Dr A.D. Buss graduated from the Royal Institute of Chemistry before receiving his MSc from the University of East Anglia and then a PhD from the University of Cambridge. He started his career in the pharmaceutical industry with Pfizer before moving to Schering Agrochemicals as a Team Leader. In 1989, he joined Glaxo as Research Manager prior to becoming Head of the Natural Products Discovery Department and, finally, Research Unit Head, Bioprocessing, in 1995. During this period, Dr. Buss served as Chairman of the GlaxoWellcome (UK) Research Management Team and Chairman of the Joint Project Team (combinatorial polyketide biosynthesis) for what was GlaxoWellcome’s largest research collaboration with the University of Cambridge. In 2000, he became head of the Centre for Natural Product Research at the Institute of Molecular and Cell Biology in Singapore and, on the Centre’s privatisation to MerLion Pharmaceuticals in May 2002, became its President and Chief Executive Officer. Dr Buss is the author or co-author of over fifty peer reviewed scientific publications. Dr Mark S. Butler received his BSc (Hons) and PhD from The University of Melbourne. After postdoctoral work at the Arizona State University, he joined the Queensland Pharmaceutical Research Institute (now Natural Product Research). He then moved to Singapore to lead the Natural Product Chemistry group at the Centre of Natural Product Research (CNPR), which was part of the Institute of Molecular and Cell Biology (IMCB). In 2002, CNPR privatized to become MerLion Pharmaceuticals where his present position is Director of Natural Product Chemistry. He has contributed to over 40 papers on various aspects of natural products chemistry and, in 2002, was awarded the Matt Suffness (Young Investigator) Award by the American Society of Pharmacognosy. His research interests include isolation, structure elucidation, absolute configuration and mechanism of natural products and development of natural product leads into pharmaceuticals, as well as separation of compounds using liquid-liquid chromatography and large-scale isolation of natural products.

Natural Product Chemistry for Drug Discovery

By Antony D. Buss, Mark S. Butler

The Royal Society of Chemistry

Copyright © 2010 Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85404-193-0

Contents

Section 1 Introduction to Natural Products for Drug Discovery,
Chapter 1 Natural Products as Drugs and Leads to Drugs: The Historical Perspective David J. Newman and Gordon M. Cragg,
Chapter 2 Chemical Space and the Difference Between Natural Products and Synthetics Sheo B. Singh and J. Chris Culberson,
Chapter 3 Mechanism of Action Studies James J. La Clair,
Section 2 Sources of Compounds,
Chapter 4 The Convention on Biological Diversity and its Impact on Natural Product Research Geoffrey A. Cordell,
Chapter 5 Plants: Revamping the Oldest Source of Medicines with Modern Science Giovanni Appendino and Federica Pollastro,
Chapter 6 Macromarines: A Selective Account of the Potential of Marine Sponges, Molluscs, Soft Corals and Tunicates as a Source of Therapeutically Important Molecular Structures Jennifer Carroll and Phillip Crews,
Chapter 7 Microorganisms: Their Role in the Discovery and Development of Medicines Cedric Pearce, Peter Eckard, Iris Gruen-Wollny and Friedrich G. Hansske,
Section 3 Advances in Technology,
Chapter 8 Advances in Biological Screening for Lead Discovery Christian N. Parker, Johannes Ottl, Daniela Gabriel and Ji-Hu Zhang,
Chapter 9 Advances in Instrumentation, Automation, Dereplication and Prefractionation Tim S. Bugni, Mary Kay Harper, Malcolm W.B. McCulloch and Emily L. Whitson,
Chapter 10 Natural Product Combinatorial Biosynthesis: Promises and Realities Daniel W. Udwary,
Section 4 Natural Products in Clinical Development,
Chapter 11 A Snapshot of Natural Product-Derived Compounds in Late Stage Clinical Development at the End of 2008 Mark S. Butler,
Chapter 12 From Natural Product to Clinical Trials: NPI-0052 (Salinosporamide A), a Marine Actinomycete-Derived Anticancer Agent Kin S. Lam, G. Kenneth Lloyd, Saskia T. C. Neuteboom, Michael A. Palladino, Kobi M. Sethna, Matthew A. Spear and Barbara C. Potts,
Chapter 13 From Natural Product to Clinical Trials: Bevirimat, a Plant-Derived Anti-AIDS Drug Keduo Qian, Theodore J. Nitz, Donglei Yu, Graham P. Allaway, Susan L. Morris-Natschke and Kuo-Hsiung Lee,
Section 5 Case Studies of Marketed Natural Product-derived Drugs,
Chapter 14 Daptomycin Richard H. Baltz,
Chapter 15 Micafungin Akihiko Fujie, Shuichi Tawara and Seiji Hashimoto,
Subject Index, 429,

