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

Moxifloxacino (fluoroquinolona de 4.ª generación), Tobramicina (aminoglucósido), Eritromicina (macrólido). Quimioterápicos tópicos.

🧪 Standard Presentations

Tobramicina 0.3% (colirio y ungüento), Moxifloxacino 0.5% (colirio estéril libre de conservantes), Eritromicina ungüento oftálmico 0.5%.

Mecanismo de Acción y Espectro Antimicrobiano

Cada grupo de antibióticos bloquea una vía metabólica o macromolecular clave en la bacteria:

Inhibición de DNA-Girasas por el Moxifloxacino: Las fluoroquinolonas de 4.ª generación inhiben competitivamente las enzimas bacterianas DNA girasa (Topoisomerasa II) and the Topoisomerase IV. Al impedir el superenrollamiento del DNA bacteriano dural durante la replicación celular, inducen la fragmentación irreversible del DNA cromosómico, provocando la muerte celular rápida de la bacteria de forma bactericida. Presenta un espectro muy amplio contra Gram-positivos (incluyendo cepas de S. pneumoniae resistentes a eritromicina) y Gram-negativos (Pseudomonas aeruginosa), idóneo para queratitis bacterianas graves.
Inhibición de la Síntesis Proteica 30S por la Tobramicina: Se une de forma selectiva e irreversible a la subunidad ribosómica 30S bacteriana, provocando una lectura errónea del código genético del RNA mensajero. Esto introduce aminoácidos incorrectos en las cadenas proteicas de la bacteria, destruyendo la integridad funcional de su membrana celular de forma bactericida dependiente de concentración. Activa contra bacterias Gram-negativas, especialmente Pseudomonas aeruginosa.
Inhibición de la Translocación Ribosómica 50S por la Eritromicina: Se une de forma reversible a la subunidad ribosómica 50S, inhibiendo la síntesis proteica de forma bacteriostática al bloquear el paso de translocación del péptido en crecimiento. Utilizada en la profilaxis oftálmica de la oftalmía neonatal 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.

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System: Ophthalmology
Cluster: Antimicrobials of the Previous Segment

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