Introduction: OCCC is a rare histotype of epithelial ovarian cancer with poor late-stage prognosis due to chemotherapy resistance brought by a unique molecular and genetic profile. There is interest in targeting the OCCC epigenome as ~50% of OCCC lesions carry truncating mutations in AT-rich interaction domain containing protein 1A (ARID1A). ARID1A loss is associated with changes to global H3K27Ac distribution and histone deacetylase (HDAC) activity, contributing to cancer progression. As such, HDACi’s are being investigated for ARID1A-mutant cancers. We aim to determine how HDACi disturbs the OCCC spheroid epigenome and how these changes alter spheroid formation and viability to determine the potential of HDACi’s as targeted therapy for OCCC.

Methods: Non-adherent, ultra-low attachment (ULA) plates allowed OCCC cell lines to autonomously form multicellular aggregates that better recapitulate patient spheroid morphology than traditional 2D culture, representing a 3D avascular tumoursphere for in vitro experimentation. Human OCCC cell lines cultured in ULA were treated with HDACi’s Entinostat (ENT) and ACY1215 (ACY) and subject to spheroid reattachment, IC50 determination, trypan blue cell counting, and whole cell and acid-extracted nuclear histone lysate collection.

Results: Both ENT and ACY treatment increased H3K27Ac levels in both adherent and spheroid OCCC cells in all cell lines tested. The IC50 of ENT was lower than ACY in both KOC-7c and 105C OCCC cell lines. KOC-7c spheroids were sensitive to both inhibitors and had a dose-dependent reduction in spheroid cell viability. 105C spheroids displayed increased cell viability and live cell counts when treated with low micromolar concentrations of ENT and ACY due to a short-term protection from detachment-associated apoptosis.

Conclusions: These findings suggest that HDACi’s can alter the epigenome of OCCC cell line spheroids, but OCCC spheroids display varying response to HDACi based on their spheroid characteristics.