Abstract 5974: Synergistic activity of the CDK4/6 antagonist narazaciclib (ON123300) with irreversible BTK inhibition in ibrutinib-resistant mantle cell lymphoma

Bruton tyrosine kinase inhibitors (BTKi) have transformed the therapeutic landscape of B-cell lymphoma including the aggressive mantle cell lymphoma (MCL) subtype, although primary and acquired resistance is frequently observed. Interestingly, preliminary results have suggested that the second-generation, orally bioavailable and clinical-stage CDK4/6 inhibitor, ON123300 (narazaciclib), may be effective in preclinical models of BTKi-resistant MCL. We compared the efficacy and safety of ON123300 vs the approved CDK inhibitors palbociclib, abemaciclib or ribociclib, in association with various BTKi, in a panel of 10 MCL cell lines with distinct sensitivity to the first-in-class BTKi, ibrutinib, or the second generation acalabrutinib. We evaluated the effects of these combinations by CellTiter-Glo proliferation assay, FACS-mediated analysis of cell cycle and apoptosis, RT-PCR and western blot. Efficacy and safety of ON123300 in vivo was evaluated in a chicken embryo chorioallantoic membrane (CAM) xenograft model of MCL. We observed that ON123300 exhibited a significant antitumor activity in MCL cell lines independently of their sensitivity to ibrutinib, with IC50 at 72h ranging from 0.7 to 7.1µM (mean: 3.61±2.1µM), while it achieved a 32% tumor growth inhibition, with no detectable toxicity, in the CAM-MCL model. In vitro, ON123300 demonstrated a superior activity than palbociclib and ribociclib, and a similar effect that abemaciclib. Interestingly, ON123300 exhibited an antitumor activity comparable to ibrutinib but was more effective than acalabrutinib. ON123300 single agent treatment repressed several positive regulators of the G2/M cell cycle phase and triggered the accumulation of the CDK inhibitors p21, p16, and phospho-p27, with the consequent loss of phospho-Histone H3 and phospho-CDK2. Accordingly, ON123300 treatment evoked a 20-35% increase in G1 cell cycle fraction at 24h, which preceded the ignition of mitochondrial apoptosis. When combined with ibrutinib, rather than with acalabrutinib, ON123300 exhibited a significant antitumor activity with synergistic combination indexes in both BTK-sensitive and BTK-resistant cell lines. This phenomenon was associated with a slight but constant (+10-15%) augmentation of G1 blockade, histone H3/CDK2 dephosphorylation and p27/p16 accumulation in cells exposed to ibrutinib-ON123300 combination, with no detectable potentiation of apoptosis. In conclusion, ON123300 is safe and effective as single agent in preclinical models of MCL, including BTKi-resistant cases. Due to its complete distinct MoA from BTKi involving the direct modulation of cell cycle, ON123300, but not abemaciclib or palbociclib, can achieve significant synergistic activity in combination with ibrutinib, and in lower extent with acalabrutinib, especially in BTKi-resistant MCL cases. Citation Format: Nuria Profitos-Peleja, Marcelo L. Ribeiro, Adar Makovski Silverstein, Stephen Cosenza, Gaël Roué. Synergistic activity of the CDK4/6 antagonist narazaciclib (ON123300) with irreversible BTK inhibition in ibrutinib-resistant mantle cell lymphoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5974.