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


METHODS
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.


RESULTS
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.


CONCLUSIONS
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|Andrew J|AJ|', 'Stubbs|Matthew|M|', 'Liu|Phillip|P|', 'Ruggeri|Bruce|B|', 'Khabele|Dineo|D|']
D000818:Animals D000971:Antineoplastic Combined Chemotherapy Protocols D019313:BRCA1 Protein D000077216:Carcinoma, Ovarian Epithelial D018797:Cell Cycle Proteins D045744:Cell Line, Tumor D015536:Down-Regulation D004357:Drug Synergism D005260:Female D059765:Homologous Recombination D006801:Humans D007211:Indoles D051379:Mice D016688:Mice, Inbred NOD D008819:Mice, Nude D016513:Mice, SCID D009375:Neoplasms, Glandular and Epithelial D009687:Nuclear Proteins D009930:Organic Chemicals D010051:Ovarian Neoplasms D010793:Phthalazines D010879:Piperazines D000067856:Poly(ADP-ribose) Polymerase Inhibitors D014157:Transcription Factors D023041:Xenograft Model Antitumor Assays
BET inhibitor Homologous recombination PARP inhibitor Ovarian cancer