Ethnopharmacology and rational evaluation of herbal remedies

Transcription

1 Research Signpost 37/661 (2), Fort P.O., Trivandrum , Kerala, India Handbook of Ethnopharmacology, 2008: ISBN: Editor: Mohamed Eddouks 2 Ethnopharmacology and rational evaluation of herbal remedies Vikas Kumar 1 and Shyam Sunder Chatterjee 2* 1 Neuropharmacology Laboratory, Department of Pharmaceutics Institute of Technology, Banaras Hindu University, Varanasi , UP, India 2 Formerly: Head of the Pharmacology Department, Dr.Willmar Schwabe Arzneimittel, Germany Abstract Historically, extrapolation of ethnopharmacological knowledge on herbal remedies has led not only to the discovery of numerous drugs, but also to many concepts and principles of modern medicine and pharmacology. Since therapeutic possibilities offered by a vast majority of such remedies have not yet been properly explored, they still remain to be promising sources for obtaining structurally and functionally novel drug leads, or for novel pharmacological * Current address: Stettiner Str. 1; D Karlsruhe, Germany Correspondence/Reprint request: Dr. Vikas Kumar, Neuropharmacology Laboratory, Department of Pharmaceutics Institute of Technology, Banaras Hindu University, Varanasi , UP, India vikas.phe@itbhu.ac.in

2 26 Vikas Kumar & Shyam Sunder Chatterjee targets suitable for drug development purposes. In addition, herbal remedies continue to be the only therapeutic possibilities for a vast majority of the global population, and during the past few decades, their uncontrolled medicinal uses have consistently increased even in the western world. Consequently, efforts to rationalize their medicinal uses continue to be a major challenge for many research laboratories and health authorities around the globe. The main objects of this chapter are to point out the problems and prospects involved in attaining such goals, and to suggest a versatile pharmacological strategy for evaluating therapeutic potentials of traditionally known herbal remedies. This strategy evolved from the know-how and experience gained from the efforts made to identify bio-active components and therapeutic potentials of Ginkgo biloba, Piper methysticum and Hypericum perforatum. The usefulness of this approach for better standardization of herbal remedies, and for drug discovery purposes, is described with the help of more popular phytomedicines manufactured from them. Introduction Despite the extensive efforts and considerable progress made during the past century, therapeutic management of many health problems remains to be an open challenge for modern medicine. Moreover, currently available drugs do not always properly meet the therapeutic demands due to socio-economic and many other reasons, a vast majority of the world population cannot afford to gain health benefits from them. Therefore, the primary goals of most modern drug discovery projects still remain to be the identification of novel therapeutic leads suitable for the development of structurally and functionally novel drugs, or of more affordable and better therapies. Such projects of most, if not all, research laboratories managed by bigger, or internationally operating, pharmaceutical companies concentrate mainly on the discovery of structurally and/or functionally novel therapeutic hits and leads only, and pay little attention to affordability related problems. Since availability of drugs and therapies developed by such companies is limited by patent laws, and by prevailing socio-economic conditions of many countries, a vast majority of the world population remain deprived of many promising therapies. Efforts to solve the affordability and availability related problems made by several commercial and non-profit organizations during the past decades have met with very limited success, and that too in a very few therapeutic areas only. Universal usefulness of these approaches still remains highly debatable. In view of the situation, traditionally known therapies and remedies continue to be the sole health care options for a vast majority of the population in economically underdeveloped, or developing countries, and for many in the more developed, or richer, parts of the globe as well. In the USA and some other industrially developed countries, all such treatment modalities are

3 Drug discovery & Ethnopharmacology 27 grouped by the main stream of medical practitioners and health care authorities under the heading alternative medicine with questionable efficacy. However, unlike in the USA, interest in therapeutic possibilities offered by traditionally known herbal remedies never vanished in Germany, France, and some other non-english speaking European countries. The recently established regulatory standard for herbal medicine in Europe is actually based on the knowledge and experience gained by the widespread uses of such remedies in these countries [1]. Interestingly, the most popular and best studied phytomedicines prescribed in Europe are also the most commercially successful alternative herbal remedies in the USA and other westernized countries [2]. In several more densely populated countries, diverse traditionally established medicinal systems are well recognized by the governments, and many medical practitioners use them also as alternative therapeutic possibilities. Amongst them, Ayurveda, Unani, Kampo, and Traditional Chinese Medicine (TCM) are perhaps the most ancient and yet widely used and recognized ones. Consequently, governments of the most populous countries like India or China not only approve regular health care uses of these systems, but also make strong investments for rationalizing and modernizing them [3,4]. In addition, numerous ethnopharmacological, ethnobotanical and many other types of ethnomedicinal studies consistently point out the existence of various other types of health care systems and principles around the globe. Widespread medicinal uses of herbs, or of different types of concoctions made from them, are common characteristics of almost all traditionally known health care systems around the globe [5]. Since both in the old and new worlds, popularity of herbal remedies has enormously increased during recent decades. Interest in more concentrated efforts to rationalize, and more properly evaluate, such remedies has considerably increased also. Consequently, reports on efforts to identify therapeutically interesting bioactive constituents of traditionally known medicinal plants, and on properly controlled clinical trials with so called standardized herbal extracts have consistently increased during the past three decades. Major goals of most such preclinical efforts are: 1) to obtain structurally novel therapeutic leads and hits, 2) to identify bioactive components suitable for standardizing medicinally used herbal preparations, 3) to experimentally verify a traditionally known medicinal use of a herb, 4) to obtain a patentable herbal extract suitable for a defined therapeutic purpose, or 5) to meet the demands of drug control authorities. Although diverse feasible strategies for achieving each of these goals are now available, most of them concentrate either on one single traditionally known therapeutic possibility offered by a given herb, or on potential bioactivities of randomly collected herbs, and/or on some of their secondary metabolites only. The choice of bioassays for such purposes depends either on the therapeutic area of interest

