Antibiotic stability over six weeks in aqueous solution at body temperature with and without heat treatment that mimics the curing of bone cement

Abstract

Objectives: Thermal stability is a key property in determining the suitability of an antibiotic agent for local application in the treatment of orthopaedic infections. Despite the fact that long-term therapy is a stated goal of novel local delivery carriers, data describing thermal stability over a long period are scarce, and studies that avoid interference from specific carrier materials are absent from the orthopaedic literature.

Methods: In this study, a total of 38 frequently used antibiotic agents were maintained at 37°C in saline solution, and degradation and antibacterial activity assessed over six weeks. The impact of an initial supplementary heat exposure mimicking exothermically curing bone cement was also tested as this material is commonly used as a local delivery vehicle. Antibiotic degradation was assessed by liquid chromatography coupled to mass spectrometry, or by immunoassays, as appropriate. Antibacterial activity over time was determined by the Kirby-Bauer disk diffusion assay.

Results: The heat exposure mimicking curing bone cement had minimal effect on stability for most antibiotics, except for gentamicin which experienced approximately 25% degradation as measured by immunoassay. Beta-lactam antibiotics were found to degrade quite rapidly at 37°C regardless of whether there was an initial heat exposure. Excellent long-term stability was observed for aminoglycosides, glycopeptides, tetracyclines and quinolones under both conditions.

Conclusions: This study provides a valuable dataset for orthopaedic surgeons considering local application of antibiotics, and for material scientists looking to develop next-generation controlled or extended-release antibiotic carriers.Cite this article: E. Samara, T. F. Moriarty, L. A. Decosterd, R. G. Richards, E. Gautier, P. Wahl. Antibiotic stability over six weeks in aqueous solution at body temperature with and without heat treatment that mimics the curing of bone cement. Bone Joint J 2017;6:296-306. DOI: 10.1302/2046-3758.65.BJR-2017-0276.R1.

Keywords: Bone cement; Local antibiotics; Thermal stability.

Fig. 1

Temperature-time curve of curing polymethyl methacrylate (PMMA) bone cement (large red circle with light grey error bars) and the simulated curve (small circles with black error bars), to which antibiotic solutions were exposed. Measurements were taken every 15 seconds, data represent mean results of a triplicate and error bars represent standard deviation.

Fig 2

Thermal stability determined for each antibiotic over time as measured by LC-MS/MS or FPIA. Antibiotic solutions were prepared and maintained at 37°C for 42 days (regular group, solid lines), or first exposed to a heat profile representative of curing bone cement, followed by exposure to body temperature for 42 days (dashed lines). Concentrations in samples taken prior to heat exposure served as 100% reference. Data are means of triplicate experiments. Statistically significant differences between measurements from the regular group and the heat exposed group are marked with an asterisk where appropriate.

Fig. 3a

Charts showing thermal stability measured for each antibiotic over time, as determined by bioassay. Results are expressed as means from triplicate experiments. Zone of inhibition (ZOI) indicates diameter of the zone of inhibition, in millimetres. Statistically significant variations of the ZOI relative to starting values are marked with an asterisk where appropriate. Antibiotic solutions were prepared and maintained at 37°C for 42 days (body temperature group). Of note, temocillin and aztreonam have been tested with E. coli NCTC 12241, and colistimethate with P. aeruginosa PAO1, whereas all other results are from inhibition of S. aureus NCTC 12973.

Fig. 3b

Charts showing thermal stability measured for each antibiotic over time, as determined by bioassay. Results are expressed as means from triplicate experiments. Zone of inhibition (ZOI) indicates diameter of the zone of inhibition, in millimetres. Statistically significant variations of the ZOI relative to starting values are marked with an asterisk where appropriate. Antibiotic solutions were prepared and maintained at 37°C for 42 days (body temperature group). Of note, temocillin and aztreonam have been tested with E. coli NCTC 12241, and colistimethate with P. aeruginosa PAO1, whereas all other results are from inhibition of S. aureus NCTC 12973.