ALDH1A1 Contributes to PARP Inhibitor Resistance via Enhancing DNA Repair in BRCA2(-/-) Ovarian Cancer Cells.

Poly (ADP-ribose) polymerase (PARP) inhibitors (PARPi) are approved to treat recurrent ovarian cancer with BRCA1 or BRCA2 mutations, and as maintenance therapy for recurrent platinum-sensitive ovarian cancer (BRCA wild-type or mutated) after treatment with platinum. However, the acquired resistance against PARPi remains a clinical hurdle. Here, we demonstrated that PARP inhibitor (olaparib)-resistant epithelial ovarian cancer (EOC) cells exhibited an elevated aldehyde dehydrogenase (ALDH) activity, mainly contributed by increased expression of ALDH1A1 due to olaparib-induced expression of BRD4, a member of bromodomain and extraterminal (BET) family protein. We also revealed that ALDH1A1 enhanced microhomology-mediated end joining (MMEJ) activity in EOC cells with inactivated BRCA2, a key protein that promotes homologous recombination (HR) by using an intrachromosomal MMEJ reporter. Moreover, NCT-501, an ALDH1A1-selective inhibitor, can synergize with olaparib in killing EOC cells carrying BRCA2 mutation in both in vitro cell culture and the in vivo xenograft animal model. Given that MMEJ activity has been reported to be responsible for PARPi resistance in HR-deficient cells, we conclude that ALDH1A1 contributes to the resistance to PARP inhibitors via enhancing MMEJ in BRCA2-/- ovarian cancer cells. Our findings provide a novel mechanism underlying PARPi resistance in BRCA2-mutated EOC cells and suggest that inhibition of ALDH1A1 could be exploited for preventing and overcoming PARPi resistance in EOC patients carrying BRCA2 mutation.

Liu L ，Cai S ，Han C ，Banerjee A ，Wu D ，Cui T ，Xie G ，Zhang J ，Zhang X ，McLaughlin E ，Yin M ，Backes FJ ，Chakravarti A ，Zheng Y ，Wang QE ... -  《-》

The BET inhibitor INCB054329 reduces homologous recombination efficiency and augments PARP inhibitor activity in ovarian cancer.

Homologous recombination (HR)-proficient ovarian tumors have poorer clinical outcomes and show resistance to poly ADP ribose polymerase inhibitors (PARPi). A subset of HR-proficient ovarian tumors show amplification in bromodomain and extra-terminal (BET) genes such as BRD4. We aimed to test the hypothesis that BRD4 inhibition sensitizes ovarian cancer cells to PARPi by reducing HR efficiency and increasing DNA damage. HR-proficient ovarian cancer cell lines (OVCAR-3, OVCAR-4, SKOV-3, UWB1.289+BRCA1) were treated with BRD4-targeting siRNA, novel (INB054329, INCB057643) and established (JQ1) BET inhibitors (BETi) and PARPi (olaparib, rucaparib). Cell growth and viability were assessed by sulforhodamine B assays in vitro, and in SKOV-3 and ovarian cancer patient-derived xenografts in vivo. DNA damage and repair (pH2AX, RAD51 and BRCA1 foci formation, and DRGFP HR reporter activity), apoptosis markers (cleaved PARP, cleaved caspase-3, Bax) and proliferation markers (PCNA, Ki67) were assessed by immunofluorescence and western blot. In cultured cells, inhibition of BRD4 by siRNA or INCB054329 reduced expression and function of BRCA1 and RAD51, reduced HR reporter activity, and sensitized the cells to olaparib-induced growth inhibition, DNA damage induction and apoptosis. Synergy was observed between all BETi tested and PARPi. INCB054329 and olaparib also co-operatively inhibited xenograft tumor growth, accompanied by reduced BRCA1 expression and proliferation, and increased apoptosis and DNA damage. These results provide strong rationale for using BETi to extend therapeutic efficacy of PARPi to HR-proficient ovarian tumors and could benefit a substantial number of women diagnosed with this devastating disease.

Wilson AJ ，Stubbs M ，Liu P ，Ruggeri B ，Khabele D ... -  《-》

Entinostat, a selective HDAC1/2 inhibitor, potentiates the effects of olaparib in homologous recombination proficient ovarian cancer.

Poly ADP ribose polymerase inhibitors (PARPi) are most effective in BRCA1/2 mutated ovarian tumors. Better treatments are needed for homologous recombination HR-proficient cancer, including CCNE1 amplified subtypes. We have shown that histone deacetylase inhibitors (HDACi) sensitize HR-proficient ovarian cancer to PARPi. In this study, we provide complementary preclinical data for an investigator-initiated phase 1/2 clinical trial of the combination of olaparib and entinostat in recurrent, HR-proficient ovarian cancer. We assessed the in vitro effects of the combination of olaparib and entinostat in SKOV-3, OVCAR-3 and primary cells derived from CCNE1 amplified high grade serous ovarian cancer (HGSOC) patients. We then tested the combination in a SKOV-3 xenograft model and in a patient-derived xenograft (PDX) model. Entinostat potentiates the effect of olaparib in reducing cell viability and clonogenicity of HR-proficient ovarian cancer cells. The combination reduces peritoneal metastases in a SKOV-3 xenograft model and prolongs survival in a CCNE1 amplified HR-proficient PDX model. Entinostat also enhances olaparib-induced DNA damage. Further, entinostat decreases BRCA1, a key HR repair protein, associated with decreased Ki-67, a proliferation marker, and increased cleaved PARP, a marker of apoptosis. Finally, entinostat perturbs replication fork progression, which increases genome instability. Entinostat inhibits HR repair by reducing BRCA1 expression and stalling replication fork progression, leading to irreparable DNA damage and ultimate cell death. This work provides preclinical support for the clinical trial of the combination of olaparib and entinostat in HR-proficient ovarian cancer and suggests potential benefit even for CCNE1 amplified subtypes.

Gupta VG ，Hirst J ，Petersen S ，Roby KF ，Kusch M ，Zhou H ，Clive ML ，Jewell A ，Pathak HB ，Godwin AK ，Wilson AJ ，Crispens MA ，Cybulla E ，Vindigni A ，Fuh KC ，Khabele D ... -  《-》

In silico screening identifies a novel small molecule inhibitor that counteracts PARP inhibitor resistance in ovarian cancer.

Lin ZP ，Al Zouabi NN ，Xu ML ，Bowen NE ，Wu TL ，Lavi ES ，Huang PH ，Zhu YL ，Kim B ，Ratner ES ... -  《Scientific Reports》

Olaparib for the treatment of relapsed ovarian cancer with a BRCA1/2 mutation.

Stewart J ，George A ，Banerjee S 《-》