Intraocular penetration of gentamicin after once-daily aminoglycoside dosing

Intraocular concentrations-particularly intravitreal concentrations-after systemic administration of gentamicin are poor. Once-daily aminoglycoside dosing of intravenous gentamicin achieves peak serum levels up to five times higher than conventional dosing. Whether these increased serum levels of gentamicin improve the aqueous or vitreous concentrations in humans has not been determined. The authors sought to determine if the intraocular penetration of gentamicin would be improved using this method.

Patients undergoing vitrectomy procedures 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, vitreous, and serum samples were collected before any intraocular surgical manipulation. The samples were analyzed by fluorescence polarization immunoassay (TDx system).

The average single gentamicin dose was 498 mg (range, 360-700 mg). The aqueous, vitreous, and serum levels averaged 1.14 microg/mL, 0.41 microg/mL, and 22.07 microg/mL, respectively. No correlation between serum level concentrations and time of administration was found for the aqueous and vitreous levels in this study.

Although the average peak serum level of gentamicin was five times higher than previously reported, the vitreous levels averaged only 1.5 times higher. The blood-retinal barrier is difficult to penetrate even when higher serum levels are achieved. Due to its poor ocular penetration, gentamicin may not be among the best drugs for prophylaxis of penetrating eye injuries, surgical prophylaxis, or treatment of endophthalmitis.