DDT Vol. 6, No. 18 September 2001 updateconference

1359-6446/01/$ – see front matter ©2001 Elsevier Science Ltd. All rights reserved. PII: S1359-6446(01)01972-9 937

The third Combinatorial Approaches toChemistry and Biology III meeting (17–18July 2001, Cambridge, UK) was attendedby 180 delegates and by several compa-nies exhibiting their latest product rangesof building blocks and equipment, mainlyfor use in combinatorial chemistry.

Protease inhibitorsThe meeting opened with a plenarypresentation from Jonathan Ellman(University of California, Berkeley, CA,USA) on ‘Combinatorial approaches toinhibitor design and catalyst develop-ment’. A key feature of his talk was thenew combinatorial method using fluoro-genic positional scanning libraries togain insights into protease subsite speci-ficity1. He highlighted the pressing needto make selective, cell-permeable pro-tease inhibitors to determine the func-tion of the 1227 mammalian proteasesthat are predicted to account for 4% ofthe human genome.

Ellman reviewed his work on inhibi-tors of Cathepsin D (a typical aspartylprotease), which might be responsiblefor tau proteolysis and contribute toAlzheimer’s disease2. The design of com-petitive, reversible inhibitors of cysteinylproteases, such as cruzipain (thought tobe essential for replication of the organ-ism responsible for Chagas disease), withgood pharmacokinetic profiles is muchmore scientifically challenging, andEllman briefly described the develop-ment of libraries of such inhibitors. Theessential ketone-binding motif, which re-acts reversibly to the enzyme catalyticsulfhydryl centre, was attached to theresin via a readily cleavable hydrazone

linkage. In the second part of his pres-entation, he described some recent workon library-enabling methodology usingolefin metathesis, and carbon monoxideinsertion using both ruthenium chem-istry3 and concomitant annulationchemistry4.

Library design and synthesisBob Boyle (Millennium Pharmaceuticals,Cambridge, UK) discussed mixed ap-proaches towards the synthesis of drug-like libraries using benzodiazepine-2,5-dione and dihydro-2H-isoquinolinonescaffolds. One thousand compoundlibraries of benzodiazepine-2,5-dionewere prepared using the Ugi reaction inconjunction with a variety of scavengerresins. Modifying the procedure byattaching the isonitrile component to aresin via a cyclohexenyl skeleton im-proved the yields considerably, evenwith unreactive aromatic aldehydes. Healso described an expedient method foroxadiazole formation using microwavetechnology and flash tubes. These wereused both as the reaction container andfor the subsequent purification to gen-erate the desired products in isolatedyields of 60%.

Neil Thomas (University of Nottingham,Nottingham, UK) described how aphage-display library of 16-residue pro-teins designed to mimic a β-hairpinmotif can select for duplex DNA motifsand so find molecules that either havethe potential to control gene expressionor to combine with specific nucleotidesequences.

An extremely entertaining lecture en-titled ‘Never enough data’ was given by

Brian Warrington (GSK, Harlow, UK), whodiscussed the philosophical meaning ofchemical diversity and how this im-pacted upon library design. Perhaps inagreement with many chemists in indus-try who now build libraries for hit gener-ation, his conclusion was that, like manythings in life, they should be done littlebut often.

It is also a fact of life that, the poorerthe experimental design, the more diffi-cult it is to analyze the data producedand the more worthless it becomes.Sean Rigby (University of Bath, Bath, UK)gave a detailed and somewhat abstracttalk on the comparative merits of geneticalgorithms, neural networks and simu-lated annealing in experimental design,especially with respect to the problemsinvolved in designing new catalysts bycombinatorial methods.

Solid-phase chemistrySolid-supported reagents combined withscavenger resins were proposed by SteveLey (University of Cambridge, Cambridge,UK) as a means of achieving high-yield-ing, complex, multi-step syntheses with-out the need for purifying intermediates.Microwave chemistry also featuredhighly in many of his syntheses andSildenafil provided a powerful illustrationof how combining these three method-ologies could be used to synthesize ahighly substituted drug5. Ley predictedthat future areas for research wouldinclude new solid-supported reagents,possibly incorporated as stirrer bars,more robust beads and higher loadingscavenger resins, possibly based upondendridized silica gel.

Future combinatorial strategies forchemistry and biologyTerry Hart, Novartis Institute for Medical Sciences, 5 Gower Place, London, UK WC1 6BN, tel: +44 207 928 9292, e-mail: terry.hart@pharma.novartis.com

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Microwave chemistry featured againin an authoritative description by ChrisBrain (Novartis Institute for MedicalSciences, London, UK) of the synthesis of heterocycles, such as oxazoles andoxadiazoles, using polymer-supportedreagents. He also highlighted the needfor workers in the field to start usingspecialized microwave-focused beammonocavity apparatus, not only forsafety reasons, but also to obtain repeat-able and publishable results6–8.

