In Vitro and In Vivo Activity of SKI-606, a Novel Abl/Src Inhibitor, Against Imatinib Resistant BCR-ABL+ Neoplastic Cells.

Resistance to imatinib represents an important scientific and clinical issue in CML and Ph+ ALL and is usually due to the selection of cells harbouring kinase point mutations of the BCR-ABL gene or presenting BCR-ABL gene amplification. In order to improve Bcr-Abl inhibition, several inhibitors with higher potency compared to imatinib have been identified including dasatinib and nilotinib, which show clinical activity in patients resistant to imatinib. In the present study the effects of the novel inhibitor SKI-606 on various models of resistance to imatinib were studied. SKI-606 is a selective dual Src/Abl inhibitor potently active against several CML cell lines and transfectants with IC₅₀ values in the low nanomolar range, 2 logs lower than those obtained with imatinib. Cells expressing activated forms of c-KIT (HMC1⁵⁶⁰ and GIST882) or PDGFR (Ba/F3 Tel-PDGFRβ) were unaffected by SKI-606, in contrast to imatinib and dasatinib. SKI-606 reduced proliferation rate of cells where resistance to imatinib was caused by BCR-ABL gene amplification, such as Lama84R and of imatinib-resistant K562R and KCL22R in which no known mechanism have been identified. D276G, Y253F and E255K kinase point mutants were affected by SKI-606, resulting in inhibition of cell proliferation with IC₅₀ values of 25, 40 and 394 nM (imatinib:1147, 1888 and 3174 nM). No activity was observed against T315I mutant. These results were confirmed in in vivo experiments, in models where resistance was not caused by mutations, as well as in cells carrying the Y253F, E255K and D276G mutations. In the first one SKI-606 was able to eradicate tumors, while significant delay in tumor growth was observed using Ba/F3 transfectants. Modelling considerations attribute the superior activity of SKI-606 to its ability to bind the intermediate (1M52) conformation of Bcr-Abl, in contrast to imatinib.

Excluding the T315 residue involved in direct hydrophilic interactions with both SKI-606 and imatinib, Y253 and E255 make electrostatic interactions stabilizing the inactive conformation of Abl nucleotide binding loop, thus affecting in a more pronounced way the binding of imatinib. Similar considerations have been made for the D276 residue affecting a-Helix C inactive conformation. The extended profile of activity performed here allows us to conclude that SKI-606 is a fairly active and specific inhibitor, with a very limited number of targets outside the Abl and Src families of TK.