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close this bookTraditional Medicinal Plants (Dar Es Salaam University Press - Ministry of Health - Tanzania, 1991, 391 p.)
close this folderPART I: USE AND PROMOTION OF TRADITIONAL MEDICINAL PLANTS IN THE AFRICAN REGION
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Chemical Evaluation of Tanzanian medicinal plants for the active constituents as a basis for the medicinal usefulness of the plants

MAYUNGA H. H. NKUNYA*, H. WEENEN**, & D. H. BRAY***

*Department of Chemistry, University of Dar es Salaam P. O. Box 36061, Dar es Salaam, Tanzania

** Quest International, P. O. Box 2, 1400 CA Bussum, The Netherlands.

***London School of Hygiene and Tropical Medicine Keppel Street, London WC 1E 7HT, U.K.

ABSTRACT

Drugs derived from medicinal plants still form the basis for rural medical care in most developing countries, apparently either because of lack of modern medical facilities in these areas, or as a supplement to the latter. In practice, most of these drugs offer effective treatment. This is not surprising because about 40% of all pharmaceutical presently in use are derived from natural sources (plants, fungi and other microorganisms, animals, etc.), either used directly as such, or with some modifications. Unfortunately, the we of crude plant extracts without any scientific evaluation, could lead to serious complications. Ineffective drugs could be used just as a matter of belief or tradition; under/over-doses could be taken; highly toxic drugs with short term, long term, or cumulative effects could be prescribed etc. The last two effects, however, are much more difficult to recognise than the others, and hence potentially more serious. In addition to these, the preparation, handling and storage of the drugs could lead to decomposition or transformation of the hitherto active constituents to ineffective and/or harmful products. Thus there is a need to evaluate and establish a scientific rationale for the use of the traditional medicinal plants, through chemical, pharmacological, toxicological and microbiological studies. In this paper, chemical investigations of medicinal plants for the active constituents and the correlation between biological activity of the crude extracts and/or the pure chemical constituents with the medicinal uses of the plants will be discussed.

Introduction

Quite a number of plants are used in different parts of the world for the treatment of various ailments. The medicinal values of most of these plants were recognised since ancient times. In fact, it can correctly be argued that the development of modern pharmaceutical is based on this ancient knowledge of medicinal plants and traditional medicines. Thus presently, about 40% of pharmaceuticals are derived from natural sources (plants, microorganisms, fungi and animals (Farnsworth, 1984). These drugs are used as such, or as derivatives. Moreover, several natural products obtained from medicinal plants, which cannot hitherto be used as such, have offered leads to the development of various pharmaceuticals, as analogues or derivatives.

In developing countries, traditional medicines from plants continue to form the basis of rural medical care. This is so because, obviously, these medicines are easily available and cheap. However, the use of such medicines in their crude forms without establishing scientifically their efficacy and safety could, in a short while or long run, be detrimental to the very health of mankind. Therefore, there is an urgent need to carry out scientific evaluations of these medicines worldwide. After all, apart from the efficacy and safety of traditional medicines, the scientific evaluation may lead to the isolation of a pure active ingredient which otherwise occurs, in minute quantities in the crude drug. And since medicinal plants depend on their geographical location, such isolated active principle can then be synthesized cheaply, so that eventually the drug is available to a larger population. Alternatively, knowledge of the structures of naturally occurring, medicinally useful compounds may give leads to the synthesis of analogues, which could be cheaper, and sometimes even more active than the naturally occurring compounds.

In 1976 we initiated a long term project on the scientific evaluation of Tanzanian medicinal plants, aimed at establishing the active constituents. So far we have studied plants which are used for the treatment of bacterial and fungal diseases (Sawhney et al., 1978a and 1978b; Khan et al., 1980), and those which are used for malaria. Occasionally we also evaluated the isolated compounds for antitumour or other activities. In this paper results of our on-going research on plants used in Tanzania for the treatment of malaria and malaria-related fevers will be discussed. Prof. Khan will present our results on the chemical investigations of plants used for bacterial and fungal diseases (Khan and Nkunya, 1990).

The malaria problem

Malaria is one of the most prevalent tropical and subtropical diseases (WHO, 1982/83). Recently it has been estimated that about 260 million people are infested annually (WHO, 1988). In tropical Africa alone about one million children under 14 years die from the disease annually (Underson, 1986). It is now over forty years since campaigns to eradicate the disease were initiated but, unfortunately, until now there is no success in eradicating this disease in the poor, developing countries. Efforts to develop an antimalarial vaccine have been futile because of the complicated stages of malaria infestation (Mgani, 1990).

