The PARP1 inhibitor BMN 673 exhibits immunoregulatory effects in a Brca1(-/-) murine model of ovarian cancer

Familial breast and ovarian cancer are often caused by inherited mutations of BRCA1. While current prognoses for such patients are rather poor, inhibition of poly-ADP ribose polymerase 1 (PARP1) induces synthetic lethality in cells that are defective in homologous recombination. BMN 673 is a potent PARP1 inhibitor that is being clinically evaluated for treatment of BRCA-mutant cancers. Using the Brca1-deficient murine epithelial ovarian cancer cell line BR5FVB1-Akt, we investigated whether the antitumor effects of BMN 673 extend beyond its known pro-apoptotic function. Administration of modest amounts of BMN 673 greatly improved the survival of mice bearing subcutaneous or intraperitoneal tumors. We thus hypothesized that BMN 673 may influence the composition and function of immune cells in the tumor microenvironment. Indeed, BMN 673 significantly increases the number of peritoneal CD8(+) T cells and NK cells as well as their production of IFN-γ and TNF-α. These data suggest that the cell stress caused by BMN 673 induces not only cancer cell-intrinsic apoptosis but also cancer cell-extrinsic antitumor immune effects in a syngeneic murine model of ovarian cancer. BMN 673 may therefore serve as a promising adjuvant therapy to immunotherapy to achieve durable responses among patients whose tumors harbor defects in homologous recombination.

Structural basis for the inhibition of poly(ADP-ribose) polymerases 1 and 2 by BMN 673, a potent inhibitor derived from dihydropyridophthalazinone

BMN 673, a novel PARP1/2 inhibitor in clinical development with substantial tumor cytotoxicity, forms extensive hydrogen-bonding and π-stacking in the nicotinamide pocket, with its unique disubstituted scaffold extending towards the less conserved edges of the pocket. These interactions might provide structural insight into the ability of BMN 673 to both inhibit catalysis and affect DNA-binding activity.

Poly(ADP-ribose) polymerases 1 and 2 (PARP1 and PARP2), which are involved in DNA damage response, are targets of anticancer therapeutics. BMN 673 is a novel PARP1/2 inhibitor with substantially increased PARP-mediated tumor cytotoxicity and is now in later-stage clinical development for BRCA-deficient breast cancers. In co-crystal structures, BMN 673 is anchored to the nicotinamide-binding pocket via an extensive network of hydrogen-bonding and π-stacking interactions, including those mediated by active-site water molecules. The novel di-branched scaffold of BMN 673 extends the binding interactions towards the outer edges of the pocket, which exhibit the least sequence homology among PARP enzymes. The crystallographic structural analyses reported here therefore not only provide critical insights into the molecular basis for the exceptionally high potency of the clinical development candidate BMN 673, but also new opportunities for increasing inhibitor selectivity.

Layer-by-layer nanoparticles for novel delivery of cisplatin and PARP inhibitors for platinum-based drug resistance therapy in ovarian cancer

Advanced staged high-grade serous ovarian cancer (HGSOC) is the leading cause of gynecological cancer death in the developed world, with 5-year survival rates of only 25-30% due to late-stage diagnosis and the shortcomings of platinum-based therapies. A Phase I clinical trial of a combination of free cisplatin and poly(ADP-ribose) polymerase inhibitors (PARPis) showed therapeutic benefit for HGSOC. In this study, we address the challenge of resistance to platinum-based therapy by developing a targeted delivery approach. Novel electrostatic layer-by-layer (LbL) liposomal nanoparticles (NPs) with a terminal hyaluronic acid layer that facilitates CD44 receptor targeting are designed for selective targeting of HGSOC cells; the liposomes can be formulated to contain both cisplatin and the PARPi drug within the liposomal core and bilayer. The therapeutic effectiveness of LbL NP-encapsulated cisplatin and PARPi alone and in combination was compared with the corresponding free drugs in luciferase and CD44-expressing OVCAR8 orthotopic xenografts in female nude mice. The NPs exhibited prolonged blood circulation half-life, mechanistic staged drug release and targeted codelivery of the therapeutic agents to HGSOC cells. Moreover, compared to the free drugs, the NPs resulted in significantly reduced tumor metastasis, extended survival, and moderated systemic toxicity.

Phenethyl Isothiocyanate Enhances the Cytotoxic Effects of PARP Inhibitors in High-Grade Serous Ovarian Cancer Cells

While PARP inhibitor (PARPi) therapies have shown promising results in the treatment of high-grade serous ovarian cancer (HGSOC) harboring homologous recombination deficiencies, primary resistance to PARPi frequently occurs and even initial responders may eventually become resistant. Therefore, the development of novel effective combinatorial strategies to treat HGSOC is urgently needed. Here, we report that H₂O₂-induced oxidative stress sensitized HGSOC cells to PARPi BMN 673. Furthermore, Phenethyl isothiocyanate (PEITC) as a ROS-inducing agent significantly enhanced the cytotoxic effects of BMN 673. Mechanistically, combined use of PEITC and BMN 673 resulted in ROS overproduction and accumulation, enhanced DNA damage, G2/M arrest and apoptosis, all of which were significantly reversed by the ROS scavenger N-Acetyl-L-cysteine. We also showed that while PEITC did not further enhance the ability of BMN 673 on PARP1 trapping in HGSOC cells, the therapeutic effects of the PEITC/BMN 673 combination were at least in part dependent on the presence of PARP1. Importantly, the PEITC/BMN 673 combination potently abrogated the growth of HGSOC tumor spheroids and patient-derived organoid models of HGSOC and cervical cancer. Our findings provide a basis for further investigation of the utility of PARPi combination regimen in HGSOC and cervical cancer through ROS-mediated mechanisms.

The PARP1 inhibitor BMN 673 exhibits immunoregulatory effects in a Brca1(-/-) murine model of ovarian cancer

Familial breast and ovarian cancer are often caused by inherited mutations of BRCA1. While current prognoses for such patients are rather poor, inhibition of poly-ADP ribose polymerase 1 (PARP1) induces synthetic lethality in cells that are defective in homologous recombination. BMN 673 is a potent PARP1 inhibitor that is being clinically evaluated for treatment of BRCA-mutant cancers. Using the Brca1-deficient murine epithelial ovarian cancer cell line BR5FVB1-Akt, we investigated whether the antitumor effects of BMN 673 extend beyond its known pro-apoptotic function. Administration of modest amounts of BMN 673 greatly improved the survival of mice bearing subcutaneous or intraperitoneal tumors. We thus hypothesized that BMN 673 may influence the composition and function of immune cells in the tumor microenvironment. Indeed, BMN 673 significantly increases the number of peritoneal CD8(+) T cells and NK cells as well as their production of IFN-γ and TNF-α. These data suggest that the cell stress caused by BMN 673 induces not only cancer cell-intrinsic apoptosis but also cancer cell-extrinsic antitumor immune effects in a syngeneic murine model of ovarian cancer. BMN 673 may therefore serve as a promising adjuvant therapy to immunotherapy to achieve durable responses among patients whose tumors harbor defects in homologous recombination.