INTRAOCULAR PENETRATION OF GENTAMICIN AFTER ONCE-DAILY AMINOGLYCOSIDEDOSING

Background: Intraocular concentrations-particularly intravitreal conce ntrations-after systemic administration of gentamicin are poor. Once-d aily aminoglycoside dosing of intravenous gentamicin achieves peak ser um levels up to five times higher than conventional dosing. Whether th ese increased serum levels of gentamicin improve the aqueous or vitreo us concentrations in humans has not been determined. The authors sough t to determine if the intraocular penetration of gentamicin would be i mproved using this method. Methods: Patients undergoing vitrectomy pro cedures were administered intravenous gentamicin in a dose of 7 mg/kg approximately 1 hour before surgery. An adjustment in dosing was made for anyone more than 20% over his or her ideal body weight. Aqueous, v itreous, and serum samples were collected before any intraocular surgi cal manipulation. The samples were analyzed by fluorescence polarizati on immunoassay (TDx system). Results: The average single gentamicin do se was 498 mg (range, 360-700 mg). The aqueous, vitreous, and serum le vels averaged 1.14 mu g/mL, 0.41 mu g/mL, and 22.07 mu g/mL, respectiv ely. No correlation between serum level concentrations and time of adm inistration was found for the aqueous and vitreous levels in this stud y. Conclusion: Although the average peak serum level of gentamicin was five times higher than previously reported, the vitreous levels avera ged only 1.5 times higher. The blood-retinal barrier is difficult to p enetrate even when higher serum levels are achieved. Due to its poor o cular penetration, gentamicin may not be among the best drugs for prop hylaxis of penetrating eye injuries, surgical prophylaxis, or treatmen t of endophthalmitis.