Synthetic lethality of LP-184, a next generation acylfulvene, in ex vivo PDX models with homologous recombination defects.

Abstract

e15064 Background: The clinical success of PARP inhibitors (PARPi) in homologous recombination (HR) deficient (HRD+) solid tumors has broadened the scope of identifying additional agents and vulnerabilities in cancers with DNA repair deficiencies. However, with more than 40% of BRCA1/2-deficient patients failing to respond to PARPi or acquiring resistance with prolonged PARPi administration, newer agents are also needed. LP-184, an acylfulvene, is a prodrug activated by PTGR1. Threshold expression levels of PTGR1 are higher in several tumors, providing a window of specificity for its cytotoxic action. DNA damage inflicted by acylfulvene (AF) agents is reliant upon HR pathway genes including BRCA1 for correction and removal. We hypothesized that tumors with high PTGR1 expression and HR deficiency will therefore be uniquely targeted and demonstrate synthetic lethality when exposed to LP-184. Methods: We evaluated ex vivo antitumor activity of LP-184 in selected PDX models representing lung, pancreatic and prostate cancers with high PTGR1 and known HR defects. Dissociated tumor fragments were treated with LP-184 across a concentration range of 5 nM to 36 uM for 5 days. Cell viability was quantified by CellTiter Glo. LP-184 IC50s were compared with PARPi efficacy. We further confirmed the dependency of PTGR1 in HR deficient tumor cells by comparing LP-184 sensitivity in the BRCA2 deficient cell line CAPAN-1 and the ATM mutant cell line PANC03.27, with and without PTGR1 suppression following an engineered CRISPR knockout of PTGR1. We also analyzed TCGA data to estimate the percentage of tumors with elevated PTGR1 and co-occurring damaging mutations in a panel of 60 HR genes. Results: The mean LP-184 IC50 across 15 HRD+ PDX models tested was 288 nM (range 31 - 2900 nM). LP-184 turned out to be 6 - 340X more potent ex vivo than the PARPi Olaparib in these models. 9 of 15 models were associated with no clinical response to or initial response followed by progression on approved standard of care (SOC) agents. 6 of 15 models showed < 10% tumor growth inhibition in vivo with SOC treatment. Regardless of cancer type, models with high-impact, loss-of-function mutations in ATM, ATR and BRCA1 showed exquisite sensitivity to LP-184 (mean IC50 ̃ 60 nM). CRISPRi-mediated stable suppression of PTGR1 in the pancreatic cancer cell lines CAPAN-1 and PANC03.27 entirely abrogated LP-184 sensitivity relative to isogenic parental cell lines. 17.6% of lung adenocarcinomas (n = 517), 4.5% of pancreatic adenocarcinomas (n = 179) and 9.6% of prostate adenocarcinomas (n = 498) displayed elevated PTGR1 along with damaging HR related mutations, and are likely to be responsive to LP-184 based on analysis of TCGA data. Conclusions: LP-184 is broadly effective in HRD+ tumors that may be less responsive to SOC including PARPi and could be useful clinically in a subset of tumors with high PTGR1 and HR defects.