Senna

Senna leaf and fruit are obtained from Cassia angustifolia (Leguminosae/Fabaceae), known as Tinnevelly senna, or less commonly from Cassia senna (syn C. acutifolia), which is described as Alexandrian senna. The plants are low, branching shrubs, C. angustifolia being cultivated in India and Pakistan, and C. senna being produced in the Sudan, much of it from wild plants. Tinnevelly senna is cultivated in wetter conditions than Alexandrian senna, which gives more luxuriant growth. Early harvests provide leaf material whilst later on, both leaf and fruit (senna pods) are obtained, a mixture which is separated by sieving (Alexandrian) or hand picking after drying (Tinnevelly). There are no significant differences in the chemical constituents of the two sennas, or between leaf and fruit drug. However, amounts of the active constituents do vary, and appear to be a consequence of cultivation conditions and the time of harvesting of the plant material.

The active constituents in both senna leaf and fruit are dianthrone glycosides, principally sennosides A and B (Figure 3.33). These compounds are both di-O-glucosides of rhein

RO O OH

RO O OH

CO2H CO2H

CO2H CO2H

CO2H OH

CO2H OH

CO2H OH

CO2H OH

R = CO2H, rhein anthrone R = CH2OH, aloe-emodin anthrone

GlcO

GlcO

R1 R2

R1 = H, R2 = Glc, cascaroside A (105) R1 = Glc, R2 = H, cascaroside B (10R)

GlcO

R = CO2H, rhein anthrone R = CH2OH, aloe-emodin anthrone

R1 R2

R1 = H, R2 = Glc, cascaroside A (105) R1 = Glc, R2 = H, cascaroside B (10R)

GlcO

R1 = H, R2 = Glc, cascaroside C (105) R1 = Glc, R2 = H, cascaroside D (10r)

H Glc barbaloin

OH OH

H Glc barbaloin

HO O

R1 "R2

R1 = H, R2 = Glc, aloin A (105) R1 = Glc, R2 = H, aloin B (10r)

R1 "R2

R1 = H, R2 = Glc, aloin A (105) R1 = Glc, R2 = H, aloin B (10r)

OH OH

Glc barbaloin (anthranol tautomer)

R1 = H, R2 = Glc, cascaroside C (105) R1 = Glc, R2 = H, cascaroside D (10r)

H Glc chrysaloin (deoxybarbaloin)

Glc barbaloin (anthranol tautomer)

H Glc chrysaloin (deoxybarbaloin)

HO O

ORha

H Glc aloinoside A (105) aloinoside B (10R)

ORha

H Glc aloinoside A (105) aloinoside B (10R)

R = Rha, glucofrangulin A R = Api, glucofrangulin B

R = Rha, glucofrangulin A R = Api, glucofrangulin B

Figure 3.33

dianthrone (sennidins A and B), and liberate these aglycones on acid hydrolysis, or the anthraquinone rhein (Figure 3.30) on oxidative hydrolysis (e.g. aq HNO3 or H2O2/HCl). Sennidins A and B are optical isomers: sennidin A is dextrorotatory (+) whilst sennidin B is the optically inactive meso form. Minor constituents include sennosides C and D (Figure 3.33), which are also a pair of optical isomers, di-O-glucosides of heterodianthrones sennidins C and D. Sennidin C is dextrorotatory, whilst sennidin D is optically inactive, approximating to a meso form in that the modest change in substituent does not noticeably affect the optical rotation. Oxidative hydrolysis of sennosides C and D would produce the anthraquinones rhein and aloe-emodin (Figure 3.30). Traces of other anthraquinone glycoside derivatives are also present in the plant material. Much of the sennoside content of the dried leaf appears to be formed by enzymic oxidation of anthrone glycosides during the drying process. Fresh leaves and fruits also seem to contain primary glycosides which are more potent than sennosides A and B, and which appear to be partially hydrolysed to sennosides A and B (the secondary glycosides) by enzymic activity during collection and drying. The primary glycosides contain additional glucose residues.

Senna leaf suitable for medicinal use should contain not less than 2.5% dianthrone glycosides calculated in terms of sennoside B. The sennoside content of Tinnevelly fruits is between 1.2 and 2.5%, that of Alexandrian fruits being 2.5-4.5%. Senna preparations, in the form of powdered leaf, powdered fruit, or extracts, are typically standardized to a given sennoside content. Non-standardized preparations have unpredictable action and should be avoided. Senna is a stimulant laxative and acts on the wall of the large intestine, increasing peristaltic movement. After oral administration, the sennosides are transformed by intestinal flora into rhein anthrone (Figure 3.33), which appears to be the ultimate purgative principle. The glycoside residues in the active constituents are necessary for water-solubility and subsequent transportation to the site of action. Although purgative action is provided by the aglycones, including anthraquinones, these materials are conjugated and excreted in the urine after oral administration rather than being transported to the colon. Senna is a purgative drug suitable for either habitual constipation, or for occasional use, and is widely prescribed.

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