The aim of this post is to review the published work on Hyoscyamus sp. plants and their in vitro-derived cultures in the context of their uses for drug and tropane alkaloid production. Hyoscyamus plants have been known to man from ancient times as a remedy for various diseases, and serve today also as a source of their pharmaceutically active constituents, the tropane alkaloids. The medicinal importance of scopolamine, hyoscyamine and atropine is illustrated by their presence in the list of the ten substances of plant origin most used as drugs in the USA in 1973. Due to their strong action on neuroreceptors, tropane alkaloids and chemically derived compounds thereof are presently employed as curative and prophylactic agents in various treatments. Recent advances in plant in vitro techniques open up new ways for plant improvement and for production of secondary metabolites. The progress in this field is given here for Hyoscyamus spp. and problems encountered with Hyoscyamus sp. cell cultures in tropane alkaloid production are discussed. This post will mainly deal with H. muticus and H. niger, the two Hyoscyamus species predominantly used in folk medicine, phytotherapy, and as a source of tropane alkaloids, and the most intensively studied at the cell culture level.
Hyoscyamus spp.: Botanical Considerations
Hyoscyamus plants are herbaceous, differing essentially from other Solanaceous genera by having a fruit composed of a thornless seed capsule with a cover. There is some uncertainty about the taxonomic classification and the number of species falling into the genus Hyoscyamus. According to D’Arcy (1979), the genus consists of 20 species, whereas the Index Kewensis () gives the following list of 14 Hyoscyamus species: H. albus L. (including var. desertorum, var. canariensis), H. arenarius Dun., H. aureus L., H. ceratophyllus Fisch., H. eminens Kunze, H. falezlez Coss., H. grandiflorus Franch., H. muticus L., H. niger L. (ineluding var. agrestis, var. bohemicus, var. pallidus), H. physaloides L., H. pseudophysaloides Roth., H. pusillus L., H. reticulatus L. and H. senecionis Willd.
H. muticus, the Egyptian henbane, native to arid and semi-arid regions of Egypt, Sudan, the Near East, Arabia, Iran, Afghanistan and Northern India, is a perennial plant 30-90 cm in height with thick, simple or sparingly branched stem, with green, fleshy, rhombic or toothed-lobed, petiolate leaves bearing thick, light-coloured veins. The leaf blade is covered on both sides with branched and non-branched glandular trichomes. Flowers (10-30) are in a one-sided spike which elongates upon ageing. The flower consists of a bell-shaped, asymmetrically lobed, white to yellow corolla with violet spots. On maturity, the capsule in the bell-shaped, elongated calyx bears a few hundred yellow to brown kidney-shaped seeds, ca. 1 mm in length.
H. niger, henbane, is an annual or biennial plant (depending on the variety), indigenous to vast regions of Europe, Asia and North Africa, naturalized also in North America, East Asia and Australia. It occurs on waysides and waste places on sandy or loamy soil in regions of temperate climate and can grow in India as high as 3000 m above sea level. H. niger resembles H. muticus morphologically in many aspects: The 20-80 cm high plant consists of an erect, simple or branched stem, covered with glandular hairs; the leaves are grey-green, egg-shaped or toothed-lobed, stalkless on the upper stem and covered with non-branched glandular hairs. The shortly stalked flowers are crowded in one-sided, leafy spikes. The funnel-shaped corolla is yellow and covered with a network of purple veins, sitting in a hairy, pitcher-shaped calyx. The seed capsule may produce over 100 1-mm-long brown seeds. H. muticus and H. niger have a characteristic heavy odour and a bitter and acrid taste. Further detailed botanical description and information about these and other Hyoscyamus species are given elsewhere.
Folk Medicine and Phytotherapy
Therapeutic, toxic and halucinogenic effects of preparations of H. muticus and H. niger have been known to man since ancient times. The psychogenic qualities of the drug henbane were often used in religious rites or by witches. Later, the medicinal applications of the drug came more to the fore. The drug henbane, used in various ways listed by Hocking (1947), was applied in past centuries as narcoticum, analgeticum, spasmolyticum and sedativum against various diseases and pains and was also employed as aphrodisiac and contraceptive. Medicine of this century uses the drug isolated from henbane leaves as a galenic for its spasmolytic (mainly against asthma and Parkinson’s disease), narcotic, mydriatic, sedative, laxative, carminative and anodyne properties. It is employed also in homoeopathic preparations in similar indications.
The value of henbane is illustrated by its presence in different Pharmacopoeias (e.g. British Pharmacopoeia: Anon 1980). Drugs of H. muticus and other Hyoscyamus species have a similar range of medicinal application, being used in doses dependent on their alkaloid contents.
