Ocular pharmacokinetics of cephalosporins using microdialysis

The purpose of this study was to delineate the ocular pharmacokinetics of cephalosporins and investigate the presence of peptide transporters in the retina. New Zealand albino rabbits were kept under anesthesia. A concentric microdialysis probe was implanted in the vitreous chamber and linear probe across the cornea in the aqueous humor. Isotonic phosphate buffer saline was perfused through the probes, and samples were collected every 20 min over a period of 10 hr. A 500 microg dose of cephalexin, cephazolin, and cephalothin was administered intravitreally. Inhibition experiments were carried out in vivo, using gly-pro and gly-sar. The vitreal half-lives of cephalexin, cefazolin, and cephalothin were 185.38 +/- 27.25 min, 111.40 +/- 17.17 min, and 146.68 +/- 47.52 min, respectively. Cephalexin generated higher aqueous humor concentrations compared to cefazolin. The pharmacokinetic parameters of cephalexin in the presence of gly-pro, i.e., AUC (44452.06 +/- 3326.55 microg x min/ml), clearance (0.0013 +/- 0.0004 ml/min) and vitreal half-life (825.12 +/- 499.95 min) were different from that of the control (14612.83 +/- 4036.47 microg x min/ml, 0.0036 +/- 0.0011 ml/min, and 187.96 +/- 65.12 min, respectively). Gly-pro did not inhibit cefazolin, and gly-sar showed no effect on the pharmacokinetics of both drugs. These studies indicate the involvement of a peptide carrier in the transport of cephalosporins across the retina. Although gly-pro inhibited the elimination of cephalexin from the vitreous, the effect of an alpha-amino group on peptide carriers was not clearly evident.