Phytochemistry and medicinal plantsJ. David Phillipson *Centre for Pharmacognosy, The School of Pharmacy, University of London, 29-39 Brunswick Square, London WC1N 1AX, UKReceived14July2000;receivedinrevisedform1August2000AbstractA truncated history of the contribution of plants to medicine is given with reference to some of the lesswell known ancestors oftheHarbornefamily.Sixofthetop20prescriptionsdispensedin1996werenaturalproductsandtheclinicaluseofdrugssuchasartemisinin, etoposideandtaxol hasoncemorefocussedattentiononplantsassourcesofnovel drugentities. Highthrough-putroboticscreenshavebeendevelopedbyindustryanditispossibletocarryout50,000testsperdayinthesearchforcompoundswhich have specicity of action against a key enzyme or a subset of receptors. Bioassay-guided fractionation of plant extracts linkedtochromatographicseparationtechniquesleadstotheisolationof biologicallyactivemoleculeswhosechemical structurescanreadily be determined by modern spectroscopic methods. The role of academics in the search for new drugs is discussed by referencetosomeofourresearchintonaturalproductswithactivityonthecentralnervoussystem,onpainreceptors,themalariaparasitePlasmodiumfalciparum,thewoundhealingpropertiesofthesapofspeciesofCroton(Dragon'sblood),andatraditionalChinesemedicineusedtotreateczema.ExpertiseinphytochemistryhasbeenessentialforthisresearchandthestrongleadshownbyPro-fessor Jerey Harborne isgratefully acknowledged. #2001 Published by Elsevier Science Ltd.Keywords: J.B. Harborne; Medicinal plants; Phytochemistry; Academics; Newdrugs; Central nervous system; Eczema; Malaria; Pain; Woundhealing1. IntroductionThe use of plants as medicines goes backtoearlyman. Certainly the great civilisations of the ancientChinese, Indians, andNorthAfricansprovidedwrittenevidenceof man's ingenuityinutilisingplants for thetreatment of a wide variety of diseases. In ancientGreece,forexample,scholarsclassiedplantsandgavedescriptionsofthemthusaidingtheidenticationpro-cess. Theophrastushasbeendescribedbysomeasthefather of botany (Fig. 1) but little, if anything, has beenrecordedonhisdistant relativeJ.B.Theophrastus1whoextolledthevirtuesofmedicinalplantsandforecastthepossibility of discovering avonoids. As Europe enteredthe dark ages much of this information would have beenlost hadit not beenfor themonasteries that actedascentres for the productionof medicinal plants whichwere used to heal the suering of mankind. There is stillmuchwecanlearnfrominvestigatingtheoldherbals,particularly those less well known such as the oneattributed to the monk J.B. Harbonus1.It wasnot until the19thcenturythat manbegantoisolatetheactiveprinciplesofmedicinalplants and oneparticularlandmarkwasthediscoveryofquininefromCinchonabarkbytheFrenchscientists CaventouandPelletier (Fig. 2). Much less is known about the isolationof quininebyJ.B. Caventou1andJ.B. Pelletier1. Suchdiscoveries ledtoaninterest inplants fromthe NewWorld and expeditions scoured the almost impenetrablejungles and forests in the quest for newmedicines (Fig. 3).Oneof the lesserknownintrepid explorerswas J.B. vanHarbon1who was never happier than when he was abletohatchethiswaythroughthejunglestrippingothebarksfromeverytreeinsight. Suchexpeditionswouldlast for years and it was not until the plants arrived at awell equippedphytochemical laboratory that the realdiscoveries could be made (Fig. 4). Laboratories such asthoseof ProfessorJ.B. deHarbonney1becamecentresfor the isolationof the active principles of medicinalplants fromaroundtheglobe. Years of toil wouldberewarded by the isolation of numerous avonoids whichwere welcomed by the cognoscenti as well as the rapidlyexpanding pharmaceuticalcompanies.0031-9422/01/$-seefrontmatter # 2001PublishedbyElsevierScienceLtd.PI I : S0031- 9422( 00) 00456- 8Phytochemistry56(2001)237243www.elsevier.com/locate/phytochem* Tel.: +44-207-753-5800; fax +44-207-753-5909.E-mail address: profjdp@msn.com1ThelecturepresentedmadereferencetoimaginaryforefathersofJerey B. Harbone.2. Newdrugs fromnaturePrior toWorldWar 2, aseries of natural productsisolated from higher plants became clinical agents and anumber arestill inusetoday. QuininefromCinchonabark, morphine and codeine from the latex of the opiumpoppy, digoxinfromDigitalisleaves, atropine(derivedfrom()-hyoscyamine) andhyoscine fromspecies oftheSolanaceaecontinuetobeinclinical use. Theanti-biotic era dawned during and after World War 2 due toFig. 1. Theophrastus father of botany.Fig. 2. First of the alkaloid chemists; Caventou, Pelletier and Quinine.238 J.D. Phillipson / Phytochemistry 56 (2001) 237243the antibacterial eects of a whole series of naturalproductsisolatedfromspeciesofPenicillium,Cephalos-porium, andStreptomyces. Inthepost-waryearstherewere relatively few discoveries of new drugs from higherplantswiththenotableexceptionofreserpinefromtheRauwolaspeciesheraldingtheageofthetranquillisersandalsovinblastineandvincristinefromCatharanthusroseus which were eective in cancer chemotherapy.Fig. 3. Wresting the Jungle's secrets.Fig. 4. The development of chemotherapy.J.D. Phillipson / Phytochemistry 56 (2001) 237243 239Despite these discoveries the impact of phytochem-istryonnewdrugdevelopment wanedandinevitablytheinnovativepharmaceutical industryturnedtosyn-thetic chemicals. Successful clinical agents emergedfrommultidisciplinary researchteams inwhichphar-macologists and synthetic chemists collaborated, e.g.atenolol (beta-blocker) and captopril (ACE-inhibitor) fortreatment of hypertension, salbutamol (adrenoceptorstimulant) for asthma andthe benzodiazepines (hyp-notics and anxiolytics) for insomnia and anxiety attacks.Duringrecentyears,theattentionofthepharmaceu-tical industry has switchedonce more tothe naturalworldandthismaybeillustratedbyreferencetothreeclinicaldrugs,taxol,etoposideandartemisinin(Phillip-son, 1999a). Taxol is obtainedfromthe barkof theWestern Pacic Yew, Taxus brevifolia. The isolationandstructuredeterminationoftaxol followedonfromexperiments that showed that a crude extract was activeagainst cancer cells inlaboratorytests. Althoughthisactivitywas discoveredintheearly1960's, it was notuntil 1971 thatthe structureelucidation of this complexditerpenewasdetermined. In1979itwasreportedthatthe mode of action was through promotion of theassembly of tubulin into microtubules. Clinical trials didnot take place until the early 1980's and it was not untilthe 1990's that taxol andits semisynthetic derivativetaxotere were showntobe clinically eective againstbreast and ovarian cancers. The long period for thedevelopmentoftaxolasaclinicalagent,itsdicultyinprocurement as a natural product and the complexity ofitschemical structureall attest tothedicultiesfacedby the pharmaceutical industry indeveloping clinicalagents fromnatural sources.Theresinpodophyllinobtainedfromtherootofthemayapple, Podophyllumpeltatum, is toxic andis usedclinically to removewarts. Themajorconstituentof theresinis thelignanpodophyllotoxinwhichinhibits celldivision. Because of its toxic properties it would seem tobe not worthwhile pursuing any medicinal activitieseventhoughits eects oncell divisionwouldindicatepotential use in cancer chemotherapy. However, a semi-synthetic modied glucoside, etoposide, which has adierent mode of action inhibiting topoisomerase II,hasfoundclinical applicationinthetreatment of lungand testicular cancers.Artemisinin is an unusual sesquiterpene endoperoxidethat