Clarithromycin

BRAND NAMES

Common brand names for Clarithromycin include:

• Biaxin (widely used in the United States)
• Klacid (common in Europe and many other countries)
• Clarithro (used in some generic markets)
• Crixan (available in select regions)
• Fromilid (used in parts of Europe)
• Klaricid (another international brand variant)

MECHANISM OF ACTION

Clarithromycin is a macrolide antibiotic that works by inhibiting bacterial protein synthesis. It binds to the 50S subunit of the bacterial ribosome, specifically the 23S rRNA, and blocks the translocation step, preventing the movement of tRNA during protein elongation. This stops bacteria from producing essential proteins, leading mainly to a bacteriostatic effect, although it can be bactericidal at higher concentrations against certain organisms.

PHARMACOKINETICS

Absorption

Clarithromycin is rapidly absorbed from the gastrointestinal tract after oral administration, with an absolute bioavailability of about 50–55% due to first-pass metabolism. It is partially metabolized in the liver to an active metabolite, 14-hydroxyclarithromycin, which also contributes to its antibacterial activity.

Distribution

Clarithromycin has a large volume of distribution (~2.5–3.5 L/kg), showing extensive tissue penetration and good intracellular accumulation, especially in respiratory tissues and macrophages.

Metabolism

Clarithromycin is extensively metabolized in the liver mainly by the CYP3A4 enzyme. It is converted into an active metabolite, 14-hydroxyclarithromycin, which contributes to its antibacterial activity. The drug can also inhibit CYP3A4, leading to important drug–drug interactions.

Elimination

Clarithromycin is eliminated mainly through hepatic metabolism and renal excretion. About 20–40% is excreted unchanged in urine, while the rest is eliminated as metabolites in both urine and feces. The elimination half-life is approximately 3–7 hours, but it can be prolonged in renal impairment.

PHARMACODYNAMICS

Clarithromycin is a macrolide antibiotic with time-dependent bacteriostatic activity. It inhibits bacterial protein synthesis by binding to the 50S ribosomal subunit (23S rRNA), blocking translocation and preventing peptide chain elongation. Its effectiveness depends on the time the drug concentration remains above the MIC (T>MIC). It also has a post-antibiotic effect and shows good intracellular activity, making it effective against atypical and intracellular pathogens.

ADMINISTRATION

Clarithromycin is administered orally as tablets, extended-release tablets, or oral suspension, usually twice daily (immediate-release) or once daily (extended-release). It can be taken with or without food, though food may reduce gastric discomfort. In some severe infections, it may also be given intravenously where available.

DOSAGE AND STRENGTH

Clarithromycin is commonly available in strengths of 250 mg and 500 mg tablets, as well as 125 mg/5 mL and 250 mg/5 mL oral suspensions. The usual adult dose is 250–500 mg twice daily, while extended-release formulations are typically 1000 mg once daily. Dosage varies depending on the type and severity of infection and patient condition.

DRUG INTERACTIONS

Clarithromycin is a CYP3A4 inhibitor, so it has many important drug interactions. It can increase levels of drugs like warfarin, statins (simvastatin, atorvastatin), carbamazepine, and benzodiazepines, raising toxicity risk. It should not be combined with drugs that cause QT prolongation (e.g., antiarrhythmics like amiodarone) due to risk of arrhythmias. It also interacts with colchicine and theophylline, requiring dose adjustment or avoidance.

FOOD INTERACTIONS

Clarithromycin has minimal clinically significant food interactions. Food may slightly delay its absorption, especially with immediate-release forms, but does not affect overall bioavailability. It can be taken with or without food; however, taking it with food may help reduce gastrointestinal side effects like nausea and stomach upset.

CONTRAINDICATIONS

 It should not be used with drugs that strongly depend on CYP3A4 metabolism and can cause serious toxicity when combined, such as cisapride, pimozide, and certain ergot alkaloids, due to risk of life-threatening cardiac arrhythmias or toxicity.

SIDE EFFECTS

• Nausea, vomiting, diarrhea

• Abdominal pain

• Metallic taste (dysgeusia)

• Headache

• Liver enzyme elevation

• Allergic reactions (rash, itching)

• QT prolongation → risk of arrhythmias (rare)

OVER DOSE

• Severe nausea, vomiting, abdominal pain

• Diarrhea and dehydration

• Hepatic dysfunction (elevated liver enzymes)

TOXICITY

Clarithromycin toxicity can cause nausea, vomiting, and diarrhea, along with liver injury and elevated liver enzymes. It may also lead to QT prolongation with risk of arrhythmias. Rare effects include reversible hearing loss, dizziness, and confusion.