Klaus Haaf (Aventis Crop Science,Parsippany, NJ, USA) described the devel-opment of a reliable solid-phase methodusing palladium-catalyzed [Pd(0)] chem-istry for the solid-supported synthesis ofphosphinates. This methodology neatlyovercomes the difficulty of purifying thefinal diacid phosphinate products.

Parallel synthesisAn interesting case study in parallel syn-thesis was given by Brian Maloney (Evotec,Hamburg, Germany), who outlined thesuccessful working collaboration be-tween a large multinational (DuPontAgriculture, Wilmington, DE, USA) and aservice company (Evotec) to providelibraries for screening. The problems oftarget confidentiality and feedback ofbiological data to the service companywere not perceived to be a problem byeither company and, apparently, con-tributed to the success of this collabor-ation. Large compound-libraries basedupon Suzuki reactions9 and nucleophilicsubstitutions of the 4,6-dichloro-5-nitro-pyrimidine scaffold were presented.

In a presentation entitled ‘A unifiedapproach to parallel synthesis and purifi-cation’, Ben Moshiri (Mettler-Toledo-Boham, Royston, UK) described theversatility of the MiniBlock™ synthesizer,which had been developed in conjunc-tion with Bristol-Myers Squibb (New York,NY, USA). Moshiri focused on its abilityto perform a variety of solution- andsolid-phase chemistries, and solid-phaseand solid–liquid extraction purification inparallel.

Mark Gardener (Pfizer, Sandwich, UK)discussed the discovery of leads throughan integrated approach to parallel syn-thesis. During their first four years, thePfizer hit-generation group used mix-and-split chemistry to generate libraries forscreening but, because of the screeningchaos these mixtures caused, they now,like most of the pharmaceutical industry,tend to make and screen single com-pounds. He exemplified the effectivenessof this new approach with the design ofinhibitors of an unspecified kinase. A625-component library of drug-like, sub-stituted pyrimidines was prepared using‘rule-of-five’ selection criteria and polarsurface-area considerations. After bio-assay, three weakly active molecules(~10 µM) were detected. Resynthesis of asimilar library, but based around the newlead motif, produced drug-like com-pounds with a tenfold increase in activity.

Claire Newton (Millennium Pharma-ceuticals, Cambridge, UK) describedparallel-processing methods, using aTecan™ robot for identifying the bestprocess for the Suzuki coupling of an in-dene boronic acid with an aryl bromide10.Rapid evaluation of a variety of conditionsusing different palladium complexes re-sulted in 25-times less catalyst being re-quired. In a similar way, an alternativezirconium catalyst for the polymerizationof ethylene was quickly identified.

Other highlightsDavid Embiata-Smith (GSK, Stevenage,UK) gave a detailed description of aprocess-chemistry case history in whichhigh-throughput experimentation (HTE),together with multivariate data analysis,was used to rapidly optimize a variety ofmulticomponent reactions, includingWittig and Sonogashira reactions. Theway that reactions moved quickly fromthe bench onto the kiloscale was ex-tremely impressive. There followed anoverview of the lead-finding process at GSK by David Hunter (GSK, Harlow,UK). In their ‘Compound Factory’, theyhave successfully integrated automated-

synthesizers, work-up stations, evapora-tors and analytical and purification sys-tems. He emphasized the importance ofscreening single compounds and theneed to design fewer libraries, but whichcontain thousands of molecules, specifi-cally targeted towards particular bio-logically important motifs, such as G-protein-coupled receptors (GPCRs),ion channels and nuclear receptors. Theaim is to provide chemists in the lead-optimization phase with high-qualitylead molecules as well as some associ-ated SAR data. The challenge for librarydesigners now, is to decide which drug-like compounds of those that can bemade need to be synthesized.

A robust, user-friendly system calledAutoChem, was described by RubenTomassi (Novartis, Summit, NJ, USA),which can automate solution-phase or-ganic reactions and purify mixtures byreverse-phase HPLC – ideal for chemistswho want to explore SARs rapidly. Thesystem, which is based on equipmentdeveloped by Gilson (Middleton, WI,USA), costs ~US$77,000 to build, iscapable of temperature control (from −25 to +100 °C) and can use both resin-bound reagents and on-line solid-phaseextraction (SPE). Its efficiency was exem-plified by the synthesis of a wide rangeof reactions, including reductive ami-nations, acylations and sulfonylations,and various palladium-mediated reactionssuch as Suzuki couplings, Mitsunobu re-actions and deallylations. In general,AutoChem can routinely prepare andpurify 48 compounds per day, rangingfrom 7 to 40 mg of product.

A charismatic and challenging pres-entation on the latest advances in micro-reactor chemistries was given by SteveHaswell (University of Hull, Hull, UK),who illustrated the rapid advances in thistechnology. He described microreactorsmade of borosilicate glass, with a net-work of channels in the 50–300 µmrange, and electric fields used to controlthe flow of reagents through the system.It was evident that a wide range of

chemistries can now be used for micro-reactors11,12. The realistic possibility thatthis technology can be used in a contin-uous-flow process for plant-scale synthe-sis probably came as a shock to most ofthe audience, who, like myself had previ-ously only considered this useful forminiature reactions. Apparently, on thepicolitre scale, a flow of reagents of 2 ml min−1 at 0.1 M concentration willproduce 1–2 tons of material per year!

