Aminophylline Injection

Description
A clear colourless or slightly yellow liquid. It contains anhydrous theophylline not less than 74% and no more than 84% of the labelled amount of Aminophylline.

Clinical pharmacology
Aminophylline relaxes bronchial smooth muscles, relieves bronchospasms and has a stimulant effect on respiration. It stimulates the myocardium and central nervous system, decreases peripheral resistance and venous pressure, and causes diuresis.

Pharmacokinetics
Aminophylline, a complex of theophylline with ethylenediamine, readily liberates theophylline in the body. Theophylline is approximately 40% bound to the plasma protein, but in neonates or adults with liver disease, binding is reduced.

Optimum therapeutic serum concentrations are generally considered to range from 10 to 20 micrograms per ml although some consider a lower dose appropriate.

Aminophylline Injection

Theophylline is metabolised in the liver to 1,3-dimethyluric acid, 1-methyluric acid, and 3-methylxanthine.

These metabolites are excreted in the urine, but in neonates about 50% is excreted unchanged and a large proposition is excreted as caffeine.

Considerable inter-individual differences in the rate of hepatic metabolism of theophylline result in large variations is clearance, serum concentrations and half-lives.

Hepatic metabolism is further affected by factors such as age, smoking disease, diet and drug interactions.

The serum half-life of theophylline in an otherwise healthy, non-smoking asthmatic adult is 6 to 12 hours, in children 1 to 5 hours, in cigerate smokers 4 to 5 hours, and in neonates and premature infants 10 to 45 hours.

The serum half-life of theophylline may be increased in the elderly and in patients with heart failure and liver disease.

Theophylline crosses the placenta; it also entres breast milk.

Indication
Aminophylline is used as a bronchodilator in the management of asthma and chronic obstructive pulmonary disease.

It was formerly used for it’s diuretic action in heart failure but has been superseded by more effective drugs.

However, it may have a role in patients with heart failure and obstructive airway disease though care is needed in those with increased myocardial excitability.

Drug interaction
The toxic effects of Aminophylline and other xanthines are additive. Concomitant use with other xanthine medications should therefore be avoided.

If Aminophylline is to be given for acute bronchospasm in patients who have been taking maintenance xanthine therapy, serum theophylline concentration should be measured first and the initial dose reduced as appropriate.

Theophylline clearance may be used by interaction with other drugs including allopurinol, some antiarrhythmics, cimetidine, disulfiram, fluvoxamine, interferon alfa, macrolide antibiotics and quinolones, oral contraceptives, thiabendazole, and viloxazine, necessitating dosage reduction.

Phenytoins and other anticonvulsants, the antiviral ritonavir, rifampicin, sulphinpyrazone, and cigerette smoking may increase theohylline clearance, necessitating and increase in dose or dosing frequency.

There is a risk of synergistic toxicity if aminophylline is given concomitantly with halothene or ketamine, and it may antagonise the effects of adenosine and of non-depolarisins muscle relaxants.

Lithium elimination may be enhanced with a consequent loss of effect.

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