Abstract 4023: The SABRE platform: A novel, unbiased technology for drug discovery and prioritization
Authors: Morris Shelli M. , Mhyre Andrew J. , Carmack Savanna S. , Burns Connor , Ferrer Marc , Ye Wenjuan , Olson James M. , Klinghoffer Richard A. 
Published: July 2017

Abstract
Targeted therapies designed to inhibit hyperactive oncogenic signaling have demonstrated some encouraging clinical responses. However, most of the time these responses are not durable as tumors “work around” pathway inhibition, often reactivating the inhibited pathway, leading to drug resistance and disease relapse. New technologies that enable more efficient discovery and development of “suites” of drugs that comprehensively inhibit multiple pathway nodes are needed. The SABRE (Splice Acceptor Brilliant Reporter) platform was developed to provide an unbiased method to screen large compound libraries and dramatically improve the speed and efficiency of which novel targeted therapeutics can be identified. SABRE is built off the premise that oncogenic output is exerted through changes in gene transcription and that such changes can be harnessed as powerful reporters of pathway activation status. SABRE utilizes the power of gene trap technology coupled with a drug selection process to isolate cells that generate a robust “off to on” signal in response to target or pathway inhibition. Via massively parallel comparative analysis, multiple traps let nature provide the best reporter for further analysis and drug discovery. In this proof-of-concept study, we employed the SABRE technology to identify insertion sites that are specifically regulated by the MAPK pathway. Experiments were performed using the human BRAFV600E mutant melanoma cell line A375. SABRE lentiviral transduced A375 cells were treated with trametinib, a MEK inhibitor, and clones were isolated that emitted a positive luciferase signal upon drug treatment. To determine if these reporters were MAPK pathway specific, the platform was miniaturized to a 1536 well format and used to screen a 6000+ compound library. Results from the screen found that 70% (28/40) of the top drug hits were known to directly modulate the MAPK pathway. Since drug resistance is a common occurrence in melanoma, we generated a “resistant” A375 SABRE reporter line to increasing concentrations of vemurafenib and interrogated the top drug candidates identified in the screen. As expected, the resistant SABRE line failed to respond to BRAFV600E specific inhibitors but continued to respond to downstream MEK inhibitors, which is helping to delineate the mechanism of resistance. We were also interested in the screening hits that were not previously known to influence the MAPK pathway including bafetinib and a Tie2 inhibitor. In follow-up testing, we found that these drugs induced robust and titratable reporter signals in both the SABRE A375 vemurafenib sensitive and resistant lines. In addition, treatment of either bafetinib or the Tie2 inhibitor reduced ERK phosphorylation in the vemurafenib resistant cells, suggesting that these drugs could be useful in the treatment of drug resistant melanomas. Studies are currently underway to determine the in vivo efficacy of these drugs in A375 SABRE reporter xenograft models.
Citation Format: Shelli M. Morris, Andrew J. Mhyre, Savanna S. Carmack, Connor Burns, Marc Ferrer, Wenjuan Ye, James M. Olson, Richard A. Klinghoffer. The SABRE platform: A novel, unbiased technology for drug discovery and prioritization [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 4023. doi:10.1158/1538-7445.AM2017-4023