Phosphatidylinositol‐3 kinase inhibitors reproduce the selective antiproliferative effects of imatinib on chronic myeloid leukaemia progenitor cells

Summary We investigated the role of the phosphatidylinositol‐3 kinase (PI‐3K) pathway in regulating the proliferation of primary chronic myeloid leukaemia (CML) progenitor cells by using imatinib to inhibit the activity of p210Bcr‐Abl. The effect of imatinib on the expression of PI‐3K pathway proteins was investigated by kinase assays and Western blotting; PI‐3K was inhibited by wortmannin or LY294002, Jak2 by AG490 and farnesylation by FTI II; progenitor cell proliferation (self‐renewal) was measured by growing myeloid colonies in vitro, then replating them to observe secondary colony formation. Suppression of p210Bcr‐Abl with imatinib indirectly suppressed the activity of PI‐3K and its downstream targets (Erk, Akt and p70S6 kinase), thereby implicating the PI‐3K pathway in p210Bcr‐Abl‐mediated signalling in primary CML progenitor cells. The PI‐3K inhibitors, wortmannin and LY294002 reproduced the differential effects of imatinib on normal and CML progenitor cell proliferation in vitro by increasing normal cell (P = 0·001) and reducing CML cell proliferation (P = 0·0003). This differential effect was attributable to dysregulated signalling by granulocyte colony‐stimulating factor in CML. The responses of individual patient's cells to wortmannin correlated with their responses to imatinib (P = 0·004) but not their responses to AG490 (Jak2 kinase inhibitor) or FTI II (farnesyltransferase inhibitor). Individual responses to wortmannin also correlated with responses to interferon α (IFNα) (P = 0·016). Imatinib‐resistant K562 cells were sensitive to LY294002. Inhibition of the PI‐3K pathway may be common to imatinib and IFNα and reflect dysregulated cytokine signalling. As imatinib‐resistant cells remained sensitive to wortmannin and LY294002, targeting the PI‐3K pathway may provide an alternative therapy for imatinib‐resistant patients.