Combination immunotherapy of ibrutinib, fludarabine, and pembrolizumab for high-risk CLL patients: clinical and immunological analysis of a pilot phase II study

Abstract Chronic lymphocytic leukemia (CLL) is due to clonal expansion of CD5+ B cells. Immunosuppression is a hallmark of CLL and includes impaired anti-tumor T cell responses. Exhaustion of PD-1+ CD8 T cells has been implicated in disease progression, providing a rationale to test checkpoint inhibitors in the clinic. However, single-agent anti-PD-1 antibodies lacked clinical efficacy. Here, we analyzed immunologic parameters in patients enrolled on a phase II study of ibrutinib (continuous administration), fludarabine (2 cycles) and pembrolizumab (for 18 cycles after fludarabine). 13 patients were enrolled, 10 patients were evaluable, 9 patients achieved partial response (90%) and 1 patient complete response (10%). Bulk RNA-seq of CLL and T cells revealed inhibition of B-cell receptor and NF-kB signaling in tumor cells, and increased proliferation and activation gene signatures in CD3+ T cells. The proportion of Ki-67+ CD8 T cells increased in 5 out of 13 patients (immune responders) 3 weeks after the first dose of pembrolizumab. Compared with immune non-responders, responders had a significantly higher percentage of CD45RO-CD27+ and CD45RO+CD27+ CD8 T cells. At 6 months, T cell proliferation in all patients was back to baseline and there was no change in TCR clonality at 12 months. In vitro stimulation with CD3 and CD28 antibodies upregulated activation markers on CD8 T cells in all patients, increases in Ki-67 were variable. Overall, responses in patients were weaker than in healthy controls. In summary, pembrolizumab in combination with ibrutinib transiently increased CD8 T cell proliferation in a subset of CLL patients but did not improve the clinical response over ibrutinib.