Interleukin-3 protects Bcr-Abl-transformed hematopoietic progenitor cells from apoptosis induced by Bcr-Abl tyrosine kinase inhibitors

Bcr-Abl tyrosine kinase has been validated as a molecular target for the treatment of chronic myelogenous leukemia (CML). More recently, it has been reported that CIVIL patients could develop resistance to the Bcr-Abl tyrosine kinase inhibitor, imatinib (ST1571, Gleevec), pointing to the need for development of additional Bcr-Abl tyrosine kinase inhibitors or other therapeutic strategies. It was also found that a significant proportion of patients who received the Bcr-Abl inhibitor did not achieve complete cytogenetic response. Mechanisms for incomplete cytogenetic response to Bcr-AbI inhibition are not entirely clear. We report here three new pyrido[2,3-d]pyrimidine Bcr-Abl tyrosine kinase inhibitors, PD164199, PD173952, PD173958, that induced apoptosis of Bcr-Abl-dependent hematopoletic cells. An interleukin-3 (IL-3) autocrine loop was observed previously in primitive CD34(+)/Bcr-Abl leukemic cells in CML patients. Using 32Dp210(Bcr-Abl) and Baf3p210(Bcr-Abl) cells as models, we tested whether IL-3 might protect Bcr-Abl-transformed, IL-3-responsive cells from apoptosis caused by Bcr-AbI tyrosine kinase inhibition. Results of trypan blue exclusion, fluoroisothlocyanate-valyi-alanyl-aspartyl-[O-methyl] -fluoromethylketone (FITC-VAD-FMK), and Annexin-V/7-aminoactinomycin D (7-AAD) binding assays indicate that IL-3 could protect l3cr-Abl-transformed, IL-3 responsive hematopoietic progenitor cells from apoptosis induced by Bcr-Abl tyrosine kinase inhibitors. This finding raises the possibility that the IL-3 autocrine loop found in primitive CD34(+)/Bcr-Abl(+) cells in CIVIL patients could contribute to the incomplete eradication of Bcr-Abl(+), cells by Bcr-Abl inhibition.