Stability of Cephalosporins in Horse Serum

Abstract

The antibiotic activity of cephalothin, cephaloridine, cephalexin, cephaloglycin, cefazolin, and cefamandole was determined after storage for up to 30 days in horse serum at -10 and 4°C. Cephalothin, cefamandole, cefazolin, and cephalexin were stable for at least 30 days at -10°C, whereas cephaloridine lost 29% of its initial activity and cephaloglycin lost more than 50%. Cefamandole, cefazolin, and cephalexin could only be stored for 3 days at 400 without significant loss in activity, whereas cephalothin, cephaloridine, and cephaloglycin could be stored for only 1 day. Repeated freezing and thawing had a detrimental effect on the stability of cephaloridine.Frequently, the nature of antibiotic bioavailability studies is such that prolonged freezing or repeated freezing and thawing of samples is required. Information is available concerning the stability of antibiotics in aqueous and frozen solutions (2, 5-10), but little is known about the stability of antibiotics in serum. Berti and Maccari (1) have recently examined the stability of 11 different antibiotics, including cephaloridine and cephalexin, stored in frozen rat plasma for 8 weeks at -20°C. Under these conditions, cephalexin was shown to be significantly more stable than cephaloridine.The present study was undertaken to examine the stability of cephalothin, cephaloridine, cephalexin, cephaloglycin, cefazolin, and cefamandole stored in horse serum under the following conditions: (i) frozen in serum at -10°C and the same sample assayed at 1-, 2-, 3-, and 30-day intervals: (ii) frozen in serum at -10°C for 30 days; and (iii) stored in serum at 40C and assayed at 1-, 2-, 3-, and 30-day intervals.Using pooled horse serum as a diluent, antibiotic solutions were prepared by volumetric dilutions to the following reference concentrations: cefazolin, 4.0 ,g/ml; cephalothin, cephaloridine, and cefamandole, 1.0 ,tg/ml; cephalexin, 0.4 ,ug/ml; and cephaloglycin, 0.2 ,ug/ml. Microbiological activity was determined by a cylinder-plate assay (4). Sarcina lutea was used as the test organism for the cephalexin and cephaloglycin plate assays, and Bacillus subtilis was used for the remaining cephalosporins. The format used for the assays is outlined in the Code ofFederal Regulations (3).Immediately after the initial assay, each antibiotic solution was divided into three portions. One sample was frozen at -10°C and then thawed, refrozen, and assayed at 1-, 2-, 3-, and 30-day intervals. The second sample was frozen at -10°C and assayed after 30 days. This sample functioned as a control for the freezethaw portion of the study. The third sample was stored at 4°C and assayed at 1-, 2-, 3-, and 30-day intervals.The microbiological activity as the precentage of initial activity of the six cephalosporins after storage at -10°C is listed in Table 1. Thawing and refreezing of the samples at 1-, 2-, and 3-day intervals had little effect on the activity of cephalothin, cephaloridine, cephalexin, cefazolin, and cefamandole. These compounds retained at least 94% of their initial antibiot...

Comparative Stability of Cephalothin and Cefazolin in Buffer or Human Serum

“…It was recently reported that cephalothin shows a significant loss in potency when held at 4°C in horse serum (1). Furthermore, Fleming and Markowsky have shown that cephalothin lost 56% activity after 6 h of incubation at 370C (Program Abstr.…”