Chronic myeloid leukemia (CML) is caused by the constitutively activated tyrosine kinase breakpoint cluster (BCR)-ABL. Current frontline therapy for CML is imatinib, an inhibitor of BCR-ABL. Although imatinib has a high rate of clinical success in early phase CML, treatment resistance is problematic, particularly in later stages of the disease, and is frequently mediated by mutations in BCR-ABL. Dasatinib (BMS-354825) is a multitargeted tyrosine kinase inhibitor that targets oncogenic pathways and is a more potent inhibitor than imatinib against wild-type BCR-ABL. It has also shown preclinical activity against all but one of the imatinib-resistant BCR-ABL mutants tested to date. Analysis of the crystal structure of dasatinib-bound ABL kinase suggests that the increased binding affinity of dasatinib over imatinib is at least partially due to its ability to recognize multiple states of BCR-ABL. The structure also provides an explanation for the activity of dasatinib against imatinib-resistant BCR-ABL mutants. (Cancer Res 2006; 66(11): 5790-7)

High throughput screening identified 2-acetamido-thiazolylthio acetic ester 1 as an inhibitor of cyclin-dependent kinase 2 (CDK2). Because this compound is inactive in cells and unstable in plasma, we have stabilized it to metabolic hydrolysis by replacing the ester moiety with a 5-ethyl-substituted oxazole as in compound 14. Combinatorial and parallel synthesis provided a rapid analysis of the structure-activity relationship (SAR) for these inhibitors of CDK2, and over 100 analogues with IC(50) values in the 1-10 nM range were rapidly prepared. The X-ray crystallographic data of the inhibitors bound to the active site of CDK2 protein provided insight into the binding modes of these inhibitors, and the SAR of this series of analogues was rationalized. Many of these analogues displayed potent and broad spectrum antiproliferative activity across a panel of tumor cell lines in vitro. In addition, A2780 ovarian carcinoma cells undergo rapid apoptosis following exposure to CDK2 inhibitors of this class. Mechanism of action studies have confirmed that the phosphorylation of CDK2 substrates such as RB, histone H1, and DNA polymerase alpha (p70 subunit) is reduced in the presence of compound 14. Further optimization led to compounds such as water soluble 45, which possesses a favorable pharmacokinetic profile in mice and demonstrates significant antitumor activity in vivo in several murine and human models, including an engineered murine mammary tumor that overexpresses cyclin E, the coactivator of CDK2.

Chronic myeloid leukemia (CML) is a stem cell disorder caused by a constitutively activated tyrosine kinase, the BCR-ABL oncoprotein. Imatinib (STI571, Gleevec) is a small-molecule inhibitor of this kinase that produces clinical remissions in CML patients and is now frontline therapy for this disease. While this agent has a high rate of clinical success in early phases of CML, development of resistance to this drug becomes increasingly problematic in later stages of the disease. BMS-354825, a small-molecule dual-function SRC/ABL tyrosine kinase inhibitor, appears to overcome many of the limitations associated with imatinib therapy. BMS-354825 is 500-fold more potent than imatinib against BCR-ABL and more importantly retains activity against 14 of 15 imatinib-resistant BCR-ABL mutants (Shah et al., Science, 2004;305(5682):399). In addition, BMS-354825 proved to be equally effective against several pre-clinically and clinically derived tumor models of imatinib resistance (Lee et al., Proceedings of the AACR, Volume 45, March 2004 abstract number 3937). In order to better understand the molecular basis of the relationship between inhibitor chemistry and biological activity, the three-dimensional structure of the kinase domain of Abl kinase complexed with BMS-354825 was determined by X-ray crystallography. The structure reveals that BMS-354825 binds in the ATP-binding site. A comparison with the imatinib-Abl complex (PDB entry 1IEP) reveals that the central cores of BMS-354825 and imatinib occupy overlapping regions but that these two inhibitors extend in opposite directions. The activation loop is observed to be in the active conformation in the presence of bound BMS-354825 in contrast to bound imatinib. There do not appear to be any steric clashes that would preclude BMS-354825 from also binding to the inactive conformation of the activation loop. This observation suggests that the increased binding affinity of BMS-354825 over imatinib is at least partially due to its apparent ability to recognize multiple states of the enzyme. The P-loop is partially disordered as indicated by high B-factors and broken electron density which suggests that interactions between this part of the protein and BMS-354825 are less critical for binding. Interestingly, several imatinib-resistant mutations occur in the P-loop. The structure was analyzed for the 15 imatinib-resistant BCR-ABL mutants and attempts are made to rationalize the activity of BMS-354825 against these mutants.