Abstract 2064: Tumor cells treated with a PBD-based antibody-drug conjugate targeting 5T4 develop acquired resistance due to 5T4 down-regulation and response to alternate therapeutics
Authors: Mao Shenlan , Garcia Andrew , Chen Tracy , Chaerkady Raghothama , Marreno Allison , Zhong Helen , Toader Dorin , Dimasi Nazzareno , Kennedy Maureen , Howard Philip , Gao Changshou , Wu Herren , Herbst Ronald , Tice David , Harper Jay 
Published: July 2017

Abstract
Resistance to antibody-drug conjugates (ADC) has been observed in both preclinical models and in the clinic. It has been previously reported that an ADC targeting CD33 with a DNA cross-linking pyrrolobenzodiazepine (PBD) dimer was active in models of multidrug-resistant (MDR) AML, suggesting that PBDs, delivered as part of a molecular-targeted cancer therapy may be effective in MDR settings. However it could still be possible to develop resistance to PBD-conjugated ADCs though other mechanisms that remain unknown. Studies were conducted to determine if tumor cells have innate resistance to PBDs or an anti-5T4 ADC delivering PBD warhead. It was found that both the ADC and its PBD warhead, SG3199, completely eradicated MDA-MB-361 breast cancer cells and NCI-N87 gastric cancer cells in culture, suggesting a lack of intrinsic resistance in these cell lines. The potential emergence of acquired resistance was investigated by culturing MDA-MB-361 cells in gradually-increasing concentrations of either the ADC or SG3199 for over a year and the variants resistant to either the ADC (361-ADCr) or the PBD warhead (361-PBDr) were established as determined by decreased sensitivity in cytotoxicity assays. Transcriptional profiling characterization of these two cell models demonstrated that neither of them had significant upregulation of P-glycoprotein (P-gp) or other drug efflux transporter genes. Significant 5T4 antigen down-regulation was observed in the 361-ADCr cells but not the 361-PBDr cells and these cells also remained sensitive to alternate treatments including ADCs targeting a different surface antigen with the same PBD warhead, suggesting that down-regulation of target was likely the main driver of resistance in 361-ADCr cells. Further transcriptional profiling analyses conducted on 361-PBDr cells suggest a multi-factorial process may be needed to develop resistance to PBD warheads as differential expression of genes associated with DNA damage and repair, drug metabolism and cell cycle was observed. Similar gene expression profiles were observed in both cultured cells and in tumors derived from these cells. Studies to validate these differentially expressed genes, including proteomic and phosphoproteomic profiling are currently being conducted. Collectively these data could identify how resistance to ADCs conjugated with PBD payloads may develop in the clinic and more importantly, provide insight into developing strategies to overcome such resistance.
Citation Format: Shenlan Mao, Andrew Garcia, Tracy Chen, Raghothama Chaerkady, Allison Marreno, Helen Zhong, Dorin Toader, Nazzareno Dimasi, Maureen Kennedy, Philip Howard, Changshou Gao, Herren Wu, Ronald Herbst, David Tice, Jay Harper. Tumor cells treated with a PBD-based antibody-drug conjugate targeting 5T4 develop acquired resistance due to 5T4 down-regulation and response to alternate therapeutics [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 2064. doi:10.1158/1538-7445.AM2017-2064