Labetalol

Last updated

Labetalol
Labetalol.svg
Clinical data
Pronunciation /ləˈbɛtəlɔːl/
Trade names Normodyne, Trandate, others
AHFS/Drugs.com Monograph
MedlinePlus a685034
License data
Pregnancy
category
  • AU:C
Routes of
administration 		By mouth, intravenous
ATC code
Legal status
Legal status
  • AU: S4 (Prescription only)
  • In general: ℞ (Prescription only)
Pharmacokinetic data
Bioavailability 25%
Protein binding 50%
Metabolism Liver pass metabolism,
Elimination half-life Tablet: 6–8 hours; IV: 5.5 hours
Excretion Excreted in urine, not removed by hemodialysis
Identifiers
  • (RS)-2-Hydroxy-5-[1-hydroxy-2-[(4-phenylbutan-2-yl)amino]ethyl]benzamide
CAS Number
PubChem CID
IUPHAR/BPS
DrugBank
ChemSpider
UNII
KEGG
ChEBI
ChEMBL
CompTox Dashboard (EPA)
ECHA InfoCard 100.048.401 OOjs UI icon edit-ltr-progressive.svg
Chemical and physical data
Formula C19H24N2O3
Molar mass 328.412 g·mol−1
3D model (JSmol)
Chirality Racemic mixture
  • c1ccccc1CCC(C)NCC(O)c2ccc(O)c(C(=O)N)c2
  • InChI=1S/C19H24N2O3/c1-13(7-8-14-5-3-2-4-6-14)21-12-18(23)15-9-10-17(22)16(11-15)19(20)24/h2-6,9-11,13,18,21-23H,7-8,12H2,1H3,(H2,20,24) Yes check.svgY
  • Key:SGUAFYQXFOLMHL-UHFFFAOYSA-N Yes check.svgY
   (verify)

Labetalol is a medication used to treat high blood pressure and in long term management of angina. [1] [2] This includes essential hypertension, hypertensive emergencies, and hypertension of pregnancy. [2] In essential hypertension it is generally less preferred than a number of other blood pressure medications. [1] It can be given by mouth or by injection into a vein. [1]

Contents

Common side effects include low blood pressure with standing, dizziness, feeling tired, and nausea. [1] Serious side effects may include low blood pressure, liver problems, heart failure, and bronchospasm. [1] Use appears safe in the latter part of pregnancy and it is not expected to cause problems during breastfeeding. [2] [3] It works by blocking the activation of β- and α-adrenergic receptors. [1]

Labetalol was patented in 1966 and came into medical use in 1977. [4] It is available as a generic medication. [2] In 2021, it was the 213th most commonly prescribed medication in the United States, with more than 2 million prescriptions. [5] [6]

Medical uses

Labetalol is effective in the management of  hypertensive emergencies, postoperative hypertension,  pheochromocytoma-associated hypertension, and  rebound hypertension  from  beta blocker  withdrawal. [7]

It has a particular indication in the treatment of  pregnancy-induced hypertension  which is commonly associated with  pre-eclampsia. [8]

It is also used as an alternative in the treatment of severe hypertension. [7]

Special populations

Pregnancy: studies in lab animals showed no harm to the baby. However, a comparable well-controlled study has not been performed in pregnant women. [9]

Nursing: breast milk has been shown to contain small amounts of labetalol (0.004% original dose). Prescribers should be cautious in the use of labetalol for nursing mothers. [9]

Pediatric: no studies have established safety or usefulness in this population. [9]

Geriatric: the elderly are more likely to experience dizziness when taking labetalol. Labetalol should be dosed with caution in the elderly and counseled on this side effect. [9]

Side effects

Common

Low blood pressure with standing is more severe and more common with IV formulation (58% vs 1% [9] ) and is often the reason larger doses of the oral formulation cannot be used. [10]

Rare

Contraindications

Labetalol is contraindicated in people with overt cardiac failure, greater-than-first-degree  heart block, severe  bradycardia,  cardiogenic shock, severe  hypotension, anyone with a history of obstructive airway disease including asthma, and those with hypersensitivity to the drug. [12]

