Fragment screening of phosphoinositide-dependent kinase-1 (PDK1) in a biochemical kinase assay afforded hits that were characterized and prioritized based on ligand efficiency and binding interactions with PDK1 as determined by NMR. Subsequent crystallography and follow-up screening led to the discovery of aminoindazole 19, a potent leadlike PDK1 inhibitor with high ligand efficiency. Well-defined structure-activity relationships and protein crystallography provide a basis for further elaboration and optimization of 19 as a PDK1 inhibitor.KEYWORDS Aminoindazole PDK1 inhibitors, fragment-based drug discovery, ligand efficiency, binding interactions F ragment-based drug discovery (FBDD) is widely recognized as an alternative to high-throughput screening for hit identification.1-3 Success in implementing FBDD to deliver clinical candidates supports FBDD as a valuable and proven strategy in medicinal chemistry. 4,5 Endeavors in FBDD begin with screening collections of small, low molecular weight molecules (generally <250 MW) for inhibition or binding to a therapeutic target. This offers a broad coverage of chemical space, high hit rates, and quality starting points for hit-to-lead campaigns. Using principles of FBDD and ligand efficiency (LE), the medicinal chemist can accurately define structure-activity relationships (SAR) and measure progress during lead optimization.6 Once a hit is confirmed in a FBDD campaign, the medicinal chemist is confronted with fragment expansion or evolution, a difficult but necessary step to bridge the gap from fragment to leadlike molecule. 7 This report is an account of our success using a FBDD approach to identify and progress fragment inhibitors of phosphoinositide-dependent kinase-1 (PDK1), an integral component of the PI3K/AKT/ mTOR pathway, which is one of the most commonly deregulated signaling pathways across all cancers.
8-11The composition of the fragment library used in the PDK1 screen was biased toward molecules with donor and/or acceptor motifs embedded in an aromatic ring, which could fill a flat lipophilic pocket and engage the kinase hinge through hydrogen-bonding interactions, similar to the adenine ring of ATP.
12The assembled library consisted of 1065 fragments originating from our proprietary compound collection and external sources. We began our screen for PDK1 inhibitors using a biochemical kinase assay at high fragment concentration (400 μM).13 We subsequently selected 193 compounds for IC 50 determination based on percent inhibition (g60%) and chemical purity14 resulting in 89 compounds with IC 50 < 400 μM. A set of 36 compounds was selected for further evaluation based on LE (LE > 0.40) and tractability. 15,16 Tractability was a subjective measure and was influenced by structural novelty, ease of synthesis, and the ability to introduce substituents in multiple vectors for optimization. Representative examples are listed in Table 1.Saturation transfer difference (STD) experiments 17 were used to confirm an interaction of the hits with PDK1. This allowed...