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Author(s): McLaren R, *McLaren A, Vernon B; Banner Good Samaritan Medical Center Phoenix, AZ

Title: Elution Generic Tobramycin from PMMA is greater than elution of NebcinR from PMMA

Purpose: The purpose was to determine if generic tobramycin (PharmaTek) has greater elution from PMMA than proprietary tobramycin (NebcinR, Lilly).

Methods: Antibiotic Loaded PMMA beads were made using PalacosR Bone Cement (Biomet) and 1.2 grams of tobramycin powder per batch, either NebcinR or generic. The PMMA polymer powder and the tobramycin powder were mixed homogeneously before adding the monomer. Seven mm beads were made using a silicone mold with the PMMA in the dough phase. Three groups of five beads of each tobramycin loaded PMMA (NebcinR or generic) were placed in 30 ml glass vials. Twenty ml of distilled water was added to each vial. The vials were kept at room temperature (24°C). They were not agitated. Partial elutant exchanges (5 of 20 ml) were carried out at regular intervals for 180 days. At the time of elutant exchanges, the elutant was vortexed. The samples were frozen at -80°C until assayed. The tobramycin concentration in the elutant was measured by UV spectroscopy (Y= 269 nm). The UV absorption data for the three groups of NebcinR and for the three groups of generic tobramycin were averaged and converted to tobramycin concentration (mg/ml), then cumulative released tobramycin was calculated and plotted vs. time in days. Statistical significance was set at p & 0.05.

Results: Recovered tobramycin was greater from the PMMA beads loaded with generic tobramycin than it was from the beads loaded with NebcinR at every time interval. The difference was statistically significant at p < 0.0000006. At one month the released tobramycin from generic tobramycin was more than double that from NebcinR (4 mg vs. 1.7 mg).

Discussion: The observation that 1.2 grams of generic tobramycin has considerably greater volume than 1.2 grams of NebcinR raised the question if it would have greater elution from PMMA due to greater induced porosity. Elution of antibiotics such as tobramycin from Antibiotic Loaded PMMA is dependent on the permeability of the bone cement. Permeability is dependent on porosity. As the fluid enters the bone cement, the tobramycin dissolves and diffuses out of the bone cement. The pores created in this process increase the elution of tobramycin from deep in the PMMA. The volume of powder per gram is greater for generic tobramycin than for NebcinR. More soluble particles result in greater porosity and greater elution. In this prophylactic dose usually accepted for revision implant fixation, a doubling of the tobramycin release may be desirable but a potential greater decrease in the material properties would not be. Relative change in material strength of tobramycin loaded PMMA has yet to be evaluated.

In conclusion, generic tobramycin elutes faster than the NebcinR by greater than 2:1.

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