Pharmacology

Mechanism of action

Labetalol is a beta blocker, or an antagonist of the β-adrenergic receptors. It is specifically a non-selective antagonist of the β1- and β2-adrenergic receptors. [13] Labetalol has intrinsic sympathomimetic activity. [13] It is also an antagonist of the α1-adrenergic receptor, and hence is additionally an alpha blocker. The antagonism of the adrenergic receptors by labetalol is competitive against other catecholamines [14] and its actions on the receptors are potent and reversible. [12]

Labetalol is about equipotent in blocking β1- and β2-adrenergic receptors. [15] The amount of α to β blockade depends on whether labetalol is administered orally or intravenously (IV). Orally, the ratio of α to β blockade is 1:3. Intravenously, α to β blockade ratio is 1:7. [15] [12] Thus, the labetalol can be thought to be a beta blocker with some α-blocking effects. [12] [14] [16] By comparison, labetalol is a weaker β-adrenergic receptor blocker than propranolol, and has a weaker affinity for α-adrenergic receptors compared to phentolamine. [15] [14]

Labetalol's dual α- and β-adrenergic antagonism has different physiological effects in short- and long-term situations. In short-term, acute situations, labetalol decreases blood pressure by decreasing systemic vascular resistance with little effect on stroke volume, heart rate and cardiac output. [17] During long-term use, labetalol can reduce heart rate during exercise while maintaining cardiac output by an increase in stroke volume. [18]

Labetalol possesses significant intrinsic sympathomimetic activity. [16] In particular, it is a partial agonist at β2-adrenergic receptors located in the vascular smooth muscle. Labetalol relaxes vascular smooth muscle by a combination of this partial β2-adrenergic receptor agonism and through α1-adrenergic receptor blockade. [16] [19] Overall, this vasodilatory effect can decrease blood pressure. [20]

Similar to local anesthetics and sodium channel blocking antiarrhythmics, labetalol also has membrane stabilizing activity. [16] [21] By decreasing sodium entry, labetalol decreases action potential firing and thus has local anesthetic activity. [22]

Physiological action

The physiological effects of labetalol when administered acutely (intravenously) are not predictable solely by their receptor blocking effect, i.e. blocking β1-adrenergic receptors should decrease heart rate, but labetalol does not. When labetalol is given in acute situations, it decreases the peripheral vascular resistance and systemic blood pressure while having little effect on the heart rate, cardiac output and stroke volume, despite its α1-, β1- and β2-adrenergic receptor blocking mechanism. [17] [18] These effects are mainly seen when the person is in the upright position. [20]

Long term labetalol use also has different effects from other beta blockers. Other beta blockers, such as propranolol, persistently reduce cardiac output during exercise. The peripheral vascular resistance decreases when labetalol is first administered. Continuous labetalol use further decreases peripheral vascular resistance. However, during exercise, cardiac output remains the same due to a compensatory mechanism that increases stroke volume. Thus, labetalol is able to reduce heart rate during exercise while maintaining cardiac output by the increase in stroke volume. [18]

Pharmacokinetics

Labetalol is classified as a beta blocker with low lipophilicity and hence lower potential for crossing the blood–brain barrier. [13] This in turn may result in fewer effects in the central nervous system as well as a lower risk of neuropsychiatric side effects. [13] In accordance, labetalol, in animal models, was found to cross the blood–brain-barrier in only negligible amounts. [23]

Chemistry

The minimum requirement for adrenergic agents is a primary or secondary amine separated from a substituted benzene ring by one or two carbons. [24] This configuration results in strong agonist activity. As the size of the substituent attached to the amine becomes greater, particularly with respect to a t-butyl group, then the molecule typically is found to have receptor affinity without intrinsic activity, and is, therefore, an antagonist. [24] Labetalol, with its 1-methyl-3-phenylpropyl substituted amine, is greater in size relative to a t-butyl group and therefore acts predominantly as an antagonist. The overall structure of labetalol is very polar. This was created by substituting the isopropyl group in the standard beta blocker structure with an aralkyl group, including a carboxamide group on the meta position, and by adding a hydroxyl group on the para position. [15]