has been isolated as the active principle of theChinese antimalarial herb Artemisia annua. Clinicaltrialshavedemonstratedthatartemisininisaneectiveantimalarial and can be used to treat infections of multi-drug resistant strains of Plasmodium falciparumthe causeof human malignant cerebral malaria. Semi-syntheticderivatives including artemether (the methyl ether ofdihydroartemisinin) have improved pharmacokineticproperties and are also of current clinical use. The activemoietyof artemisininis 1,2,4-trioxaneandaseries ofsynthetic analogues showremarkable activity againstPlasmodium species in vitro and in vivo. Whether or notthese will prove to be eective clinical agents or will leadto new clinical drugsis a matterfor future research.Theprospectofnewdrugsandmedicinesfromplantsources is discussedfurther withreferencetosomeofour research investigations (Phillipson,1995,1999a,b).3. Will further newdrugs bedevelopedfromnaturalproduct research?Theclinicalapplicationsoftaxol,etoposideandarte-misinin have helped to revive an interest in higher plantsas sourcesof newdrugs(Phillipson,1999a).Despitethebelief thatthemajorityof clinicaldrugs are syntheticinorigin, itisinterestingtonotethat6outofthetop20pharmaceutical prescription drugs dispensed in 1996werenaturalproductsandthatover50%ofthetop20drugs couldbelinkedtonatural product research. Inrecent years the development of sensitive biologicaltestingsystems,mainlybyindustry,hasledtothepro-cedureofhighthrough-putscreening. Suchscreensarecarriedoutroboticallyanditispossibleforapharma-ceutical or biotechnological company to run 50,000biological testsperday. Thetest screensarebasedonspecicenzymeswithinananimal ormicrobial biosyn-theticpathwayoronreceptorsorsubsetsofreceptors.Newscreensarecontinuallybeingintroducedandbat-teries of compounds, synthetic and natural, are tested asscreens comeonline. Hence, banks of compounds orextractsareneededforindustrial biological tests. It isestimatedthattherearesome250,000speciesofhigherplants and the majority of these have not been examinedin detail for their pharmacological activities. Specicplantsmayhavebeensubjectedtoparticulartests, e.g.for cardiacactivity, but theyhavenot beenexaminedfor anyother type of activity. The major screens forbiological activitiesofplant extractshavebeencarriedoutinthesearchfornewanticancer,antiviralandanti-fertilitydrugs. Thedevelopmentoftherapidscreeningtestsnowinuseinindustryhasmeantthatmanymoreplantscanbeevaluatedforawiderangeof biologicalactivities.Unfortunatelytheresultsofsuchtestsdonotnecessarily reach the public domain and are kept inlockedindustrial les.There still remains anurgent needtodevelopnewclinical drugs and this can be exemplied by thenumerous diseases which result from the malfunction ofthecentral nervoussystem(CNS), e.g. AlzheimersandParkinsons disease, epilepsy, migraine, pain, schizo-phrenia, sleeping disorders. Natural products alreadyhaveaproventrackrecordforCNSactivities, e.g.caf-feine, codeine, morphine, nicotine, reserpine andit ispossible that there are further suchdrugs still tobefound fromnature (Phillipson,1999b).240 J.D. Phillipson / Phytochemistry 56 (2001) 237243Withthis inmind, we collaboratedwithtwomajorinternational pharmaceutical companies, Glaxo (nowGlaxoWellcome) and Pzer. In one investigation, some10 Chinese plants were assessed for their activities against18radioligand-receptorbindingassayswhichareimpli-catedwithCNS.Theotherinvestigationwasconcernedwith pain and some 600 species of plant were tested. Thepain receptors used were bradykinin II, neurokinin I, anda calcitonin gene related peptide. Half of the plants wereselected from the ethnobotanical literature as being usedfor the