Comparative aminoglycoside inactivation by .BETA.-lactam antibiotics. Effect of a Cephalosporin and six Penicillins on five aminoglycosides.

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Abstract
Gentamicin, tobramycin, netilmicin, kanamycin and amikacin were evaluated over time for biologic activity in human serum, in combination with 6 .beta.-lactams. Simple addition of aminoglycoside and 250 .mu.g/ml penicillin produced aminoglycoside inactivation at 8-48 h. All .beta.-lactam antibiotics exhibited decay in human serum at 37.degree. C, even when present as a single component. All aminoglycosides could be inactivated by penicillins but differed markedly in their susceptibility. Amikacin, at 20 .mu.g/ml, was the least inactivated by any penicillin; netilmicin, at 10 .mu.g/ml, was the next least inactivated. Tobramycin had pronounced loss of biological activity exceeding that of any aminoglycoside, appearing as early as 8 h. The ability of the various penicillins to produce aminoglycoside inactivation, in approximate descending order, was carbenicillin, ticarcillin, penicillin G, oxacillin, methicillin, ampicillin. Cephalothin produced minimal inactivation. Aminoglycoside inactivation also occurred at 25.degree. C and with many samples stored at 4.degree. C, although at proportionately slower rates. For samples stored at -20.degree. C, only tobramycin had substantial loss of activity. Adequate handling and prompt assay of the specimen are important.