Evaluation of antimicrobial-loaded calcium sulfate composites for the management of resistant Gram-negative diabetic foot osteomyelitis

Background and objective: One of the most serious complications of diabetes is diabetic foot osteomyelitis (DFO) which can lead to limb amputation, reduced quality of life, and early mortality (International Diabetes Federation, 2020). Antimicrobial resistance (AMR) is a leading cause of death globally and is an increasing problem within DFO. We investigated the antimicrobial and pharmaceutical properties of antimicrobial-loaded calcium sulfate composites for the targeted treatment of DFO.

Methods: Calcium sulphate (Stimulan® Rapid Cure) beads containing gentamicin, ciprofloxacin, amoxicillin or colistin were tested against Staphylococcus aureus (NCTC6571), Pseudomonas aeruginosa (NCTC6750 and an extensively drug-resistant clinical isolate from DFO) and Escherichia coli (NCTC8196) over time using an adapted EUCAST disk-diffusion methodology. MIC testing and standard disk-diffusion testing were undertaken to determine susceptibility. Synergy testing, drug-release studies, dose uniformity and hygroscopicity testing were undertaken to further characterise these composites.

Results: Amoxicillin and ciprofloxacin released continuously and zones of inhibition (ZOI) remained consistent, over a XX week period. Whereas gentamicin and colistin underwent burst-release, with ongoing release and ZOI decreasing over time. Calcium sulfate had no antimicrobial effect and no synergy was observed between any of the antimicrobials.

Amoxicillin and ciprofloxacin had a more uniform dose (4% and 7% variation, respectively) whereas for gentamicin and colistin were not uniform ( 48.0% and 133.6% variation, respectively). A decrease in mass of beads was seen in hygroscopicity testing, suggesting release of moisture from the composites.

Conclusions: Antibiotic-loaded calcium sulfate beads release drug and inhibit bacterial growth over time, suggesting utility in the management of difficult to treat DFO. Future work should focus on different mixing methods for making calcium sulfate beads to determine effects on dose uniformity