a Institute of Medical Microbiology, University Hospital, Paul-Ehrlich-Strasse 40, 60596 Frankfurt am Main; b Coripharm, Dieburg; c Orthopaedics, University Hospital Frankfurt, Germany
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Abstract |
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Introduction |
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Systemic treatment is an inefficient method for achieving high local tissue concentrations of drug and therefore, local deposition of antimicrobial agents has become increasingly popular in the management of osteomyelitis or open fractures.1 Several biodegradable and non-biodegradable substances have been employed as the vehicle for delivery.25 Polymethylmethacrylate (PMMA) is a non-resorbable antibiotic-loaded bone cement that has been used clinically for more than 20 years, but with the use of commercially available PMMA additional surgery is needed for its removal, and individualized chemotherapy in terms of administration of different antibiotics is not possible.6 Absorbable carrier materials with a collagen basis stimulate the formation of seroma and do not guarantee a prolonged release of the antibiotic. The hemihydrate of calcium sulphate (CaSO4), commonly known as plaster of Paris (POP), is a resorbable vehicle that does not require removal and releases its entire antibiotic load on resorption.7 The aim of this study was to evaluate in vitro the release kinetics of vancomycin, teicoplanin, gentamicin and clindamycin from biodegradable CaSO4 carrier beads.
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Materials and methods |
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Vancomycin 80, 160 and 320 g/L (Eli-Lilly, Bad Homburg, Germany), teicoplanin 80 g/L (Aventis, Frankfurt, Germany), gentamicin 80 g/L (Sigma, Deisenhofen, Germany) and clindamycin 80 g/L (Sigma) stock solutions were prepared according to the manufacturer's recommendations and stored at 20°C.
Carrier beads
The CaSO4 beads were manufactured by Coripharm (Dieburg, Germany) using a special vacuum technique. POP 50 g, Gips Special 40 (Heidelberger Cement AG, Heidelberg, Germany), was mixed with 30 mL of deionized water to yield optimal porosity. This liquid plaster was then poured into a mould that produced spherical beads of 6 or 4 mm in diameter. The beads were sterilized with radiation 25 kGy.
In vitro elution studies
Dry beads were initially soaked in 1 mL of the antibiotic solution for 3 min at room temperature and thereafter re-dried. The release of antibiotic by diffusion from a single bead was investigated by elution in 5 mL phosphate-buffered saline (PBS) at pH 7.4 and 37°C. After each 24 h, the bead was removed and transferred to a tube containing 5 mL of fresh PBS. The eluates for each day were stored at 20°C and assayed within 10 days. Each elution series was replicated six times.
Determination of the antibiotic concentration
The elution samples of PBS were assayed by agar diffusion microbiological assay. Indicator organisms were Bacillus subtilis ATCC 6633 for vancomycin, teicoplanin and gentamicin, and Micrococcus luteus ATCC 9341 for clindamycin. Fifty millilitres of sterile nutrient agar (Antibiotika-Agar Nr.5; Merck, Germany) was seeded with the appropriate bacteria at a concentration of 108 cfu/L and poured into 22 cm diameter round Petri dishes. The agar was allowed to set, and 14 wells (10 mm diameter) were punched into the agar and then filled with 100 µL of the elution samples and antibiotic calibrators. Calibrator concentrations were 0.625, 1.25, 2.5 and 5.0 mg/L, prepared in PBS. After incubation at 37°C for 18 h, the zones of inhibition were read using a micrometer. The antibiotic concentration of the eluates was determined by computer-assisted regression analysis. Each sample and calibrator was analysed on two separate agar plates, giving two readings per sample, with the mean being taken for calculation.
Statistical analysis
Statistical analysis was performed with Student's t-test.
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Results and discussion |
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CaSO4 as a filler for bone defects was first described by Dreesmann in 1892.7 After local treatment of bone infection was established as an alternative to systemic treatment, CaSO4 was also loaded with antibiotics.810 The antibiotic, however, was usually added before hardening of the CaSO4.10 Here, we report for the first time the possibility as well as advantage of loading hardened CaSO4 beads with various antibiotic solutions. This means, first, that selecting appropriate antibiotics facilitates individualized therapy and secondly, that no degradation of the antibiotic owing to the sterilization process or theromoinstability reduces the activity of the antibiotic.10 With regard to the total amount of antibiotic eluted within 10 days the kinetics demonstrated that in an initial phase the beads released approximately 45% of the glycopeptide antibiotic and about 80% of gentamicin and clindamycin within the first 24 h (Figure 1). In a second phase, a more gradual release over a period of 10 days could be demonstrated for all of the antibiotics. This study further revealed a direct relationship between the antibiotic load in the beads and the amount released daily. As shown in Figure 2
, doubling the antibiotic load of the bead gave a more prolonged elution and a two-fold increment in antibiotic release. Therefore, it may be expected that the MIC for relevant bacteria might be exceeded locally during treatment and that, if need be, the antibiotic concentration of the solution used to soak the carrier material initially is capable of being increased. Sufficiently high antibiotic concentration in the infected tissue especially during the first days of therapy is of major importance in preventing infection. Local carrier systems based on CaSO4, PMMA or high molecular weight lactic acid polymers demonstrate adequate high peak concentrations on the first day of release. With elution kinetics as shown in this study revealing antibiotic release over at least 10 days, the CaSO4 carrier system offers characteristics that, with regard to the duration of release, fall between those of collagen carriers, i.e. 2 days, on the one hand and PMMA or polylactate carriers, i.e. 30350 days, on the other.
In conclusion, CaSO4 as a vehicle for local antibiotic therapy displays several advantages. First, prolonged release of the antibiotic at the site of infection achieves elevated local concentrations while minimizing any risk of systemic toxicity. Secondly, a variety of antibiotics can be selected to load the beads, thereby facilitating individualized chemotherapy. Thirdly, the biodegradability of the carrier beads eliminates the need for a second surgical procedure for their removal.
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Acknowledgments |
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Notes |
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References |
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Received 16 November 2000; returned 20 February 2001; revised 16 March 2001; accepted 11 April 2001