| Summary: | Ibrutinib and other targeted inhibitors of B-cell receptor signaling achieve impressive clinical results for patients with chronic lymphocytic leukemia (CLL). A treatment-induced rise in absolute lymphocyte count (ALC) has emerged as a class effect of kinase inhibitors in CLL and warrants further investigation. We here report correlative studies in 64 patients with CLL treated with ibrutinib. We quantified tumor burden in blood, lymph nodes, spleen, and bone marrow, assessed phenotypic changes of circulating cells, and measured whole blood viscosity. With just one dose of ibrutinib the average increase in ALC was 66%, and in over 40% of patients the ALC peaked within 24 hours of initiating treatment. Circulating CLL cells on day 2 showed increased Ki67 and CD38 expression, indicating an efflux of tumor cells from the tissue compartments into the blood. The kinetics and degree of the treatment-induced lymphocytosis was highly variable; interestingly in patients with a high baseline ALC the relative increase was mild and resolution rapid. After two cycles of treatment the disease burden in lymph node, bone marrow, and spleen decreased irrespective of the relative change in ALC. Whole blood viscosity was dependent on both ALC and hemoglobin. No adverse events were attributed to the lymphocytosis.
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