Rhamnus spp.

Dried bark of Rhamnus purshiana and Rhamnus frangula (family Rhamnaceae), stored for at least 1 year before use, is used medicinally as a purgative. Rhamnus purshiana DC (cascara buckthorn, chittem bark, sacred bark, bitter bark, bearwood) is a small tree (1.5 to 12 m) or large shrub indigenous to the Pacific Coast of North America. The bark of R. purshiana is light- to dark brown, marked with lenticels and yellow on the inside. The transversely cut surface of the bark shows a yellowish-grey cortex in which darker translucent points (groups of sclereids) are present. Most of the present-day market supply comes from wild trees in Oregon, Washington and British Columbia. Efforts to cultivate the tree commercially in Canada, Kenya, and the western United States have been largely unsuccessful. Rhamnus frangula L. (Frangula alnus Mill., buckthorn, alder buckthorn) is a shrub (1 to 3 m) growing in Europe and Western Asia. The bark of R. frangula is grey-brown and bears many transversely elongated whitish lenticels. The structure of the bark is very similar to that of R. purshiana, from which it is distinguished, however, by the absence of groups of sclereids. The commercial supply is obtained from Russia and the southern Balkan countries. The main cathartic principles of both barks are known to be 1,8-dihydroxyanthraquinone glycosides. For medical application usually aqueous or dry extracts are used. However, not only anthraquinones but also other 1,8-dihydroxyanthracene derivatives, i.e., anthrones and dianthrones, are found in these barks. The structural relationships between 1,8-dihydroxyanthraquinones and their corresponding anthrones and dianthrones are given. 1,8-Dihydroxyanthracene derivatives accumulating in Rhamnus species are biosynthesized by the acetate-malonate pathway (polyketides).

In the medicinal bark of R. frangula, Rhamni Frangulae Cortex, the major glycosides are glucofrangulin A and B, frangulin A and B and emodin-8-mono-glucoside. Of the free aglycones emodin is the main component, besides traces of chrysophanol and physcion. Rhamni Frangulae Cortex contains not less than 6% of 1,8-dihydroxyanthracene derivatives, calculated as glucofrangulin (Martindale). In the undefined bark of R. frangula also dianthrones and heterodianthrones of emodin, chrysophanol, and physcion have been found. In the fresh bark, however, anthracene derivatives occur exclusively in their anthrone forms.

In the medicinally used bark of R. purshiana, Rhamni Purshianae Cortex (Cascara sagrada), anthracene derivatives occur in a complex mixture. Of the totally present anthra-derivatives 10% -20% occur as O-glycosides. Some authors mention the presence of emodin-oxyanthrone-10-glucoside and emodin-oxyanthrone-9-glucoside. Also monoglucosides of chrysophanol, physcion, and aloe-emodin have been found. The occurrence of several dianthrones as O-glycosides was reported by Kinget (1966). The major part of totally present an-thra-derivatives (80%-90%), however, consists of C-glycosides being aloin and 11-desoxyaloin and combined C- and O-glycosides, the cascarosides. The cascarosides A and B consist of aloin and the cascarosides C and D of desoxyaloin, each with an extra sugar moiety in O-glycosidic linkage. Rhamni Purshianae Cortex contains not less than 8% of hydroxyanthracene derivatives of which not less than 60% is constituted of cascarosides, calculated as cascaroside A (Martindale).

The importance of the medicinal value of R. purshiana bark is best illustrated by data in the National Prescription Audit (NPA) of the United States. Analysis of pharmaceutical prescriptions for 1973 and 1980 showed that the use of extracts from R. purshiana ranks with those from Digitalis purpurea (). The yearly production of R. purshiana bark is about 2 million kg. Many preparations containing extracts from the medicinal bark are on the market (e.g. Cas-Evac). Often the extract is combined with other laxative ingredients (e.g., Oxothalein, Stimulax, Almax, Nature’s Remedy). Casanthranol, a purified mixture of anthraglycosides extracted from R. purshiana bark, is marketed as such (Peristim Forte) or in combination with superfactant drugs and/or hydrocolloids, including Disanthrol, Afko-Lube Lax and Disolan. A specialty product containing extract from R. frangula bark is Saraka.

To produce polyketide anthracene derivatives by plant cell cultures, callus cultures of R. frangula and callus and suspension cultures of R. purshiana were set up. The obtained plant cell cultures were phytochemically examined and for the suspension cultures also growth and production kinetics were determined.

Prospects

To obtain callus of Rhamnus frangula and Rhamnus purshiana the best explant for callus induction was the cambial zone of mature branches. Both tissue cultures were satisfactory subcultivated on the medium according to Murashige and Skoog (1962), modified by Gamborg (1975) and supplemented with 1 mg/1 2,4-D and 0.1 mg/1 kin. Also callus-derived suspension cultures of R. purshiana could well be subcultured by the use of this medium, however, with 1 mg/1 2,4-D and 1 mg/1 NAA present. In general, extraction and purification of anthra-quinones pose no problems, since this type of compound is chemically stable. An-thrones and dianthrones, however, are far less resistant to oxidative conditions. If a true picture of genuinely occurring anthra-derivatives (i.e., anthrones and dianthrones) is to be obtained, extraction procedures preventing oxidation have to be used.

Although the patterns of polyketide anthracene derivatives in callus and suspension cultures differ from those in the intact barks, our experimental results show that a large range of desirable compounds differing in substitution pattern can be produced by cell cultures of R. frangula and R. purshiana. The anthracene derivative content of the cultures, however, was rather low. Experiments to optimize anthra-derivative production, including precursor-feeding and immobilization, have already been performed for suspension cultures of R. purshiana and led to a significant increase of the anthracene derivative content. A high yielding callus of R. purshiana which accumulates 1.25% (dry weight) anthra-derivatives, obtained through a process of visual selection, provide even more promising prospects. Results of these studies will be published elsewhere. More experiments in terms of fundamental correlations between biochemical factors governing production yields, and experimental optimization procedures, however, are needed to reach commercialization of the production of polyketide anthracene derivatives by plant cell cultures.

A. J. J. van den Berg and R.P. Labadie, Medicinal and Aromatic Plants I (1988)