Antimicrobials in Acne and Rosacea: Doxycycline, Minocycline and Topical Metronidazole

Mechanism

Pharmacological Class and Group

Systemic antimicrobials of the second generation tetracycline class (Doxycycline and Minocycline) and nitroimidazoles for topical use (Metronidazole).

Mechanism of Action

Tetracyclines exert a double synergistic therapeutic effect on acne and rosacea:

• Direct Antimicrobial Effect: They diffuse through the bacterial wall of Cutibacterium acnes in the follicle. They bind reversibly to the bacterial 30S ribosomal subunit, mechanically blocking the access of the aminoacyl-tRNA to the acceptor site (A) of the ribosome, which stops bacterial protein synthesis (bacteriostatic effect).
• Sub-Antimicrobial Anti-inflammatory Effect (Fundamental in Rosacea): They inhibit the chemotaxis of neutrophils, block the expression of inducible nitric oxide synthase (iNOS), suppress the activation of the matrix metalloproteinase cascade (MMP-1, MMP-9, MMP-13) decreasing collagen tissue damage follicular, and decrease the synthesis of proinflammatory cytokines such as IL-1β, IL-6 and TNF-α.
• Topical Metronidazole: Penetrates the cell and undergoes intracellular chemical reduction by bacterial and protozoan electron transport proteins, forming active intermediates that directly damage the DNA strand. Additionally, it has local anti-inflammatory properties by capturing free oxygen radicals from the neutrophilic infiltrate of rosacea.

Anti-inflammatory Mechanism of Tetracyclines in Rosacea

In rosacea, the pathogenesis is linked to the deregulation of innate immunity, with local hyperproduction of the protein Catelicidin (LL-37) by keratinocytes after the stimulation of active serine proteases of kallikrein-5 (KLK5):

Doxycycline (Low Doses) Metalloproteinases and KLK5 ↓ Active cathelicidin LL-37 ↓ Erythema and Papules

Tetracyclines administered at low doses (sub-antimicrobial, e.g., doxycycline 40 mg/day modified release) inhibit the enzymatic activation of KLK5, decreasing the generation of the vasoactive peptide LL-37 and attenuating persistent erythema and papules without inducing intestinal bacterial resistance.

Pharmacokinetics

High Resolution Pharmacokinetics

• Absorption: Excellent oral absorption (>90-95%) for doxycycline and minocycline. Absorption is substantially reduced if they are administered with milk, dairy products or antacids containing divalent or trivalent cations (calcium, iron, aluminum, magnesium) due to the formation of stable insoluble chelates in the gastric lumen.
• Distribution and Lipophilia: Highly lipophilic. They are selectively concentrated in the sebaceous secretions and lipid-rich tissues of the pilosebaceous follicle, reaching optimal therapeutic concentrations. Minocycline significantly crosses the blood-brain barrier, which explains its neurological adverse effects.
• Metabolism and Elimination: Doxycycline is eliminated unchanged in a mixed biliary and intestinal manner (it does not require adjustment in renal failure). Minocycline is partially metabolized in the liver.
• Half-life (t_1/2): Prolonged plasma half-life of 16 to 22 hours, allowing once or twice daily dosing.

Indicators and dose

Clinical Indications and Off-Label Uses

• Moderate to Severe Inflammatory Vulgar Acne: Systemic treatment combined with topical retinoids or benzyl peroxide. *Note:* Strictly limit the use of antibiotics to a maximum period of 12 continuous weeks to avoid the induction of bacterial resistance.
• Moderate to Severe Papulopustular Rosacea: Rapid control of papular and pustular skin lesions.
• Perioral Dermatitis (off-label): Systemic treatment of choice (Doxycycline).
• Hidradenitis Suppurativa Hurley I (off-label): Initial follicular bacterial and inflammatory control.

