ADENOCOR

Indications

• Paroxysmal supraventricular tachycardia (PSVT)
• Myocardial perfusion scintigraphy adjunctive use

Dosage

Adults: For the acute termination of PSVT, the initial dose is 6 mg given as a

Mechanism of action

Adenosine exerts its pharmacological effects through the agonism of adenosine receptors A1 and A2. This action reduces conduction time in the atrioventricular (AV) node by inducing potassium efflux and inhibiting calcium influx, leading to hyperpolarization of nerve cells and an increased threshold for calcium-dependent action potentials. This results in an antiarrhythmic effect, particularly through the interruption of AV nodal reentry pathways, thereby restoring normal sinus rhythm in patients with PSVT. Furthermore, adenosine relaxes vascular smooth muscle by decreasing calcium influx, which enhances blood flow in normal coronary arteries while not affecting stenotic arteries.

Pharmacodynamics

Adenosine is indicated for the conversion of sinus rhythm in paroxysmal supraventricular tachycardia and as an adjunct for thallium-201 in myocardial perfusion scintigraphy. It has a short half-life of less than 10 seconds, resulting in a brief duration of action and a wide therapeutic window. Care must be taken as it can cause cardiovascular side effects, bronchoconstriction, seizures, and hypersensitivity reactions.

Pharmacokinetics

Adenosine has a rapid onset of action due to its short half-life, which is less than 10 seconds, allowing for quick therapeutic effects. It is distributed widely in body tissues as it is an endogenous compound. The drug is metabolized primarily by cellular uptake and enzymatic degradation, leading to its effects being transient. Its pharmacokinetic profile necessitates careful monitoring during administration to avoid potential adverse effects.

Contra-indications

• Second or third degree AV block
• Sick sinus syndrome
• Bronchospastic lung disease
• Hypersensitivity to adenosine

Adverse effects

• Flushing
• Chest pain
• Shortness of breath
• Hypotension
• Nausea
• Headache
• Dizziness

Interactions

• dipyridamole: Moderate (increases exposure)
• aminophylline: Unknown (decreases efficacy)
• caffeine citrate: Unknown (decreases efficacy)
• theophylline: Unknown (decreases efficacy)

Precautions

• Use with caution in patients with asthma or other bronchospastic conditions
• Monitor for cardiovascular side effects
• Consider potential effects on patients with known heart conditions

Pregnancy

Adenosine should only be used in pregnancy if clearly needed, as the effects on the fetus are not well established.

Breast-feeding

It is not known whether adenosine is excreted in human milk; caution should be exercised when administering to nursing mothers.

Storage

Store at room temperature, away from light and moisture. Keep out of reach of children.

Formulations

• Adenosine injection

Mechanism of action

Agonism of adenosine receptors A1 and A2 reduces conduction time in the atrioventricular node of the heart. Conduction time is decreased by inducing potassium efflux and inhibiting calcium influx through channels in nerve cells, leading to hyperpolarization and and increased threshold for calcium dependent action potentials. Decreased conduction time leads to an antiarrhythmic effect. Inhibition of calcium influx, reduces the activity of adenylate cyclase, relaxing vascular smooth muscle. Relaxed vascular smooth muscle leads to increased blood flow through normal coronary arteries but not stenotic arteries, allowing thallium-201 to be more readily uptaken in normal coronary arteries. Adenosine is an endogenous nucleoside present in all cells of the body. Adenosine may exert its pharmacologic effects by activation of purine (cell-surface A1 and A2 adenosine) receptors; relaxation of vascular smooth muscle may be mediated by reduction in calcium uptake through inhibition of slow inward calcium current and activation of adenylate cyclase in smooth muscle cells. Adenosine may reduce vascular tone by modulation of sympathetic neurotransmission. Adenosine has negative chronotropic, dromotropic, and inotropic effects on the heart. The drug slows conduction time through the AV node and can interrupt AV nodal reentry pathways, leading to restoration of normal sinus rhythm in patients with PSVT, including that associated with Wolff-Parkinson-White syndrome. Adenosine is a potent vasodilator in most vascular beds, except in renal afferent arterioles and hepatic veins where it produces vasoconstriction. Adenosine is thought to exert its pharmacological effects through activation of purine receptors (cell-surface A1 and A2 adenosine receptors). Although the exact mechanism by which adenosine receptor activation relaxes vascular smooth muscle is not known, there is evidence to support both inhibition of the slow inward calcium current reducing calcium uptake, and activation of adenylate cyclase through A2 receptors in smooth muscle cells. Adenosine may also lessen vascular tone by modulating sympathetic neurotransmission. The intracellular uptake of adenosine is mediated by a specific transmembrane nucleoside transport system. Once inside the cell, adenosine is rapidly phosphorylated by adenosine kinase to adenosine monophosphate, or deaminated by adenosine deaminase to inosine. These intracellular metabolites of adenosine are not vasoactive. Myocardial uptake of thallium-201 is directly proportional to coronary blood flow. Since Adenoscan significantly increases blood flow in normal coronary arteries with little or no increase in stenotic arteries, Adenoscan causes relatively less thallium-201 uptake in vascular territories supplied by stenotic coronary arteries ie, a greater difference is seen after Adenoscan between areas served by normal and areas served by stenotic vessels than is seen prior to Adenoscan. ... Current evidence suggests that ADO-metabolizing enzymes such as ecto-5'-nucleotidase or ADO deaminase, as well as enzymes that degrade ATP to adenosine, play an important role in the vasoconstrictor signals sent from the macula densa to the afferent arterioles when tubuloglomerular feedback is activated; increased ADO concentration induced by temporal infusion of AngII results in downregulation of A2 ADO receptors, leading to a predominant effect of A1 receptors; the alteration in the ADO receptors balance further contributes to the synergic interaction between ADO and AngII. ... The ADO-metabolizing enzymes have become important regulators of the effects of ADO on the tone of the afferent and efferent arterioles. As AngII is able to increase de-novo renal ADO content through decrease of ADO-metabolizing enzymes, accumulation of ADO induces downregulation of ADO A2 receptor population without modifying ADO A1 receptor, thereby enhancing the constrictive effects of AngII in the renal vasculature. For more Mechanism of Action (Com

Pharmacodynamics

Adenosine is indicated as an adjunct to thallium-201 in myocardial perfusion scintigraphy and also indicated for conversion of sinus rhythm of paroxysmal supraventricular tachycardia. Adenosine has a short duration of action as the half life is <10 seconds, and a wide therapeutic window. Patients should be counselled regarding the risk of cardiovascular side effects, bronchoconstriction, seizures, and hypersensitivity.