Prolonged cell cycle arrest by the CDK4/6 antagonist narazaciclib restores ibrutinib response in preclinical models of BTKi‐resistant mantle cell lymphoma

Introduction: Bruton tyrosine kinase inhibitors (BTKi) have transformed the therapeutic landscape of mantle cell lymphoma (MCL); however, primary and acquired resistance to these agents remains a challenge. Previous studies have suggested that narazaciclib (ON123300), a second-generation, orally bioavailable and clinical-stage CDK4/6 inhibitor (CDKi), may trigger cell cycle arrest and significant tumor growth inhibition (TGI) in BTKi-resistant MCL models. Methods: We compared the efficacy and safety of narazaciclib with the health authority approved CDKi in association with different BTKi, in a panel of 10 MCL cell lines with distinct sensitivity to the first-in-class BTKi, ibrutinib. We used the CellTiter-Glo proliferation assay, FACS-mediated quantification of cell cycle and apoptosis, and RNA sequencing followed by gene set enrichment analysis (GSEA), RT-PCR and western blot validations. In addition, we evaluated the safety and efficacy of narazaciclib/ibrutinib combo in vivo in an immune-competent chicken embryo chorioallantoic membrane (CAM) MCL xenograft. Results: Narazaciclib exhibited the highest antitumor activity among MCL cell lines (mean IC50: 3.61 ± 2.1 µM), regardless of their sensitivity to ibrutinib. Although there was no correlation between CDKi sensitivity and activation of the CDK4/CDK6-pRb pathway in MCL, transcriptomic and phenotypic analyses revealed a predominant downregulation of E2F target genes and G2/M checkpoint response (NES > 2.5) upon narazaciclib treatment. This feature was associated to intracellular accumulation of p21, p16, and phospho-p27, decreased mitotic index, G1 cell cycle blockade, and apoptosis onset. When combined with ibrutinib, but not with the second generation therapeutic acalabrutinib, narazaciclib achieved significant synergistic antitumor activity in both BTK-sensitive and BTK-resistant cells. The combination was not associated with improved apoptosis, but rather with a slight but constant (+10%–15%) augmentation in G1 phase blockade and the down-modulation of cell cycle-associated transcriptome. Both the downregulation of phospho-histone H3 and the upregulation of p-p27/p27 and p16, also underwent a 10%–15% improvement in combination-treated cells. In vivo, while narazaciclib single agent achieved a 28% TGI in the CAM model, the narazaciclib-ibrutinib combination reduced tumor spreading by 65% and allowed a 50% reduction in malignant B cell infiltration into the bone marrow, with no detectable toxicity. Conclusions: Narazaciclib, due to its completely distinct MoA from BTKi involving the direct modulation of the cell cycle, can achieve significant synergistic activity with ibrutinib in vitro and in vivo, especially in BTKi-resistant models of MCL. Ongoing phospho-proteomics and genetic edition assays will help deciphering the molecular bases of this unique drug cooperation at the cell cycle level. Encore Abstract - previously submitted to AACR 2023 The research was funded by: This study was financially supported by Onconova Therapeutics, Spanish Ministry of Science and Innovation (grant PID2021-123039OB-C21) (to GR). MFS holds a predoctoral fellowship (FI19/00338) from Instituto de Salud Carlos III. This work was carried out under the CERCA Program (Generalitat de Catalunya). Keywords: Aggressive B-cell non-Hodgkin lymphoma, Combination Therapies, Molecular Targeted Therapies Conflicts of interests pertinent to the abstract. A. Makovski-Silverstein Employment or leadership position: Onconova Therapeutics S. Cosenza Employment or leadership position: Onconova Therapeutics G. Roué Research funding: Onconova Therapeutics