NAPLEX Study Guide Pharmacology

August 1st, 2019

Drug Profile – Beta-2 Agonists

beta 2 agonists pharmacology

Introduction

When it comes to your pharmacy board exam, it’s imperative that you review all major drug classes. Here, we review the fundamental clinical details about beta-2 agonists that you need to know.

Beta-2 agonists are a widely employed drug class used to treat asthma, COPD and hyperkalemia. Though they are used to treat asthma, some of these drugs carry significant risks, including an elevated risk of asthma-related deaths. We learn more about these risks below.

Here, we talk more about these clinical factors and how best these risks can be averted – indications, pharmacology, mechanism, drug classification, side effects and clinical pharmacology.

Indications

Beta-2 agonists are used in the treatment of the following indications:

  • Asthma – short-acting drugs are used to relieve breathless. For example, they are widely used for exercise-induced bronchospasm. Long-acting beta-2 agonists are used in later stages of chronic obstructive pulmonary disease (COPD), usually alongside an inhaled corticosteroid (see below).
  • COPD – again, short-acting beta-2 agonists are effective for symptom relief, such as breathlessness. Long-acting agents are also used to treat COPD, often as second-line therapy.
  • Hyperkalemia – nebulized albuterol may be used to treat urgent hyperkalemia until a more long-term therapeutic solution is determined. It may be used alongside other drugs such as insulin, glucose and calcium gluconate.

Beta-2 agonists may also be used for purposes not listed in this guide.

Mechanism of action

Beta-2 receptors are found in the smooth muscle of the respiratory tract (bronchi), uterus, blood vessels and gastrointestinal tract.

Beta-2 receptors are G-protein coupled receptors which, when stimulated, lead to a signalling cascade that eventuates in smooth muscle relaxation – enhancing the passage of air through constricted respiratory passages and alleviating symptoms of breathlessness.

As we have learned, beta-2 agonists may also be used to treat hyperkalemia.

  • In hyperkalemia, beta-2 agonists cause a shift of potassium ions from the extracellular compartment to the intracellular compartment. This rapidly reduces potassium levels, hence why beta-2 agonists are used for emergency treatment whilst more long-term therapeutic solutions are found. Beta-2 agonists are also effective when other administrative routes, such as IV route access, becomes challenging.

For hyperkalemia, they are not used in isolation but are, instead, used alongside other agents such as insulin, glucose and calcium gluconate.

Beta-2 Agonist Classification

Beta-2 agonists are divided into three classes, depending on their duration of effect:

  • Short-acting
    • Albuterol (Ventolin)
    • Levalbuterol (Xopenex)
    • Terbutaline (Bricanyl)
  • Long-acting
    • Salmeterol (Serevent)
    • Formoterol (Foradil)

There are also ultra-long-acting drugs:

  • Indacaterol (Arcapta Neohaler)
  • Olodaterol (Striverdi Respimat)
  • Vilanterol
    • With umeclidinium bromide (Anoro Ellipta)
    • With fluticasone furoate (Breo Ellipta)

Of course, combination treatments are not limited to the above. Long-term beta-2 agonists – such as salmeterol – are often combined with an inhaled corticosteroid, such as fluticasone. Such a combination is available as the medicine Advair.

Inhaled short-acting drugs are prescribed for ‘as needed’ administration. Long-acting drugs are used for maintenance therapy and are therefore prescribed as a schedule (for example – twice daily etc.)

Side effects

Side effects associated with beta-2 agonists include:

  • Tremors
  • Tachycardia
  • Palpitations
  • Anxiety
  • Agitation
  • Sweating
  • Insomnia

Long-acting drugs are associated with muscle cramps.

Glucose levels may rise because they also stimulate glycogenolysis. Lactate levels may rise when taken in high doses.

Clinical factors

Here are some of the essential factors to consider when prescribing beta-2 agonists:

  • Long-acting beta-2 agonists should not be taken alone but must, instead, be taken alongside an inhaled corticosteroid. Taking long-acting beta-2 agonists without corticosteroids increases risk of asthma-related deaths.
  • Potassium levels should be monitored if high-dose nebulised beta-2 agonists are taken alongside theophylline and corticosteroids. This combination can lead to pronounced hypokalemia.
  • Beta-2 agonists are administered in aerosol form (metered-dose inhaler – MDI) or in dry powder form. In addition, solution-based formulations are also available from which nebulization is achieved. Nebulised form is used more than inhalers for emergency treatment.
  • Albuterol and terbutaline are also available in IV forms, but these forms have only mostly been used for tocolytic purposes – to suppress premature labor.
  • Inhaled short-acting drugs are prescribed for ‘as required’
  • In asthma and COPD exacerbations, nebulised forms are typically used.
  • Long-acting drugs are prescribed for maintenance therapy.
  • Patients should be trained how to use their inhaler / technique, at each consultation. Patients are often advised to hold their breath for at least 10 seconds to ensure the drug permeates and reaches the target respiratory site.
  • Patients with asthma may monitor disease severity via serial measurements of peak expiratory flow rate. Treatment may be adjusted with guidance from their healthcare provider / action plan.
  • Nebulisers can be driven by oxygen or air. Typically, oxygen is used in asthma whereas medical air is used in COPD (due to the risk of carbon dioxide retention).
  • Recall that they cause tachycardia and so caution is warranted in patients with established cardiovascular disease. This risk is augmented in patients with both CVD and hyperkalemia who may receive high doses of beta-2 agonists.

This drug class remains one of the most important, and most effective, drug classes in the treatment of asthma and COPD. You should expect to receive NAPLEX questions on this subject. For your NAPLEX exam, or other pharmacy board exam, you should have an intimate knowledge of the details mined in this article.

Register with NAPLEX Study Guide today to receive instant access to hundreds of exam-like questions and every other class used in to treat respiratory diseases. Check back to our NAPLEX blog soon for even more great tips, tricks and tutorials to help you become a professional pharmacist.

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