Potent and cellularly active 4-aminoimidazole inhibitors of cyclin-dependent kinase 5/p25 for the treatment of Alzheimer’s disease Christopher J. Helal a, * , Zhijun Kang a , John C. Lucas a , Thomas Gant a , Michael K. Ahlijanian b , Joel B. Schachter b , Karl E. G. Richter b , James M. Cook b , Frank S. Menniti b , Kristin Kelly b , Scot Mente c , Jay Pandit d , Natalie Hosea e a Neuroscience Medicinal Chemistry, Pfizer Global Research and Development, Eastern Point Road, Groton, CT 06340, USA b Neuroscience Biology, Pfizer Global Research and Development, Eastern Point Road, Groton, CT 06340, USA c Computational Chemistry, Pfizer Global Research and Development, Eastern Point Road, Groton, CT 06340, USA d Structural Biology, Pfizer Global Research and Development, Eastern Point Road, Groton, CT 06340, USA e Pharmacokinetics and Drug Metabolism, Pfizer Global Research and Development, Eastern Point Road, Groton, CT 06340, USA article info Article history: Received 10 June 2009 Revised 31 July 2009 Accepted 4 August 2009 Available online 8 August 2009 Keywords: Cyclin-dependent kinase 5 p25 Cyclin-dependent kinase 2 Alzheimer’s disease abstract Utilizing structure-based drug design, a 4-aminoimidazole heterocyclic core was synthesized as a replacement for a 2-aminothiazole due to potential metabolically mediated toxicity. The synthetic route utilized allowed for ready synthesis of 1-substituted-4-aminoimidazoles. SAR exploration resulted in the identification of a novel cis-substituted cyclobutyl group that gave improved enzyme and cellular potency against cdk5/p25 with up to 30-fold selectivity over cdk2/cyclin E. Ó 2009 Elsevier Ltd. All rights reserved. Cyclin dependent kinases (cdk) carry out critical roles in cell cy- cling, and cdk inhibitors have been extensively researched as ther- apeutic agents for the treatment of cancer. 1 Binding of cdk5 by the protein p35 results in kinase activation and the ability to phos- phorylate the cytoskeletal stabilizing protein tau, a critical compo- nent of cell structure regulation. 2 The membrane-bound p35 can be proteolytically cleaved by calpain to the more stable, cytosolic p25 which has led to the hypothesis that cdk5/p25 may play an integral role in Alzheimer’s disease development. The longer-lived cdk5/p25 complex is believed to over-phosphorylate tau, resulting in the formation of paired helical filaments and deposition of cyto- toxic neurofibrillary tangles. 3 Thus, inhibition of the aberrant cdk5/ p25 complex is a viable target for treating Alzheimer’s disease by preventing tau hyperphosphorylation and subsequent neurofibril- lary tangle formation. 4 In addition to being a target for Alzheimer’s disease, recent evidence suggests that inhibition of cdk5 could also be relevant for the treatment of type-II diabetes, pain, and stroke, furthering interest in this enzymatic target. 5,6 We reported on the optimization of potency and cdk2/cyclin E selectivity in a series of 2-aminothiazole cdk5/p25 inhibitors that bind in the ATP binding pocket. 7 Selectivity over cdk2 is desired due to its role in modulating the cell cycle and potential side effects. This is a challenging task, considering 93% (27/29) of residues are conserved in the respective ATP pockets of cdk5 and cdk2 and that the two differing amino acid residues (Cys83 and Asp84 in cdk5; Leu83 and His84 in cdk2) have sidechains that project away from the ATP pocket, thereby reducing their impact on inhibitor binding. In furthering our exploration of this series, alternative heterocy- clic cores to the 2-aminothiazole (1) were sought, based upon the metabolism-induced toxicity potential of this class of heterocycles 8 ( Fig. 1). Considering the binding mode of inhibitors to cdk5, as observed in computer modeling in addition to X-ray crystal struc- tures in cdk2, key H-bond acceptor and donor interactions with Cys 83 in the hinge region (Leu 83 in cdk2) were to be maintained along with necessary lipophilic groups. The 4-aminoimidazole core (2) was appropriately disposed to make the requisite hydrogen bonding and hydrophobic interactions and was thus deemed a suitable target for biological evaluation. The synthesis of 4-aminoimidazole analogs was carried out as shown in Scheme 1. 9 The treatment of 1,4-dinitroimidazole 10 (3) with a primary amine afforded 1-substituted 4-nitroimidazoles 4. CAUTION: thermodynamic testing showed that 3 was a highly ener- getic substance with the potential for explosion. 11 Catalytic hydroge- 0960-894X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.bmcl.2009.08.019 * Corresponding author. Tel.: +1 860 715 5064. E-mail address: chris.j.helal@pfizer.com (C.J. Helal). Bioorganic & Medicinal Chemistry Letters 19 (2009) 5703–5707 Contents lists available at ScienceDirect Bioorganic & Medicinal Chemistry Letters journal homepage: www.elsevier.com/locate/bmcl