4 28 Vikas Kumar & Shyam Sunder Chatterjee of the project, or on the available facilities and know-how of the laboratories concerned. Hence, well known drugs and other experimental tools are regularly used as standards for evaluating and interpreting the bioactivities of the herbal extracts and their constituents. Ethnopharmacological literature surveys reveal though, that traditionally known medicinal uses of most, if not all, known herbs are, in general, not restricted to one single health problem, and that their conventionally described medicinal indications are not always clearly definable in terms of modern medicinal terminology and definitions. Moreover, most currently used strategies commonly neglect the facts that, in general, medicinal herbs contain multiple constituents with varying physicochemical properties and stability, and with synergistic and/or antagonistic bioactivities. In addition, available pharmacological models used to evaluate their therapeutic potentials evolved from the knowledge gained by studies of known drugs and other bio-active agents only. Thus, currently available strategies seem to be particularly suitable neither for evaluating the most appropriate therapeutic potential of a given herb, nor for the search of potentially novel mode(s) of action(s) plausible for traditionally known herbal remedies and their bio-active constituents. The main aim of this chapter is to describe the situation with the example of three popular and more extensively studied medicinal plants, and to point out the usefulness of a modified strategy for a more rational, or ethnopharmacological knowledge based, evaluation of traditionally known herbal remedies. Know-how, or experience, gained by efforts to define the active constituents and therapeutic possibilities offered by Ginkgo biloba and Piper methysticum led eventually to the strategy, and feasibility and usefulness of the strategy were tested by the use of Hypericum perforatum. Therefore, some more important information and observations on these three herbs, relevant for developing the strategy and to test its validity, are summarized at first. For more detailed available information on these three medicinal plants, consultation of the cited monographs and reviews is recommended. Ginkgo biloba Diverse medicinal uses of different types of concoctions from various parts of Ginkgo biloba have been known since long in China. However, it was only during the mid 1960s that the very first reports on therapeutic trials or experiences made with extracts obtained from its dried green leaves appeared in the western literature [6,7]. These earliest clinical reports suggested beneficial effects of some, not very precisely described, Ginkgo extracts on cerebral and peripheral circulatory disturbances. The very first pharmacological reports on Ginkgo biloba [8] describe some pharmacological properties of Ginkgo flavonoids, and concentrated mainly on their effects on smooth muscles and blood vessels. Since during this period flavonoids were considered

5 Drug discovery & Ethnopharmacology 29 to be novel therapeutically interesting lead molecules, efforts were consequently made to obtain therapeutically useful Ginkgo biloba extracts, enriched and standardized on their flavones contents (20 to 30%) only, which were subsequently patented. One of these patented extracts, now referred to as EGb 761, was fortunately enriched in bilobalide and Ginkgolides also. Till the 1990s, almost all preclinical and clinical studies reported in the literature on Ginkgo biloba extracts were conducted with this extract only. During the late 1970s, it became apparent though, that flavonoids cannot be the sole therapy relevant bioactive constituents of EGb 761, and that its clinical efficacy and pharmacological activity profiles are not easily interpretable in terms of its effects on blood vessels, or on smooth muscles, only. Consequently, more concentrated and extensive efforts were made in several German and French laboratories. Results of these efforts made during early 1980s, strongly suggested that the Ginkgo biloba specific terpenic lactones, i.e. of bilobalide and ginkgolides present in high concentrations in EGb, 761, could be the major CNS function modulating bioactive constituents of the extract observed in preclinical and clinical studies [9]. Although the observed pharmacological activity profiles of EGb 761 and its known active components were unlike those of any known drug, some of their CNS functions modulating activities were like those of the so called nootropic agents like piracetam. Thus, during the last two decades of the past century, EGb 761 was often pharmacologically referred to as a novel type of nootropic agent with cognitive function modulating as well as neuroprotective properties. Moreover, pre-clinical and clinical efficacy data on the extract generated till the late 1970s were considered by the German and French health authorities as sufficient enough for approving EGb 761, and some other flavonoids and terpenic lactones enriched Ginkgo biloba leaf extracts (EGbs), as phytotherapeutic drugs, which could be prescribed also by registered medicinal practitioners. EGb 761 was actually one of the very first phytotherapeutic drugs; enriched and analytically standardized for its contents of several bioactive constituents with diverse activity profiles; approved by the German health authorities under its current law, which went into effect in The idea that the use of extraction procedures could be a better means for obtaining therapeutically more interesting uses of medicinal herbs has been known to mankind for ages. One of the very early reports [10], in 1907, clearly pointed out diverse, not easily controllable factors dictating the bioactivities of herbal extracts, and pointed out the therapeutic importance of more precisely standardized herbal extracts for their bioactive constituents. Despite such knowledge, it was only during the late 1970s, i.e. only after the introduction of Ginkgo biloba extracts to the western world, that several laboratories and health authorities around the globe became more attentive to rationally solve the problems concerning standardization of herbal extracts for their bioactive