Efforts to eradicate the mosquito vector, the Anopheles mosquito, have been futile because of financial and management problems of the eradication programmes. Furthermore, the mosquitoes are now known to be developing resistance against the cheap insecticides, such as DDT, fenitrothin, proppoxur, malathion, clorfoxin, and synthetic pyrethrins, which are generally used in these programmes (WHO, 1984). The use of large quantities of these insecticides also poses an environmental problem, since some of them, such as DDT, are non-biodegradable. The economic difficulties being faced by the affected countries, coupled with the emergence of other killer diseases, such as AIDS, will, most likely, hamper financial commitments in the fight against malaria, particularly the massive mosquito eradication programmes, since these involve huge financial requirements.

Due to the above constraints, at the moment, malaria chemotherapy should be given due attention. But again sad news have emerged in this direction. That is, the most dangerous human malaria parasite, Plasmodium falciparum, is developing resistance against the commonly used cheap drugs such as quinine and chloroquine (Breman and Campbell, 1984). The use of the new drugs, mefloquine, fansidar, amodiaquine, primaquine, etc, in malaria chemotherapy, poses other problems. These drugs are quite expensive and some have serious side effects. They particularly affect human liver, kidneys and the nervous system (Mtulia, 1976). Hence, at present, chloroquine and quinine continue to be prescribed to malaria patients. Larger doses of chloroquine are now being recommended for drug resistant strains of P. falciparum. However, long-term effects of such large doses of chloroquine we still unknown, but could be significant.

Due to the shortcomings discussed above, efforts are now being directed in obtaining drugs which have structural features that are different from those of chloroquine and related drags, and those of sulfa drugs, either synthetically or from plants.

Antimalarials from plants

After the isolation of quinine from Cinchona trees (Sterling, 1977), and artemisinine from Artemisia annua L. (Compositae) (Xu-Ren et al., 1985), it has become apparent that plants are a potential source of antimalarial drugs. Artemisinine (also known as ginghaosu) is one of the most potent antimalarial drugs known at present, which is toxicologically the safest (Xu-Ren et al., 1985). Since this compound has a structural feature which is different from that of any other known antimalarial, parasite resistance to this compound is unlikely to take place in the near future.

The drug is still obtained from the plant where it occurs in small quantities, since its synthesis is still very cumbersome (Gavagan, 1988). This makes the drug to be very expensive. It is, therefore, worthwhile to put more efforts in searching for other potent and abundant antimalarials from medicinal plants, or other sources, while efficient and cheap synthetic methods for artemisinine and its derivatives are being developed. That is why at present enormous efforts are being exerted in searching for antimalarials from medicinal plants, and several leads have so far been obtained. Thus, the vascular plant famines Amaryllidaceae, Meliaceae, Rubiaceae and Simaroubaceae, have been found to include plant species which are active against malaria parasites (Spencer et al., 1947), and several active compounds have been isolated from some of these plants. Several quassinoids, which were isolated from some plants of the family Simaroubaceae, showed potent antimalarial activity in vitro (e.g., see WHO, 1984; Thaithong et al., 1983). The compounds also - owed a strong mammalian cytotoxicity. However, preliminary studies on the structure- activity relationship of quassinoids have shown that the structural requirements for antimalarial activity and cytotoxicity are different (e.g. see Bray et al., 1987). Therefore, one can expect that structural modifications of these compounds to suppress cytotoxicity, if feasible, can be performed to give modified compounds which might be safe antimalarials However, up to now such modifications have not been performed (Phillipson, 1990).

Recently, Prof. Hostettmann from Switzerland has found that the crude extract from Psorospermum febrifugum (Guttiferae) possesses an antimalarial activity at a level similar to that of artemisinine (Hostettmann, 1990). He has isolated the active constituents from the plant, and further evaluation of this compound for its potency as an antimalarial drug is in progress.

