Targeting the Hedgehog (Hh) pathway represents a potential leukaemia stem cell (LSC)-directed
therapy which may compliment tyrosine kinase inhibitors (TKIs) to eradicate LSC in chronic phase
(CP) chronic myeloid leukaemia (CML). We set out to elucidate the role of Hh signaling in CP-CML and
determine if inhibition of Hh signaling, through inhibition of smoothened (SMO), was an effective
strategy to target CP-CML LSC. Assessment of Hh pathway gene and protein expression demonstrated
that the Hh pathway is activated in CD34+ CP-CML stem/progenitor cells. LDE225 (Sonidegib), a
small molecule, clinically investigated SMO inhibitor, used alone and in combination with nilotinib,
inhibited the Hh pathway in CD34+ CP-CML cells, reducing the number and self-renewal capacity of
CML LSC in vitro. The combination had no effect on normal haemopoietic stem cells. When combined,
LDE225+nilotinib reduced CD34+ CP-CML cell engraftment in NSG mice and, upon administration
to EGFP+ /SCLtTA/TRE-BCR-ABL mice, the combination enhanced survival with reduced leukaemia
development in secondary transplant recipients. In conclusion, the Hh pathway is deregulated in
CML stem and progenitor cells. We identify Hh pathway inhibition, in combination with nilotinib, as a
potentially effective therapeutic strategy to improve responses in CP-CML by targeting both stem and
progenitor cells.