The protein kinase Chk2 (checkpoint kinase 2) is a major effector of the replication checkpoint. Chk2 activation is initiated by phosphorylation of Thr68, in the serineglutamine/threonine-glutamine cluster domain (SCD), by ATM. The phosphorylated SCD-segment binds to the FHA domain of a second Chk2 molecule, promoting dimerisation of the protein and triggering phosphorylation of the activation segment/T-loop in the kinase domain. We have now determined the structure of the kinase domain of human Chk2 in complexes with ADP and a small-molecule inhibitor debromohymenialdisine. The structure reveals a remarkable dimeric arrangement in which T-loops are exchanged between protomers, to form an active kinase conformation in trans. Biochemical data suggest that this dimer is the biologically active state promoted by ATM-phosphorylation, and also suggests a mechanism for dimerisation-driven activation of Chk2 by trans-phosphorylation.

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CYC202 showed therapeutic activity on human cancer cell lines in vitro and on xenografts. Pharmacodynamic markers are altered in vitro and in vivo, consistent with the inhibition of cyclin-dependent kinases. Such markers may be potentially useful in the clinical development of CYC202 and other cyclin-dependent kinase inhibitors.