CML is caused by a consistent genetic abnormality, termed the Philadelphia chromosome, that results from a reciprocal (9;22) translocation leading to the expression of the BCR-ABL fusion protein. Although treatment has been revolutionized by the introduction of tyrosine kinase inhibitors which target Abl activity, reactivation of Abl signaling via several different point mutations remains problematic. In particular the mutation of Threonine 315 to Isoleucine (T315I) confers resistance to all existing therapies with tyrosine kinase inhibitors in the clinical settings. We describe the in vitro and in vivo effects of AT9283, a potent inhibitor of several protein kinases, including Abl kinase (wild type BCR-ABL and several of the drug resistant mutant variants that have arisen in clinical practice e.g. T315I), JAK2, JAK3 and Aurora kinases A and B, on imatinib-resistant CML cells including those harboring BCR-ABL (T315I). AT9283 has potent anti-proliferative activity in a panel of BaF3 and human cell lines expressing the BCR-ABL or its mutant forms. In BaF3 BCR-ABL wild-type and T315I mutant cells and K562 CML cells we observed inhibition of substrates of both BCR-ABL (STAT5) and Aurora B (Histone H3) at concentrations >300nM and <100nM, respectively, suggesting that AT9283 is capable of inhibiting Aurora and BCR-ABL simultaneously in these cell lines. The in vivo effects of AT9283 were examined in several mouse models engrafted either subcutaneously or intravenously with BaF3, human CML cell lines or primary CML patient samples expressing the BCR-ABL or its mutant forms. Specifically AT9283 prolonged the survival of mice engrafted intravenously with either BaF3 BCR-ABL T315I, or E255K cells when administered intraperitoneally twice daily at doses of either 6.25 or 10mg/kg or once daily at 15mg/kg when administered 5 days in every week repeated twice. Maximal survival advantage was conferred at either 10mg/kg twice daily or 15mg/kg once a day. Similar data were obtained in an intravenous model using primary CML cells taken from a patient harbouring the BCR-ABL E255K mutation. We also present data from ongoing studies showing increased survival rates in these in vivo model systems following multiple cycles of AT9283 administered on the 15mg/kg once daily schedule. These data together support further clinical investigation of AT9283 in patients with treatment resistant CML.

Disclosures: Squires:Astex Therapeutics Ltd.: Employment, Research Funding. Mallett:Astex Therapeutics Ltd.: Employment, Research Funding. Smyth:Astex Therapeutics Ltd.: Research Funding. Thompson:Astex Therapeutics Ltd.: Employment, Research Funding. Lyons:Astex Therapeutics Ltd.: Employment, Research Funding.

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