ANTIMIcROBIAL AGENTS AND CHEMarHERAPY, Jan. 1977, p. 174-175 Vol. 11, No. 1 Copyright © 1977 American Society for Microbiology Printed in U.S.A. Stability of Cephalosporins in Horse Serum M. A. FOGLESONG The Lilly Research Laboratories, Eli Lilly and Company, Indianapolis, Indiana 46206 Received for publication 30 July 1976 The antibiotic activity of cephalothin, cephaloridine, cephalexin, cephalogly- cin, cefazolin, and cefamandole was determined after storage for up to 30 days in horse serum at -10 and 4°C. Cephalothin, cefamandole, cefazolin, and cepha- lexin 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, cefazo- lin, 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 bioavail- frozen at -10°C and assayed after 30 days. This ability studies is such that prolonged freezing sample functioned as a control for the freeze- or repeated freezing and thawing of samples is thaw portion of the study. The third sample required. Information is available concerning was stored at 4°C and assayed at 1-, 2-, 3-, and the stability of antibiotics in aqueous and fro- 30-day intervals. zen solutions (2, 5-10), but little is known about The microbiological activity as the precen- the stability of antibiotics in serum. Berti and tage of initial activity of the six cephalosporins Maccari (1) have recently examined the stabil- after storage at -10°C is listed in Table 1. ity of 11 different antibiotics, including cepha- Thawing and refreezing ofthe samples at 1-, 2-, loridine and cephalexin, stored in frozen rat and 3-day intervals had little effect on the ac- plasma for 8 weeks at -20°C. Under these con- tivity of cephalothin, cephaloridine, cepha- ditions, cephalexin was shown to be signifi- lexin, cefazolin, and cefamandole. These com- cantly more stable than cephaloridine. pounds retained at least 94% of their initial The present study was undertaken to exam- antibiotic activity for 3 days. Cephaloglycin, ine the stability of cephalothin, cephaloridine, however, retained only 75% of its activity after cephalexin, cephaloglycin, cefazolin, and cefa- 1 day and 50% of its activity after 3 days. After mandole stored in horse serum under the fol- 30 days at -10°C, cephalothin, cephalexin, lowing conditions: (i) frozen in serum at -10°C cefazolin, and cefamandole showed no loss in and the same sample assayed at 1-, 2-, 3-, and 30-day intervals: (ii) frozen in serum at -10°C TABLE 1. Antibiotic activity as the percentage of initial activity ofcephalosporins stored at -10C for 30 days; and (iii) stored in serum at 40C and in pooled horse serum and assayed at various assayed at 1-, 2-, 3-, and 30-day intervals. time intervalsa Using pooled horse serum as a diluent, anti- biotic solutions were prepared by volumetric Antibiotic activity (%) after Anti- dilutions to the following reference concentra- storage (days): biotic ac- tivity (%) tions: cefazolin, 4.0 ,g/ml; cephalothin, cepha- Antibiotic of freeze- loridine, and cefamandole, 1.0 ,tg/ml; cepha- 0 1 2 3 30 thaw con- lexin, 0.4 ,ug/ml; and cephaloglycin, 0.2 ,ug/ml. trol Microbiological activity was determined by a Cephalothin 100 93 92 94 91 93 cylinder-plate assay (4). Sarcina lutea was used (±8) (±4) (±6) (±4) (±8) (±3) Cephaloridine 100 97 94 94 71 90 as the test organism for the cephalexin and (±3) (±3) (±5) (±1) (±3) (±8) cephaloglycin plate assays, and Bacillus sub- Cephalexin 100 105 NSb 95 98 98 tilis was used for the remaining cephalospo- (±1) (±7) (±1) (±1) (±1) rins. The format used for the assays is outlined Cephaloglycin 100 75 55 <50 <50 <50 (±2) (± 1) (± 1) in the Code ofFederal Regulations (3). Cefazolin 100 101 94 97 100 101 Immediately after the initial assay, each an- (±8) (±6) (±4) (±4) (±2) (±5) tibiotic solution was divided into three por- Cefamandole 100 100 94 99 100 104 tions. One sample was frozen at -10°C and (±3) (±10) (±2) (±8) (±3) (±6) then thawed, refrozen, and assayed at 1-, 2-, 3-, a Average of six determinations ± standard deviation. and 30-day intervals. The second sample was b NS, Not sampled. 