Combination of Ibrutinib with CD19 Chimeric Antigen Receptor (CAR) T Cells Suggests Greater Contribution to CAR T-Cell Efficacy Than Other Approved BTK Inhibitors in the Preclinical Setting

Background: CD19-directed CAR T-cell therapy has demonstrated remarkable clinical benefits for patients with B-cell malignancies, but impaired CAR T-cell fitness can result in lack of response or relapse. Rational combinations that enhance T-cell fitness may overcome primary and secondary resistance. Bruton tyrosine kinase inhibitors (BTKi), such as Ibrutinib (Ibr) have been shown to enhance CAR T-cell efficacy (Fraietta et al, Blood 2016; Liu et al, Cancer Sci 2020), but the precise mechanism remains unclear. In addition to BTK, Ibr potently inhibits IL-2-inducible tyrosine kinase (ITK), an important mediator of T-cell signaling. Ibr has been shown to enhance Th1 polarization through suppression of the Th2 differentiation pathway, thereby potentiating a cytotoxic effector response (Qin et al, J Immunother 2020). Consistent with this finding, the Th1 phenotype was more frequent in peripheral blood mononuclear cells of Ibr-exposed patients than in acalabrutinib (Acala)-exposed patients (Scarfò et al, Blood 2021). Therefore, we aimed to assess the potential impact of ITK inhibition (ITKi) on CD19 CAR T-cell function by comparing Ibr to other approved BTKi that display low or no ITKi activity (Estupiñán et al, Front Cell Dev Biol 2021).

Methods: CD19-41bb CAR T cells (CAR19) were generated by lentiviral transduction of healthy donor T cells. BTKi additivity to CAR19 function was evaluated in a Luciferase (Luc)+ Ramos, BTKi resistant systemic tumor model in NSG mice co-treated with covalent BTKis Ibr, Acala, zanubrutinib (Zanu), and CAR T cells. Next, to evaluate the impact of ITKi on CAR19 fitness and cytotoxicity, repeated stimulation in an in vitro cytotoxicity assay was performed at low CAR19 effector-to-tumor ratio (1:20) to mimic higher tumor burden stress conditions in the presence of each BTKi, including non-covalent BTKi pirtobrutinib (Pirto) (500 nM). Tumor cells were replenished every 3 days and cytotoxicity was measured on day 7 by luciferase assay. For phenotypic characterization, CAR T cells were repeatedly stimulated by plate-bound anti-Whitlow linker every 3 days. Viability and proliferation were measured by cell counter. Flow cytometry (FACS) was performed on surface and intracellular T-cell phenotypic, activation, and exhaustion markers. Metabolic fitness (glycolysis and ATP production) was characterized using Seahorse (Van der Windt, 2012). Impact of Ibr on signal strength was evaluated in CD19-expressing Jurkat reporter cells (CAR-J19) with knock-in of mKate in frame with Nur77. Ibr was added to coculture of CAR-J19 and tumor cells and upregulation of Nur77 was measured by FACS. Downstream ITK signaling pathway was measured by Western blot.

Results: Preliminary in vivo experiments demonstrated that co-administration of each BTKi with CD19 CAR T cells improved tumor control at day 26 (N=5 per group; P≤0.0004 vs CAR T cells alone). In serial killing experiments, addition of Ibr to the coculture increased proliferation (n=2; P<0.05), viability (n=2; P=0.01), and the cytotoxicity of CAR T cells against Ramos cells (P=0.02 vs CAR19 alone) to a greater extent than other BTKis tested (Fig. 1A). Prolonged stimulation of CAR19 with Ibr led to downregulation of activation and inhibitory markers on CAR19 and maintenance of a naive T-cell phenotype (Fig. 1B) that was confirmed by increased metabolic fitness (n=5; P<0.05). Addition of Ibr to CAR-J19 resulted in a 72% reduction in CAR activation and reduced phosphorylation of ITK, PLCG, and Zap70. Additional data including the impact of ITK knockout on CAR-T cells will be presented.

Conclusions: Our study expands on the hypothesized mechanism of action underlying the clinical benefit observed with Ibr in CD19 CAR T-cell-treated patients with chronic lymphocytic leukemia or mantle cell lymphoma (Liu et al, Front Immunol 2022; Wang et al, NEJM 2020). Our data suggest that Ibr enhances CAR T-cell fitness to a greater extent than both covalent and noncovalent BTKi that display reduced or no ITKi activity. Through ITKi, Ibr tempers CAR T-cell activation induced cell death, maintains a naive state and prevents premature exhaustion, thereby enabling more efficacious tumor cell killing over time. Together, these data support the further exploration of Ibr to enhance CD19 CAR T-cell efficacy.