Tetracycline: Action as a broad-spectrum antibiotic

Tetracycline is the classical parent compound of an entire antibiotic class that was introduced into therapy in the 1940s and 1950s. Modern representatives such as doxycycline, minocycline, and tigecycline evolved from tetracycline and dominate clinical practice today. In Germany, tetracycline as a substance remains available, but its clinical significance has diminished in favor of better-tolerated successor preparations. Tetracycline continues to be used in ophthalmic ointments (Mydocalm ophthalmic ointment and generics) as well as in selected dermatological and ophthalmological indications.

In everyday clinical practice, tetracycline is used as a broad-spectrum antibiotic against gram-positive and gram-negative pathogens as well as against some intracellular organisms such as Chlamydia, Mycoplasma, and Rickettsia. The resistance problem is significant because tetracycline has selected for resistance over decades. Careful indication assessment with organism identification and antibiogram is now standard. For many infections, doxycycline is the first choice due to better pharmacokinetics and fewer side effects.

Mechanism of Action

Tetracyclines bind reversibly to the 30S subunit of bacterial ribosomes and block the attachment of aminoacyl tRNA to the A site. This results in inhibition of bacterial protein synthesis. Clinically, tetracyclines act bacteriostatically. Because of their wide distribution in tissues, including intracellular spaces, they are particularly suitable for pathogens that multiply within cells.

The spectrum of activity includes streptococci, staphylococci (with increasing resistance), Haemophilus influenzae, Chlamydia trachomatis and pneumoniae, Mycoplasma pneumoniae, Rickettsia, Borrelia burgdorferi (with doxycycline as first-line), Treponema pallidum in penicillin allergy, Brucella, Yersinia, some anaerobes, Vibrio cholerae, and Helicobacter pylori in combination therapies. Pseudomonas aeruginosa and many enterobacteria are frequently resistant.

The oral bioavailability of tetracycline is 60 to 80 percent but is significantly reduced by food containing di- or trivalent cations (calcium in dairy products, iron, magnesium, aluminum). The half-life is approximately 8 to 11 hours. Tetracycline is predominantly excreted renally and bilially. With impaired renal function, it accumulates, which is why doxycycline is preferred as an alternative.

Indications

• Acne vulgaris of moderate severity, orally as long-term therapy supplementing topical agents such as adapalene or benzoyl peroxide
• Perioral dermatitis and rosacea, orally and topically in specific indications
• Atypical pneumonia caused by Mycoplasma or Chlamydia
• Bullous pemphigoid, orally in combination with nicotinamide
• Ophthalmic ointment for bacterial conjunctivitis or trachoma
• Cholera, brucellosis, tularemia, Q fever, rickettsial infections, in specialized settings

For most classic tetracycline indications, doxycycline is now preferred because it has a more favorable half-life, better tolerability, and less influence from food. Self-medication is not available.

Dosage and Administration

Adults orally: 250 to 500 mg four times daily, depending on the indication. Acne indication often 500 mg twice daily initially, later reduction.

Ophthalmic ointment: apply one to two times daily to the lower conjunctival sac, treatment duration typically 5 to 7 days.

Pediatric: Use in children under 8 years is generally contraindicated due to tooth and bone damage, but possible in life-threatening indications with careful consideration.

Administration: on an empty stomach, at least one hour before or two hours after a meal, with plenty of water. Maintain upright position and do not take directly before bedtime, as risk of esophageal erosions exists. Maintain at least two hours distance from dairy products, antacids, iron preparations, calcium, or magnesium-containing dietary supplements.

Renal insufficiency: dosage adjustment required if eGFR is below 60 ml per minute, if eGFR is below 30, tetracycline should be avoided due to risk of accumulation. Doxycycline is a safer alternative here.

Hepatic insufficiency: use with caution in severe functional impairment.

Side Effects

Very common: nausea, vomiting, diarrhea, abdominal pain, glossitis, stomatitis.

Common: photosensitivity with sunburn-like reactions, allergic skin reactions, vaginal mycosis, esophageal irritation to ulcers, increased liver transaminases.

Occasional: pseudotumor cerebri (idiopathic intracranial hypertension) with headache and visual disturbances, transient renal dysfunction with increased urea and creatinine (antianabolism).

In children and pregnancy: incorporation into teeth and bones, tooth discoloration, transient growth disturbance. Contraindicated for this reason.

Rare: pancreatitis, hemolytic anemia, Stevens Johnson syndrome, DRESS syndrome, Clostridioides difficile associated diarrhea.

Jarisch Herxheimer reaction: when treating spirochete-induced infections such as Lyme disease or syphilis, a systemic reaction with fever, chills, and headache may occur after therapy initiation. It is usually self-limiting.

Drug Interactions

• Calcium, magnesium, aluminum, iron, zinc: formation of insoluble complexes, significant reduction of tetracycline absorption, maintain minimum distance of two hours.
• Dairy products: also reduced absorption.
• Antacids and sucralfate: relevant binding, maintain minimum distance.
• Bismuth preparations: component of quadruple therapy in Helicobacter pylori eradication, binding can influence effect.
• Vitamin K antagonists (warfarin, phenprocoumon): increased anticoagulation, INR monitoring.
• Hormonal contraceptives: potentially reduced efficacy with recurrent diarrhea, clinical significance controversial, individual counseling.
• Methotrexate: possible alteration of MTX toxicity with high-dose administration.
• Penicillins: theoretical antagonism, clinical relevance controversial, in practice avoid combination with clear indication.
• Retinoids (isotretinoin, acitretin): increased risk for pseudotumor cerebri, avoid combination.

Special Precautions

Pregnancy: contraindicated from the second trimester because tetracyclines are incorporated into fetal teeth and bones and cause discoloration, hypoplasia, and growth disturbances. Breastfeeding: passes into breast milk, binding to calcium reduces bioavailability in the infant, nonetheless use should be avoided if possible.

Children under 8 years: contraindicated due to tooth and bone effects, except in life-threatening situations without alternatives.

Elderly patients: increased risk for esophageal irritation and renal impairment, prefer doxycycline if eGFR is below 30.

Before therapy initiation: history of allergies, previous antibiotics, pregnancy test in women of childbearing age, liver and kidney values. Counseling on sun exposure and administration rules.

Sun protection: during therapy and for several days afterward, avoid intense sun exposure, use sun protection factor 30 or higher, because severe phototoxic reactions are possible.

Esophageal protection: do not take tablets while lying down, remain upright for at least 30 minutes, adequate water. Esophageal ulcers are a typical complication with improper administration.

Antibiotic responsibility: no antibiotic without clear indication. For acne, topical therapy and lifestyle measures are the basis, oral antibiotics a time-limited option.

Driving ability: generally maintained, use caution if dizziness or visual disturbances occur.

You may also be interested in

• Doxycycline, modern tetracycline with better pharmacokinetics
• Minocycline, another tetracycline in dermatological use
• Clarithromycin, macrolide for atypical pneumonia
• Erythromycin, classic macrolide
• Adapalene, topical retinoid in acne therapy

Frequently Asked Questions

Sources

• Gelbe Liste, tetracycline active ingredient profile
• BfArM, Federal Institute for Drugs and Medical Devices
• AWMF, guidelines on antibiotic therapy and acne
• Robert Koch Institute, antibacterial resistance surveillance