174 VOL. 11, 1977 NOTES 175 activity. However, under the same conditions, 90% of their initial activity, cephalothin and cephaloridine lost 29% of its initial activity. cephaloridine retained approximately 80%, and Cephaloglycin possessed less than 50% ofits ac- cephaloglycin possessed less than 50%. After 30 tivity when stored for 30 days at -10°C. days of storage at 400, cefamandole retained Of the cephalosporins tested, only cephalori- 70% of its initial activity, cephalexin retained dine appeared to be affected by repeated freez- 56%, and cefazolin retained 42%. Cephalothin, ing and thawing. The cephaloridine control cephaloridine, and cephaloglycin all possessed sample, which was thawed only once prior to less than 50% of their initial activity. being assayed, contained 90% of the initial ac- tivity after 30 days. The cephaloridine sample The technical assistance of Jack Scott is gratefully ac- that was thawed and refrozen on days 1, 2, 3, knowledged. and 30 possessed only 71% of its initial activity LITERATURE CITED after 30 days. 1. Berti, M. A., and M. Maccari. 1975. Stability of frozen Storage at 40C (Table 2) had a more drastic rat plasma containing different antibiotics. Antimi- effect on the stability of the antibiotics than crob. Agents Chemother. 8:633-637. storage at -10°C or repeated thawing and 2. Boylan, J. C., J. L. Simmons, and C. L. Winely. 1972. Stability of frozen solutions of sodium cephalothin freezing. After 3 days at 4°C, cephalexin, cefa- and cephaloridine. Am. J. Hosp. Pharm. 29:687-689. zolin, and cefamandole retained greater than 3. Code of federal regulations. Vol. 39, no. 105, part III, §436.105(a)(b)(c)(d). 4. Grove, D. C., and W. A. Randall. 1955. Penicillin section TABLE 2. Antibiotic activity as the percentage of A: microbiologic assay methods for penicillin, p. 7-14. initial activity ofcephalosporins stored at 4°C Assay methods of antibiotics. Medical Encyclopedia, in pooled horse serum and assayed at various Inc., New York. time intervals a 5. Hou, J. P., and J. W. Poole. 1969. Kinetics and mecha- nism of degradation of ampicillin in solution. J. Antibiotic activity (%) after storage (days): Pharm. Sci. 58:447-454. Antibiotic 6. Kuchinskas, E. J., and G. N. Levy. 1972. Comparative 0 1 2 3 30 stabilities of ampicillin and hetacillin in aqueous so- Cephalothin 100 94 85 79 <25 lution. J. Pharm. Sci. 61:727-729. (±8) (±3) (±3) (±3) 7. Mann, J. M., D. L. Coleman, and J.'C. Boylan. 1971. Cephaloridine 100 102 78 80 <25 Stability of parenteral solutions of sodium cephalo- (±3) (±5) (±4) (±2) thin, cephaloridine, potassium penicillin G Cephalexin 100 98 NSb 95 56 (buffered), and vancomycin hydrochloride. Am. J. (±2) (±5) (±3) (±2) Hosp. Pharm. 28:760-763. Cephaloglycin 100 65 <50 <50 <50 8. Sauello, D. R., and R. F. Shangrau. 1971. Stability of (±10) (±5) sodium ampicillin solutions in the frozen and liquid Cefazolin 100 100 98 99 42 states. Am. J. Hosp. Pharm. 28:754-759. (±8) (±5) (±2) (±7) (±1) 9. Stolar, M. H., H. S. Carlin, and M. I. Blake. 1968. Cefamandole 100 95 95 92 70 Effect of freezing on the stability of sodium methicil- (±6) (±4) (±2) (±5) (±2) lin injection. Am. J. Hosp. Pharm. 25:32-35. 10. Yamana, T., A. Tsuji, K. Kanayama, and 0. Nakano. a Average of six determinations ± standard deviation. 1974. Comparative stabilities of cephalosporins in NS, Not sampled. aqueous solutions. J. Antibiot. 27:1000-1002..