The fusion protein BCR-ABL is a hallmark of chronic myelogenous leukemia (CML) and Philadelphia-positive acute lymphocytic leukemia (Ph+-ALL), and has been demonstrated as the primary driver of these diseases. Control of CML for considerable periods of time has been achieved through use of selective ABL kinase inhibitors, particularly imatinib. Once patients fail imatinib therapy, they are commonly found to harbor a mutated and activated form of BCR-ABL which is unable to bind the inhibitor. A more recent ABL inhibitor, dasatinib, can block growth of cells harboring most of the imatinib-resistant mutations, but T315I and F317L mutations are often seen in patients relapsing after dasatinib therapy. Thus once a patient develops CML harboring these mutations, there are few therapeutic options available. XL228 is a potent multi-targeted protein kinase inhibitor with activity against IGF1R, src, and Abl. It displays low nanomolar biochemical activity against wild type Abl kinase (Ki = 5 nM), as well as the T315I form of Abl resistant to imatinib and dasatinib (Ki = 1.4 nM). XL228 also inhibits Aurora A with an IC50 of approximately 3 nM, demonstrating a more balanced inhibition profile compared to other dual Abl /Aurora inhibitors. CML and ALL cell lines were evaluated for sensitivity to XL228, and in each case the IC50 for inhibition of proliferation was less than 100 nM. XL228 inhibits phosphorylation of BCR-ABL and its substrate STAT5 in K562 cells in vitro with IC50s of 33 and 43 nM, respectively. Single-dose pharmacodynamics studies demonstrate a potent effect of XL228 on BCR-ABL signaling in K562 xenograft tumors. Phosphorylation of BCR-ABL was decreased by 50% at XL228 plasma concentrations of 3.5 μM; a similar decrease in phospho-STAT5 occurred at 0.8 μM plasma concentration. XL228 showed clear superiority to MK-0457, imatinib, and dasatinib in downregulating BCR-ABL phosphorylation in BaF3 cells expressing the T315I form of BCR-ABL in vitro (406 nM, 6912 nM, >10,000 nM, >10,000 nM, respectively), and in xenograft experiments in vivo. These results indicate that XL228 potently inhibits wild type and T315I forms of BCR-ABL, and provide a rationale for the clinical development of this agent for the treatment of patients with drug-resistant disease. A phase I dose escalation study of XL228 in subjects with CML or Ph+-ALL who have failed prior imatinib and dasatinib therapy has been initiated, focusing on safety/tolerability, pharmacodynamics, and pharmacokinetics. Pharmacodynamic assessments include a flow cytometry-based phospho-CrkL assay, quantitative PCR for BCR-ABL, and plasma markers of XL228 activity. An update on our clinical experience with XL228 in subjects resistant or intolerant to imatinib and dasatinib will be presented.

Author notes

Disclosure:Employment: Douglas Clary and Lynne Bui are employees of Exelixis, Inc. Consultancy: Neil Shah has been a consultant of BMS, Novartis, and Exelixis. Ownership Interests:; Douglas Clary and Lynne Bui have ownership interest in Exelixis. Research Funding: Neil Shah is receiving funding from Exelixis for translational research. Membership Information: Neil Shah has participated on an Exelixis advisory committee.

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