Pharmacology for Technicians PTCB Test Prep

PTCB Guide to Macrolide Antibacterials.

Jul 8th, 2024
ptcb guide to macrolide antibacterials

What are macrolide drugs?

Macrolides are drugs used in the treatment of bacterial infections. They are also widely prescribed and routinely tested on the PTCB exam. Here, we review the core details you need to know about these important medicines.

As with other antibacterial drug classes, pharmacy technicians are only expected to know the broad outline of the drug class. In this case, it means knowing:

  • Major indications
  • Side effects
  • Mechanism of action
  • Any significant drug or food interactions.
  • You should also be able to identify prominent brand names.

Note that you will not be expected to memorize the entire list of indications that macrolides are used to treat. What you need to know is that macrolides are used to treat bacterial infections. Not viral or protozoal or fungal infections, but bacterial infections.

That said, for the purpose of context it’s nonetheless worth listing what kind of infections these drugs are used to treat.

  • Respiratory infections
  • Skin and soft tissue infections
  • Severe pneumonia – often in combination with a penicillin.
  • Eradication of Helicobacter pylori infection – a bacteria implicated in peptic ulcer disease. A macrolide is typically used alongside a proton pump inhibitor and either amoxicillin or metronidazole. This is known as “triple therapy” in the treatment of this common infection.

With these indications in mind, let’s learn more about the different types of macrolide drug you need to know, as well as their most widely dispensed medicine brand names.

Macrolides to know for the PTCB Exam.

There are three key FDA-approved macrolide antibacterial drugs.

ErythromycinE.E.S 400; and E.E.S 200

Note that all macrolide drugs end in the suffix –mycin, though this is of relatively limited utility because many other antibacterial drugs carry precisely the same suffix. That said, knowing this detail can be useful in eliminating incorrect answers on a given PTCB question. Think of the mnemonic ACE to help you remember mACrolidEs – where;

  • A = Azithromycin
  • C = Clarithromycin
  • E = Erythromycin

Erythromycin is an older macrolide and carries a higher risk of gastrointestinal side effects, such as nausea and vomiting (that’s because it also works as a motilin agonist – though you don’t need to know this for the PTCB test!). Clarithromycin is by far the most prescribed macrolide. It is far more stable than erythromycin and has fewer adverse effects.

Azithromycin is unique in that, unlike the other two macrolides, it does not extensively inhibit CYP 3A4. In other words, it means that azithromycin has fewer drug-drug interactions compared to erythromycin or clarithromycin.

How do Macrolides work?

Put simply, macrolide antibacterials work as protein synthesis inhibitors.

That’s the fundamental detail you need to know. Pharmacy technicians are not expected to understand the specific nature by which this protein synthesis inhibition is achieved.

You should know that bacteria produce proteins inside organelles called ribosomes; and this is done for a range of essential internal processes that keep the bacteria functioning in the way that it does. Interfering with protein production inside ribosomes disrupts these essential processes, leading to a cessation or slowing down of bacterial growth (bacteriostatic as opposed to bactericidal).

This then allows the body to eliminate any remaining bacteria.

As we learned, erythromycin was the first macrolide. It has a broad spectrum of activity against Gram-positive and Gram-negative bacteria. Later macrolides – such as azithromycin and clarithromycin – have more enhanced activity against Gram-negative organisms.

Quick notebacteria are typically divided into Gram-positive bacteria and Gram-negative bacteria, depending on the thickness of the bacterial cell wall. The “Gram-positive” simply refers to a positive laboratory test result (i.e. Gram-positive organisms have the thicker cell wall compared to Gram-negative organisms). Antibacterial drugs work differently depending on whether bacteria have a thick or thin cell wall; where they might be effective against only one type, or possibly both types.

Side Effects and Drug Interactions.

Most side effects of macrolides are GI-related, and include:

  • Nausea
  • Vomiting
  • Abdominal pain
  • Diarrhea

Where macrolides are given intravenously, thrombophlebitis may occur.

As we learned earlier, erythromycin is associated with a higher incidence of GI-related side effects than either clarithromycin or azithromycin.

Other adverse effects of macrolides include:

  • Ototoxicity – toxicity to the ear; often at higher doses.
  • QT prolongation – meaning an increased risk of cardiac arrythmias.
  • Antibiotic-associated colitis – inflammation of the inner lining of the colon.

Earlier, we briefly talked about the CYP 3A4 enzyme. Recall that this enzyme is responsible for most drug-drug interactions. Clarithromycin and erythromycin extensively inhibit the enzyme, leading to a wide range of possible interactions with other drugs.

You do not need to know all drug interactions but you should know that both drugs extensively inhibit the CYP3A4 enzyme, whereas azithromycin does not.