treatment of pain and the other half were from arandom sample. The results showed that there were morepositive hits for activity inthebiologicalscreensfortheselectedgroup of plants (Phillipson,1999b).4. MalariaIn1996it wasreportedthat therewerebetween1.5and 2.7 million deaths annually and that the majority ofthesewerechildren. Thereareintheorderof500mil-lionnewincidences of malariaannually. It is withoutdoubtoneofthemajorthreatstomankindandchemo-therapy is hindered by the increase indrug resistantstrains, particularlyof Plasmodiumfalciparum. Inthemid 1980's we posed the question `` Are newanti-malarial drugs awaitingdiscoveryfromplants?'' Qui-nine, the rst eective antimalarial drug is still inclinical use and the more recently discovered artemisininhasprovedtobeanincentiveforfurtherresearchintoplants. Theonlymajorscienticpaperof antimalarialtesting of plant extracts by the mid 1980's dated back to1947 when it was reportedthat some 600 species ofhigher plant representingsome 126families hadbeentested against avian malarias. Several plants were activebuttheresearchpointedtotwoparticularplantfamil-ies, Simaroubaceae and Amaryllidaceae which hadnumerous active species. It is pertinent toaskwhyittook more than 30 years for this research to be followedthrough. The answer probably lies in the techniqueswhich were available for carrying out this type ofresearch. Avianmalarias wereusedbecausetheywerethe onlytests for activityagainst Plasmodiumspeciesapartfromthoseusingmonkeys.Theaviantestswhichusedlivechickensandducklingswerenotoriouslydi-cult to carry out and were not thought to be necessarilypredictiveofactivityagainstPlasmodiumspecieswhichaected humans. These tests were not suitable forbioassay-guided fractionation of plant extracts. Fur-thermore, the chemical techniques available werealsonotsuitableforthistypeofresearch.Inthe1940'sand1950's there were not the sophisticated chromatographicseparation techniques which are available today. Even ifanactiveprinciplewereisolatedtherewerenoneofthespectroscopic techniques available for structure deter-minationsuchas nuclear magneticresonancespectro-scopy, massspectrometryorX-raycrystallography.Bythemid1980'snotonlywerethesechemicaltechniquesavailable but alsoit was possible totest for activityagainst P. falciparuminvitroandtherewas areliabletest in mice against P. berghei (Phillipson,1995).Followingthe leadfromthe 1947paper, we testedactivitiesof 5 speciesof SimaroubaceaeagainstP. falci-paruminvitroandutilisedbioassay-guidedfractiona-tiontechniques toisolateaseries of activeterpenoids(quassinoids). Some 40 quassinoids became available forstructureactivity studies and this led to the preparationofsemi-syntheticandsyntheticanalogues.Despitecon-siderable research eort, no new clinical drug was devel-opedfromthiswork.Investigationofarangeofplantsused in traditional medicine for the treatment of malarialedtotheisolationofaseriesofothercompoundswithactivity against P. falciparum including isoquinoline andindole alkaloids, avonoids, mono-, di- and sesquiterpe-noids. These results providedsome scientic evidencewhichhelpedtowardsthejusticationofclaimsfortheuseofanumberoftraditional medicinesandtheyalsoprovidedtemplate molecules for synthetic approachestonewantimalarial drugs. Wideningtherangeof bio-logical tests to include other species of protozoademonstratedactivityofnaturalproductsagainstothertropical diseaseswhichaectmankind, e.g. trypanoso-miasis and leishmaniasis (Phillipson,1995 and 1999a).5. Dotraditional medicines necessarilycontainasingleactiveingredient?The isolation and use of natural products such asdigoxin, morphine and quinine has resulted in replacingthe plant extracts used with single