💊 Macrolides

Macrolides are a class of bacteriostatic antibiotics characterised by a large macrocyclic lactone ring. They are effective against a range of Gram-positive bacteria, as well as atypical pathogens.

Macrolides inhibit bacterial protein synthesis by reversibly binding to the 50S ribosomal subunit. This prevents translocation of the peptidyl-tRNA, thereby blocking the elongation of the polypeptide chain.

They are commonly used for respiratory tract infections, including community-acquired pneumonia (especially atypical pathogens like Mycoplasma and Chlamydia). They are also indicated for skin and soft tissue infections, sexually transmitted infections (e.g., chlamydia), and as an alternative for penicillin-allergic patients.

Known hypersensitivity to macrolides is an absolute contraindication. They are contraindicated in patients with a history of QT prolongation or those taking other drugs known to prolong the QT interval (e.g., certain antiarrhythmics, antipsychotics). Severe hepatic impairment requires caution.

Common side effects include gastrointestinal disturbances (nausea, vomiting, abdominal pain, diarrhoea), which can be dose-limiting. Less common but serious effects include QT prolongation and associated arrhythmias (e.g., Torsades de Pointes), cholestatic hepatitis, and allergic reactions.

Liver function tests should be monitored in patients with pre-existing hepatic impairment or during prolonged treatment. ECG monitoring may be considered in patients at risk of QT prolongation or those on concomitant QT-prolonging drugs. Clinical response and resolution of symptoms should be assessed.

Always check for a history of QT prolongation or concomitant medications that prolong the QT interval (e.g., amiodarone, sotalol, some antipsychotics). Counsel patients on potential GI upset and to report any palpitations or fainting. Be aware of significant interactions with statins (risk of rhabdomyolysis) and warfarin (increased INR).

Erythromycin, clarithromycin, and azithromycin are common macrolides. Azithromycin has a longer half-life allowing for shorter courses. They are excellent for atypical respiratory pathogens. Key adverse effects are GI upset and QT prolongation. Significant CYP3A4 inhibition is a major drug interaction point.

SBA questions frequently test knowledge of macrolides' role in atypical pneumonia. Another common theme is drug interactions, particularly with drugs metabolised by CYP3A4, leading to increased levels of the co-administered drug. QT prolongation and its implications are also high-yield.