Effects of the Btk Inhibitor Ibrutinib on Monocyte Responses to Antibodies

The novel and irreversible tyrosine kinase inhibitor, Ibrutinib, has shown significant activities across a variety of B-cell tumors such as Chronic Lymphocytic Leukemia (CLL) and B-cell Non-Hodgkin lymphoma, either alone or in combination with antibody therapy. Fcγ receptors (FcγR) on effector immune cells such as macrophages play an important role in tumor-specific antibody-mediated immune responses. Here we tested the effect of Ibrutinib on FcγR-mediated activities in monocytes.

Our results indicated that Ibrutinib does not affect monocyte FcγR-mediated phagocytosis even at concentrations higher than those achieved physiologically, but suppresses FcγR-mediated cytokine production. This phenomenon was also seen in macrophages from Xid mice, in which there is a decrease in FcγR-mediated cytokine production but not phagocytosis.

One major signaling event downstream of Btk that is blocked by Ibrutinib is calcium flux. Thus, we next tested whether phagocytosis was independent of calcium flux in order to help explain the lack of effect that Ibrutinib had on monocyte phagocytic ability. Results showed that intracellular calcium flux is required for FcγR-mediated cytokine production but not for phagocytosis. In order to verify this finding, we then measured the activation of the GTPase Rac, which is responsible for actin polymerization needed for phagocytosis. Results showed that Ibrutinib did not inhibit Rac activation, nor did inhibition of calcium flux by BAPTA-AM.

We next asked whether the effect of Ibrutinib on cytokine production could be reversed by IFNγ priming since NK cells produce significant levels of IFNγ in response to antibody therapy. Treatment of monocytes with IFNγ prior to Ibrutinib treatment resulted in a lack of inhibition of cytokine production, suggesting that IFNγ signaling can overcome Ibrutinib effects. We are currently in the process of examining the mechanisms by which IFNγ can protect from Ibrutinib effects on monocytes.

These results suggest that combing Ibrutinib with monoclonal antibodies can enhance B-CLL killing while not affecting macrophage effector function.