Penetration of Cefazolin, Cephaloridine, and Cefamandole into Interstitial Fluid in Rabbits

We compared the penetration of three cephalosporins into interstitial fluid. Interstitial fluid was obtained in rabbits from Silastic tissue cages. Cefazolin, cephaloridine, and cefamandole were administered by the intramuscular route (30 mg/kg per injection). Peak blood levels and interstitial concentrations were studied after a single injection. Interstitial levels were also compared in a three-injection study (one injection every 12 h) and in a cumulative effect study (six injections), in which the interval between injections was established for each drug on the basis of its common therapeutic use. After a single injection, cephaloridine activity was detected more rapidly and attained higher levels than the other two drugs within the first 4 h. However, 2 h after the third injection, cefazolin levels in tissue fluid were higher than with cephaloridine. Cefamandole consistently gave the lowest interstitial levels. With all three drugs, detectable concentrations were present in interstitial fluid at a time when no detectable antibiotic was found in serum. In the six-injection study, the interstitial levels obtained with cefazolin were significantly higher than those observed with the other drugs. Our data suggest that cefazolin is a drug of choice due to its high extravascular levels.