Justin Gor

 Introduction.  Ovarian cancer is the most lethal gynecologic cancer among women worldwide, while breast cancer has the highest incidence^1,2.  The breast cancer susceptibility genes 1 and 2 (BRCA 1 and 2) are linked to production of tumor suppressor proteins that play a key role in homologous recombination and replication fork protection^1,2.  Mutations in those genes have been associated with increased risk of ovarian, breast, prostate and pancreatic cancers^2-6.  Poly-ADP-ribose polymerase (PARP) inhibitors (PARPi) target the DNA double strand break repair mechanisms in BRCA-mutant tumor cells, but resistance lends to combination therapy for increased efficacy^1-5.  Ataxia telangiectasia and Rad3-related protein kinase (ATR) is responsible for a separate repair mechanism from PARP, making it a possible candidate for combination therapy in addressing PARPi resistance^2,3,5,6.  Methods.  High grade serous ovarian cancer (HGSOC) cells were cultured in vivo using a BRCA2^MUT, sourced from a patient-derived xenograft (PDX) model⁵.  In addition, cell lines observed included UWB1.289, UWB1.289 + BRCA1, PEO1 (BRCA2^MUT) and PEO4 (BRCA2 reversion mutation), which were grown in RPMI media^2,5.   Utilizing a combination of DNA fiber assay and colony formation assay, the effects of PARPi (olaparib), CHK1 inhibition (CHK1i; MK-8776), and ATR inhibition (ATRi; VE-821, VE-822, AZ20, AZD6738) monotherapy was compared to combination therapy with regards to genomic instability, colony formation, tumor cell survival and apoptosis^2,5.  Using ultrasound (Sonosite Edge II Ultrasound System), tumor length and width were measured to calculate tumor volume in the PDX model⁵.  Grown cell lines were cultured, incubated and analyzed^2,5.  Results. PARPi monotherapy displayed decreased BRCA^MUT survival, colony formation and tumor growth via mechanisms of genomic instability, tumor suppression and increased apoptosis^2,5.  However, the efficacy of PARPi monotherapy for BRCA 1 and BRCA 2 mutant cells was limited due to mutations and other mechanisms of resistance^1-5. The BRCA mutant cells displayed an increased reliance on the ATR-CHK1 fork protection pathways while being treated with PARPi^2,4,5.  ATRi and CHK1i displayed a synergistic effect when combined with PARPi, resulting in an increase in chromosomal gaps and breaks within the cancer cell lines that were treated with combination therapy^2,5.  BRCA mutants treated with combination therapy displayed increased sensitivity to PARPi when previously displaying resistance with monotherapy, suggesting a re-sensitization effect when combined with ATRi/CHKi^2,4,5,6.  Conclusions.  The combination of PARPi with ATRi/CHK1i displayed increased efficacy in suppressing and eliminating BRCA1 and BRCA 2 mutant cells derived from ovarian cancer cell lines when compared to PARPi monotherapy^2-6.

1. Han Y, Chen M, et al. Synergism of PARP inhibitor fluzoparib (HS10160) and MET inhibitor HS10241 in breast and ovarian cancer cells. American Journal of Cancer Research. 2019;9(3):608-618. Published 2019 Mar 1.
2. Yazinski S, Comaills V, et al. ATR inhibition disrupts rewired homologous recombination and fork protection pathways in PARP inhibitor resistant BRCA-deficient cancer cells. Genes & Development. 2017;31:318-322. doi: 10.1101/gad.290957.116.
3. Murai J, Feng Y, et al. Resistance to PARP inhibitors by SLFN11 inactivation can be overcome by ATR inhibition. 2016;7(47)76534-76550. doi: 10.18632/oncotarget.12266.
4. Michelena J, Lezaja A, et al. Analysis of PARP inhibitor toxicity by multidimentional fluorescence microscopy reveals mechanisms of sensitivity and resistance. Nature Communications. 2018;9(2678). doi: 10.1038/s41467-018-05031-9.
5. Kim H, George E, Ryan L, et al. Targeting the ATR/CHK1 Axis with PARP Inhibition Results in Tumor Regression in BRCA-Mutant Ovarian Cancer Models. Clincal Cancer Research. 2017;23(12):3097-3108. doi: 10.1158/1078-0432.CCR-16-2273.
6. Haynes B, Murai J, Lee J. Restored replication fork stabilization, a mechanism of PARP inhibitor resistance, can be overcome by cell cycle checkpoint inhibition. Cancer Treatment. 2018;71:1-7. doi: 10.1016/j.ctrv.2018.09.003.