Other β Blockers

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Chapter: Essential pharmacology : Antiadrenergic Drugs (Adrenergic Receptor Antagonists) And Drugs For Glaucoma

A number of β blockers have been developed having some special features. The associated properties with their significance can be summarized as:


OTHER β BLOCKERS

 

A number of β blockers have been developed having some special features. Their comparative properties are presented in Table 10.2. The associated properties with their significance can be summarized as:

 

Cardioselectivity (in metoprolol, atenolol, acebutolol, bisoprolol, nebivolol).

 

These drugs are more potent in blocking cardiac (β1) than bronchial (β2) receptors. However, selectivity is only relative and is lost at high doses. Their features are:

 



 

·     Lower propensity to cause bronchoconstriction, but even these drugs should be avoided, if possible, in asthmatics.

 

·     Less interference with carbohydrate metabolism and less inhibition of glycogenolysis during hypoglycaemia—safer in diabetics. However, tachycardia in response to hypoglycaemia is blocked.

 

·     Lower incidence of cold hands and feet, less chances of precipitating Raynaud’s phenomenon.

 

·     No/less deleterious effect on blood lipid profile.

 

·     Ineffective in suppressing essential tremor (it occurs through β2 action on muscle fibres).

 

·     Less liable to impair exercise capacity.

 

Partial Agonistic (Intrinsic Sympathomimetic) Action (in pindolol, acebutolol). These drugs themselves activate β1 and/or β2 receptors submaximally. The benefits of this property are controversial.

 

·        Bradycardia and depression of contractility at rest are not prominent, but exercise tachycardia is blocked; may be preferred in those prone to severe bradycardia (elderly patients; sick sinus) or with low cardiac reserve.

·        Withdrawal is less likely to exacerbate hypertension or angina; continued agonistic action on receptors (of the drug itself) prevents development of supersensitivity.

·        Plasma lipid profile is not/less worsened.

·        Not effective in migraine prophylaxis—they dilate cerebral vessels.

·        Not suitable for secondary prophylaxis of MI.

 

Membrane Stabilizing Activity (in propranolol, oxprenolol, acebutolol). This activity is claimed to contribute to the antiarrhythmic action, but appears to be significant only at high doses.

 

Lipid Insolubility   (atenolol, sotalol)

 

·      They are less likely to produce central effects.

 

·      They are incompletely absorbed orally, but do not undergo first pass metabolism and are primarily excreted unchanged in urine: are longer acting (t½ 6–20 hours) and tend to be effective in a narrow dose range. In contrast, the lipid soluble agents are primarily metabolized in liver and have shorter t½ (2–6 hours).

 

Salient features of important β blockers are given below.

 

1. Sotalol Nonselective β blocker with lower lipid solubility. It has additional K+ channel blocking and class III antiarrhythmic property.

 

SOTAGARD 40, 80 mg tab.

 

2. Timolol It is the β blocker preferred for topical use in eye (see p. 144).

Orally it is a potent β blocker—has been used in hypertension, angina and prophylaxis of myocardial infarction.

 

Betaxolol, Levobunolol, Cartiolol and Metipranolol are β blockers employed exclusively for topical application to the eye (see p. 145).

 

3. Pindolol A potent β blocker with prominent intrinsic sympathomimetic activity. It has been used primarily as antihypertensive: may be advantageous in patients who develop marked bradycardia with propranolol. Chances of rebound hypertension on withdrawal are also less. The effective dose range is rather narrow.

 

PINADOL 5 mg tab, VISKEN 10, 15 mg tab.

 

4. Metoprolol It is the prototype of cardioselective (β1) blockers; nearly 50 times higher dose is needed to block isoprenaline induced vasodilatation. It is less likely to worsen asthma, but is not entirely safe. It may be preferred in diabetics receiving insulin or oral hypoglycaemics. Patients who complain of cold hands and feet while on propranolol do better on metoprolol.

 

First pass metabolism of metoprolol is less marked than propranolol, but 90% or more is ultimately metabolized before excretion. There are slow and fast hydroxylators of metoprolol (CYP2D6 alleles); the former may require a lower dose.

 

Side effects of metoprolol are milder. It is generally given orally, but i.v. injection (5–15 mg) has been used in myocardial infarction provided bradycardia is absent.

 

BETALOC 25, 50, 100 mg tab, 5 mg/ml inj. LOPRESOR, METOLAR 50, 100 mg tab.

 

S(–) Metoprolol is the active enantiomer, now available as a single enantiomer product. It is to be used at half the dose as the recemate.

Dose: 12.5–50 mg OD–BD.

 

METPUREXL 12.5, 25, 50 mg extended release tabs.

 

5. Atenolol A relatively selective β1 blocker having low lipid solubility. It is incompletely absorbed orally, but first pass metabolism is not significant. Because of longer duration of action, once daily dose is often sufficient. Side effects related to CNS action are less likely. No deleterious effects on lipid profile have been noted. Effective dose for most individuals falls in a narrow range. It is one of the most commonly used β blockers for hypertension and angina.

 

BETACARD, ATEN, TENORMIN 25, 50, 100 mg tab.

 

S(–) Atenolol This pure active enantiomer is effective at half the dose and may be better tolerated.

 

Dose: 12.5–50 mg OD;

 

ATPURE 12.5, 25, 50 mg tabs.

 

6. Acebutolol Another cardioselective agent with significant partial agonistic and membrane stabilizing properties. Effect on resting heart rate is less. The side effect profile is like that of metoprolol. Acebutolol is rapidly metabolized to an active metabolite diacetolol which is primarily excreted by kidney and has a longer t½ (8–12 hours). As such, a single daily dose is sufficient in many patients.

 

SECTRAL 200,   400 mg       tab.,    10      mg/2  ml      amp.

 

Intravenous dose for arrhythmias 20–40 mg.

 

7. Bisoprolol A cardioselective β blocker lacking intrinsic sympathomimetic activity; suitable for once daily administration in angina, hypertension and CHF.

 

CONCOR, CORBIS 5 mg tab; ½ to 2 tab OD.

 

8. Esmolol It is an ultrashort acting β1 blocker devoid of partial agonistic or membrane stabilizing actions. It is inactivated by esterases in blood; plasma t½ is < 10 min; action disappears 15–20 min after terminating i.v. infusion—degree of β blockade can be titrated by regulating rate of infusion. Rapid onset, short lasting fall in BP attends i.v. infusion of esmolol.

 

A loading dose of 0.5 mg/kg is given followed by 0.05–0.2 mg/kg/min infusion. It has been used to terminate supraventricular tachycardia, episodic atrial fibrillation or flutter, arrhythmia during anaesthesia, to reduce HR and BP during and after cardiac surgery, and in early treatment of myocardial infarction.

 

MINIBLOCK 100 mg/10 ml, 250 mg/10 ml inj.

 

9. Celiprolol It is a selective β1 blocker having additional weak β2 agonistic activity which reduces vascular resistance and holds promise of safety in asthmatics. Nonadrenoceptor media ted vasodilatation (probably due to NO production) adds to its antihypertensive action.

 

Dose: 200–600 mg OD; CELIPRES 100, 200 mg tab.

 

10. Nebivolol This highly selective β1 blocker also acts as a NO donor, produces vasodilatation and has the potential to improve endothelial function, which may delay atherosclerosis. In contrast to older β blockers, hypotensive response to nebivolol has a rapid onset. It has been used in hypertension and CHF.

 

Dose: 5 mg (elderly 2.5 mg) OD;

 

NEBICARD 2.5, 5 mg tabs, NODON 5 mg tab.

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