Labetalol has two chiral carbons and consequently exists as four stereoisomers. [25] Two of these isomers, the (S,S)- and (R,S)- forms are inactive. The third, the (S,R)-isomer, is a powerful α1-adrenergic receptor blocker. The fourth isomer, the (R,R)-isomer which is also known as dilevalol, is a mixed non-selective β-adrenergic receptor blocker and selective α1 blocker. [15] Labetalol is typically given as a racemic mixture to achieve both α- and β-adrenergic receptor blocking activity. [26]

Stereoisomers of labetalol
(RR)-Labetalol Structural Formula V1.svg
(R,R)-Labetalol
CAS number: 75659-07-3		 (SS)-Labetalol Structural Formula V1.svg
(S,S)-Labetalol
CAS number: 83167-24-2
(RS)-Labetalol Structural Formula V1.svg
(R,S)-Labetalol
CAS number: 83167-32-2		 (SR)-Labetalol Structural Formula V1.svg
(S,R)-Labetalol
CAS number: 83167-31-1

Labetalol acts by blocking α- and β-adrenergic receptors, resulting in decreased peripheral vascular resistance without significant alteration of heart rate or cardiac output.

The β:α antagonism of labetalol is approximately 3:1. [27] [28]

It is chemically designated in International Union of Pure and Applied Chemistry (IUPAC) nomenclature as 2-hydroxy-5-[1-hydroxy-2-[(1-methyl-3-phenylpropyl)amino]ethyl]benzamide monohydrochloride. [26] [29]

History

Labetalol was the first drug created that combined both α- and β-adrenergic receptor blocking properties. It was created to potentially fix the compensatory reflex issue that occurred when blocking a single receptor subtype, i.e. vasoconstriction after blocking β-adrenergic receptors or tachycardia after blocking α-adrenergic receptors. Because the reflex from blocking the single receptor subtypes acted to prevent the lowering of blood pressure, it was postulated that weak blocking of both α- and β-adrenergic receptors could work together to decrease blood pressure. [15] [18]

Related Research Articles

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<span class="mw-page-title-main">Propranolol</span> Beta blocker drug

Propranolol, sold under the brand name Inderal among others, is a medication of the beta blocker class. It is used to treat high blood pressure, a number of types of irregular heart rate, thyrotoxicosis, capillary hemangiomas, performance anxiety, and essential tremors, as well to prevent migraine headaches, and to prevent further heart problems in those with angina or previous heart attacks. It can be taken orally or by intravenous injection. The formulation that is taken orally comes in short-acting and long-acting versions. Propranolol appears in the blood after 30 minutes and has a maximum effect between 60 and 90 minutes when taken orally.

<span class="mw-page-title-main">Adrenergic receptor</span> Class of G protein-coupled receptors

The adrenergic receptors or adrenoceptors are a class of G protein-coupled receptors that are targets of many catecholamines like norepinephrine (noradrenaline) and epinephrine (adrenaline) produced by the body, but also many medications like beta blockers, beta-2 (β2) agonists and alpha-2 (α2) agonists, which are used to treat high blood pressure and asthma, for example.

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<span class="mw-page-title-main">Isoprenaline</span> Medication for slow heart rate

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<span class="mw-page-title-main">Dobutamine</span> Medication which strengthens heart contractions

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<span class="mw-page-title-main">Phentolamine</span> An α-adrenergic antagonist medication

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<span class="mw-page-title-main">Bisoprolol</span> Beta-1 selective adrenenergic blocker medication used to treat cardiovascular diseases

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<span class="mw-page-title-main">Nebivolol</span> Chemical compound

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<span class="mw-page-title-main">Adrenergic antagonist</span> Type of drug

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<span class="mw-page-title-main">Alpha blocker</span> Class of pharmacological agents

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<span class="mw-page-title-main">Beta-adrenergic agonist</span> Medications that relax muscles of the airways

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<span class="mw-page-title-main">Discovery and development of beta-blockers</span>

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The β3 adrenergic receptor agonist or β3-adrenoceptor agonist, also known as β3-AR agonist, are a class of medicine that bind selectively to β3-adrenergic receptors.

References

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