Half-life of Intravitreal Vancomycin in Humans

Abstract

Abstract: : Purpose: To calculate the half-life of intravitreal vancomycin in infected human eyes. Methods: Vitreous samples were harvested from human eyes with persistent inflammation following intravitreal antibiotics for endophthalmitis. Samples were cultured as per the Endophthalmitis Vitrectomy Study protocol and used to measure the intravitreal vancomycin concentration. Using log₁₀, the half-life = 0.30103/Z, where Z = slope of the line in the formula log C_f =log C_i – Z(t_f – t_i). C_f = vancomycin concentration measured in vitreous at time final, t_f. C_i = vancomycin concentration at time initial, t_i. For eyes receiving their first vancomycin injection, C_i = micrograms of vancomycin injected divided by the vitreous volume (assumed to be 5.0 ml in pseudophakic eyes and 4.5 ml in phakic eyes). In eyes receiving a repeat vancomycin injection at t_i, C_i was increased by the vitreal vancomycin level measured just prior to injection. Vitrectomies done just prior to repeat vancomycin injections were assumed to clear all previously injected vancomycin. Results: The half-life was calculated 18 times in 9 eyes (once in 5 eyes, 2 times in 2 eyes, 3 times in 1 eye, and 6 times in 1 eye). In the eye with six determinations, one value was suspect because it was based on a low concentration of intravitreal vancomycin (1.7 mcg/ml). This low level could have been significantly influenced by the intravenous vancomycin the patient was receiving. Eliminating this suspect value, the half-life of intravitreal vancomycin in all eyes varied from 16.2 to 241.7 hours with a median of 30.2 hours. In the eye with six determinations, the value of the five non-suspect half-lives varied from 24.1 to 32.7 hours. The eye with three determinations was the only phakic eye; the half-lives varied from 23 to 38.9 hours. In one eye with two measurements, the first half-life was 16.2 hours and the second 25 hours. In the other eye with two measurements, the first half-life was 241.7 hours and the second was 72.7 hours. Seven specimens from 5 patients yielded the same organism as initially cultured. Conclusions: The half-life of intraviteal vancomycin varied widely both between eyes and in the same eye. The vancomycin in the one phakic vitrectomized eye did not have a significantly longer half-life than the vancomycin in the eight pseudophakic eyes. The high rate of growth from subsequent cultures suggests repeat intravitreal vancomycin injections were justified in eyes with persistent intraocular inflammation.

Keywords: endophthalmitis • bacterial disease • antibiotics/antifungals/antiparasitics