6 30 Vikas Kumar & Shyam Sunder Chatterjee components. Thus, although strictly speaking, EGb 761 cannot be considered to be a traditionally known herbal remedy, the strategy used to develop and obtain better acceptance of medical practitioners of a Ginkgo biloba extract seems to be an appropriate one for better exploration of herbal remedies. Unfortunately, even today most commercialized herbal remedies are seldom standardized for their active constituents, and little efforts are made to identify and bioassay them for their activity profiles and potencies. More definitive knowledge on a few bioactive constituents of EGb 761, and on its therapeutic usefulness for helping patients with circulatory disturbances, not only led to widespread popularity and commercial interest in exploiting medicinal possibilities offered by Ginkgo biloba leaves, but also triggered the interests of many laboratories around the globe to define its modes of action and most appropriate therapeutic indication. Results of more concentrated preclinical and clinical efforts made to clarify the situation, revealed that EGb 761 is a therapeutically useful remedy for coping with some age related mental health problems of the elderly, including those associated with AD, and consistently reconfirmed that patients with peripheral circulatory disturbances also benefit from therapies with EGb 761. However, many questions concerning active constituents and modes of action of EGb 761 cannot yet be answered with certainty. Experimental evidences acculmulated during more recent years have mainly reconfirmed and further elaborated the facts established during the 1990s, and have opened up some avenues of potential interest for drug discovery purposes. Since describing all published information on Ginkgo biloba leaf extracts and their active constituents is beyond the scope of this chapter, in the following discussion an attempt is made to summarize only those potentially useful for obtaining structurally and functionally novel hits, leads or drugs useful for meeting the therapeutic demands of patients with not so precisely definable CNS disorders. Several reviews and monographs [11-22] can be consulted for more detailed and exhaustive information. The neuroprotective and CNS-function modulating effects of EGbs have attracted special attention of many research groups. In general, most of them concentrated their efforts either on free radical scavenging and/or the antioxidative potentials of the extracts and their flavonic constituents, or on the modulating effects of ginkgolides on the receptors of the platelet activating factor (PAF). Therefore, reports on therapy relevant bioactivities of other extract constituents have been rare, the only major exception being those dealing with bilobalide, i.e. quantitatively the major bioactive molecule present in EGb 761. One of the very first bioactivities of EGb 761, and of bilobalide, observed after oral administration to animals, were their protective effects in a triethytin (TET) induced rat cerebral edema model [23]. TET is a well known uncoupler of oxidative phosphorylation, and has since long been known to be a

7 Drug discovery & Ethnopharmacology 31 very potent cerebral edema inducer in rats and humans. Since some early efforts to establish bioactivity profiles of EGb 761 led to observations suggesting mitochondrion as the site of action of the extract, the TET-induced cerebral edema model was also included in the test battery designed to screen for its bioactivity profile. These efforts revealed not only one of the most potent bioactivities of the extract observed to date after oral administration to experimental animals, but also led to the identification of bilobalide as EGb 761 major bioactive component in the model [24,25]. However, quantitatively the high potency of EGb 761 in the model cannot yet be explained by its bilobalide contents only, or by its interactions with ginkgolides and any other known constituents of the extract, which are all inactive in this model. Since several other experimental problems encountered during efforts to identify the very first bioactivity of bilobalide are analogous to those commonly encountered by many engaged in the identification of herbal bioactive components using extraction, fractionation and sub-fractionation procedures, they are described here in short. Although efficacy of EGB 761 in the TET-model could be reproducibly quantified in several laboratories, despite concentrated efforts and using numerous methods and procedures, it was impossible to obtain an extract fraction with any activity in the model. This was because its major active component in the model, i.e. bilobalide, is unstable under conditions generally used for fractionating herbal extracts. Thus, as a desperate ultimate effort, attempts were made to collect, or isolate, and test individually all known pure compounds known to exist in green Ginkgo biloba leaves. These efforts not only led to the identification of the very first bioactivity of bilobalide, but also to the facts that although isolated pure bilobalide and the ginkgolides are stable molecules, they are very unstable in solutions and suspensions with ph higher than ca Only by appropriate uses of this know-how, it became possible to better standardize EGb 761 and other Ginkgo biloba extracts and fractions, and to isolate, identify and characterize ginkgolides and several other naturally occurring secondary metabolites of the herb. Soon after sufficient quantities of ginkgolides became available, the French laboratories identified their PAF-receptor antagonistic activities. Since during this period, i.e. during the mid 1980s, PAF-receptors were newly discovered, and diverse speculative suggestions were made on the potential importance of PAF in diverse pathologies, ginkgolides and EGbs were extensively studied in numerous laboratories around the globe. Despite such efforts, no very definitive statements can yet be made on the role of PAFreceptors in the modes of actions involved in the observed effects of EGbs. Numerous reports now available on the neuroprotective and CNS-function regulating effects of EGb 761 and its known constituents strongly suggest that bilobalide is not only quantitatively the major single constituent of EGb 761, but is also the most potent neuronal and/or astrocytic function regulating

