Combinatorial Chemistry & High Throughput Screening - Volume 8, Issue 7, 2005

Since the appearance of the first reports during the early nineties on non-peptidal small molecule libraries synthesized on solid phase supports, the fields of combinatorial chemistry and high throughput screening have experienced enormous progress and development. These techniques have become an essential part of the modern discovery process within the pharmaceutical industry and many pharmaceutical companies, including AstraZeneca, GlaxoSmithKline and Pfizer, are currently making major investments in the area. This progress did not come without setbacks and the biological screening results obtained from early combinatorial libraries were often so disappointing that many companies went though a period during which they seriously doubted whether it would be possible to utilise these new techniques within industry. However, even these bad results had value in that they showed what not to do and today the pharmaceutical industry seems to have learned how to more successfully apply these powerful new tools to the drug discovery process. The current situation and progress within the agrochemical industry with regards combinatorial chemistry and high throughput screening is not easy to determine. This is at least partly because comparatively little has been published in the area to date and what has been published is rather widely scattered throughout the chemical literature. Progress in the area was reviewed at the 2nd Pan-Pacific Conference on Pesticide Science held in October 1999 in Honolulu, Hawaii [1] but since that time no really comprehensive review has been published. This special issue of Combinatorial Chemistry and High Throughput Screening brings together a series of five review articles whose purpose is to provide an up to date overview of the current state of the art as relevant and applicable to the agrosciences. In addition, seven research papers are being published describing recent applications of high throughput synthesis within the agrochemical discovery process. It is hoped that the publication of these collected works in a single issue will help facilitate informed debate about how best to direct the vital research resources needed to ensure the long term success of the agrochemical industry. [1] Baker, D. R.; Umetsu, N. K.; Eds., ACS Symposium Series 774, "Agrochemical Discovery", American Chemical Society, Washington, DC, 2001. For a more recent review article see, Smith, S. Pesticide Outlook, 2003, February, 21-25.

The recent progress and future prospects for the successful application of combinatorial chemistry and high throughput screening within the agrochemical lead discovery process are outlined and discussed. Solid and solution phase library synthesis technologies are reviewed and compared, and the role and importance of bioavailability, diversity and virtual screening in rational library design are detailed.

During the past ten years combinatorial chemistry developed from a powerful synthetic methodology, providing large libraries of usually simple new chemical entities, to a comprehensive strategy presently covering a multitude of technologies across the whole workflow from hit generation to lead optimization. Thus combinatorial chemistry had a major impact not only on the pharmaceutical research but also with some delay on the agrochemical research. The agrochemical discovery environment is different from that of the pharmaceutical research in that it relies mainly on whole organism screenings. This review summarizes some recent applications of combinatorial chemistry in the agrosciences, covering all the three major fields of research: Fungicides, herbicides, and insecticides. The article focuses on libraries with published biological activities and thus highlights some characteristic features of successful agrochemical libraries, which may be fundamentally different from pharmaceutical libraries.

In vivo high throughput screening (HTS) has been adopted by most of the larger crop protection companies as an important tool for the discovery of new agrochemicals. There has been a paradigm shift in capabilities from screening a few thousand compounds a year to several hundred thousand and the quantity of screening sample required has fallen dramatically. The unifying goal now bringing together screens and inputs is the need to maximise the flow of useful information from HTS and thereby minimise the time taken to discover robust leads and new products. This review examines the positive changes that have occurred towards targeted design and selection of chemical inputs for agrochemical discovery over the last ten years and corresponding developments in HTS assays, data analysis and the logistics of compound storage and dispensing.

The demand for new herbicides, insecticides and fungicides led to a steady increase in the number of compounds being tested to find novel market products. To keep pace with the rising workload, high throughput screening (HTS) technologies have been introduced. In agrochemical research miniaturised in vivo tests on whole real target organisms are now possible and are an integral part of the screening cascade. A complementary target based in vitro HTS has also been established in agrochemical research. Target based HTS allows a directed approach towards untouched market shares by novel modes of action. Selection of the best suited targets is the most crucial issue in this approach. Genomic methods thereby deliver many essential genes as candidate targets. Consideration of further criteria such as druggability notably narrows down the number of promising targets. Though target to hit to lead progression still is as in pharmaceutical research a complex and therefore risky process, the implementation of novel bioscience technologies has entailed the transition to an integrated innovative agrochemical research perspective.

Progress and developments made in microwave-assisted combinatorial synthesis and library production since 2002 are reviewed. The use of microwave technology in both solution and solid phase synthesis is discussed with special reference to agrochemical applications where appropriate.

A general route to a series of differentially substituted bicyclo[2,2,2]octenones has been developed, making use of the in situ intramolecular Diels Alder reaction of masked ortho-benzoquinones. This approach was used to synthesize a series of thirteen key acid-containing templates from which a solution phase discovery library of 1126 diverse amides was then constructed. The rigid polycyclic nature of the templates and the prevalence of oxygenated functionality confer natural product-like qualities and three-dimensional diversity. The library was screened in HTS in vivo against a number of weed, insect and fungal model organisms leading to the discovery of a novel series of herbicidally active compounds. The development, production and biological activity of the library are described.

