Penetration of antibiotic into interstitial tissue fluid following parenteral administration of lysine cephalexin.

Serum and interstitial fluid levels of antibiotic have been measured at various times following i.m. administration of equivalent doses of lysine cephalexin and sodium ampicillin, in a cross-over design, to dogs fitted with s.c. implanted silastic "tissue cages" and the results subjected to pharmacokinetic analysis. Serum concentrations of cephalexin were consistently higher than those of ampicillin and elimination half-lives were calculated as 1.73 h and 0.87 h, respectively. The rate of appearance of cephalexin in the interstitial fluid (Ka 1.741 h-1 was faster than that for ampicillin (Ka 0.946 h-1) and the maximum concentration obtained also proved to be higher. The half-life of cephalexin in the interstitial fluid (6.79 h) was almost four times longer than that in serum whereas that of ampicillin (2.03 h) was only a little more than twice as long. As a consequence a greater concentration and retention in interstitial fluid was obtained with cephalexin at all times tested. Since the interstitial fluid levels of cephalexin were still relatively high 8-12 h after the administration of a single dose repeated administration of lysine cephalexin 2 or 3 times daily is expected to result in a beneficial accumulation of cephalexin in the interstitial fluid.