In recent times, the importance of the henbane drug has decreased, being often replaced by the pure tropane alkaloids, which allow a more controlled and specific application.
Both H. muticus and H. niger have been cultivated for drug and alkaloid production for many decades. Already at the end of the last century henbane was being processed by the chemical company Merck, Darmstadt. H. niger, which is mainly used as a source of the drug, was cultivated before World War II predominantly in European countries like Belgium, France, Germany, Hungary, Italy and the Soviet Union, whereas, more recently, cultivation has taken place in Australia, Brazil, Great Britain, Japan, India, Pakistan, the Soviet Union, Sudan, the USA and the countries of Central Europe. Reported yields for H. niger are in the order of 600-1800 kg/ha on a dry herb basis, which would correspond to 0.4-1.8 kg/ha tropane alkaloid. H. muticus plants cultivated in Bulgaria, Egypt, Greece, India, Yugoslavia, Pakistan, S. California and the Sudan are said to be mostly used for hyoscyamine production. This plant, which is especially rich in tropane alkaloids, is one of the important crops of the semi-arid and arid regions, e.g. in parts of Egypt. Published yields for H. muticus vary in the range 800-3800 kg/ha on dry herb basis corresponding to 2.5-23.4kg/ha tropane alkaloid. However, most tropane alkaloids are produced today not from Hyoscyamus species but from other Solanaceae like Datura stramonium, Datura innoxia, Duboisia myoporoides and Atropa belladonna.
Atropine, hyoscyamine and scopolamine are still produced exclusively from plants; chemical synthesis of these compounds is possible but not profitable. Atropine, hyoscyamine and scopolamine rank in Farnsworth’s list among the ten compounds of plant origin most used in medicinal prescriptions in the USA in 1973. Of all prescriptions sold in 1973 in the USA, 25% contained compounds of plant origin, corresponding to an estimated value of 3000 million $, 1.50% contained atropine, 0.75% hyoscyamine and 0.66% scopolamine. Atropine sulphate and scopolamine were sold in 1984 on the world market at prices of ca. 350 $/kg and 1400 $/kg, respectively. The price for hyoscyamine may fall between these two values. Although no exact data are available, the total world market volume for pure tropane alkaloids may be at present in the order of 100 million $/year. A large part of the tropane alkaloids is used today for chemical derivatization. For instance, more than 30% of all spasmolytica sold in 1983 in Japan, corresponding to a value of ca. 19 million $, were derivates of tropane alkaloids. In order to guarantee the supply of scopolamine for the production of derivatives the company Boehringer Ingelheim recently established new plantations of Datura sp. in Brazil. New production centres for tropane alkaloids were also set up recently in India, Iran and South Africa.
Hyoscyamus spp.: Conclusions and Prospects
Man has exploited the medicinal value of H. muticus and H. niger plants and their products for a long time. Fresh plants, plant extracts, their pharmaceutically active constituents, the tropane alkaloids, as well as their derivatives, have been employed in the treatment of several diseases in different medicinal indications. Because of its high content in hyoscyamine, H. muticus was used more for tropane alkaloid production, while H. niger served rather as source for the crude drug and its galenical preparations. The importance of Hyoscyamus plants for tropane alkaloid production decreased, as now other Solanaceous plants from the genera Atropa, Datura and Duboisia are used mostly for this purpose. By breeding for high tropane alkaloid-producing strains with interesting cultivation features, it should be possible to stimulate renewed interest in Hyoscyamus for tropane alkaloid production.
Alternatively, tropane alkaloids might also be produced by in vitro cultures, by biotransformation using biological systems or by chemical synthesis. Up to now, all attempts to use cell cultures of Hyoscyamus and other Solanaceous species gave disappointing results, as tropane alkaloid contents in cultured plant cells were low or unstable. More encouraging results were achieved by root cultures of Hyoscyamus sp. which stably accumulate tropane alkaloids in relatively high amounts. With further genetical and physiological optimizations using empirical or more rational approaches it should be possible in future to reach production performances with these root cultures which come close to a profitable process. The progress in improving tropane alkaloid production in vitro is dependent also on a better understanding of the biosynthesis of tropane alkaloids and its regulation. Genetic engineering techniques will certainly also be integrated into the efforts for improving tropane alkaloid production by Hyoscyamus plants and their in vitro cultures.
Independent of any progress in the in vitro production of tropane alkaloids, crude preparations from the plant drug of Hyoscyamus will always be of importance as a cheap source of valuable pharmaceuticals.
Selections from the book: “Medicinal and Aromatic Plants II”, 1989.