The discovery of potent ribosomal S6 kinase inhibitors by high-throughput screening and structure-guided drug design

// Sylvain Couty 1,* , Isaac M. Westwood 1,2,* , Andrew Kalusa 1 , Celine Cano 3 , Jon Travers 1 , Kathy Boxall 1 , Chiau Ling Chow 1,2 , Sam Burns 1 , Jessica Schmitt 1 , Lisa Pickard 1 , Caterina Barillari 1,2 , P. Craig McAndrew 1 , Paul A. Clarke 1 , Spiros Linardopoulos 1 , Roger J. Griffin 3 , G. Wynne Aherne 1 , Florence I. Raynaud 1 , Paul Workman 1 , Keith Jones 1 , Rob L.M. van Montfort 1,2 1 Cancer Research UK Cancer Therapeutics Unit, The Institute of Cancer Research, London, SM2 5NG, UK 2 Division of Structural Biology, The Institute of Cancer Research, London, SW3 6JB, UK 3 Newcastle Cancer Centre, Northern Institute for Cancer Research, School of Chemistry, Bedson Building, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK * Authors contributed equally to this work Correspondence: Rob L.M. van Montfort, email: // Keith Jones, email: // Keywords : S6 kinase, P70S6K, cancer, inhibitor, structure-based drug design Received : August 7, 2013 Accepted : August 23, 2013 Published : August 25, 2013 Abstract The ribosomal P70 S6 kinases play a crucial role in PI3K/mTOR regulated signalling pathways and are therefore potential targets for the treatment of a variety of diseases including diabetes and cancer. In this study we describe the identification of three series of chemically distinct S6K1 inhibitors. In addition, we report a novel PKA-S6K1 chimeric protein with five mutations in or near its ATP-binding site, which was used to determine the binding mode of two of the three inhibitor series, and provided a robust system to aid the optimisation of the oxadiazole-substituted benzimidazole inhibitor series. We show that the resulting oxadiazole-substituted aza-benzimidazole is a potent and ligand efficient S6 kinase inhibitor, which blocks the phosphorylation of RPS6 at Ser235/236 in TSC negative HCV29 human bladder cancer cells by inhibiting S6 kinase activity and thus provides a useful tool compound to investigate the function of S6 kinases.