Antimalarials from Tanzanian medicinal plants

In our on-going research on Tanzania antimalarial plants, we have screened crude extracts from leaves, stem and root bark of sixty medicinal plants. The results are shown in Table 1 (Weenen et al., 1990). Some of the most active plants were the tubers of Cyperus rotundus L. (Cyperaceae), and the root bark of Hoslundia opposita Vahl. (Labiatae). Chemical studies of the C. rotundus extracts led to the isolation of a number of compounds, some of which were active against the multidrug resistant K1 strain of P. falciparum malarial parasite in vitro. These included a-cyperone (1) and (+)-b-selinene (2) (Weenen et al., 1990b). However, the activity of 2 appeared to be due to decomposition products. Thus, whereas the undercomposed compound was inactive, the decomposed material was active.

We have isolated three new compounds from the root bark of H. opposita which we have named hoslunone (3), hoslundione (4) and hoslundin A (5) (Marandu, 1990). All these compounds were active against P. falciparum malaria parasites in vitro. The crude H. opposita extract also gave several other active compounds, which were in minute quantities, and hence their structures could not be determined. We are now re-investigating the plant in order to obtain larger quantities of the compounds so that their structures can be identified.

Other active plants in our investigation were Margaritaria discoidea (Baill.) Webster (Euphorbiaceae), from which securinine (6) was obtained and found to be the active principle, and Zanthoxylum gilletii (De Wild) Waterm. (Rutaceae), which contains two active compounds, pellitorine (N- isobutyldec-2, 4-dienamide) (7), and fagaramide (8) (Weenen et al., 1990b). Another compound (9) was obtained from the latter plant as well, but this metabolite, despite its novel chemical structure, was inactive (Kinabo, 1990).

All the compounds 1, 3-7 shown above, contain an a,b-unsaturated carbonyl moiety. It is believed that their antimalarial activity is due to the ability of the nucleic acids of P. falciparum malaria parasites to react with the a,b-unsaturated carbonyl moiety, in a Michael addition fashion (Weenen et al., 1990).

We also isolated several compounds from the crude root bark extract of Artemisia afra Wild (Composite) (same genus as Artemisia annua, the source of artemisinine) but none of the isolated compounds had any marked activity (Kinabo, 1989).

Azidarachta indica A. Juss. (Mwarobaini in Swahili)

Azidarachta indica is widely used in East and West Africa for the treatment of malaria and malaria related fevers. We therefore included this plant in our investigations. Results on the antimalarial activity of this plant are given in Table 1 (Weenen et al., 1990a). As it can be noted, the plant showed only a mild activity. Apparently, the active component from this plant, which has recently been isolated in India, occurs in very minute quantities (Philipson, 1990). This might be the reason for the mild activity of the crude extract.

Antimalarials from plants of the genus Uvaria

Uvaria species have proved to be rich in a variety of compounds, some of which exhibit a wide range of biological properties, such as antibacterial, antifungal, and anticancer activities, and pharmacological properties (Leboef et al., 1982). The chemistry and biological activities of these compounds have attracted interests in investigating these plants phytochemically. That is why in the course of our investigations on antimalarial plants, we decided to screen the Uvaria species, which grow in Tanzania, for their antimalarial activity, and ultimately isolate the active principles and/or any other chemically interesting compounds. After all, most of these Uvaria species (commonly known is Mshofu or Msofu) are used for the treatment of malaria (Kokwaro, 1976).

We have screened nine Uvaria species which were collected from different parts of Tanzania. Their activities are summarised in Table 1 (Nkunya, et al., 1990). It can be noted from Table 1 that all nine plants are active against the multidrug resistant K1 strain of P. falciparum malarial parasite, leaf extracts being the least active. Table 1 also shows that most of the activity is concentrated in the less polar or medium polar compounds, which are soluble in petroleum ether or chloroform.

Several compounds have been isolated from the most active extracts, and these have been assayed for their activity against the multidrug resistant K1 strain of P. falciparum malaria parasites (Nkunya, et al., 1990a). C-Benzylated dihydrochalcones (the uvaretins) (Mgani, 1990, Nkunya, 1985), and sesquiterpeneindoles (Nkunya et al., 1987a, Nkunya and Weenen, 1989, Nkunya et al., 1990b) have been found to be the active components of these plants. The activity of the dihydrochalcones was found to depend on the presence of free hydroxyl groups, and on the molecular size of the compounds (Nkunya et al, 1990a). That is, small molecules showed a higher activity than large ones. The activity of the sesquiterpeneindoles appears to be due to the sesquiterpene side chain and not the indole moiety. The presence of an a,b-unsaturated alcohol moiety on the sesquiterpene side chain is also essential for the activity (Nkunya et al., 1990a).

