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The phosphatidylinositol 3‑kinase/protein kinase B (PI3K/AKT) signaling pathway is critically active in many cell types, both normal and neoplastic. Many small-molecule inhibitors targeting different levels of the PI3K/AKT pathway have been developed for cancer therapy, but their efficacy is reduced by compensatory pathway re-activation mechanisms, and their tolerability by toxic side effects. We studied this problem using cell lines representing diffuse large B-cell lymphoma (SUDHL-4 and OCI-Ly7), a genetically-encoded live-cell reporter of AKT activity, and 3 small-molecule inhibitors targeting different levels of the pathway: idelalisib (PI3Kδ), GSK2334470 (PDPK1), and ipatasertib (AKT). Half-maximal (IC50) concentrations of these inhibitors for AKT activity inhibition at 1 h, when used individually, were much lower than their IC50 values for reduction of viable cell number after 4 days. Time-course studies explained this discrepancy: AKT activity in the continuous presence of the inhibitors returned to normal after 24 h, and was supranormal after inhibitor removal. Combining all 3 inhibitors produced sustained inhibition of AKT activity, was broadly synergistic at reducing viable cell number, enabled substantially lower doses of each inhibitor to be used, and was enhanced further by the mTOR inhibitor rapamycin. Moreover, combined PDPK1 and AKT inhibition showed synergy with multiple different PI3K inhibitors. In a syngeneic mouse cell line model of lymphoma (A20), the triple combination showed antitumor activity and no evidence of toxicity. Our findings provide proof of concept suggesting further study of the safety and efficacy of low-dose multilevel PI3K/AKT pathway inhibition, for lymphoma and perhaps other cancers., (© 2024. The Author(s).)

Background: Mantle cell lymphoma (MCL) is a chronically relapsing malignancy with deregulated cell cycle progression. We analyzed efficacy, mode of action, and predictive markers of susceptibility to palbociclib, an approved CDK 4/6 inhibitor, and its combination with venetoclax, a BCL2 inhibitor., Methods: A panel of nine MCL cell lines were used for in vitro experiments. Four patient derived xenografts (PDX) obtained from patients with chemotherapy and ibrutinib-refractory MCL were used for in vivo proof-of-concept studies. Changes of the mitochondrial membrane potential, energy-metabolic pathways, AKT activity, and pro-apoptotic priming of MCL cells were evaluated by JC-1 staining, Seahorse XF analyser, genetically encoded fluorescent AKT reporter, and BH3 profiling, respectively. MCL clones with gene knockout or transgenic (over)expression of CDKN2A, MYC, CDK4, and RB1 were used to estimate impact of these aberrations on sensitivity to palbociclib, and venetoclax., Results: Co-targeting MCL cells with palbociclib and venetoclax induced cytotoxic synergy in vitro and in vivo. Molecular mechanisms responsible for the observed synthetic lethality comprised palbociclib-mediated downregulation of anti-apoptotic MCL1, increased levels of proapoptotic BIM bound on both BCL2, and BCL-XL and increased pro-apoptotic priming of MCL cells mediated by BCL2-independent mechanisms, predominantly palbociclib-triggered metabolic and mitochondrial stress. Loss of RB1 resulted in palbociclib resistance, while deletion of CDKN2A or overexpression of CDK4, and MYC genes did not change sensitivity to palbociclib., Conclusions: Our data strongly support investigation of the chemotherapy-free palbociclib and venetoclax combination as an innovative treatment strategy for post-ibrutinib MCL patients without RB1 deletion., (© 2024. The Author(s).)