Imatinib mesylate has an inhibitory effect on hematopoiesis in a mice model

Imatinib mesylate (Gleevec or STI-571), a tyrosine kinase inhibitor, can potently block both platelet-derived growth factor (PDGF) receptor-α and β by inhibiting the receptor tyrosine kinase activity. Imatinib mesylate has been successfully used to treat patients with chronic myeloid leukemia (CML) and Ph positive acute lymphoid leukemia (ALL). However, a significant number of patients with imatinib mesylate treatment have developed neutropenia, thrombocytopenia and anemia (van Deventer, Hall et al. 2002). Our previous studies have demonstrated that PDGF/PDGFR plays an important role on thrombopoiesis, and blockage of PDGFR by imatinib inhibited the downstream signal pathway, which led to reduce of megakaryocyte production (Ye, et al. Haematologica, 2010). This may partially explain the imatinib-related thrombocytopenia. However, the role of imatinib on normal hematopoiesis remains to be investigated. In the present study, we identified the expression of PDGFR-β on the human megakaryoblastic cells: Meg-01, CHRF-288-11, Dami and M-07e, human erythromyeloblastoid leukemia cells: K-562 and human myeloblastic cells: HL-60 by RT-PCR, western blot and flow cytometry. We then demonstrated the effect of imatinib on normal hematopoiesis in the 8 wk old C57BL/6 mice model. Gleevec (25 mg/kg/day) dissolved in distilled water or water only were given orally for up to 2 wks. 15ul whole blood was taken from postorbital weekly to perform blood cell counts. The Gleevec -treated group showed a reduction of platelet count as compared to the control at day 14 (543.3±71.95×10⁹/L vs. 330.0±62.23×10⁹/L, n=5, P<0.05). Similarly, Gleevec caused a significant decrease in the number of white blood cells on day 14 (9.667×±0.8511×10⁹/L vs. 6.080±0.8015×10⁹/L, n=5, P<0.05). While no significant differences were found on red blood cell count between Gleevec and control groups. The mice bone marrow was harvested from each group to perform CFU assays. Gleevec significantly inhibited the formation of bone marrow CFU-MK (15.33±1.764 vs 3.333±0.6667, n=3), and CFU-GM(13.50±2.784 vs 6.333±0.7265, n=3) compared with the control. There were no significant differences found in CFU-GEMM and CFU/BFU-E formation between the Gleevec and control treatment. Our studies suggested that PDGFR were expressed on hematopoietic cells. Imatinib mesylate, an inhibitor of PDGFR, not only played an important role in malignant hematopoiesis, but also in normal hematopoiesis. Administration of Gleevec reduced white blood cells and megakaryocytes/platelets production, which implied its potential therapeutic role in treating myeloproliferative diseases, such as Essential Thrombocytosis.