A novel class of highly active dihydropyridine miticides was prepared using a multicomponent reaction process. The initial lead was rapidly optimized using solution phase parallel synthesis techniques and a positional scanning approach. Detailed structure-activity relationships were developed for the amino and carbonyl components of the molecule and used to select the best candidates for broad field testing. The chemistry, biology and toxicology of these compounds will be presented along with numerous structural variants of the reaction products.

Amino acids were immobilised by attaching them via a carbamate linker to Wang resin. These intermediates were converted to1-(1,3-benzoxazol-2-yl)alkanamines over three steps, followed by coupling with 4-alkyl-6-chloro-1,3,5-triazine-2-amines to furnish the desired N-[1-(1,3-benzoxazol-2-yl)alkyl]-6-alkyl- 1,3,5-triazine-2,4-diamines. Physico-chemical property profiles were used to support design and development of a combinatorial library. The synthetic methodology described herein was validated with the production of a herbicide targeted library of 300 members.

The Automated Synthesis & Purification team at Bayer CropScience in Frankfurt provides services and support in the areas of synthesis and post synthesis activities for chemists. This article describes our workflow and the special robotic systems used. We produce small to medium sized compound libraries using liquid phase techniques. An example of a compound library taken from the herbicide area is given.

In order to gain a broad access to phosphinic acid derivatives, a palladium catalysed coupling reaction of aryl iodides with hypophosphorous acid derivatives has been developed on the solid phase. The resulting arylphosphorous acids (or esters) were derivatised using addition reactions with aldehydes, imines and isocyanates, to give phosphinic acids (or esters) with α-hydroxy , α-amino or aminoacyl groups attached to the aryl phosphorus moiety. This approach provided a broad chemical entry into a class of polar phosphinates compounds which were rather difficult to handle using normal solution phase synthesis. The synthetic potential of this solid phase based methodology was demonstrated by the synthesis of targeted libraries against the enzyme dihydrodipicolinate synthase (DHDPS).

The polymer assisted solution phase (PASP) synthesis of 4,5-dihydro-1,4-benzoxazepin-3(2H)-ones is described. Using salicylic aldehydes, α-bromo acetic acid esters, and primary amines as broadly variable building blocks, the target molecules were obtained in a straightforward manner. The use of polymer bound reagents and scavengers greatly simplified workup, and avoided the use of protecting groups. A small library was prepared, showing the feasibility of the synthetic concept for the generation of larger sets of screening compounds.

Potent new agonists of the insect muscarinic acetylcholine receptor (mAChR) have been discovered by synthesizing and screening a library of 225 oxime ether amines. Library evaluation was facilitated by the development of a high throughput test enabling the rapid determination of muscarinic agonist activity. The most interesting compounds were the thiadiazole 17 and the isoxazole 24 which were potent muscarinic agonists (EC50 13 and 21 nM, respectively) and showed lead levels of insecticidal activity.

Jurgen Scherkenbeck was born in Remscheid, Germany in 1960 and obtained his diploma degree in chemistry from the Ruhr- University Bochum in 1985. His doctoral work in the field of natural product synthesis was performed under the supervision of Professor Peter Welzel at Bochum. In 1988 he took up a position within Central Research at Bayer AG in Leverkusen. During his industrial research career he has been involved in diverse research areas, including pheromone synthesis, peptide chemistry and combinatorial chemistry. From 1999 to 2000, he spent one year as an Associate Professor at the Hong Kong University of Science and Technology and following his return to Central Research, he became head of the Combinatorial Chemistry Department. In 2002 he moved to Bayer CropScience where he is currently head of an insecticide chemistry group. He is author or co-author of around 70 scientific papers and patents and holds an honorary professorship at the University of Leipzig.

Stephen Lindell was born in Ipswich, England in 1955 and obtained his bachelors degree in chemistry from Imperial College, London in 1977. His doctoral work was performed at the Research Institute for Medicine and Chemistry in Cambridge, Massachusetts under the supervision of Professor Sir Derek Barton and Dr. Robert Hesse. In 1982 he moved to Stanford University in Palo Alto, California to undertake post doctoral research with Professor William Johnson. He returned to England in 1985 to take up a position with Schering Agrochemicals at their Chesterford Park research site near Cambridge. In 1996, he moved to the Companies Frankfurt-Hoechst research site in Germany to work first with AgrEvo, then Aventis CropScience and since 2002, with Bayer CropScience. During his industrial research career he has been involved in the design and synthesis of novel biologically active molecules in all three indications; fungicides, herbicides and insecticides. He is author or co-author of over 50 scientific papers and patents and is currently head of the Hit Generation Chemistry Group at Bayer CropScience in Frankfurt.