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to investigate the signaling pathways altered upon drug treatment. An orthotoppic PDX model was established from a MCL patient with dual resistance to ibrutinib-venetoclax. Single agent copanlisib at 6 mg/kg/2d, idelalisib at 50 mg/kg/2d, ibrutinib at 50 mg/kg/d, volasertib at 10 mg/kg/week, or vehicle control, were administrated in mice carrying the orthotopic PDX model to assess their in vivo efficacies. Results: Our data revealed that PI3K-AKT signaling closely associated with ibrutinib resistance, venetoclax resistance and dual resistance. Constitutive PI3K-AKT-mTOR signaling overcomes the G2/M cell cycle checkpoint and promote continuous cell proliferation in tumor cells. All four isoforms of PI3K, α, β, γ and δ are expressed in all the MCL cell lines tested. Unlike idelalisib, a PI3Kδ-specific inhibitor, copanlisib is a pan-PI3K inhibitor, which targets all PI3K isoforms, and was approved by FDA to treat follicular lymphoma. Compared to idelalisib, copanlisib is more potent in targeting MCL cells in vitro with IC50 at a nanomolar range. Copanlisib significantly inhibited in vivo tumor growth of an ibrutinib-venetoclax dual-resistant PDX model (Fig. 1A). PLK1, a central player in regulating G2/M transition, acts upstream of PI3K/AKT signaling via phosphorylating PTEN to cause a tumor-promoting metabolic state. Volasertib, a specific PLK1 inhibitor, is under a phase III clinical trial for patients with Acute Myeloid Leukemia. Volasertib dramatically arrested MCL tumor cells at G2/M phase, which leads to cell apoptosis at a low nanomolar range. Volasertib at 10mg/kg/week significantly inhibited in vivo tumor growth of the ibrutinib-venetoclax dual -resistant PDX model (Fig. 1B). More interestingly, the combination of copanlisib and volasertib induced synergistic effect in ibrutinib-resistant, venetoclax-resistant, and ibrutinib-venetoclax dual resistant cell lines. Detailed mechanism of the observed synergy is still under investigation in vitro and in vivo. Conclusion: Copanlisib and Volasertib are potent agents in targeting MCL cells in vitro and in vivo, and have great potential to overcome ibrutinib and venetoclax resistance in MCL.