8 32 Vikas Kumar & Shyam Sunder Chatterjee component of the extract. Although the sites and primary targets involved in its observed effects have not yet been precisely defined, several more recent reports encourage us to suggest that regulating mitochondrial functions, and modulation of chloride channel functions are most probably involved in its mode(s) of action [26]. Results of some ongoing studies are awaited for better clarification of the situation. Taken together, reported bioactivities and therapeutic observations made with EGb 761 add strong experimental evidence to the conviction that they are multicomponent therapeutic principles acting simultaneously, with different potencies and durations of actions, not only on diverse organs and blood vessels, but also on circulating body fluids and many other processes controlling circulation [27]. Thus, such therapeutically widely used extracts seem to be more rationally definable in terms of the principles of many traditionally known and older medicinal systems like Ayurveda, TCM etc., than by those of modern pharmacology and medicine. It must be pointed out though, that all currently commercialized EGbs must not, necessarily, be therapeutically equivalent. This is because all bioactive components of such extracts are still not known, and they are, in general, analytically standardized for their contents of only a few known and more easily quantifiable constituents. Further clarification of the situation is necessary not only for better standardization of therapeutically used extracts, but also for better defining their targets and modes of action necessary for eventually designing or obtaining more specific, and/or better, therapeutics than EGb 761. Numerous properly controlled clinical trials have repeatedly reconfirmed many therapeutic potentials of EGb 761 predictable from its pharmacological activity profile. Amongst them, the ones dealing with dementia, cognitive disorders and Alzheimer s disease (AD) are currently of major therapeutic, socioeconomic and scientific interest. It has been pointed out indeed, that efficacy of EGb761 for treating patients with AD is almost equivalent to that obtainable by some known single component drugs [28], and that due to safety reasons, EGbs could be a better therapeutic option [29]. Since efficacy of all so far available drugs, including EGb 761, for AD and other cognitive disorders and dementia are not very satisfactory, attempts to obtain a more effective product based on the knowledge accumulating from efforts and medicinal uses of EGb 761 seems to be warrantable. Availability of proper pharmacological screening models is an essential prerequisite for initiating such efforts. Attempts to identify a screening model suitable for identifying bilobalide-like bioactivities in mice led to the observation that unlike in rats, after oral administration to mice, it not only dose dependently induced sedation, hypothermia and analgesia, but also revealed anxiolytics like effects [30]. Since γ-butyrolactone has a similar activity profile in mice, and a bilobalide molecule contains three condensed

9 Drug discovery & Ethnopharmacology 33 γ-butyrolactone moieties, efforts were made to clarify the importance of this ring system in its observed effects by the use of structure activity studies. However, the pharmacophore moiety speculated from the results of such efforts turned out to be the t-butyl substituted cyclopentane ring system, which is also a unique structural characteristic of all ginkgolides isolated to date from leaves and other parts of Ginkgo biloba [31]. Eventually, these findings led to the identification of modulating effects of bilobalide and ginkgolides on glycine- or GABA-gated chloride channels, and also to a structurally unique Ginkgolide X with much higher efficacy than the terpenic-lactones present in EGb 761 [32,33]. These reports not only triggered interest of many in clarifying the role of chloride channels in the modes of action of EGb 761, but also led to the identification of NV-31 as a neuroprotective agent with therapeutically interesting radical scavenging, memory enhancing and antidepressant-like activities in animal models with a broad safety margin. Detailed discussion on these more recent findings is beyond the scope of this chapter. For keeping up with such developments, more recent literature has to be consulted. During the past three decades, almost all more successful strategies to evaluate EGb 761 were conceived by appropriate uses of existing knowledge on the structure activity relationship between known bioactive molecules. Hence, special attention was paid to the effects of the extract observed in vivo in animals, patients and volunteers. Since, in general, almost all problems encountered during these efforts are not unique to this traditionally known medicinal plant; more widespread uses of the indicated features of these strategies could be more rational and rapid means for more properly evaluating other traditionally known herbal remedies as well. Indeed, that such could be the case is indicated also by the progress made, to date, to more rationally use the traditional knowledge on Piper methysticum for better standardizing its therapeutically used extracts, and for identifying structurally and functionally novel therapeutic leads, hits and pharmacological targets. It is therefore described, in short, in the following text. Piper methysticum Piper methysticum G. Forster. (Family: Piperaceae) is a shrub indigenous to the South Pacific islands, where it has occupied a central place in the everyday life of the habitants. Terms like kava, kawakawa or kawa are traditionally used to describe the plant and beverages and/or concoctions prepared from its roots according to the traditions of the aborigines living in the islands. The very first pharmacological observations made with its extracts and of some of their isolated secondary metabolites, now often collectively referred to as kava pyrones were published as early as 1886 [34]. Since then, many European laboratories have sporadically reported diverse bioactivities of