Tony Czarnik (Sensors for Medicineand Science, Germantown, MD, USA)gave an overview, in his inimitable style,of the advantages of radiofrequency tagsfor communicating information in a non-destructive way, and how they can be usedin conjunction with implanted microflu-orimeters as sensors to monitor fluctuatingglucose levels in type 1 diabetic patients.

A rather complex talk was given byHicham Fenniri (Purdue University, WestLafayette, IN, USA) on dual-recursivedeconvolution (DRED), a spectroscopicmethod for screening large, resin-supported compounds. The concept isbased on the observation that polymericresin beads can be ‘barcoded’ by varyingthe amount and type of monomer presentbefore polymerization. The subsequentdecoding of the bead, which would revealthe identity of the attached compound,can then be performed using near-IRand Raman spectroscopy and standardlaser-based barcode readers13–16.

ConclusionIt is evident that a combination of power-ful, cutting-edge technologies, such asmicrowave technology with polymer-supported reagents and palladium cata-lysts, will rapidly change laboratorychemistry. In the case of microreactors, thismight well have an effect on process-and plant-scale chemistry. In terms oflibrary design, the current trend withinthe industry appears to be the need tomake smaller, targeted libraries (1000s)of drug-like molecules as singles for hitgeneration, and even smaller, iterativelibraries for lead explosion (100s).

It is now approximately 10 years sincethe concept of combinatorial chemistryor parallel processing first entered thepharmaceutical arena. Judging from thenumber of library hits that are now inthe optimization phase, and allowing forthe time lag in publishing commerciallysensitive preclinical research, we can re-alistically expect to hear the first, fullcase-histories of ‘from library hit to clinic’to enter the public domain in the nearfuture.

References1 Harris, J.L. et al. (2000) Rapid and general

profiling of protease specificity by usingcombinatorial fluorogenic substrate libraries.Proc. Natl. Acad. Sci. U. S. A. 97, 7754–7759

2 Bi, X. et al. (2000) Novel Cathepsin Dinhibitors block the formation ofhyperphosphorylated tau fragments inhippocampus J. Neurochem. 74, 1469–1477

3 Szewczyk, J.W. et al. (2001) A Massspectrometric labeling strategy for high-throughput reaction evaluation andoptimization: Exploring C-H activationAngew. Chem., Int. Ed. Engl. 40, 216–219

4 Tan, K.L. (2001) Annulation of alkenyl-substituted heterocycles via rhodium-catalyzed intramolecular C-H activatedcoupling reactions. J. Am. Chem. Soc.123, 2685–2686

5 Baxendale, I.R. and Ley, S.V. (2000) Polymer-supported reagents for multi-step organicsynthesis: application to the synthesis ofsildenafil. Bioorg. Med. Chem Lett. 10, 1983–1986

6 Larhed, M. and Hallberg, A. (2001)Microwave-assisted high-speed chemistry: a new technology in drug discovery. Drug Discov. Today 6, 406–416

7 Edwards, P. (2001) More microwave reactorsrequired. Drug Discov. Today 6, 614

8 Brain, C.T. (2001) Are microwaveinstruments just expensive hot-plate stirrers?Drug Discov. Today 6, 663–664

9 Havelkova, M. (1999) The Suzuki–Miyauracross coupling reactions of 6-halopurineswith boronic acids leading to 6-aryl and 6-alkenylpurines. Syn. Lett. 7, 1145–1147

10 van Beek, J.A.M. et al. (1997) Indenylcompounds and catalyst components for thepolymerization of olefine. US5,646,322

11 Haswell, S.J. et al. (2001) The application ofmicro reactors to synthetic chemistry. J. Chem. Soc. Ser. Chem. Commun. 391–398

12 Watts, P. et al. (2001) The synthesis ofpeptides using micro reactors. J. Chem. Soc.Ser. Chem. Commun. 991–998

13 Fenniri, H. et al. Barcoded resins: A newconcept for polymer-supportedcombinatorial library self-deconvolution. J. Am. Chem. Soc. (in press)

14 Ghose, A.K. et al. (1999) A knowledge-basedapproach in designing combinatorial ormedicinal chemistry libraries for drugdiscovery. 1. A qualitative and quantitativecharacterization of known drug databases. J. Combi. Chem. 1, 55–68

15 Fenniri, H. et al. (2001) Towards the DRED of resin-supported combinatorial libraries: A non-invasive methodology based on beadself-encoding and multispectral imaging.Angew. Chem., Int. Ed. Engl. 39, 4483–4485

16 Lawrence, R.N. (2001) Recursive deconvolutionfor real-time lead identification. Drug Discov.Today 6 (Suppl.) S2–S3

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