br The serine threonine kinase haspin ploid Germ

The serine/threonine kinase haspin (ploid Germ Cell-pecific Nuclear rote Kinase, also known as erm Cell-pecific ene-2; Gsg2) functions in mitosis, where it phosphorylates histone H3 at Thr-3 (H3T3ph). During mitosis, this phosphorylation generates a binding site on H3 for Survivin and thereby positions the Chromosome Passenger Complex at centromeres to regulate chromosome segregation,, and it also displaces proteins such as TFIID that normally bind to H3 through methylated Lys-4. Depletion of haspin by RNA interference, or microinjection of H3T3ph antibodies, causes chromosome alignment defects and failure of normal mitosis., , Human haspin has ATP-binding and catalytic sites structurally similar to other members of the eukaryotic protein kinase (ePK) superfamily with several notable exceptions. For example, the highly conserved DFG motif involved in ATP-binding and the APE motif involved in stabilizing the C-terminal lobe among ePKs are altered or absent and the activation loop region is substantially rearranged in haspin compared to other ePKs., Haspin kinase inhibitors are expected to be useful probes for elucidating the cellular roles of this protein and may have therapeutic utility in treating cancer. A recently described small molecule, CHR-6494 (), that inhibits haspin displayed anti-tumor activity in a mouse xenograft model. Also, 5-iodotubercidin () has been reported as an effective haspin kinase inhibitor., We previously utilized a time-resolved fluorescence resonance LY-411575 synthesis transfer (TR-FRET) high throughput screening (HTS) assay to identify the acridine derivative (LDN-192960) as another potent haspin inhibitor (; IC=0.010μM)., This assay has now also been used to discover the beta-carbolines harmine, , and harmol, , as moderately potent haspin inhibitors with IC values of 0.59 and 0.77μM, respectively. Harmine has previously been identified as an inhibitor of DYRK family kinases, with IC values between 0.03 and 0.35μM reported for DYRK1A, and approximately 50-fold lower potency toward DYRK2. Herein, we describe the design, synthesis and improved potency of the beta-carboline series for haspin inhibition utilizing the structure–activity relationships previously determined for the acridine series combined with in silico docking using a recently disclosed crystal structure of the kinase. In addition, in vitro mouse liver microsome stability and kinase profiling of a representative beta-carboline analog are presented. A crystal structure of haspin bound to AMP was used for docking calculation., Analysis of this structure revealed key hydrogen bonds between nitrogen atoms of the adenine ring of AMP and protein backbone atoms of residues E606 and G608 (). A 12Å docking grid was generated using the AMP center of mass as the point of origin with a single hydrogen bond constraint on the backbone amide of G608. Docking calculations were performed on , which demonstrated that this inhibitor was well accommodated within the binding site and satisfied the hydrogen bonding constraint on G608 (). In addition, the inhibitor also made a hydrogen bond with K511, which likely disrupts a key salt bridge between this residue and E535 that is necessary for closure of the ATP-binding cleft enabling kinase activity. A metadynamic simulation sampling the two torsion angles (X and X) of the alkylamine as collective variables was also conducted. One low energy conformation was found () that allowed a hydrogen bond between the amine and D611 (). Next, docking calculations were performed on four harmine analogs (– and –) that incorporate at two different positions alkylamines similar to that present in . The two derivatives with the alkylamine on the N-position ( and ) were well accommodated within the binding site making a hydrogen bond with D687 as well as the hydrogen bonding constraint on G608 (B and C). In contrast, compounds and resulted in steric hindrance with the region around F607 and G608 and were not well situated within the ATP-binding site, suggesting that they would unlikely be haspin inhibitors. A similar metadynamic simulation for sampling two torsion angles (X and X) of the alkylamine as collective variables found two low energy conformations (A). One of these allowed a hydrogen bond between the amine and D611 (B), similar to . However, the second conformation permitted a hydrogen bond between the amine and the backbone carbonyl of I490 in the glycine-rich loop (C).