Hit-to-Lead Studies: The Discovery of Potent Adamantane Amide P2X 7 Receptor Antagonists Andrew Baxter,* JaniceBent,KeithBowers,MartinBraddock,SteveBrough, Malbinder Fagura, Mandy Lawson, Tom McInally, Mike Mortimore, Mark Robertson, RichardWeaverandPeterWebborn AstraZeneca R&D Charnwood, Bakewell Road, Loughborough LE11 5RH, UK Received 17 July 2003; revised 19 August 2003; accepted 19 August 2003 Abstract—AHit-to-Leadoptimisationprogrammewascarriedoutontheadamantanehighthroughputscreeninghit 1 resultingin thediscoveryofanumberofpotentP2X 7 antagonists. # 2003ElsevierLtd.Allrightsreserved. The use of High Throughput Screening (HTS) is now widespread in the pharmaceutical industry. There was an expectation that once a screen was established for a particular target then potent lead compounds or candi- datedrugswouldbefound.Therealityisoftenfarfrom this.BridgingthegapbetweentheendofaHTSandthe start of a full Lead Optimisation (LO) project has been described as Hit-to-Lead (HtL). 1 Hits from HTS are profiled and compared to a generic target lead criteria. The lead target profile used is shown in Figure 1. Lead series then have a balance of properties — potency and SARaswellasanencouragingmetabolicandselectivity profile — such that rapid (less than two years) further optimisation should provide Candidate Drugs (CDs) suitable for progression into clinical development. TheP2X 7 receptorisaligand-gatedionchannelpresent on a variety of cell types involved in the inflammatory/ immune process, specifically macrophages, mast cells and lymphocytes (T and B). 2 À5 Activation of the P2X 7 receptor by the extracellular nucleotide adenosine tri- phosphate(ATP),leadstotheprocessingandreleaseof the proinflammatory cytokine interleukin-1b (IL-1b) from monocytes and macrophages, 6 degranulation of mast cells and the shedding of surface molecules, l- selectin and CD23 from lymphocytes. 7 P2X 7 receptors are also located on antigen-presenting cells, keratino- cytes, salivary acinar cells and hepatocytes. It was desirabletomakecompoundseffectiveasP2X 7 receptor antagonists for use in the treatment of chronic inflam- matory diseases where the P2X 7 receptor may mediate the release of IL-1b. Recently, evidence for the role of this receptor in disease processes was published; P2X 7 - KOmiceshowingareducedseverityinananti-collagen antibody arthritis model. 8 Few classes of P2X 7 antagonists are known in the lit- erature. KN-62, a bis-isoquinolinesulphonyltyrosine derivative, was first described by Gargett and Wiley 9 and has an IC 50 of 51 nM (see Fig. 2). Subsequently various groups have explored this area and have found more potent analogues such as MRS2306 (IC 50 40 nM) 10 and a phenylpiperazine derivative (IC 50 1nM). 11 Whilst being interesting and useful tools to investigate the biology of P2X 7 antagonism, they do not represent good starting points for the discovery of oral drugs. Thesecompoundshavelargemolecularweights( > 700), 0960-894X/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.bmcl.2003.08.034 Bioorganic & Medicinal Chemistry Letters 13 (2003) 4047–4050 Figure 1. Hit-to-Leadgenericleadtargetprofile. *Corresponding author. Tel.: +44-1509-644772; fax: +44-1509- 645513;e-mail: andrew.jg.baxter@astrazeneca.com