Targeting PI3K and PLK1 to Overcome Ibrutinib-Venetoclax Resistance in Mantle Cell Lymphoma

BTK inhibitors such as ibrutinib and BCL-2 inhibitors such as venetoclax are two of the most effective therapies used to treat patients with refractory/relapsed mantle cell lymphoma (MCL). However, MCL patients treated with these therapies alone or in combination frequently develop therapeutic resistance, which results in poor clinical outcome. Therefore, targeting the signaling pathways responsible for therapeutic resistance and disease progression will potentially overcome primary and acquired resistance in ibrutinib-venetoclax-refractory/relapsed MCL. Our previous study on a MCL patient cohort comparing ibrutinib-sensitive and -resistant patients revealed that PI3K-ATK signaling is closely associated with ibrutinib resistance. Our recent single-cell transcriptomic profiling of another MCL patient cohort comparing ibrutinib responders and ibrutinib-venetoclax non-responders revealed that PI3K-AKT-mTOR signaling and the G2/M checkpoint were significantly upregulated in ibrutinib-venetoclax non-responders and were highly associated with disease progression. The constitutive PI3K-AKT-mTOR signaling in ibrutinib-resistant or ibrutinib-venetoclax dual-resistant MCL cells may be important in overcoming the G2/M checkpoint and promoting cell survival and cell proliferation. All PI3K isoforms - α, β, γ and δ - are expressed in all the MCL cell lines tested. Copanlisib is a pan-PI3K inhibitor, which targets all PI3K isoforms, and was FDA-approved to treat follicular lymphoma. Compared to idelalisib, a PI3Kδ inhibitor, copanlisib is more potent in targeting MCL cells in vitro with IC₅₀ at a nanomolar range. Copanlisib at low dosage of 6 mg/kg/2d significantly inhibited in vivo tumor growth of an ibrutinib-venetoclax dual-resistant PDX model. 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 the G2/M phase, which caused cell apoptosis at a low nanomolar range. Volasertib at low dosage of 10 mg/kg/week diminished in vivo tumor growth of the ibrutinib-venetoclax dual-resistant PDX model. More interestingly, the combination of copanlisib and volasertib induced synergistic effects in ibrutinib-resistant, venetoclax-resistant, and ibrutinib-venetoclax dual-resistant cell lines in vitro. Moreover, combined treatment with copanlisib (6mg/kg/2d) and volasertib (5mg/kg/week) significantly enhanced anti-MCL activity in the same PDX model, compared to that of either single-agent treatment. Our data demonstrated that pan-PI3K inhibitor copanlisib and PLK1 inhibitor volasertib are potent agents in targeting MCL cells in vitro and in vivo, and they have great potential to overcome ibrutinib and venetoclax resistance in MCL. When combined, copanlisib and volasertib have even greater potential to overcome ibrutinib and venetoclax resistance in MCL.