chemical entities.Thereisabasicsuppositionthat anyplant possessingclinical eectiveness must contain an active principlewhichcancompletelyreplacetheplant extract. Threeexamplesfromourresearchhaveshownthat thismaynot necessarily be true (Phillipson,1995).Artemisininiswithoutdoubtthepotentantimalarialactive principle of Artemisia annua. Crude extracts of A.artemisia contain a plethora of other compoundsincluding a series of avonoids and some of theseenhance the activity of artemisinin against P. falciparuminvitro. Whetherthesendingshaveclinical relevancehas not been determined but they do lend support to theviewthattheremaybesomeadvantagestothemedicaluse of extracts as opposed to isolated single entities.Dragon'sbloodisatermusedforthebloodredsapobtainedfromthe barkof anumber of S. AmericanCroton species which are used for the treatmentofwounds.Themajorconstituentsofthesaparepoly-mericanthocyanidinswhichco-occurwithmanyminorconstituents including diterpenes and simple phenols.Chemical andbiological investigationofthepropertiesJ.D. Phillipson / Phytochemistry 56 (2001) 237243 241ofDragon'sbloodledustoconcludethatthereisnotone single woundhealing principle. Whenthe sapisusedtocoverawoundit formsaprotectiveocclusivelayer whilst someof thesimplephenols act as potentantimicrobial agents andother compounds exert anti-inammatory eects.In the 1980's it was noted by clinical dermatologists atGreat Ormond Street Hospital for Sick Children inLondonthat someof theiryoungpatientswithsevereatopic eczemawere showingsigns of improvement intheir disease state. These improvements were not due tohospital therapybuttotheco-administrationofatra-ditional Chinese medicine (TCM). The patients hadvisitedaTCMpractitionerincentral Londonandhadbeenprescribedamulti-herbalprescriptionfromwhichan aqueous extract was prepared for oral use. In 1992, itwas reported that a double blind placebo controlledclinical trial ofatenherbmixturefororal useinchil-drenwithnon-exudativeeczemaconrmedsubstantialclinical benet as assessedbycurrentlyacceptedWes-tern orthodox medical practitioners. Our scienticinvestigations utilising an anti-inammatory/analgesictest withmiceshowedthat four of thetenherbs pos-sessed signicant activity in the mice but they proved tobeinactiveclinicallyinchildren. After someconsider-ableinvestigationweconcludedthatnotonlywastherenosingleactiveingredient but alsothat it requiredalltenherbstobepresent forclinical eectiveness. Therearemorethan12dierentbiologicalactivitiesfromtheherbs inthis TCMprescriptionincludinganti-inam-matory, immuno-modulatory, anti-allergic, sedative andanti-pruritic.Thechemicalcompositionofthe10herbsis a complexmixtureof natural productmolecules.6. ConclusionsPlantscontinuetobeusedworld-wideforthetreat-mentofdiseaseandnovel drugentitiescontinuetobedevelopedthroughresearchintotheir constituents. Inthe developed countries, high-throughput screening testsare usedfor bioassay-guidedfractionationleading totheisolationofactiveprinciplesthatmaybedevelopedintoclinical agents either as thenatural product or asynthetic modicationor asynthesisedanalogue withenhancedclinicalactionorreducedadversesideeects.Despitethemassivearsenalofclinicalagentsdevelopedbythepharmaceuticalindustrytherehasbeenanaver-sionbymanymembersofthepublicandherbal reme-dies have proved to be popular as alternative orcomplementary treatments of disease. There is a need toevaluateherbal treatments byclinical trials usingcur-rentlyacceptedprotocols. InthedevelopingcountrieslargenumbersoftheWorld'spopulationareunabletoaordpharmaceutical drugs andtheycontinuetousetheir own systems of indigenous medicine that aremainlyplant based. There is agreat needtoharnessscientic and clinical