Abstract Epithelial ovarian cancer (EOC) is the most lethal gynaecological cancer. Amongst the five histotypes (high-grade serous, clear-cell, endometrioid, mucinous, and low-grade serous), mucinous epithelial ovarian cancer (mEOC) is a rare subset and represents approximately 3% of EOC. Beside these different histotypes are unique clinical entities with a different prognosis and response to therapy, they are still treated as a homogeneous group with a similar chemotherapeutic approach (platinum-paclitaxel combination), to which however mEOC is generally poor responsive. New specific and active therapeutic strategies are needed for mEOC. To find new therapeutic options for mEOC, we have performed a high-throughput screening using a siRNA library directed against 719 human protein kinases in the mEOC MCAS cell line. After two independent screenings and validation experiments with specific esiRNAs, Polo-like kinase1 (PLK1), a mitotic serine/threonine kinase, was identified as the most significant kinase whose downregulation interfered with both cell proliferation and survival. Both PLK1 siRNA and nanomolar concentrations of commercially available inhibitors against PLK1 (onvansertib (NMS-P937) and volasertib) inhibited cell growth in other two additional mEOC cell lines (EFO-27 and JHOM-1). In addition, in the mEOC cell lines PLK1 inhibition was able to induce apoptosis and to block cells in the G2-M phase of the cell cycle. We then evaluated in vitro combinations between the PLK1 inhibitors and different chemotherapeutic drugs used in mEOC, including cisplatin and paclitaxel used in front line therapy in all the available mEOC cell lines. No synergism could be found in any of the cell lines, when both PLK1 inhibitors were combined with cisplatin. On the contrary, strong synergism could be observed in two out the three mEOC cell lines investigated when onvansertib and volasertib were combined with both paclitaxel and eribulin. On the basis of the strong in vitro synergistic effect between onvansertib and paclitaxel, this combination was tested in vivo in nude mice transplanted with MCAS cells. Onvansertib as single agent was inactive, while paclitaxel at the dose used was active. Interestingly enough, the combination of the two drugs was well tolerated and was much more active than paclitaxel single agent, with both a stronger tumor regression and more sustained tumor growth inhibition that translated in a longer survival time. These data suggest that onvansertib and paclitaxel could represent a new active therapeutic option in mEOC. Citation Format: Roberta Affatato, Rosaria Chilà, Monica Lupi, Valentina Restelli, Mark Erlander, Laura Carrassa, Giovanna Damia. PLK1: a new therapeutic target in mucinous ovarian carcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3062.