HARNESSING BTKI THERAPY BY CDK4/6I CONTROL OF T CELL SURVEILLANCE

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Introduction: Drug resistance remains a formidable challenge in mantle cell lymphoma (MCL). Cell cycle dysregulation driven by aberrant Cyclin D1 and CDK4 expression is a hallmark for MCL. By inhibition of CDK4/6, we have developed a novel strategy that both inhibits proliferation of MCL cells and reprograms them for cytotoxic killing. In a phase I clinical trial we investigated if inhibition of cyclin D1/CDK4 with palbociclib could reprogram recurrent MCL to deepen and prolong the ibrutinib response, and the mechanism for resistance by longitudinal genomic analysis of sequential samples from individual patients in the context of the clinical response. Methods: Palbociclib was administered to recurrent MCL patients on days 1–21 of a 28-day cycle; ibrutinib was given continuously. For longitudinal genomic analysis, sequential tissue and blood specimens from 27 evaluable MCL patients before, during therapy and on progression were collected. Single cell RNA-seq (scRNA-seq) of PBMC or the monocytic fractions from bone marrow and lymph node was performed using a unique in house MCL-specific library. The data were then subject to multiplex analysis with whole transcriptome sequencing and whole exome sequencing of purified MCL cells and flowcytometry of the same samples in conjunction with IHC. Results: Palbociclib appears to deepen and prolong the ibrutinib response; the CR rate was 42% CR, and 5 patients (2 CR and 3 PR) remained on therapy for >8 years. Longitudinal scRNA-seq further revealed that MCL cells comprise 4 major transcriptomically distinct clusters (C)s. Primary resistance and progression on therapy were associated with a marked expansion of either long-live non-proliferating C3 cells or C2 cells that fuel the proliferating C4 cells. Resistance was associated with a profound reduction in MHC I and MHC II expression and disease progression was further accompanied by a rapid loss of both CD8+ and CD+ T cells. This suggests T cell surveillance plays a critical role in maintaining a durable ibrutinib response. Guided by our discoveries, we have restored ibrutinib sensitivity by target patient-specific expansion of C2 or C3 MCL cells ex vivo. In one patient, this led to restoration of CD4+ and CD8+T cells and a CR over 2.5 years. Conclusion: T cell surveillance plays a critical role in prolonging the ibrutinib response by CDK4/6 inhibition, implicating genome-guided combination therapy to overcome ibrutinib resistance in MCL. The research was funded by: PO1 grant from the National Cancer Institute MCL-RI grant from Leukemia and Lymphoma Society Keywords: combination therapies, genomics, epigenomics, and other-omics, tumor biology and heterogeneity No conflicts of interests pertinent to the abstract.

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