Screening Methods of Bronchodilators

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Guided By: Presented By: Mr. Rupesh Gautam Prafulla Chandra TiwariSenior Lecturer, Dept. of Pharmacology M.Pharm PharmacologyJaipur College of Pharmacy Jaipur College of Pharmacy

Bronchodilators

The pharmacological therapy of asthma employsdrugs aimed more directly at decreasingbronchospasm (i.e., bronchodilators).

The main drugs used as bronchodilators are β2-adrenoceptor agonists; others include xanthines,cysteinyl leukotriene receptor antagonists andmuscarinic receptor antagonists.

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β2 Adrenergic Receptor Agonists

The βadrenergic receptor agonists available for the treatment of asthma are selective for the β2 -receptor subtype.

The mechanism of the antiasthmatic action of β2

adrenergic receptor agonists the direct relaxation of airway smooth muscle and consequent bronchodilation

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Theophylline

• It inhibits phosphodiesterase and blocks adenosinereceptors.

• Theophylline has a narrow therapeutic window:unwanted effects include cardiac dysrhythmia,seizures and gastrointestinal disturbances.

• It is given intravenously (by slow infusion) forstatus asthmaticus, or orally (as a sustained-releasepreparation) as add-on therapy to inhaledcorticosteroids and long-acting β2 agonists

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Screening Models of Bronchodilators

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1.Bronchospasmolytic activity in anesthetized guineapigs (Konzett-Rössler method)

2. Effect of arachidonic acid or PAF on respiratoryfunction in vivo

3. Bronchial hyperreactivity

4.Body plethysmography and respiratory parametersafter histamine-induced bronchoconstriction inanesthetized guinea pigs.

5. Pneumotachography in anesthetized guinea pigs

Bronchospasmolytic activity in anesthetizedguinea pigs (Konzett-Rössler method)

PURPOSE AND RATIONALE:•The method is based on registration of air volume changes of aliving animal in a closed system consisting of the respirationpump.

•Bronchospasm decreases the volume of inspired air and increasesthe volume of excess air.

•Thus, the degree of bronchospasm can be quantified by recordingthe volume ofexcess air.

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Procedure:

•Guinea-pigs of either sex weighing 250–500 g areanaesthetized with 1.25 g/kg i.p. urethane.

•The trachea is cannulated by means of a two way cannula,one arm of which is connected to the respiratory pump andthe other to a Statham P23 Db transducer.

•The animal is artificially respired using a Starling pumpwith an inspiratory pressure set at 90–120 mm of water, anadequate tidal volume of 3 ml/100 g body weight and afrequency of 60 strokes per minute.

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•Excess air, not taken up by the lungs, ismeasured and recorded on a polygraph

• The internal jugular vein is cannulatedfor the administration of spasmogens andtest compounds.

• The carotid artery is cannulated for measuring blood pressure.

Effect of arachidonic acid or PAF onrespiratory function in vivo

PURPOSE AND RATIONALE:

The test allows to evaluate the sites of action of drugs,which interfere with the mechanisms of broncho-constriction and thrombocytopenia; in an in vivo-model guinea pigs are challenged with the spasmogensand platelet-aggregating substances arachidonic acidor PAF (platelet activating factor).

Procedure:

1.Male guinea pigs (Pirbright White) weighing 300–600 g areanesthetized with 60 mg/kg pentobarbital sodium (i.p.).

2. One of the jugular veins is cannulated for the administration of spasmogen and test compound.

3. Both external carotid arteries are cannulated; one is connectedto a pressure transducer to register blood pressure, the other isused for blood withdrawal.

4.The trachea is connected to a Starling pump with aninspiratory pressure set of 80 mm H2O, an adequate tidal volumeof approx. 10 ml/kg body weight and a frequency of 70–75strokes/ min.

5. Spontaneous respiration is inhibited by intravenous injectionof pancuronium (4 mg/kg) or gallamine (2 mg/kg) on time.

6. In some experiments, pulmonal ß-receptors are blocked byintraperitoneal administration of propranolol (2 mg/kg).

7. Excess air, not taken up by the lungs, is conducted to a transducer with bronchotimer, which translates changes in air flow to an electrical signal.

8. Changes in air flow and arterial blood pressure are recorded continuously.

Bronchial hyper reactivity

PURPOSE AND RATIONALE:

• Symptoms like asphyctic convulsions resembling bronchialasthma in patients can be induced by inhalation of histamine orother bronchospasm inducing agents in guinea pigs.

•The challenging agents are applied as aerosols produced by an ultra-sound nebulizer

• The first symptoms are increased breathing frequency, forcedinspiration, and finally asphyctic convulsions.

•The occurrence of these symptoms can be delayed byantagonistic drugs.

• Pre-convulsion time, i.e. time until asphycticconvulsions, can be measured.

PROCEDURE•Ten male albino guinea pigs weighing 300–400 g pergroup are used. The inhalation cages consist of 3boxes each ventilated with an air flow of 1.5 l/min.

• The animal is placed into box A to which the test drug or the standard is applied using an ultra-sound nebulizer.

•Alternatively, the animal is treated orally orsubcutaneously with the test drug or the standard.

Box B serves as a sluice through which the animal is passed into box C.

There, the guinea pig is exposed to an aerosol of a 0.1%solution of histamine hydrochloride provided by anultra-sound nebulizer.

Time until appearance of asphyctic convulsions ismeasured. Then, the animal is immediately withdrawnfrom the the inhalation box. The aerosols are removedfrom the back wall of the boxes by applyinglow pressure.

Bibliography:

1.Goodman Gilman, Pharmacological Basis of

Therapeutics,11th Edition. Published by Mc-Graw Hill.2.Vogel H. Gerhard, “Drug Discovery and Evaluation”,

Second Edition, Published by Springer. Page No. 359-368.

3.Kagoshima M, Tomomatsu N, Iwahisha Y, YamaguchiS, Matsuura(1997) Suppressive effects of Y-24180, areceptor antagonist to platelet activating factor (PAF),on antigen-induced asthmatic responses in guinea pigs.Inflamm Res 46:147–153.