Genacyn

Approved indications

• Serious gram-negative bacterial infections: Pseudomonas aeruginosa, Escherichia coli, Klebsiella spp., Enterobacter spp., Proteus spp., and Serratia spp.
• Gram-positive infections (limited use): Staphylococcus aureus (including penicillin-resistant strains), typically in combination therapy.
• Severe systemic infections: Septicemia, complicated urinary tract infections, intra-abdominal infections, bone and joint infections.
• Respiratory tract infections: Nosocomial pneumonia, especially in ventilated patients.
• Skin and soft tissue infections: Severe cellulitis, burns with bacterial colonization.
• Ophthalmic infections: Conjunctivitis, keratitis, corneal ulcers (topical formulations).
• Otic infections: Otitis externa, in combination with corticosteroids for inflammatory bacterial ear conditions.

Off-label / clinically accepted uses

• Endocarditis (in combination with other antibiotics).
• Meningitis (with other antibiotics, due to poor CNS penetration of gentamicin alone).
• Neonatal sepsis.

Adults (parenteral)

• IV/IM: 3–5 mg/kg/day, divided into 2–3 doses (usual total 5 mg/kg/day).
• Severe infections: 5–7 mg/kg/day, adjusted based on renal function and serum drug levels.

Once-daily dosing (extended interval)

• 5–7 mg/kg IV/IM once daily is commonly used in adults with normal renal function.

Pediatrics

• Neonates: 2.5 mg/kg IV/IM every 12–24 hours (dose interval depends on gestational and postnatal age).
• Infants & children: 2.5 mg/kg IV/IM every 8 hours; once-daily dosing may be used with monitoring.

Elderly

• Start with standard adult dose; adjust based on renal function.

Renal impairment

• Dose adjustment based on creatinine clearance; extend dosing interval rather than reducing single dose.
• Serum gentamicin levels must be monitored.

Topical administration (ophthalmic / otic)

• Instill 1–2 drops in the affected eye 2–4 times daily.
• For otic use, instill 3–4 drops 2–3 times daily into the affected ear.

Duration

• Typically 7–10 days for systemic infections; duration may vary depending on infection severity and site.

Gentamicin is an aminoglycoside antibiotic that binds irreversibly to the 30S ribosomal subunit of susceptible bacteria. This binding inhibits the initiation complex of protein synthesis, causes misreading of mRNA, and produces defective bacterial proteins, leading to cell membrane disruption and bacterial death. It exhibits bactericidal activity against aerobic gram-negative bacteria and some gram-positive organisms. Its action is concentration-dependent and exhibits a post-antibiotic effect, allowing bacterial suppression even after plasma levels fall below the MIC (minimum inhibitory concentration).

• Absorption: Poor oral absorption; usually administered IV, IM, or topically.
• Distribution: Widely distributed in extracellular fluid; minimal CNS penetration. High concentrations in kidney, inner ear, and urine.
• Protein binding: Approximately 30%.
• Metabolism: Minimal; gentamicin remains mostly unchanged.
• Elimination: Primarily renal via glomerular filtration; 90–95% excreted unchanged in urine.
• Half-life: 2–3 hours in adults with normal renal function; prolonged in renal impairment.
• Onset: Rapid bactericidal activity once therapeutic levels are reached.

• Pregnancy: Category D; use only if benefits outweigh risk. Can cause fetal ototoxicity with systemic use.
• Lactation: Excreted in small amounts in breast milk; generally considered safe for topical use but caution with systemic therapy.

• Primary class: Aminoglycoside antibiotic
• Subclass: Broad-spectrum, bactericidal aminoglycoside

• Known hypersensitivity to gentamicin or other aminoglycosides.
• Pre-existing severe renal impairment without appropriate monitoring.
• History of aminoglycoside-induced ototoxicity or nephrotoxicity.
• Use in pregnancy unless no safer alternatives exist.

• Ototoxicity: Risk of irreversible hearing loss, vestibular toxicity; increased with prolonged therapy, high doses, or renal impairment.
• Nephrotoxicity: Risk of acute tubular necrosis; monitor serum creatinine and urine output.
• Neuromuscular blockade: Rare, may exacerbate myasthenia gravis.
• Monitoring: Serum drug levels recommended for prolonged therapy, elderly, neonates, and renal impairment.
• High-risk groups: Elderly, neonates, patients with renal impairment.

Common

• Mild nausea, vomiting
• Local irritation (IV/IM injection)
• Rash, pruritus

Serious / rare

• Ototoxicity: tinnitus, hearing loss, vertigo
• Nephrotoxicity: proteinuria, elevated creatinine, acute renal failure
• Hypersensitivity reactions: anaphylaxis, angioedema
• Neuromuscular blockade: respiratory depression

Onset

• Systemic toxicity usually appears after days to weeks of therapy.

• Loop diuretics (e.g., furosemide): Increased risk of ototoxicity.
• Other nephrotoxic drugs (e.g., vancomycin, amphotericin B): Increased risk of renal injury.
• Muscle relaxants: Potentiation of neuromuscular blockade.
• Other antibiotics: May have additive or synergistic effects against certain pathogens.

• Once-daily dosing is now preferred for most adults due to equivalent efficacy and reduced nephrotoxicity.
• Serum drug monitoring recommended for prolonged therapy and patients with renal impairment.
• Avoid unnecessary use to reduce development of bacterial resistance.

• Store at 20°C to 25°C (68°F to 77°F).
• Protect from light and moisture.
• Keep the bottle tightly closed when not in use.
• For multi-dose vials, follow manufacturer instructions regarding stability after opening.
• Keep out of reach of children.