CHAPTER 1

Natural Products as Drugs and Leads to Drugs: The Historical Perspective

DAVID J. NEWMAN AND GORDON M. CRAGG

Natural Products Branch, Developmental Therapeutics Program, NCI-Frederick, PO Box B, Frederick, Maryland, 21702, USA

1 Ancient History (> 2900 BCE to 1800 CE)

It is always a little difficult to know where to start and when to stop in time when discussing the historical influence of natural products upon drug discovery because, even today, materials that were identified as late as the 1970s (though used for many centuries as a mixture) are still influencing chemists and biological scientists to use the “native product” and/or a modification as either probes for specific targets or as a treatment in its own right. Later in the chapter, we will demonstrate how natural product structures are still valid models upon which to base 21st century drugs.

Throughout the ages, humans have relied on Nature to cater for their basic needs — not the least of which are medicines for the treatment of a wide spectrum of diseases. Plants, in particular, have formed the basis of sophisticated traditional medicine systems, with the earliest records, dating from around 2900–2600 BCE, documenting the uses of approximately 1000 plant-derived substances in Mesopotamia and the active transportation of medicinal plants and oils around what is now known as Southwest Asia. These include oils of Cedrus species (cedar) and Cupressus sempervirens (cypress), Glycyrrhiza glabra (liquorice), Commiphora species (myrrh) and Papaver somniferum (poppy juice), all of which are still used today for the treatment of ailments ranging from coughs and colds to parasitic infections and inflammation. In addition to plants, around 120 mineral substances were also listed as “medicinal in nature” including materials now identified as arsenic sulfide, sulfur, lime, potassium permanganate and even rock salt. In most cases, the materials were delivered as infusions (teas), ointments, medicated wines, enemas and even by fumigation — methods still in use in pharmaceutical delivery systems even today. By approximately 700 BCE, the concept of “contagion” was developing, though it would be millennia before the relationship of microbes to plagues, etc., was formally established. Although what is interesting is a description of the use of “rotten grain” in treating wounds; it is tempting to speculate that this might have been a method of administering a crude antibiotic formulation to a patient.

Egyptian medicine dates from about 2900 BCE with the best known record being the “Ebers Papyrus” dating from 1500 BCE, documenting over 850 drugs, mostly of plant origin including opium, cannabis, linseed, aloe and garlic. At around the same time, the Chinese Materia Medica was being extensively documented, with the first record dating from about 1100 BCE (Wu Shi Er Bing Fang, containing 52 prescriptions), although records from the Pent’sao are reputed to date from ~2700 BCE. These were followed by works such as the Shennong Herbal (~100 BCE, 365 drugs) and the Tang Herbal (659 CE, 850 drugs).Likewise, documentation of the Indian Ayurvedic system dates from before 1000 BCE (Charaka; Sushruta and Samhitas with 341 and 516 drugs respectively).

The Greeks and Romans contributed substantially to the rational development of the use of herbal drugs in the ancient Western world with Hippocrates (~460 to 377 BCE) being considered the father of medicine through an anonymous treatise known as Corpus Hippocraticum, which covered the usage of mainly plant-based mixtures but with an emphasis on the correct diet. Sources included extracts of poppy, henbane and mandrake, alongside juniper and saffron. Entertainingly, one might well argue that establishing a potential resistance to poisoning (assassination rather than happenstance) also contributed to the evolution of Greek pharmacy around 100 BCE, with the preparation of Mithridaticum, a combination of 54 ingredients, made for Mithridates, who was the King of Pontus at that time. The mixture was “improved” by Andromachus, Nero’s physician to contain 70 ingredients and was still available under the name Theriac in various European pharmacopoeias until the 19th century. Dioscorides, a Greek physician (100 CE), accurately recorded the collection, storage and use of medicinal herbs during his travels with Roman armies throughout the then “known world”, publishing his famous five volume botanical work, De Materia Medica at that time; details are available from the US National Library of Medicine (NLM). The majority of his “drugs” (80%) were based on plant sources, with animal and mineral sources making up ~10% each. Almost at the same time, Galen (130–200 CE.), a practitioner and teacher of pharmacy and medicine in Rome, was well-known for his complex prescriptions and formulae used in compounding drugs, with details given in his herbal, De Simplicibus, of 473 drug entities.

