¹Molecular Oncology and Experimental Therapeutics Programs, H Lee Moffitt Cancer Center and Research Institute, University of South Florida College of Medicine, Tampa, FL, USA

²Department of Interdisciplinary Oncology, University of South Florida College of Medicine, Tampa, FL, USA

³Department of Medical Microbiology and Immunology, University of South Florida College of Medicine, Tampa, FL, USA

⁴Cancer Pharmacology, Pfizer Global Research and Development, Ann Arbor Laboratories, Ann Arbor, MI, USA

⁵Department of Biochemistry and Molecular Biology, University of South Florida College of Medicine, Tampa, FL, USA

Correspondence to: J Wu, Molecular Oncology Program, MRC 3-East, H Lee Moffitt Cancer Center and Research Institute, 12902 Magnolia Drive, Tampa, FL 33612, USA; Fax: (813) 903-6817

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 CML patients could develop resistance to the Bcr-Abl tyrosine kinase inhibitor, imatinib (STI571, 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-Abl 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 hematopoietic cells. An interleukin-3 (IL-3) autocrine loop was observed previously in primitive CD34⁺/Bcr-Abl⁺ leukemic cells in CML patients. Using 32Dp210^Bcr-Abland Baf3p210^Bcr-Abl cells as models, we tested whether IL-3 might protect Bcr-Abltransformed, IL-3-responsive cells from apoptosis caused by Bcr-Abl tyrosine kinase inhibition. Results of trypan blue exclusion, fluoroisothiocyanate-valyl-alanyl-aspartyl-[O-methyl] -fluoromethylketone (FITC-VAD-FMK), and Annexin-V/7-amino-actinomycin D (7-AAD) binding assays indicate that IL-3 could protect Bcr-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 CML patients could contribute to the incomplete eradication of Bcr-Abl⁺ cells by Bcr-Abl inhibition.

Leukemia (2002) 16, 1589-1595. doi:10.1038/sj.leu.2402678

Bcr-Abl; leukemia; tyrosine kinase inhibitor; interleukin-3; apoptosis