Let’s take two clinical applications of this theory:

  • Warfarin is metabolized by the CYP 3A4 enzyme. Clarithromycin inhibits this enzyme. This means that taking the two drugs together increases the risk of bleeding with warfarin.
  • Statins are also metabolized by the CYP 3A4 enzyme. Taking erythromycin (or clarithromycin) with statins therefore increases the risk of adverse effects of statins, including myopathies (disease of muscle fibers).

However, these two drug interactions would not occur with azithromycin precisely because it does not extensively inhibit the CYP 3A4 enzyme.

PTCB Exam Questions.

That concludes our PTCB guide to macrolide antibacterials.

Below, we have put together some sample questions for you to practice. Don’t worry if you don’t get all answers correct. This is about slowly building your knowledge to a point where you do. Answer explanations are provided later in this study guide.

Q1. Which of these medicines contains the active ingredient, clarithromycin?

a. E.E.S 400

b. Zithromax

c. Biaxin

d. Vibramycin

Q2. What is the mechanism of action of macrolide antibacterials?

a. Protein synthesis inhibition

b. Cell wall synthesis inhibition

c. Folate inhibition

d. DNA synthesis inhibition

Q3. Macrolides are frequently used as part of triple therapy in the treatment of which condition?

a. Severe pneumonia


c. Folliculitis

d. Eradication of H. pylori infection

Q4. Which macrolide antibacterial does not extensively inhibit the hepatic enzyme, CYP 3A4?

a. Clarithromycin

b. Azithromycin

c. Erythromycin

d. No macrolide extensively inhibits CYP 3A4

Q5. Which macrolide is most associated with causing GI-related side effects?

a. Clarithromycin

b. Azithromycin

c. Erythromycin

d. Gentamicin

Answer Explanations.

Question 1 – Biaxin

Clarithromycin is the active ingredient of the medicine Biaxin.

Question 2 – Protein synthesis inhibition

Macrolide antibacterials work by inhibiting protein synthesis inside the ribosomes of bacterial cells. They are effective against many Gram-positive and Gram-negative organisms.

Question 3 – Eradication of H. pylori infection

H. pylori is a bacterium that can cause peptic ulcer disease in the stomach. Triple therapy is considered the most effective option and consists of treating the patient with three medicines concurrently: a macrolide, a proton-pump inhibitor, and either amoxicillin or metronidazole (metronidazole is typically used if the patient has a penicillin allergy).

Question 4 – Azithromycin

Azithromycin does not extensively inhibit the CYP 3A4 enzyme, whereas both erythromycin and clarithromycin do – and so interact with drugs such as warfarin and statins (i.e. causing adverse effects of these drugs).

Question 5 – Erythromycin

Erythromycin is the oldest and first macrolide and is associated with significantly greater GI-related side effects compared to other macrolides. That’s also because erythromycin works as a motilin agonist.

Topic Review.

Let’s take a final review through our PTCB guide to macrolide antibacterials.

Over the course of this review, we have learned:

  • Macrolides are drugs used in the treatment of bacterial infections. Indications include pneumonia, skin and soft tissue infections, and eradication of H. pylori infection.
  • Examples of macrolides include azithromycin, clarithromycin, and erythromycin (recall the mnemonic: ACE – mACrolidEs).
  • Macrolides work as protein synthesis inhibitors.
  • Erythromycin was the first macrolide. Later macrolides – such as clarithromycin and azithromycin – have greater activity against Gram-negative organisms.
  • Bacteria are divided into how thick their cell wall is. Gram-positive organisms have a thick cell wall, whereas Gram-negative organisms have a thinner cell wall.
  • Erythromycin is associated with a higher incidence of GI-related side effects (because it is a motilin agonist).
  • At high doses, macrolides can cause ototoxicity and QT prolongation.
  • Both clarithromycin and erythromycin extensively inhibit the hepatic enzyme, CYP 3A4 – whereas azithromycin does not. This means that the former two drugs increase the risk of adverse effects with drugs metabolized by CYP 3A4, such as statins and warfarin.

We hope you found this PTCB study of macrolides helpful! Check back to our PTCB blog soon for more exclusive content to help you study and prepare for your pharmacy technician examination.

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PTCB Test Prep Author


Elaine Walker

Elaine joined PTCB Test Prep in 2017, currently serving as the lead product development manager overseeing both course development and quality improvement. Mrs. Walker is a graduate of California State University and has worked as a pharmacy technician for over twenty years – with particular interests in pediatric pharmacy, extemporaneous compounding, and hospital pharmacy. Over the past 8-years, she has helped prepare thousands of students for the PTCB examination.