Abstract Background: Mesothelioma is a cancer that arises out of the pleura or the peritoneum and is considered incurable in the majority of patients afflicted. Abnormalities of the cell cycle pathway are among the most common molecular alterations in mesothelioma. Previously, we have demonstrated that inhibition of CDK4/6 by abemaciclib or palbociclib is effective against mesothelioma in vitro. Increased expression of Cyclin E has been demonstrated to be one mechanism of resistance to CDK4/6 inhibition in other cancers. We hypothesized that select cell cycle-related proteins may compensate for inhibition of one part of the cell cycle machinery and that the CDK2 inhibitor dinaciclib may act synergistically in combination with abemaciclib. Methods: Dinaciclib was evaluated for activity against mesothelioma cells in vitro with regard to cell proliferation and colony formation. In addition, dinaciclib was evaluated in combination with abemaciclib at several concentration combinations in a cell proliferation assay to determine if the combination may be additive or synergistic. Combination indices (CI) were calculated via the Chou-Talalay method. Immunoblotting was used to evaluate for expression of key survival proteins after treatment with abemaciclib. Results: The IC50 of dinaciclib was determined to be 18 nM, 23 nM, 32 nM, and 33 nM in mesothelioma cell lines H2373, H2052, H2452, and H2461, respectively. Colony formation was decreased significantly after 72 hours incubation with 50 nM or 100 nM dinaciclib in Bap1-expressing (35.1 ± 2%; 22.8 ± 2%) and Bap1-null cell lines (14.5 ± 3%; 8.1 ± 2%) versus untreated controls. CI < 1 (denoting synergism) was identified for several combinations of varied concentrations of abemaciclib and dinaciclib for both cell lines evaluated. Further, treatment with abemaciclib for 72 hrs in both Bap1-expressing and Bap 1-null cells at 1x the IC50 resulted in complete loss of FoxM1 expression and pro-survival survivin and significantly decreased the expression of Mcl-1 and XIAP. Conclusions: Dinaciclib is a potent inhibitor of mesothelioma cell growth. The combination of abemaciclib and dinaciclib is synergistic across a spectrum of concentrations in vitro. Further experiments are currently underway to determine which CDKs and related proteins may be more effectively targeted in mesothelioma. As the cell cycle is altered in the vast majority of mesothelioma tumors, select combinations of CDK inhibition may be a feasible future therapeutic strategy against mesothelioma. Citation Format: Marian Kratzke, Betsy Kren, Stephen Porter, George Scaria, Mark A. Klein. Cyclin-dependent kinase inhibition in mesothelioma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1961.