Abstract #5359: Synthesis, stereochemical characterization and biological evaluation of C2-aryl pyrrolo[2,1-c][1,4]benzodiazepine (PBD) C11-(sodium bisulphite) addition products

The chemistry of pyrrolo[2,1-c][1,4]benzodiazepines (PBDs) has been extensively investigated due to their potential in the treatment of cancer. The PBDs are produced as secondary metabolites by Streptomyces species and are isolated in their monomeric form (e.g., anthramycin). Many semi- and fully-synthetic PBDs have been produced, and the fully-synthetic PBD dimer SG-2000 (SJG-136) will move into Phase II clinical trials in 2009. The mechanism of action of the PBDs involves binding in the minor groove of DNA followed by nucleophilic attack of a guanine C2-NH2 on the electrophilic N10-C11 imine moiety, thus forming a covalent aminal bond with the molecule spanning three DNA base pairs. We have previously reported that insertion of an aryl group at the C2-position of monomeric PBDs can dramatically increase their DNA-binding affinity and cytotoxicity by enhancing hydrophobic contacts within the DNA minor groove (Biochemistry, 47(45), p11818, 2008). However, one disadvantage of adding C2-aryl substituents to the PBD core is that water solubility can be adversely affected. To maintain and improve the water solubility of compounds of this type, we have investigated the formation of C11-(sodium bisulphite) addition products. A small library (6 members) of C2-aryl PBD C11-(sodium bisulphite) addition products has been synthesized, and the structures and stereochemical characteristics of members studied using 1D and 2D high-field (500MHz) NMR techniques. Interestingly, we discovered that the C11-stereochemistry of library members could be influenced by both solvent composition and by controlling the time of reaction of the parent N10-C11 PBD imine with sodium bisulphite. For example, quenching the reaction after just one minute afforded mainly the (S)-C11-(sodium bisulphite) addition product, while a longer reaction time (i.e., 80 minutes) gave predominantly the (R)-C11 form. These two stereochemical forms were identified as the kinetic (S) and thermodynamic (R) diastereomers, respectively. As anticipated, all library members were significantly more water soluble than their PBD N10-C11 imine precursors. However, with the exception of the C2-naphthyl (more active) and C2-quinolinyl (equipotent) substituted molecules, all other C2-aryl PBD C11-(sodium bisulphite) addition products were less cytotoxic in both the NCI 60 cell-line panel (LC50 range = 2.6-12.0 µM) and a K562 leukaemia cell line (IC50 range =1.2-70.0 nM) compared to the parent N10-C11 imine PBDs from which they were formed (NCI 60 cell-line: LC50 range = 0.2-5.1 µM; K562: LC50 range = 0.4-48.0 nM). Studies are presently underway to determine the precise mechanism by which these C11-(sodium bisulphite) addition products react with DNA, with a focus on establishing whether initial hydrolysis to the N10-C11 carbinolamine species occurs in biological media. Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 5359.