Tumor sensitization to PARP inhibitors by DNA damaging synthetically lethal acylfulvenes.

Abstract

e15123 Background: 20-30% of solid tumors (ovarian, breast, pancreatic, prostate cancers) harbor some level of deficiency in DNA Damage Repair (DDR) pathways. Such cancers become dependent on alternative DDR pathways to maintain genome integrity, making them selectively vulnerable to DNA damaging agents. The concept of synthetic lethality has been leveraged to treat subsets of such tumors. Clinically, PARP inhibitors (PARPi) have been successful in Homologous Recombination Deficiency (HRD) cancers. LP-100 and LP-184 are acylfulvene (AF) class small molecule prodrugs that are metabolized to active compounds by prostaglandin reductase 1 (PTGR1) that is often elevated in solid tumors. AF-induced DNA lesions are likely selectively processed by repair pathways including BRCA1/2 mediated Homologous Recombination (HR). Methods: Cell Titer Glo based 10 day cell viability assays were used to generate drug IC50 values in vitro. Bliss synergy scores were computed with scores ≥ 10 reflecting synergy. Drug-induced DNA double strand breaks (DSB) were detected and quantified with STRIDE (SensiTive Recognition of Individual DNA Ends) assay in cancer cells. A subcutaneous tumor xenograft model was used to determine tumor volumes in control and drug treated mice. Results: In a BRCA2 mutant ovarian cancer cell line OVCAR3, LP-184 and olaparib co-treatment in vitro showed synergistic effects (Bliss score 10). Similarly in a BRCA2 mutant prostate cancer cell line 22Rv1, LP-100 combination with rucaparib was synergistic (Bliss score 18). 400 nM LP-184 showed a time dependent increase in DNA DSB signals in vitro, 1.7X more so in BRCA2 mutant vs wild type (WT) DLD-1 isogenic colon cancer cell lines up to 24 hours. To validate the hypothesis that AF-induced DSBs remain unresolved if exposed to and further synergize with PARPi in vivo, we tested LP-100 plus olaparib in a DU145 (BRCA2 mutant) prostate cancer xenograft model. LP-100 + olaparib treatment resulted in sustained, durable tumor regression indicating in vivo synergy whereas either agent alone did not produce tumor shrinkage. Day 42 tumor growth inhibition was 66% for 2.5 mg/kg LP-100 alone, 34% for 40 mg/kg olaparib alone and 108% for the combination. Moreover, LP-184 treatment resulted in early onset and complete tumor regression in 7 PDX models of PARPi resistant triple negative breast cancers (TNBC) harboring BRCA1/2 loss whereas olaparib/ niraparib remained entirely ineffective in these. Conclusions: AF response is influenced by tumor DNA repair competence. LP-100 and LP-184 are synthetically lethal resulting from their abilities to cause unresolvable DNA damage where tumor cells express high PTGR1 and are deficient in the HR pathway. LP-184 single agent was superior to PARPi in TNBC and LP-100 combination made an olaparib resistant tumor highly sensitive, thereby facilitating a path to extend the opportunities for both AFs and PARPis and enhance the clinical utilization of PARPi.