Dosage and Clinical Adjustment

Doxycycline (Acne):

• Standard dose of 100 mg orally once daily, or 50 mg twice daily, taken with main meals (avoiding dairy).
• Rosacea: Doses of 50 mg to 100 mg per day, or modified-release doxycycline monohydrate in a sub-antimicrobial dose of 40 mg per day in the morning to reduce digestive side effects and bacterial resistance.

Minocycline (Acne):

• Dose 50 mg to 100 mg orally once daily.

Topical Metronidazole (Rosacea):

• Apply metronidazole gel or cream 0.75% or 1% in a very thin layer to the face once or twice a day after gently cleansing.

Adjustment in Renal Failure: Doxycycline does not require dose adjustment in advanced chronic renal failure, since it is excreted in an inactive form through the fecal route. Avoid the use of classic tetracycline hydrochloride due to its potential general catabolic effect.

Security

Absolute and Relative Contraindications

Absolute Contraindications

• Known hypersensitivity to tetracyclines or nitroimidazoles.
• Pregnancy, lactation and pediatrics (under 8 years of age): Tetracyclines avidly bind to calcium in bone tissue, causing permanent discoloration and yellowish staining of fetal and infant teeth, as well as hypoplasia of dental enamel and temporary delay in the growth of long bones.
• Concomitant use with systemic retinoids (Isotretinoin).

Relative Contraindications

• Intense direct exposure to solar UV radiation.
• Previous inner ear vestibular dysfunction (for minocycline).

Adverse Effects (ADR) and Specific Toxicity

• Frequent (1%-10%): Gastrointestinal disorders (nausea, epigastric burning, diarrhea due to intestinal dysbiosis), ulcerative erosive esophagitis (due to physical impaction of the acid doxycycline capsule on the esophageal mucosa, avoidable by ingesting with plenty of water), candidal vaginitis due to yeast overgrowth.
• Uncommon (0.1%-1%): Phototoxic skin photosensitivity (rapid bullous erythema on areas exposed to the sun after minimal periods of radiation), symptoms of vertigo, dizziness of vestibular origin and ataxia (common with the use of minocycline due to penetration of the drug into the labyrinth of the inner ear).
• Rare (<0.1%): Irreversible bluish-gray mucocutaneous hyperpigmentation in acne scars, pimples or gums (with prolonged use of minocycline due to deposits of insoluble ferro-melanic complexes), Minocycline-induced lupus (with positive ANCA antibodies), Stevens-Johnson syndrome.

Alert on Good Taking Practices for Doxycycline

Doxycycline in hydrochloride form is highly acidic and corrosive to the mucosa of the esophagus if the tablet dissolves incompletely before reaching the stomach. To prevent the appearance of retrosternal pain or tightness due to ulcerative esophagitis due to pills, the patient must receive strict instructions for taking:

Intake of Doxycycline Accompany with a large glass of water (250 mL) Do not lie down for 30-60 minutes

The tablet should be ingested standing or sitting, with a full large glass of plain water, and avoid lying in bed or lying on the couch for the next 30 to 60 continuous minutes to ensure complete gastric transit by gravity.

Drug Interactions of Clinical Relevance

• Iron, Calcium, Antacids and Dairy: They chelate tetracycline molecules and prevent their absorption in the duodenum. Its intake should be physically separated at least 2 hours before or 4 hours after the administration of the antibiotic.
• Oral Contraceptives: A potential reduction in estrogenic enterohepatic circulation due to dysbiosis of the intestinal microflora has been theoretically described, although actual clinical data suggest minimal clinical impact. Caution is advised.

Safety in Pregnancy and Breastfeeding

• Pregnancy: Absolutely contraindicated in the second and third trimester of pregnancy due to fetal bone and dental toxicity. In the first trimester, it is strongly discouraged due to general potential risk.
• Breastfeeding: Not recommended. Although they are excreted in low quantities and are partially chelated with the calcium in breast milk, reducing their absorption by the infant, it is advisable to avoid their use to prevent any theoretical risk of skeletal alteration or neonatal intestinal dysbiosis.