10 34 Vikas Kumar & Shyam Sunder Chatterjee different types of extracts obtained from its roots, or of purified individual kava pyrones isolated from such extracts. Observations made during these efforts eventually led not only to more widespread uses of kawa extracts as anxiolytics, tranquilizers, sedatives hypnotics, narcotics etc. in Europe, but also to the introduction (in Germany) of a synthetically produced racemic kava pyrone, i.e. ± kawain, as a single component plant derived anxiolytic. Thus, as is the case for Ginkgo biloba, designating Piper methysticum as a traditionally known herbal remedy does not seem to be very appropriate. Its popularity in the western world is based, actually, on the observations made by the scientific curiosity of a few researchers interested in clarifying the widespread social uses, and pleasurable intoxicating effects, of kava. For detailed information on early efforts leading to the current popularity of kava extracts, the reader is referred to some excellent and exhaustive reviews [35-37]. After 1990, several phytopharmaceuticals containing diverse types of Kava Kava (defined as Piper methystici Rhizoma) extracts, containing mg kava pyrones as active principles, were approved by German health authorities as suitable remedies for treating conditions of nervous anxiety, stress and restlessness. This was based on the recommendation of the so called Commission E established by the German Ministry of Health in 1978 for evaluating the 300 most popular herbal remedies commercialized in the country till the 1990s. This commission consisted of many well recognized physicians, pharmacists, pharmacologists, toxicologists, biostatisticians and practitioners of herbal remedies, and thoroughly evaluated not only the traditional uses and case reports from physicians files, but also all the chemical, biochemical, pharmacological, toxicological and clinical data available, by electronic as well as manual search, on a given herb. Thus, the monographs of the commission are sometimes referred to as the most accurate information available in the entire world on the safety and efficacy of herbs and phytomedicine [38]. However, such monographs are useful for a more rational regulation of herbal remedies only, and are of very little, if any, help for evaluating them for their more appropriate, or better, or more rational uses. As exemplified by the following discussion, even choices of more promising herbal extracts on the basis of such monographs, reports and reviews could, indeed, be very misleading. Although diverse therapeutically interesting pharmacological properties of kava pyrones (i.e. 4-methoxy-alpha-pyrone derivatives encountered in the roots and other parts of the herb) have been known for a long time, till recently, most reports on clinical trials on kava extracts have concentrated mainly on the three indications approved recently by the German health authorities. In addition, except for the so called standardized kava extract WS 1490; almost all clinical and preclinical studies were conducted with hydro-alcoholic extracts prepared by different procedures and conditions.

11 Drug discovery & Ethnopharmacology 35 Since kava pyrones are considered to be the pharmacologically active principles of kava, and its chalconic constituents [39] and yangonin [40] are responsible for some observed undesirable effects of kava drink consumptions [41], the more lipophilic extract WS 1490 was conceived. The goal was to obtain well standardized phytotherapeutically enriched kava pyrones, devoid of chalcones and yangonin. Indeed, numerous preclinical and clinical studies (including many properly designed placebo-controlled double-blind ones) have consistently reconfirmed efficacy of WS 1490 and diverse other types of kava extracts against anxiety disorders and sleep disturbances. Although these reports are in agreement with the recommendations of health authorities, it cannot be ignored that more recent reports have consistently been successful in demonstrating diverse other medicinal possibilities offered by the herb. For more detailed information, the cited references and a more recent review [11] can be consulted. In the following discussion, only some such information, potentially useful for more rational evaluation and exploitation of traditionally known herbal remedies, is summarized. The fact that kavapyrones are involved in many therapeutically interesting pharmacological activities of Kava extracts has repeatedly been reconfirmed during more recent years [42-44]. Amongst them, their anticonvulsant activities are better defined, and an early structure activity study has also revealed that amongst all naturally occurring kavapyrones, methysticine is the most potent one [45]. A very short report summarizing some efforts made during [46], concluded that a crude kava extract, or a synthetic α-pyrone called ethysticin, afforded only limited protection against grandmal seizure after several weeks treatment. However, since neither the concentration of kavapyrones in the extract was tested, nor the optical purity of the tested pyrone derivative was defined, validity of this report for judging therapeutic potentials of kava extracts and kavapyrones as anti-convulsants remains questionable. Later observations, made in collaboration with Prof. Achenbach (Erlangen University; Germany) and others, clearly indicated that potencies of all racemic kavapyrones, resulting from available synthetic methods, as anticonvulsants are always lower than those of naturally occurring ones (unpublished observations). More concentrated structure activity studies for better clarification of the situation led to the observation that pharmacological activity profiles of some pyranone derivatives are almost identical to those of kavapyrones [47]. Some such pyranone derivatives were several folds more potent and much longer acting ones than any naturally occurring kavapyrone. In addition, collaborative efforts with late Prof. S.K. Bhattacharya (BHU; India) revealed a unique pharmacological activity profile of one such newly synthesised pyranone derivative. As an offshoot of these efforts, several therapeutically interesting pharmacological properties of the kava extract WS 1490 could be unravelled. Amongst them, its antidepressant-like activity, and