research in order to investigate thequality, safetyand ecacyof these herbaltherapies.Theaimofthepharmaceuticalindustryistodevelopnovel drugentities for the treatment of disease. Suchdrugs require specicity of action and are, for e.g. aimedat a particular subset of receptor. Although natural pro-ductscontinuetosupplybanksofcompoundsfornewscreens, thefocusofindustryiscurrentlyoncombina-torial synthesis for new drug development. It must not beforgottenthat natural productswhichresult frommil-lenniaof biosyntheticpathwaysmodiedbyevolutionhavea wellestablishedtrackrecordas medicinalagentsandpresent awiderangeof structural diversity. Drugdevelopment throughnatural product researchis notwithout its problems andthere is, for e.g. aneedtoeliminatecommonnatural products suchas saponins,tannins,etc.fromplantextractspriortotestingbybio-logical screening procedures.Academics canplaya use-ful role inthis area of research. They cannot matchindustryinthewiderangeofscreensbuttheycanuseselective targets and collaborate with industry. This typeof research needs a multi-disciplinary approach and thisincludesexpertise in phytochemistry.Itisapleasureandanhonourtopresentthislectureandtoacknowledge the leadwhichProfessor JereyHarborne has giventoPhytochemistryover somanyyears. Iamoneofthosewhooweagreatdebttohimandtotheexamplewhichhehasset.Myownspeciali-sationofPharmacognosywasvirtuallywipedoutfromPharmacyundergraduatecurriculaandformanyyearshas been considered to be an outmoded area ofresearch.TechniquesinPhytochemistryhaverevolutio-nised our ability to investigate the medicinal agentspresent in plants and this is acknowledged by theindustrial interest in plants over recent years. Thanks tothe hard work and tenacity of Jerey Harborne we havebeenabletopublishresearcharticlesinPhytochemistryand to continue working on the wealth of chemicaldiversity that existsin the plantkingdom.AcknowledgementsIamgrateful toParke,DavisandCompanyforper-missiontoreproducethegures fromthebookGreatMomentsinPharmacy byG.A.Bender,Detroit, North-woodInstitute Press, 2nd Edition, 1967.ReferencesPhillipson, J.D., 1995. Amatterof somesensitivity. Phytochemistry38, 13191343.Phillipson, J.D., 1999a. Newdrugsfromnatureit couldbeyew.Phytotherapy Research 13, 28.Phillipson, J.D., 1999b. Radioligand-receptor bindingassays inthesearchfor bioactiveprinciples fromplants. J. Pharm. Pharmacol.51, 493503.242 J.D. Phillipson / Phytochemistry 56 (2001) 237243DavidPhillipsonisEmeritusProfessorof Pharmacognosy at the Centre forPharmacognosyandPhytotherapyatThe School of Pharmacy, The Uni-versity of London. He was formerlyProfessorandHeadofDepartmentofPharmacognosyat TheSchool beforeretiring in 1994. In 1995, he wasappointedfor 6months at The Chi-nese University of Hong Kong asWilson T.S. Wang DistinguishedInternationalVisitingProfessor.HeisanHonoraryProfessorattheChineseAcademy of Medical Sciences, Insti-tute of Medicinal Plant Development, Beijing. For many years, he hasbeenanactivememberofthePhytochemical SocietyofEuropeandbetween 1977 to 1988 heheld oces of Secretary, Vice-Chairman andChairman.Hisresearchinterestsincludethechemistryandbiologicalactivities of plants used in traditional medicine. He has receivedawardsfromthePhytochemicalSocietyofEuropeincludingtheTateand Lyle Award (1992), Medal (1994) and Pergamon Prize forcreativityinplant biochemistry(1996). In1989, he andfour otherEuropeanscientists incollaborationwithProfessor Meinhart Zenk(thenoftheUniversityofMunich)wereawardedtheKorberFoun-dation Prize for achievement in European Science. The Pharma-ceutical Societyof Great Britainpresentedhimwiththeir HarrisonMemorial medal in 1999.J.D. Phillipson / Phytochemistry 56 (2001) 237243 243