Understanding the mechanisms of action of pyrrolobenzodiazepine dimer-based antibody-drug conjugates

DNA interstrand cross-linking (ICL) Pyrrolobenzodiazepine (PBD) dimers are being investigated clinically as warheads for antibody-drug conjugates (ADCs). Understanding their molecular mechanisms of action will help inform the clinical application and future development of this class of targeted anti-cancer agents. PBD dimer-containing ADCs, targeting CD19 or CD22, were investigated in vitro by comparing cell-surface receptor expression, binding and internalisation kinetics, ICL formation and how these correlated with growth inhibition and apoptosis. The effect of mouse strain immunodeficiency on antibody biodistribution and ADC efficacy was investigated using in vivo imaging, and the contribution of the tumour microenvironment on protease-cleavable ADC efficacy was explored by depletion of tumour-associated macrophages (TAMs), or replacing the cleavable linker. Cells with acquired resistance to PBD dimer or PBD dimer-delivering ADCs were generated using sub-lethal, pulsed treatments. The mechanisms of resistance were investigated by measuring DNA ICLs, transporter gene expression, and antibody internalisation. Combinations of ADC and conventional drugs were studied for cytotoxic synergy and effects on antigen expression and DNA ICL formation. The anti-CD19 ADC was more cytotoxic than the anti-CD22 ADC, and growth inhibition correlated with DNA ICL formation, but not cell surface target antigen expression. ADCs were sequestered in non-target organs in severely immunodeficient mouse strains, reducing in vivo efficacy. Target-independent interaction of protease-cleavable ADCs with TAMs in the tumour microenvironment contributed to the anti-tumour response. Specific ABC transporters were upregulated in all cell lines with acquired resistance to either ADC or PBD, while antigen expression and internalisation were unaffected. Inhibitors or siRNA knockdown of ABCC2 and ABCG2 were able to restore sensitivity. Combinations of ADC with chemotherapeutic drugs that increase target antigen expression or hypomethylate DNA were synergistic in vitro. These data include a number of potentially clinically relevant findings that could influence treatment strategy, as well as future ADC design and pre-clinical development