by egpat 29 May 2024
Beta-blockers can be classified as non-selective as well as selective beta-blockers based on how they interact with beta-adrenergic receptors. A few of the non-selective beta-blockers include propranolol, timolol, nadolol, and pindolol. They have no selectivity for different types of beta-adrenergic receptors. Esmolol is a selective beta-1 blocker. Other selective beta blockers include atenolol, metoprolol, acebutolol, nebivolol, and bisoprolol. These drugs selectively act on beta-1-adrenergic receptors.
Among the selective betablockers, esmolol is the only drug that is given by IV infusion. Many of the selective beta blockers are given by oral route, and a few of these selective beta blockers, like atenolol and metoprolol, can also begin by IV route, but as an IV bolus administration, that is a single administration by IV route. On the other hand, esmolol is given by IV infusion, that is, by the continuous administration of the drug by the IV route.
Drugs can be given by the intravenous route in two ways. Either as an IV bolus administration or as an IV infusion. Intravenous bolus administration is preferred in a few cases where a drug has a high first-pass metabolism so that it is not suitable for the oral route. In such cases, it can begin with IV bolus administration. Otherwise, if the drug action is required immediately, particularly in emergency conditions, then it can be achieved by IV bolus administration.
Most of the selective beta1 blockers do not have significant first-pass metabolism, so they can be given by the oral route, and whenever they require an emergency treatment, they can be given by IV bolus administration. But in the case of esmolol, it is given by IV infusion.
Normally, an IV infusion is preferred when a drug is short-acting. Esmolol has a short half-life of around nine minutes. That means half of the drug is going to be cleared every nine minutes, so it is very short-acting. Therefore, it should be administered continuously by IV infusion in order to retain its pharmacological actions.
Before going to see why this esmolol is short-acting, let us first look at the structure of the esmolol. Beta-blockers are chemically called aryloxypropanol amines. Esmolol has a phenyl ring as an aryl group. To these aryloxy propanol amines, whatever the group at the para position is, is very important. This paragroup will dictate the pharmacokinetic parameters of the beta blockers. Esmolol has three carbon chains in the para position that form an ester. Therefore, it has methyl propanoate at the paraposition. This ester group is responsible for the short duration of action of esmolol.
Due to the ester group in the para position, esmolol is rapidly metabolized in the liver. This ester functional group can be cleaved by the esterases within the erythrocytes so that it can be converted into free carboxylic acid. Now, this free carboxylic acid metabolite of esmolol has no significant activity on beta receptors. Because of this metabolism, esmolol is very short-acting, so it has a half-life of around nine minutes.
Esmolol has an onset of action of around two to ten minutes, and the onset of action of esmolol can be improved by adding a loading dose along with the IV infusion. The duration of action of esmolol is very short, with a duration of around 10 to 20 minutes. Because of the short duration of action, esmolol is given by IV infusion. So esmolol can be used in conditions like the treatment of atrial fibrillation and flutter and in the emergency treatment of tachycardia.
Esmolol can block beta 1 receptors selectively, thereby inhibiting both rate and force of contraction in the heart. In this way, esmolol is one of the drugs of choice in clinical settings for the emergency treatment of tachycardia as well as atrial fibrillation and flutter.