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Nuelin SR

 

COMPOSITION
Nuelin SR contains 125 mg or 250 mg anhydrous theophylline (1,3-dimethyl xanthine, also named 3,7-dihydro 1,3-dimethyl-purine-2,6 (1H)-dione) in the form of sustained release tablets.


PHARMACOLOGY
Site and Mode of Action:
Theophylline has a direct relaxant effect on the smooth muscle of bronchial airways and pulmonary blood vessels, serving as a bronchodilator and pulmonary vasodilator.


It also exhibits activities typical of xanthines such as CNS stimulation including the respiratory centre, cardiac stimulation, coronary vasodilatation, diuresis and increased gastric secretion.


The mechanism of action of theophylline in viva has not been fully elucidated. One mechanism of smooth muscle relaxation may be inhibition of phosphodiesterase that reduces intracellular hydrolysis of cyclic AMP. Increased intracellular concentrations of cyclic AMP have been associated with relaxation of bronchial smooth muscle.


There is no evidence that tolerance develops with continued use of theophylline.


Relationship to Other Drugs:
Theophylline is closely related to the other xanthines, caffeine and theobromine. Generally, the xanthines relax smooth muscle, act on the kidney to produce diuresis, stimulate the central nervous system and stimulate cardiac muscle.


PHARMACOKINETICS
Nuelin SR is a sustained release formulation appropriate for long term use. Steady-state conditions are usually achieved after 4 days' therapy.


It is now generally believed that plasma concentrations of 10-20 μg/mL constitute a therapeutic range, although some patients may benefit from levels below this.


Absorption
Theophylline is well absorbed throughout the gastrointestinal tract. The bioavailability of theophylline from Nuelin SR is approximately 100%. Peak levels after administration of Nuelin SR usually occur at 4 to 6 hours post-dose.


Total bioavailability is not altered by food intake. Single dose studies with Nuelin SR show that food delays the rate of absorption slightly, especially in children. In multiple dosing situations, a slower rate of theophylline absorption leads to lower peak-trough fluctuation.


The plasma half-life of theophylline in adults varies considerably. In healthy adults it ranges from 3 to 12 hours. The half-life is shortened by smoking.


The half-life of theophylline is prolonged by reduced hepatic function, congestive heart failure, pulmonary disease, severe hypoxia, reduced thyroid function, acute febrile states, viral infections and administration of some drugs. (See Drug Interactions.) Patients with a prolonged half-life of theophylline, from whatever cause, require a reduced dosage.


In children aged 1-9 years, the half-life is usually significantly shorter than in adults, averaging about 3.5 hours. In newborns and neonates, clearance is extremely slow.


Distribution
Approximately 50-70% of circulating theophylline is bound to the plasma proteins of adults, but binding is decreased to about 40% in newborn infants and in adults with hepatic cirrhosis. Theophylline partitions into saliva and breast milk and crosses the placental barrier.


Metabolism
Theophylline is metabolised in the liver, principally to 1,3-dimethyluric acid with other metabolites being 3-methylxanthine and 1-methyluric acid. 3-Methylxanthine has some pharmacological activity, but less than theophylline.


Excretion
Theophylline and its metabolites are excreted by the kidney. About 10% of the administered dose is excreted unchanged in the urine.


INDICATIONS
For the relief and prophylaxis of reversible bronchospasm associated with bronchial asthma, bronchitis, emphysema and related conditions.


CONTRAINDICATIONS
Nuelin SR should not be used where hypersensitivity to its constituents or to xanthines generally is known or has been demonstrated.

 

PRECAUTIONS
1. As there is a correlation between plasma levels of theophylline and therapeutic effect and as patient response can vary considerably due to variable rates of elimination, monitoring plasma levels in individual patients is strongly recommended (see THEOPHYLLINE MONITORING). Dosage should be individualised if optimal therapeutic effect is to be achieved. However, individual patients also have a widely variable tolerance to adverse effects and so symptomatology should be considered as well as monitored levels.
2. Acute symptoms of asthma requiring rapid treatment: Sustained release products are therapeutically inappropriate for acute asthma requiring prompt treatment.
3. Theophylline should not be administered concurrently with other xanthine medications and caution should be exercised when sympathomimetic agents are also part of the regimen.
4. Theophylline clearance decreases in patients with reduced thyroid function, congestive heart failure, acute pulmonary oedema, chronic obstructive pulmonary disease, severe hypoxia, pneumonia, acute febrile episodes and during acute viral infection. Clearance is markedly decreased in patients with impaired liver function, such as hepatic cirrhosis. Also refer to Drug Interactions section.
5. Because of its cardiac side effects, use theophylline with caution in patients with cardiac arrhythmias, coronary artery disease, unstable angina, cardiomyopathy and severe hypertension. Theophylline increases gastric acid secretion and should be used with caution in patients with peptic ulcer or gastro-oesophageal reflex.
6. Smoking may increase theophylline clearance and increased doses of theophylline may be required. (See Drug Interactions.)
7. There is some evidence that theophylline exhibits dose-dependent kinetics, at least in sick and elderly patients. Care should be exercised by titration of dosage requirements in small increments and by monitoring serum theophylline levels.
8. Xanthine containing beverages leg tea, coffee, cola, cocoa) may interfere with some serum theophylline assays.


Use in Pregnancy
Theophylline does cross the placental barrier. The effect on foetal development is not known.


Theophylline clearance is significantly decreased in premature infants. Therefore, if this drug is administered to the mother near the time of delivery, the neonate should be monitored closely for the pharmacological effects of theophylline. Hence the use of theophylline in pregnant women should be balanced against the risk of uncontrolled asthma.


Use in Lactation
Theophylline is excreted in breast milk and irritability has been reported in infants of nursing mothers taking theophylline. It is advisable to keep serum theophylline concentrations as low as possible in nursing mothers while maintaining adequate asthma control.
 

Drug Interactions
The following drugs have been shown to decrease the hepatic clearance of theophylline, thus increasing its serum concentration: Cimetidine, high dose allopurinol, propranolol, macrolide antibiotics (eg. erythromycin, clarithromycin) quinolone antibiotics (eg. ciprofloxacin and enoxacin), alcohol, oral contraceptives, mexilitene, tacrine, thiabendazole, disulfiram, Interferon alpha and verapamil.
 

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