12 36 Vikas Kumar & Shyam Sunder Chatterjee protective effects against extrapyramidal side effects of narcoleptics, or against drug-withdrawal syndromes of addictive agents, could be of major therapeutic interest. The results of some therapeutic trials designed to test these possibilities are encouraging and some others are still awaited. During efforts to identify the pharmacophore moiety of the kavapyrone methysticine, and to design a more specific and potent anti-epileptic agent, it was observed that the 4-metoxylated-α-pyone moiety present in the molecule could be replaced by 4-methoxy-2(5H)-furanone moiety without complete loss of its bioactivities. Subsequent literature surveys revealed that some since long known secondary plant metabolites like fadynolide, isolated from Piper fadyenii, and pipeperolide, isolated from Piper sanctum, also possess seizure protecting activities [48]. However, despite several attempts, sufficient quantities of authentic samples of these two piper species could not be acquired. Consequently, they were synthesised and pharmacologically screened for their anticonvulsant-like and other therapeutically interesting pharmacological activities. An offshoot of these efforts was the identification and further development of losigamone as an antiepileptic agent [49]. Although its broad CNS-activity profile is almost identical to those of kava pyrones, till now, all clinical trials with losigamone have concentrated on its anticonvulsant-like therapeutic potentials only. This is mainly because properly controlled clinical trials for detecting antiepileptics are much easier, or convenient, to design than those necessary for evaluating other therapeutic possibilities offered by the agent. Taken together, the results of more concentrated efforts made to reveal therapeutic possibilities offered by Piper methysticum have consistently demonstrated that, in comparison to most therapeutically used psychotherapeutics; its bioactive constituents possess a broader spectrum of neuronal function modulating properties. Thus, kava extracts seem to be a mixture of CNS active agents, each of which has an analogous, but not identical, activity spectrum with varying potencies and durations of actions. Since, more often than not, most patients suffer simultaneously from more than one mental health problem, more properly standardized kavapyrone enriched extracts could, indeed, be a better therapeutic alternative available currently. Although during recent years much concern has been raised on the adverse effect potentials of kava extracts, available preclinical and clinical evidences strongly suggest that such potentials of kava extracts cannot be higher than those of any known psychotherapeutic. Rational efforts to evaluate and precisely define the most appropriate therapeutic possibility offered by the herb are still necessary for achieving this goal. It must be pointed out though, that kava roots must not, necessarily, be the most appropriate starting material for such purposes. This is because only mature roots are good sources of kavapyrones, and one has to wait a few years before such roots can be harvested. Since kava leaves are also

13 Drug discovery & Ethnopharmacology 37 enriched in kavapyrones, they could as well be an alternative. Since almost all kava products are obtained from the roots harvested in the far eastern islands, environmental consequences of such practices have to be given due consideration. To our judgement, such consideration should be an integral part of any rational strategy for evaluating any traditionally known herbal remedies. Hypericum perforatum or St. John s Wort Unlike in the case of Ginkgo biloba or Piper methysticum, European medicinal history of Hypericum perforatum (HP) can be traced back to the beginning of the 16 th century when the famous German physician Paracelsus ( ) recommended its uses for the treatment of wounds, parasites and phantasmata [50]. Although analogous and diverse other therapeutic uses of the herbs have been known in other parts of the globe, till more recently, it was considered to be a weed in the USA. During recent decades though, scientific as well as therapeutic and commercial interests in HP-extracts (HPEs) have increased enormously not only in Europe, but also in the USA. Such gain in popularity paralleled the number of reports describing antidepressant-like clinical efficacy and the preclinical observations made with diverse types of HPEs. Although more efforts to exploit other therapeutic potentials offered by the herb and its known bioactive secondary metabolites are now being made, the major therapeutic interest in the herb continues to be its antidepressant-like efficacy. The very first therapeutic report describing the beneficial effect of the herb in depressed patients dates back to much earlier than the fortuitous discovery of modern antidepressants. This report also mentioned a rat study revealing photo-sensitizing and some neuropsychological effects of the red pigment of the herb [51]. However, it was only after the 1980s that reports on the usefulness of diverse types of HPEs in treating patients with mild to moderately severe symptoms of depression started appearing regularly. Observations resulting from more recent clinical and other studies conducted till 2001 have been chronologically published [52-55], and two more recent books summarize our current knowledge on the CNS function modulating effects of HPEs [56,57]. Diverse medicinal uses of HPEs known traditionally, or indicated by more recent findings, are summarized in a recent review [58], in addition to a more recent report nicely pointing out the necessity of a closer chemical examination of the starting material for judging the medicinal values of the herb and its sub-species [59]. However, as is the case for Ginkgo biloba, more modern literature has to be consulted for keeping up with the potential therapeutic options offered by the herb. In the following text, some more important developments leading to the identification of currently known bioactive constituents of HPEs are summarized.

