Reprogramming the Mechanism of Action of Chlorambucil by Coupling to a GQuadruplex Ligand

Abstract

The nitrogen mustard Chlorambucil (Chl) generates covalent adducts with double-helical DNA and inhibits cell proliferation. Among these adducts inter-strand crosslinks (ICLs) are the most toxic, as they stall replication by generating DNA double strand breaks (DSBs). Intra-strand crosslinks generated by Chl are repaired by a dedicated Nucleotide Excision Repair (NER) enzyme. We synthesized a novel crosslinking agent that combines Chl with the G-quadruplex (G4) ligand PDS (PDS-Chl). We demonstrated that PDS-Chl alkylates G4 structures at low M doses, without reac-tivity towards double- or single-stranded DNA. Since intra-molecular G4s arise from a single DNA strand, we reasoned that preferential alkylation of such structures might prevent the generation of ICLs, whilst favoring intra-strand crosslinks. We observed that PDS-Chl selec-tively impairs growth in cells genetically deficient in NER, but did not show any sensitivity to the repair gene BRCA2, involved in double-stranded break repair. Our findings suggest that G4 targeting of this clinically im-portant alkylating agent alters the overall mechanism of action. These insights may inspire new opportunities for intervention in diseases specifically characterised by genetic impairment of NER, such as skin and testicular cancers.