During the Dark and Middle Ages (5th to 12th centuries), the Arabs (covering Southwest and Central Asia) preserved much of the Greco-Roman expertise and expanded it to include the use of their own resources, together with Chinese and Indian herbs unknown to the Greco-Roman world. Thus Rhazes, a Persian physician in the early 900s, gave the first accurate descriptions of measles and smallpox and Avicenna, an Arab physician of the late 900–early 1000 era, codified the then current knowledge with his epic and encyclopaedic work, Canon Medicinae, a book that influenced the practice of medicine for the next 600 plus years. This was subsequently superseded by the work of Ibn al-Baitar (whose full name was Abu Muhammad Abdallah Ibn Ahmad Ibn al-Baitar Dhiya al-Din al-Malaqi) an Arab, born in Malaga towards the end of the 12th Century (died 1248 CE), but who travelled extensively over the Muslim world and produced two extremely well known treatises, one on botany, that described over 1400 plants, over 200 of which had never previously been recorded (Kitab al-Jamifi al-Adwiya al-Mufrada) and the other, a comprehensive compilation known as the Corpus of Simples in English (Kitab al-Mlughni fi al-Adwiya al-Mufrada in Arabic). Both books were translated into Western languages in later centuries. For those readers wanting to further investigate the influence of the Arabic schools, details can be found at the NLM.

From a Western perspective, following on from the ~1500 CE time frame, the person whose ideas permeated the West for the next two hundred or so years was Paracelsus, whose real name was Theophrastus Phillipus Aureolus Bombastus von Hohenheim, born in Switzerland in 1493. He attempted to “modernise” the then existing works of his forerunners by perhaps the first use of alchemy to separate “good from bad” effects of treatments. Although he was probably responsible for the derivation of what later became known as the “doctrine of signatures”, he did place pharmacy on a relatively sound chemical footing and may best be known for the use of mercury as a treatment for syphilis and for the value of mineral waters, plus substituting more simple herbal remedies for some of the complex mixtures handed down from the time of Galen. However, pharmacy was still an empiric science, as shown by publication of the herbal The London Pharmacopoeia in 1618 and then in 1676, the book, Observationes Medicae, by the English physician Thomas Sydenham. This latter publication was used for close to two centuries as a standard textbook and contained such “observational remedies” as the use of laudanum (opium in alcohol), quinquina (Jesuit bark preparations for malaria) and iron for iron-deficiency anaemias.

2 The Initial Influence of Chemistry upon Drug Discovery

The subtitle for this section could easily be “The experimental chemist discovers the active principles of major drug preparations”. Although it is not often realised, the initial discoveries that can be considered to have revolutionised drug discovery and development were made by European chemists (known as apothecaries at that time) in the 1803–1805 time frame, building upon the physico-chemical principles evolving in the recent past from the work of experimental and theoretical chemists such as Proust, Davy, Gay-Lussac, Berzelius, Dalton, etc. This body of theory and experiment led away from “polypharmacy” towards the “pharmacology” of single (pure) agents’ which was probably first enunciated by Cadet de Gassicourtin 1809.

2.1 Alkaloids

An excellent example of this change would be the story of morphine 1. The initial report of the isolation of fractions from the opium poppy was reputedly made by Derosne in 1803 at the Institute of France and then published in 1814. However, this preparation had no narcotic properties whatsoever and was probably noscapine with a little meconic acid extracted by the ethanol-water system that he used. A controversy arose because the German pharmacist Seturner then published his work in 1805, claiming that he had commenced work before Derosne; however inspection of this title implies investigation of the acidic and not the basic fractions of opium, probably meconic acid, as shown in a paper the following year. In 1817, Seturner’s use of a different extraction technique — hot water extraction followed by precipitation with ammonia — led to colourless crystals that had the narcotic properties of opium. What surprised the scientists reading this publication at the time was that the material obtained was alkaline, not acidic; thus this was the first non-acidic compound with biological properties purified from a plant.

Subsequent conversion into heroin 2 was first reported in 1874 by Wright in the UK as a result of boiling morphine acetate; the process was commercialised by Bayer AG in 1898. The subsequent use and abuse of these compounds is much too complex to discuss here, but one major discovery came in the early 1970s when Pert and Synder reported the identification of opioid receptors in brain tissue. This report was followed closely by the identification of “endogenous morphine-like substances” in 1975 by Kosterlitz and Hughes, which over the next few years led to the identification of enkephalins, endor-phins and dynorphins — all of which had the common N-terminal sequence of Tyr-Gly-Gly-Phe-(Met/Leu), leading to the concept that morphine actually mimics this sequence.