14 38 Vikas Kumar & Shyam Sunder Chatterjee The red pigment of HP has been the curiosity of many researchers since 1830 [60], and the name hypericin was given to one such pigment isolated from the herb in 1911 [61]. Since then, this red pigment had enjoyed widespread acceptance as the CNS active principle of the herb, and up until the mid 1990s little efforts were made to identify its other CNS function modulating components. During 1984, it was wrongly reported that hypericin inhibits MAO-A [62], and this report further encouraged the idea that hypericin(s) could, indeed, be the major antidepressant-like component of HPEs. Even today, many therapeutically used HPEs are characterized, or standardized, in terms of their hypericin(s) contents only. Later reports indicated though, that the use of an impure hypericin sample could have been responsible for the wrong interpretation of the observations [63], and that some xanthones present in the herb could be the MAO-inhibitory components of the herb [64]. Although the question concerning the presence of such xanthones in the herb has not yet been clarified, several later reports continued to evaluate this enzyme as a potential target for their observed antidepressant-like activities [65-67]. On the contrary, despite the fact that a xanthone and a biflavone, and not hypericin, have been reported to be the red pigments of some traditionally known oily preparations obtained from the herb [68], efforts are seldom made to define and characterize the bioactivities of these pigments in terms of the traditionally known medicinal uses of such preparations. Although several reports do indicate that some xanthones are genuine secondary metabolites of the herb, most of them dealing with the chemical constituents and bioactivities of HPEs mention that their concentrations in the herb are very low, or negligible [69]. These reports deal mainly with the extracts from herbs commercialized in Europe, and sometimes they mention (without analytical and other details) that xanthones are present mainly in its roots [70]. A more recent patent [71] mentions though that the upper one-third portion of Indian Hypericum perforatum plants is enriched in xanthones, and that HPEs containing 3-5% xanthones can be obtained by aqueous ethanolic or methanolic extraction of the flowering upper part of the herb. This patent also mentions that such extracts possess antidepressant-like efficacy in animal models. However, since 1999 many other reports have consistently reported antidepressant and other CNS function modulating effects of Indian HPEs [67, 72-81]. In addition, unlike in the case of other HPES, anti-inflammatory, analgesic and anti-stress effects of Indian HPEs have also been demonstrated in animal models [76,77]. Taken together, these reports do not yet allow any definitive suggestions on the possible roles of xanthones in the pharmacological activity profiles of therapeutically used HPEs. It must be mentioned though, that hydroxylated xanthones have also been isolated from the upper part of HP collected in China [82], and it has recently been pointed out that fungal infections of the herb can as well increase its xanthone contents

15 Drug discovery & Ethnopharmacology 39 [83]. Since several bioactivities of xanthones encountered in HP are known, proper control of their contents in HPEs seems to be an urgent necessity. Since the end of the 1990s, some more concentrated efforts to evaluate many secondary metabolites as potential antidepressant components of the herb have been made [84,85]. Information accumulating from these efforts and many other circumstantial evidences strongly suggests that clinically observed antidepressant-like efficacy of HPEs could be due to the presence of many CNS active components in them; and that the phloroglucinol derivative hyperforin is quantitatively the major neuroactive component of the herb. Consequently, many commercialized extracts are now standardized for their contents of hypericin as well as of hyperforin. It must be noted though, that these two secondary metabolites do not necessarily co-exist in all subspecies of Hypericum perforatum [59], and that antidepressant-like efficacy of different types of HPEs in animal models cannot be predicted by their contents of these two components only. In addition, it cannot be ignored that clinical efficacy of HPEs devoid of hyperforin has also been reported [86], and that the extracts used in numerous clinical trials reported till the mid 1990s [87] were never controlled for their hyperforin contents. These and many other facts demonstrate clearly that as in the case of most herbal extracts, therapeutic potentials of HPEs cannot yet be properly defined by the contents of a few bioactive components only. During the past decade, some reports dealing with antibacterial, antiretroviral and anticancer activities have appeared. Most of them concentrate their efforts either on hyperforin or on hypericin only, and little efforts are made to rationally evaluate the therapeutic potentials of other secondary metabolites of the herb. Although antiviral activities of HPEs have been known since long, it was only during the 1990s that reports on antiretroviral and other therapeutically interesting bioactivities started appearing regularly. The report that hypericin and pseudohypericin isolated from another Hypericum species (Hypericum triquetrifolium) possess antiretroviral activity with unique modes of action [88] stimulated the interest of many to more properly evaluate the therapeutic potentials offered by synthetically obtainable pure hyperforin. Although these efforts soon led to many currently well known bioactivities and uses of hypericin as an experimental tool [89], very little efforts have till now been made to rationally evaluate the therapeutic potentials of other naphthodianthrones of HP, or of HPEs. Till recently, somewhat analogous was the case with hyperforin too. Although the antimicrobial activity of hyperforin and HPEs was known during the 1960s [90], it was only after 1996 that reports on diverse therapeutic potentials of hyperforin started appearing regularly [91]. More modern reports on hyperforin do not deal with its antimicrobial activities only, but also with its broad spectrum of CNS function modulating, anti-inflammatory, antitumoral,

