Tobramycin, Moxifloxacin, Erythromycin

Antimicrobials of the Previous Segment

Topical ophthalmic antibiotics provide high bactericidal or bacteriostatic concentrations on the ocular surface and corneal stroma, allowing the effective treatment of conjunctivitis and serious bacterial ulcers.

Mechanism

💊 Commercial Names

Tobrex, Vigamox, Moxofoftal, Ilotycin Ophthalmic, Exocin, Oftacilox.

🔬 Pharmacological Group

Moxifloxacin (4th-generation fluoroquinolone), tobramycin (aminoglycoside), erythromycin (macrolide). Topical chemotherapeutic agents.

🧪 Standard Formulations

Tobramycin 0.3% (eye drops and ointment), moxifloxacin 0.5% (sterile, preservative-free eye drops), erythromycin ophthalmic ointment 0.5%.

Mechanism of Action and Antimicrobial Spectrum

Each antibiotic group blocks a key metabolic or macromolecular pathway in the bacterium:

Inhibition of DNA Gyrases by Moxifloxacin: 4th generation fluoroquinolones competitively inhibit bacterial enzymes DNA gyrase (topoisomerase II) and the Topoisomerase IV. By preventing supercoiling of bacterial DNA during cell replication, they induce irreversible fragmentation of chromosomal DNA, causing rapid cell death of the bacteria in a bactericidal manner. It has a very broad spectrum against Gram-positive (including strains of S. pneumoniae resistant to erythromycin) and Gram-negative (Pseudomonas aeruginosa), ideal for severe bacterial keratitis.
Inhibition of 30S Protein Synthesis by Tobramycin: It binds selectively and irreversibly to the bacterial 30S ribosomal subunit, causing an erroneous reading of the genetic code of the messenger RNA. This introduces incorrect amino acids into the bacteria's protein chains, destroying the functional integrity of its cell membrane in a concentration-dependent bactericidal manner. Active against Gram-negative bacteria, especially Pseudomonas aeruginosa.
Inhibition of 50S Ribosomal Translocation by Erythromycin: It binds reversibly to the 50S ribosomal subunit, inhibiting protein synthesis in a bacteriostatic manner by blocking the translocation step of the growing peptide. Used in ophthalmic prophylaxis of ophthalmia neonatorum por Neisseria gonorrhoeae y Chlamydia trachomatis.

Pharmacokinetics

Quantitative Ocular and Systemic Pharmacokinetics

Parameter	Moxifloxacin 0.5% (Eye drops)	Tobramycin 0.3% (Eye drops)	Erythromycin 0.5% (Ointment)
Cornea Penetration	Excellent stromal penetration; passes through intact and inflamed corneas rapidly.	Low stromal penetration in intact corneas; increases in epithelial defects.	Action strictly localized on the conjunctival external ocular surface.
C_max in Aqueous Humor	1.8 - 2.5 µg/mL (enough to exceed the MIC90 of pathogens).	<1.0 µg/mL on average.	No intraocular penetration.
Plasma Bioavailability	Negligible; does not associate relevant systemic effects.	No free systemic absorption at conjunctival therapeutic doses.	No absorption; prolonged eyelid tissue retention due to ointment.
Half-life in Previous Segment	t_1/2 ≈ 2 - 3 hours for normal flow of aqueous humor.	t_1/2 elimination ≈ 1.5 - 2 h from the conjunctival surface.	Long residence time thanks to the vehicle fatty.

Security

Aminoglycoside Epitheliotoxicity: The Danger of Tobramycin Abuse

Topical ophthalmic aminoglycosides (especially tobramycin and neomycin) are highly hydrophilic and insoluble in the lipids of corneal epithelial junctions. However, its uninterrupted application for more than 7 to 10 days or dosing at very short intervals can induce severe direct chemical corneal epitheliotoxicity. The drug accumulated in the tear damages the microvilli of the glycocalyx of the basal corneal epithelium, causing a confluent superficial punctate keratopathy that mimics an active bacterial keratitis. The inexperienced clinician often mistakenly interprets this as ineffectiveness of the antibiotic, increasing the dose and perpetuating the corneal damage in an iatrogenous manner.

Contraindications

Absolute: Documented hypersensitivity to aminoglycosides (tobramycin), quinolones (moxifloxacin) or macrolides (erythromycin), respectively.
Relative: Prolonged prophylactic use without infectious indication (risk of induction of chromosomal bacterial resistance and superinfection by opportunistic fungi).

Adverse Effects (ADR)

Eye Locals: Irritation, burning and stinging immediately after instillation; conjunctival erythema; allergic contact blepharoconjunctivitis; toxic keratopathy; moderate delay in corneal healing (especially with ointments due to physical mechanical barrier).
Systemic: Scarce. Very rare generalized hypersensitivity reactions.

Epistemis is educational review material. It is not a medical device, does not diagnose or prescribe treatment, and does not replace formal medical training, current clinical guidelines, or professional clinical judgment.

PreviousCiclosporin A, Lifitegrast NextGanciclovir, Acyclovir

System: Ophthalmology
Group: Antimicrobials of the Previous Segment

Download Epistemis