduct formation with inosine-containing polynucleotides, confirming that the N2 amino group of guanine is the site for covalent attachment of the drug. Mass spectrometric analysis of oligonucleotide-drug complexes confirmed that formation of covalent pixantrone-DNA adducts is mediated by a single methylene linkage provided by formaldehyde, and that this occurs only with guanine-containing double stranded oligonucleotide substrates. CpG methylation, an epigenetic modification of the mammalian genome, significantly enhanced the generation of pixantrone-DNA adducts within a methylated DNA substrate, indicating that the methylated dinucleotide may be a favoured target in a cellular environment.

Key words: Mass Spectroscopy, DNA damage and repair, Pharmacokinetics, metabolism and activation, DNA intercalation