2518 Background: AT9283 inhibits aurora kinase A and B and targets other tyrosine and serine/threonine kinases associated with myeloid cell proliferation. Exposure of leukemia cell lines to AT9283 in vitro induces an “aurora inhibitory” phenotype with creation of large aneuploidal cells with limited replication potential which ultimately undergo mitotic catastrophe and apoptosis. Methods: AT9283 was administered by 72-hour IV continuous infusion (IVCI) to pts with refractory AML, ALL, high risk MDS, and imatinib- and dasatinib-refractory CML. A 3+3 dose escalation phase I design was employed to determine MTD and biological activity, and standard DLT definitions were employed. Results: A total of n=29 patients (med. age 54 years, range 22 to 86; 15M/14F) have been treated at 8 different dose levels: 3 (n=3 pts); 6 (n=3); 12 (n=7); 24 (n=3); 48 (n=4); 72 (n=3); 108 (n=3); and 162 (n=3) mg/m2 daily x 3. Median number of prior treatments was 2 (range 1–5); n=3 pts had prior allografts. Biological activity was...