16 40 Vikas Kumar & Shyam Sunder Chatterjee antiangiogenic, NMDA receptor-antagonistic and other diverse therapeutically interesting bioactivities; for more details see [92,93]. However, unlike in the case of hypericin, the very first neuronal function modulating effects of hyperforin were detected only after concentrated efforts made to obtain a well standardized herbal extract suitable treatment of dyspeptic complaints and other infectious gastro-intestinal disorders. Choice of HP for such purposes was made not only on the basis of its traditionally known uses for gastrointestinal disorders, but also after an exhaustive and critical analysis of all information on the herb available during the 1990s. The strategy used to select the herb, extraction procedures and pharmacological models for the purpose have been described elsewhere [94] and the observations leading to the revelation of the very first neuronal function regulating activities of hyperforin have also been reported [54]. Extensive use of existing information on all medicinal uses of a herb and its concoctions, as well as on its bioactivities and chemical constituents was made in all steps of the strategy. Thus, special attention was paid to the available knowledge on the bioactivities of structural analogs of known secondary metabolites of the herb and on the quantitative data available on them. In short, since the antispasmodic and antibacterial properties of phloroglucinol and its derivatives have been known for a long time, and hyperforin, i.e. a phloroglucinol derivative with antimicrobial activity, has been known to be quantitatively the major component of HP, it was considered to be an interesting starting point for further evaluation. Some other considerations leading to the choice of the herb and pharmacological models were the facts that antidepressants are often prescribed for dyspeptic disorders, and that bacterial and viral infections are often diagnosed as causative factors for many gastrointestinal disorders. Although potential therapeutic use of HPEs for gastrointestinal disorders has never been seriously tested, our current understanding on the bioactivities of the herb suggests that some patients suffering from such disorders could, indeed, benefit from certain commercialized HPEs. Discussion and concluding remarks Medicinal uses of plants and diverse types of concoctions obtainable from them are important features of almost all traditionally known health care systems. Extensive efforts made to properly understand and rationalize the most appropriate therapeutic uses of many traditionally known medicinal and/or poisonous plants led to the discovery not only of numerous currently therapeutically used drugs, but also of many pharmacological principles and targets currently widely used for drug discovery purposes. Unfortunately, since the mid 1950s onwards, i.e. along with the advent of combinatorial chemistry, high throughput screening, molecular biology and many other more modern technologies and methods, little concentrated efforts have been made to

17 Drug discovery & Ethnopharmacology 41 identify novel therapeutically interesting pharmacological targets by the use of existing knowledge on traditionally known herbal remedies. Although plants and other natural products have continued to be the major sources of therapeutically useful drugs and pharmacological targets [95,96], almost all more recent drug discovery efforts exploit them only for obtaining structurally novel hits, leads and drugs for some pre-determined therapeutic indication only. For such purposes, arbitrarily chosen extraction and fractionation procedures, and pharmacological screening models are used routinely to evaluate their therapeutic potentials. Using diverse modern technologies and equipments such as in vitro screening procedures are often automated and miniaturized, so that large numbers of randomly collected samples are rapidly screened or evaluated. Unfortunately, despite the extensive efforts and high costs involved in such modern strategies, the numbers of new drug approvals have consistently declined [97], and amongst the new approvals, the numbers of higher priced new chemical entities (NCE) with marginal therapeutic advantages have consistently increased. This situation has recently led to a reconsideration of the more traditional approaches using systems pharmacology and in vivo screening models for drug discovery purposes. According to our judgment, at present such models should be the preferred ones not only for identifying the bioactivities and active constituents of traditionally known herbal remedies, but also for the evaluation of their most appropriate therapeutic potentials. It cannot be ignored though, that due to diverse political, socio-economic and/or scientific reasons, numerous laboratories and health authorities around the globe have continued to have vested interests in properly evaluating and standardizing herbal extracts for therapeutic purposes. Thus, reports on successful clinical trials with herbal extracts, and on novel therapeutically interesting bioactivities of their since long known chemical constituents, have constantly increased during the past three decades. In general, major goals of most concentrated efforts made by manufacturers of herbal remedies, or in laboratories sponsored by them, are to obtain evidence supporting one of the various therapeutic possibilities traditionally known for a given herb, or to define the active constituents and modes of actions, or to define and compare analytical profiles of diverse types of herbal extracts. In addition, literature is now overloaded with reports revealing antiviral, antimicrobial, anti-inflammatory, antifungal, anticancer, anti-diabetic and various other therapeutically interesting pharmacological activities of specific types of extracts from specific plants. However, it is not uncommon to encounter contradictory findings and conclusions on the same plant scrutinized in different laboratories, using different extraction procedures and strategies. Unlike in other medicinal sciences, attempts are seldom made to reconfirm the reported observations, or to clarify the situation more properly by the use of isolated organs, or in vivo animal models.