Pharmacological Characterization Of The Antiasthmatic Constituent Of The Leaves Of Asystasia Gangetica (L.) Anderson (ACANTHACEAE)

ABSTRACT

Asthma is currently a worldwide problem, with increasing prevalence in both

?r children and adults; a prevalence rate of 5 - 10% has been reported for Nigeria.

The pathophysiological features of the disease are bronchoconstriction, airway

hyperresponsiveness and chronic inflammation. Drugs used in the management

include bronchodilators which are short-term relievers and anti-inflammatory

drugs which are long-term controllers. Despite the availability of oral and inhaled

medications, the prevalence of asthma is on the rise. The challenge of developing

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new effective, safe and long lasting antiasthmatic drugs from natural products

appears inevitable. The leaves of Asystasia gangetica L. (T). Anderson sub species

micrantha (Nees) Ensermu (Acanthaceae), a traditional anti-asthma remedy, offer

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great potential for the development of a novel anti-asthmatic agent. The leaves

have been shown to possess antihistaminic, bronchospasmolytic and antiinflammatory

properties. The aim of this research was to isolate and

pharmacologically characterize the anti-asthmatic constituent of the leaves of this

plant using bioactivity-guided fractionation of the leaf extract.

The leaves of A. gangetica were sun dried in open air for 48 h and pulverized to

coarse powder. The leaf powder was extracted by cold maceration in 100%

methanol for 48 h. The extract obtained was concentrated in a rotary evaporator

under reduced pressure to afford the methanol extract (ME). The median lethal

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dose (LDSd) of ME was determined in mice using the intraperitoneal route. The

ME was fractionated in a silica gel (70 - 230 mesh) column successively eluted

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with hexane: ethylacetate (7:3) and methanol (100%) to obtain fractions A, B and

C. Screening of the three fractions for bronchospasmolytic activity using

inhibition of histamine induced contraction of the guinea pig trachea and

relaxation of pre-contracted trachea (pathological tissue) as bioactivity-guides

showed that fraction B exhibited the greatest effect in both tests. Based on the

results, fraction B was hrther separated using gradient solvent mixtures of hexane:

ethyl acetate (9.5:0.5, 9:1, 8:2, 7:3, 6:4, 5:5, 0:l) to obtain eight fractions (Fr I, Fr

11, Fr 111, Fr IV, Fr V, Fr VI, Fr VII, Fr VIII). Activity-guided tests on these

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fractions revealed greatest potency in Fr VIII, which on chromatographic

separation successively eluted with petroleum ether: ethyl acetate (7:3) and

ethylacetate (100%) yielded two fractions (Fr 1 and Fr 2). Fr 2 yielded a brown ' amorphous powder AG-1 which at 0.4 mg/ml completely inhibited the

contractions of the guinea pig trachea induced by histamine. The methanol extract

(ME), fraction B, Fr VIII and Fr 2 were subjected to phytochemical analysis for

identification of constituents. The effect of the extract and fractions (200 and 400

mgkg) on systemic acute inflammation was studied in rats using the paw edema

induced by carrageenan. Also, the effect of the extract and fractions (400 mg/kg)

on carrageenan-induced leukocyte infiltration into the pleural cavity as well as

pleural exudate formation was evaluated in rats. The data obtained was analysed

using One Way Analysis of Variance in SPSS Version 1 1. +

Phytochemical tests showed that the ME tested positive to alkaloids, glycosides,

saponins, reducing sugars, terpenes and carbohydrate. Fraction B gave positive

reactions for flavonoids and terpenes while Fr VIII tested positive to flavonoids,

terpenes and steroids. Fr 2 gave positive reactions for terpenes and steroids. The

9 intraperitoneal median lethal dose (LDSo) of ME in mice was estimated to be

greater than 5,000 mglkg. In the isolated tissue tests, the ME, Fr B, Fr VIII and

AG-1 exhibited varying degrees of dose-dependent inhibition of contractions of

the guinea pig trachea induced by histamine. The ME and the fractions also

relaxed the guinea pig trachea pre-contracted with histamine. In the whole animal

experiments, ME, Fr B and Fr VIII significantly (P