A-420983: a potent, orally active inhibitor of lck with efficacy in a model of transplant rejection David W. Borhani, David J. Calderwood, * Michael M. Friedman, Gavin C. Hirst, * Biqin Li, Adelaine K. W. Leung,   Brad McRae, Sheldon Ratnofsky, Kurt Ritter à and Wendy Waegell Abbott Bioresearch Center, 100 Research Drive, Worcester, MA 01605-5314, USA Received 6 February 2004; accepted 17 February 2004 Abstract—We have identified the pyrazolo[3,4-d]pyrimidine A-420983 (compound 7) as a potent inhibitor of lck. A-420983 exhibits oral efficacy in animal models of delayed-type hypersensitivity and organ transplant rejection. Ó 2004 Elsevier Ltd. All rights reserved. Lck, a src family tyrosine kinase expressed primarily in T lymphocytes, provides a critical function during the initial steps of T-cell receptor (TCR) signaling. 1 A cas- cade of downstream signaling pathways ultimately leads to T-cell activation and the production of cytokines such as interleukin-2 (IL-2) and IFNc. 2;3 A selective inhibitor of lck should prevent T-cell activation and thus has broad application for the treatment of T-cell dependent processes such as autoimmune and inflammatory dis- eases as well as allogeneic organ transplant rejection. Of importance to transplant rejection, lck = mice are inca- pable of rejecting either major or minor MHC-incom- patible skin grafts despite the presence of peripheral T-cells. 4 Early work from our laboratories described the syn- thesis and SAR of a series of pyrazolo[3,4-d]pyrimidines as lck inhibitors, exemplified by compound 1. 5 This molecule accesses the lipophilic pocket in lck, a feature desirable for binding and therefore potency. This com- munication describes the exploration of this region with the goal of retaining potency versus lck whilst improving the in vivo properties and pharmacokinetic profile of the class. Compounds represented by general structure 2 (R ¼ COHet, CH 2 Ar) were synthesized using acylation or reductive amination protocols starting from amine 2 (R ¼ H). Other chemistries accessible from 2 (R ¼ H) have been reported elsewhere. 6 N N N N NH 2 N H O Ph O N N 2 1 1 N N N N NH 2 N H O N N R 2 1 2 Inhibitors were screened against a nonphosphorylated construct of human lck, lck (64-509) in HTRF format using 1 mM ATP and biotinylated lck peptide as sub- strates. 7 The closely related kinase src and two receptor tyrosine kinases, kdr and tie-2, served as counterscreens. Potent compounds were progressed for profiling in cel- lular settings and ultimately in vivo. Data are presented here for the inhibition of anti-CD3 mAb induced IL-2 * Corresponding authors. Tel.: +1-508-688-8003; fax: +1-508-688-8100 (D.J.C.); tel.: +1-508-688-8016; fax: +1-508-688-8100 (G.C.H.); e-mail addresses: david.calderwood@abbott.com; gavin.hirst@ abbott.com   Present address: MRC Laboratory of Molecular Biology, Hills Road, Cambridge CB2 2QH, UK. à Present address: Aventis Pharma Deutschland GmbH, Frankfurt am Main, Germany. 0960-894X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.bmcl.2004.02.101 Bioorganic & Medicinal Chemistry Letters 14 (2004) 2613–2616