| Class | Adenosine receptor antagonist |
|---|---|
| Chemistry |
Methylxanthine |
| Routes of administration |
Oral |
| Absorption |
Rapidly and completely absorbed (99-96% bioavailability) |
| Solubility |
pKa = 8.77, slightly soluble in water |
| Distribution |
VOD= 0.45L/kg, 58-82% protein-bound |
| Metabolism |
90% of theophylline is metabolised by CYP 450 enzymes which have significant polymorphism, with about fourfold interindividual variation, and which are susceptible to indiuction or inhibition by other agents, which means theophylline interacts with everything (erythromycin, rifampicin, nicotine, etc) |
| Elimination |
Minimal free drug is eliminated in the urine |
| Time course of action |
Half-life is 8 hours, or 4-5 hours in smokers |
| Target receptor |
Adenosine A1 and A2 receptors, which inhibit neurotransmitter releease by regulating presynaptic potassium and calcium traffic |
| Mechanism of action |
Increases neurotransmitter (dopamine and noradrenaline) release from presynaptic nerve terminals. Adenosine is a regulatory neurotransmitter that is normally responsible for hyperpolarising presynaptic terminals, mainly through its activity on potassium channel and voltage-gated calcium channel activity. Neurotransmitter release from these terminals is therefore inhibited by adenosine; and to reverse this inhibition allows increased neurotransmitter release. |
| Clinical effects |
Bronchodilation, pulmonary arterial vasodilation, increased diaphragmatic contractility, tachycardia, increased cardiac output, arrhythmias, diuresis, increased gastric acid secretion, and rhabdomyolysis and seizures in overdose |
| Literature reference |
Weinberger, 1984 |
| CICM details of understanding | Level 2 |
| Mentioned around Deranged Physiology |
Pharmacology of drugs used to treat asthma Also, Theophylline toxicity and Sympathomimetic toxicity, because toxic. |
| Related SAQs |
Question 11 from the second paper of 2023 (as aminophylline) Question 29 from the first paper of 2017 (Second Part Exam) Question 11(p.2) from the second paper of 2007 |