Abstract 5242: Further evidence that the DNA-interactive Pyrrolobenzodiazepine (PBD) Dimer SJG-136 works through a transcription factor inhibition mechanism
Authors: Mantaj Julia , Jackson Paul J. , Thurston David E. , Rahman Khondaker Miraz 
Published: July 2017

The pyrrolo[2,1-c][1,4]benzodiazepines (PBDs) are sequence-selective DNA minor-groove interacting agents. The PBD dimer SJG-136 has currently been investigated in Phase II clinical trials in ovarian cancer and leukaemia in the UK and the USA. More recently, PBD dimer analogues are being attached to tumour-targeting antibodies to create Antibody-Drug Conjugates (ADCs), some which are now in Phase 3 clinical trials with many others in pre-clinical and clinical development. Transcription factors (TFs) are sequence-specific DNA-interacting proteins that bind to consensus DNA sequences, thereby controlling transcription. TFs regulate processes such as cell differentiation, proliferation and apoptosis. The interaction of a small-molecule with the consensus DNA recognition sequences of TFs can prevent a TF from interacting with its cognate sequence, thereby inhibiting the expression of genes critical for the survival and proliferation of cancer cells. There is growing evidence that PBDs may exert, at least in part, their pharmacological effect through TF inhibition in addition to the arrest of the replication fork, DNA strand breakage, and inhibition of enzymes including endonucleases and RNA polymerases. For this reason, there is now interest in using PBDs as the basis for a small-molecule strategy to target specific DNA sequences for TF inhibition as a novel anticancer therapy. We have developed a reversed-phase HPLC/MS method as a tool to evaluate the interaction of DNA-binding PBD molecules with oligonucleotides of varying lengths and sequences. Using this methodology, we have demonstrated that the PBD dimer SJG-136 binds to the cognate sequences of the oncogenic transcription factors NF-κB, EGR-1, AP-1 and STAT3. Surprisingly, significant differences in the rate and extent of adduct formation between the different cognate sequences were observed which may explain, at least in part, the differences in potency of SJG-136 in various tumour cell lines. Furthermore, an RT PCR study has been carried out using the human tumour cell lines MDA-MB-231 (breast) and HT-29 (colon) to see whether these transcription factors are affected in vitro. The results were consistent with the HPLC-MS studies in that SJG-136 was shown to significantly down-regulate a number of AP-1- and STAT3-dependent genes such as Bcl-2, VEGF, p53 and survivin. These findings add significantly to knowledge of the mechanism of action of SJG-136 and related PBD compounds, and could be relevant for the correct interpretation of clinical activity of molecules of this type both as standalone agents and as ADC payloads.

Citation Format: Julia Mantaj, Paul J. Jackson, David E. Thurston, Khondaker Miraz Rahman. Further evidence that the DNA-interactive Pyrrolobenzodiazepine (PBD) Dimer SJG-136 works through a transcription factor inhibition mechanism [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5242. doi:10.1158/1538-7445.AM2017-5242