Irrespective of the exact timing of the isolation of morphine, alkaloids were discovered at an ever increasing rate from plant sources over the next 50 or so years, confirming the influence of chemistry on pharmacology and drug development in its simplest form from 1817 to roughly the middle of the 19th century. Thus, emetine 3 was probably the first alkaloid to be purified and reported by Pelletier and Magendie in 1817 from Ipecacuanha, closely followed the same year by the isolation of strychnine 4 from Strychnos by Pelletier, now working with Caventou. Then in 1820, the same workers reported the isolation of quinine 5 from Cinchona species. They developed a commercial process for the preparation of quinine and, in addition to its subsequent use in the treatment of malaria, it was also used extensively as a “tonic” and an antifever drug. Though not documented specifically, the realisation of its use for malaria probably followed after the usage for other “illnesses” in northern Europe.

Over the next few years, a veritable “treasure trove” of potential drug structures was reported, though one must realise that the structures were not identified for many years if not many decades following their initial isolation. Thus in 1819, brucine 6 and caffeine 7 were purified, followed in 1820 by colchicine 8, in 1820, codeine 9, in 1833, atropine 10 and papaverine 11 in 1848. During this time frame, the first plant-derived alkaloid to be purified, have its structure elucidated and finally synthesised was coniine 12. The compound was extracted in 1826, followed by determination of its structure in 1870 and then synthesised by Ladenberg in 1881. Even today, over 180 years since it initial isolation, the compound is still a candidate for drug discovery, this time being a model for induction of apoptosis in trypanosomal infections.

2.2 Aspirin

No historical perspective of natural product derived drugs would be complete without a discussion of aspirin (acetylsalicylic acid) — probably the most widely utilised drug of all time when the numbers of tablets consumed worldwide on an annual basis are considered. Even today, where presumably the major pharmacological effect is modulation of the cyclooxygenase isoforms, its full activity is still not fully defined.

Salicin 13 was first introduced into medicinal use by Maclagan in 1876 as the single agent, although as a part of “herbals” the use of extracts of Salix or Spiraea ulmaria (the source of “spirin” in “aspirin”) for treatments of fevers and pain dated from the days of Hippocrates. It is probable that the formal identification of salicin in more “modern” medicine would be the letter from the Reverend Edmund Stone to the President of the Royal Society in 1763 covering the use of the compound for treatment of “fever”. There are various “stories and/or anecdotes” over the transition from salicin to acetylsalicylic acid (aspirin), but the most probable steps were as follows.

Piria, working with Spiraea species, first isolated salicylaldehyde in 1839 while working in Dumas’ laboratory and then prepared salicylic acid. This was followed by the first synthesis of acetylsalicylic acid in 1853 by Gerhardt but without any pharmacological investigation. The story that Hoffmann at Bayer synthesised acetylsalicylic acid to overcome the taste problems with salicylic acid in order to help his father “take his medicine” has been revised relatively recently by Sneader. It would appear, in spite of some discussion on Sneader’s paper in later issues of the British Medical Journal, that acetylsalicylic acid was synthesised by Hoffmann, but “under the direction of” Arthur Eichengrün at Bayer and that the compound was in fact under “impromptu clinical testing” before the 1898 time frame. Eichengriin left Bayer in 1908 and proceeded to develop materials and processes such as flame-retardant fibres and also injection moulding of plastics. However, when the Hoffmann story was published as a footnote in a book on chemical engineering in 1934, Eichengrün, being of Jewish descent, could not comment due to the political climate of 1930s Germany and it was not until 1949 that he was able to publish his story of the discovery.

2.3 Digitalis

In 1775, the English physician Withering reported his extensive work on a potential treatment for “dropsy” that he had developed as a result of studies on a plant decoction that the local inhabitants were using for their own treatment. He subsequently investigated the methods used and identified the foxglove, Digitalis purpurea, as the potential source and also demonstrated what today would be known as a “narrow therapeutic index” for the preparation. It was realised subsequently that purification would have to be performed, but in spite of significant efforts, it was not until 1867 that Nativelle was able to produce an effective crystalline preparation that he named “crystallised digitalin”. A few years later in 1875, the German pharmacologist, Schmiedeberg was able to produce digitoxin 14. Subsequently, other compounds with a similar pharmacology were discovered by a combination of what would now be known as ethnopharmacology/ethnobotany — yielding ouabain from Acocanthera bark and roots and strophantin from Strophantus species. Both of these agents were used as arrow poisons in Africa, albeit in very crude form.

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