Despite their novel structures, both the benzopyranyl sesquiterpenes, lucidene (13) and tanzanene (14) isolated from U. lucida ssp. Lucida (Weenen et al., 1990c) and U. tanzaniae, respectively (Weenen et al., 1991) and the schefflerins 15 and 16 from U. scheffleri (Nkunya et al., 1990b) are virtually inactive.

The three cyclohexene epoxides, (+) -pandoxide (17), (+)-b-senepoxide (18) and (-)-pipoxide (19), isolated from U. pandensis (compound 18 was also isolated from U. faulknerae), are weakly active. However, these compounds have been found to possess marked antibacterial, antifungal and antitumour activities (Nkunya et al., 1986).

We would like to emphasize that the compounds isolated in our investigations were the major ones. We are presently investigating whether more active minor components are present and whether these compounds can be isolated.

Conclusion

Our studies have indicated that most of the plants which are used for the treatment of malaria show at least some activity against the multidrug resistant K1 strain of P. falciparum malaria parasites. This, thus verifies the scientific basis for the traditional uses of these plants. However, these studies are only preliminary. More investigations for the in vivo activity and toxicity of the active plant extracts and pure compounds, are required for any definitive conclusions.

The results from our studies, and those reported by others, indicate that most of the active components are weakly, or medium polar compounds, which are soluble in petroleum ether or chloroform. However, in traditional medicines, water is the solvent which is used to prepare the extracts and concoctions. This is obviously so because the traditional healer has only water as the solvent for the preparation of his medicines. Thus in most cases the active ingredients in traditional medicines may be in minute concentrations, due to their low solubility in water. Therefore, larger quantities of these medicines are invariably needed for any curative effects. This appears to be the general practice with traditional medical practitioners.

The lack of suitable solvents means that many useful plants may not show any curative properties in traditional medicines, despite some of them containing highly potent compound(s), albeit in minute quantities. Therefore this calls for a massive scientific evaluation of plants so that should there be any potent, but minor component(s) in these plants, they should be characterised, so that efforts to synthesize them, or their analogues, can be initiated, with the objective of getting the compounds in larger quantities.

Acknowledgements

Financial support for this research, for which we are grateful, was obtained from the University of Dar es Salaam, the Norwegian Agency for International Development (NORAD), the Netherlands Universities Foundation for International Cooperation (NUFFIC), and the German Academic Exchange Service (DAAD). We are also grateful to the following people for providing spectral facilities: Prof. Dr. H. Achenbach (University of Erlangen, Germany); Prof. Dr. B. Zwanenburg (University of Nijmegen, The Netherlands); Prof. Dr. P. Waterman (University of Strathclyde, U.K.) and Dr. J. Wijnberg (University of Wageningen, The Netherlands). The plants used in this study were located and identified by Mr. L. B. Mwasumbi (The Herbarium, Botany Department, University of Dar es Salaam). We are grateful to Mr. F. Sung'hwa of the Department of Chemistry, University of Dar es Salaam who skillfully carried out most of the extractions and isolations of the pure compounds.

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Table 1: Antimalarial activity of extracts of Tanzanian plants

Family

Species

Part useda

Activityb c




PE

CH2Cl2

MeOH

Amarylidaceae

Crinum stuhlmannii

W.P.

N.D.

N.D.

**


C. portifolium

W.P.

N.D.

N.D

-


C. papilosum

W.P.

N.D.

N.D.

***


Scadoxus multiflorus

W.P.

N.D.

N.D.

**

Anacardiaceae

Ozoroa insignis

R.B.

***

***

-


Sclerocarya cafra

S.B.

-

-

-


Sorindeia madagascariensis

R.B.

*

*

-

Annonaceae

Enantia kumeriae

R.B.

**

**

***


Uvaria dependens Eng&Diels

R.B.

***

*

-



S.B.

*

***

-



leaves

*

*

-


U. faulknerea Verdc.

R.B.

*

*

-



S.B.

**

**

*



leaves

-

*

-


U. kirkii Hook. f.

R.B.

***

**

-



S.B.

***

***

-



leaves

-

*

-


U. leptocladon Oliv.

R.B.

***

***

**



S.B.

***

***

*



leaves

-

*

*


U. lucida ssp. lucida Benth.

R.B.

***

****

**



S.B.

***

****

****



leaves

**

***

*


Uvaria sp. (Pande)

R.B.

****

***

*



S.B.

****

***

*



leaves

*

**

**


U. pandensis Verdc.

R.B.

