Synergism between bosutinib (SKI-606) and the Chk1 inhibitor (PF-00477736) in highly imatinib-resistant BCR/ABL+ leukemia cells

• Bosutinib/PF co-treatment synergically induces apoptosis in imatinib-resistant cells.
• Bosutinib blocks PF-induced ERK1/2 activation and sharply increases apoptosis.
• Bosutinib/PF suppresses BaF3/T315I tumor growth and prolongs survival.
• Bosutinib/PF potentiates apoptosis in CML patient samples but not normal cells.

Interactions between the dual BCR/ABL and Src inhibitor bosutinib and the Chk1 inhibitor PF-00477736 were examined in BCR/ABL+ leukemia cells, particularly imatinib-resistant cells, including those with the T315I mutation. Bosutinib blocked PF-00477736-induced ERK1/2 activation and sharply increased apoptosis in association with Mcl-1 inhibition, p34(cdc2) dephosphorylation, BimEL up-regulation, and DNA damage in imatinib-resistant CML or Ph+ ALL cell lines. Inhibition of Src or MEK1 by shRNA significantly enhanced PF-0047736 lethality. Bosutinib/PF-00477736 co-treatment also potentiated cell death in CD34+ CML patient samples, including dasatinib-resistant blast crisis cells exhibiting both T315I and E355G mutations, but was minimally toxic to normal CD34+ cells. Finally, combined in vivo treatment significantly suppressed BaF3/T315I tumor growth and prolonged survival in an allogeneic mouse model. Together, these findings suggest that this targeted combination strategy warrants attention in IM-resistant CML or Ph+ ALL.