GC-t8-linked pyrrolobenzodiazepine (PBD)-biaryl conjugates with femptomolar in vitro cytotoxicity and in vivo antitumour activity in mouse models of pancreatic and breast cancer.

Proceedings: AACR 104th Annual Meeting 2013; Apr 6-10, 2013; Washington, DC As a class, minor-groove non-covalent DNA-binding small molecules generally have A/T rather than G/C selectivity, thought to be due to the narrower minor groove in A/T regions of DNA, and the presence of exocyclic guanine C2-amino groups which project into the minor groove and prevent multiple close van der Waals contacts in GC-rich regions. We have developed a set of biaryl building blocks based on phenyl-substituted heterocycles with significant GC-selectivity and sufficient length to span two DNA base pairs. These have been conjugated to DNA minor-groove covalent-binding pyrrolobenzodiazepine (PBD) molecules via a four-carbon linker to produce C8-linked PBD-MPB hybrid molecules. In particular, the 4-(1-methyl-1H-pyrrol-3-yl)benzenamine (MPB) biaryl motif either alone or conjugated to a PBD molecule has a strong preference for GC-rich sequences as demonstrated by the results of FID, HPLC-MS, FRET-based and DNA footprinting assays. Molecular modeling studies support these observations, suggesting that the high GC-affinity may be due to a combination of overall shape and the formation of key hydrogen bonds. Some PBD-MPB conjugates have sub-picomolar IC50 values in MCF7, A431, A2780, A549, MIA PaCa2 and MDA-MB-231 human tumour cell lines in vitro, while being up to six orders of magnitude less cytotoxic in the non-tumour cell line WI38, suggesting that key DNA sequences may be relevant targets in these ultra-sensitive cancer cell lines. One conjugate, which has femptomolar activity in the breast cancer cell line MDA-MB-231 (IC50 = 0.065 picomolar), has significant dose-dependent antitumour activity in MDA-MB-231 (breast) and MIA PaCa2 (pancreatic) human tumour xenografts in nude mice. It is well tolerated at concentrations up to 350 μg/kg, with no signs of toxicity at this dose level. Preliminary results based on cell culture and Western blotting experiments, and on histology studies on xenograft biopsies, have led to a suggested mechanism of action involving selective transcription factor inhibition, which is supported by molecular modeling studies. Citation Format: Khondaker M. Rahman, Paul J.M. Jackson, Colin H. James, B. Piku Basu, John A. Hartley, Maria de la Fuente, Andreas Schatzlein, Matthew Robson, Barbara Pedley, Chris Pepper, Keith R. Fox, Philip W. Howard, David E. Thurston. GC-t8-linked pyrrolobenzodiazepine (PBD)-biaryl conjugates with femptomolar i n vitro cytotoxicity and in vivo antitumour activity in mouse models of pancreatic and breast cancer. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1129. doi:10.1158/1538-7445.AM2013-1129