*

**

*



S.B.

**

***

-



leaves

*

*

*


U. scheffleri Diels.

R.B.

**

****

****



S.B.

**

**

*



leaves

**

**

*


U. tanzaniae Verdc.

R.B.

**

***

**



S.B.

***

***

**

Apocynaceae

Rauvolfia mombasiana

R.B.

*

***

***



S.B.

N.D.

N.D.

-

Araliaceae

Cussonia arborea

R.B.

***

***

-

Bignoniaceae

Kigelia africana

S.B.

-

***

-



leaves

-

-

*

Caesalpinaceae

Caesalpinia bonduc

W.P.

N.D.

N.D.

-


Cassia abbreviata

R.B.

**

*

*


C. occidentalis

W.P.

-

-

-


Tamarindus indica

fruits

N.D.

N.D.

-

Celastraceae

Catha edulis

aerial

N.D.

N.D.

-

Compositae

Artemisia afra

R.B.

**

***

*



aerial

***

***

*


Conyza pyrrhopappa

leaves

*

***

**


Crassocephalum bojeri

aerial

*

***

**


Tridax procumbens

W.P.

*

*

-


Vernonia amygdalina

leaves

N.D.

N.D.

-


V. colorata

R.B.

*

**

*



S.B.

-

**

-



leaves

-

**

-

Cyperaceae

Cyperus rotundus

tubers

***

****

***



aerial

N.D.

*

N.D.

Ebenaceae

Diospyros natalensis

R.B.

**

*

N.D.


D. zombensis

R.B.

*

*

N.D.


D. greenwayii

R.B.

-

**

N.D.



S.B.

-

**

N.D.



leaves

-

**

N.D.

Euphorbiaceae

Bridelia cathartica

R.B.

*

**

-


Clutia robusta

R.B.

-

-

-


Margaritaria discoidea

R.B.

***

***

*

Guttiferae

Vismia orientale

S.B.

N.D.

N.D.

-



leaves

N.D.

N.D.

-

Labiatae

Hoslundia opposita

R.B.

****

***

*



S.B.

**

-

-

Lauraceae

Ocotea usambarensis

R.B.

***

***

*

Leguminosae

Acacia clavigera

S.B.

*

*

-


Albizia anthelmintica

S.B.

-

-

-


Piliostigma thonningii

S.B.

*

*

***



leaves

*

*

***

Meliaceae

Azadirachta indica

S.B.

N.D.

N.D.

-



leaves

*

**

-


Entandrophragma bussei

S.B.

***

***

*

Myrtaceae

Psidium guajava

leaves

***

*

**

Olacaceae

Ximenia caffra

leaves

-

*

*

Plantaginaceae

Plantago major

W.P.

*

***

-

Rhizophoraceae

Anisophylia obtusifolia

R.B.

****

-

-



S.B.

-

*

-

Rosaceae

Parinari exelsa sabin

S.B.

***

***

-

Rubiaceae

Crossopterix febrifuga

S.B.

*

*

*


Gardenia jovis-tonantis

S.B.

N.D.

N.D.

-



leaves

N.D.

N.D.

-



fruit

-

**

-


Vangueria infausta

R.B.

-

***

***



S.B.

N.D.

N.D.

*

Rutaceae

Clausena anisata

R.B.

-

*

-



leaves

*

**

*


Todalia asiatica

R.B.

**

-

***



S.B.

***

*

***


Zanthoxylum gilletii

R.B.

***

***

**



R.B.

**

**

***


Z. xylubeum

S.B.

*

*

*

Tiliaceae

Grewia egglingii

S.B.

**

N.D.

N.D.


G. forbesii

leaves

-

*

*

Verbenaceae

Lantana camara

R.B.

****

***

*

Zygophyaceae

Balanites aegyptica

S.B.

-

***

**

Key

a) W.P. = whole plant; R.B. = root bark; S.B. = stem bark.
b) Antimalarial activities are given in IC50 values and these have been categorized as follows:

****: IC50 = 5 to 9 mg/ml
***: IC50 = 10 to 49 mg/ml
**: IC50 = 50 to 99 mg/ml
*: IC50 = 100 to 499 mg/ml
-: IC50 > 499 mg/ml

N.D.: Not determined.
c) P.E. = petroleum ether (boiling range 40-60°C);
CH2Cl2 = dichloromethane; MeOH = methanol.


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Figure