Abstract 5232: Tyrosine kinase inhibitor, gefitinib targets ZAP-70 over expressing chronic lymphocytic leukemia cells

Abstract

Abstract Most patients with chronic lymphocytic leukemia (CLL) initially respond to chemotherapy but relapse. There is a subset of CLL patients that have aggressive disease characterized by an over expression of the tyrosine kinase ZAP-70. These ZAP-70+ patients have a poorer survival and are more resistant to chemotherapy. The biological effects of ZAP-70 in CLL are believed to be related to its ability to enhance activation of Syk with subsequent triggering of down-stream modulators, such as ERK and AKT. Normally, Syk is activated by increased phosphorylation by Lyn, which is increased in CLL, or by activation of the B-cell receptor. Gefitinib is a tyrosine kinase inhibitor that is presently used to treat lung cancer. The drug inhibits epidermal growth factor receptor (EGFR) tyrosine kinase activity but has activity against >20 other kinase targets, including members of the Src family of kinases, such as Lyn and Syk. We thus evaluated the cytotoxic activity of gefitinib against primary CLL cells using the MTT assay. Gefitinib was cytotoxic to ZAP-70+ (≥20% cells positive) CLL samples (median IC50, ∼3.0 µM) while ZAP-70- patients failed to respond to gefitinib with a median IC50 of >15.0 µM. Jurkat cells, which are T cells that express high levels of ZAP-70, were more sensitive to gefitinib than B cell lines that did not express ZAP-70 (BJAB and NALM6). Further studies confirmed that gefitinib was inducing cell death through apoptosis. Gefitinib was also effective against ZAP-70+ CLL cells that were resistant to fludarabine and chlorambucil indicating a novel mechanism of action to standard chemotherapy. Enhanced apoptosis was observed in ZAP70+ CLL and Jurkat cells exposed to gefitinib and fludarabine or chlorambucil, but this was not observed in ZAP-70- or the B cell lines. In contrast, another EGFR inhibitor, erlotinib, had no effect against ZAP-70+ CLL cells indicating that gefitinib was inhibiting a kinase unaffected by erlotinib. When Jurakat and ZAP-70+ CLL cells were treated with gefitinib, there was reduced total tyrosine phosphorylation and decreased tyrosine phosphorylation of ZAP-70, and Syk but no difference in Lyn phosphorylation. Furthermore, gefitinib blocked B cell receptor activation and mediated cell survival. Taken together, these results indicate that gefitinib may be useful agent in the treatment of ZAP-70+ CLL, either alone or in combination with fludarabine or chlorambucil. Ongoing studies are determining the precise mechanism of action of gefitinib in ZAP-70+ CLL and assessing its effects on the CLL microenvironment. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 5232. doi:1538-7445.AM2012-5232