PLLA Coating of Lyophilized Human Bone Allograft for Long-term Release of Antibiotics

Abstract

Background/aim: Mineralized allogeneic bone substitutes are ideal biomaterials for bone regeneration in surgeries. Moreover, they are also suitable for releasing antibiotics, delivering initial concentrations many times higher than the minimal inhibitory concentration for relevant bacterial strains, without delaying bone formation. In the present study, the potential of sustained-release long-term antibiotic delivery using allografts was investigated. A broad spectrum of antibiotics, including gentamicin, vancomycin, rifampicin, and clindamycin, was incorporated into the bone blocks using a poly-L-lactic acid (PLLA) polymer coating.

Materials and methods: An in-vivo model of implantation within the rabbit tibia plateau was used to track the sustained release of single/combined antibiotics for up to 120 days. Bony integration and tissue responses to the PLLA-coated antibiotic-delivery systems were analyzed at 4 months post implantation using histopathological analysis.

Results: The variant loaded with both vancomycin and rifampicin demonstrated the highest activity against methicillin-sensitive Staphylococcus aureus and Staphylococcus epidermidis. Histopathological analysis revealed that the tissue responses to the coated allogeneic bone substitutes were comparable in all study groups, with no observable histopathological differences. The coated bone blocks induced a strong foreign-body reaction, including high numbers of multinucleated giant cells and other immune cells but no material-associated bone growth.

Conclusion: Based on these results, future optimization can focus on selecting more efficient release of antibiotics and increasing the encapsulated concentration to sustain antibiotic release over 4 months, thereby improving the bacteriostatic effect in vivo. Furthermore, biocompatibility and osteoconductivity must be improved.

Keywords: Biocompatibility; MIC90; allograft; antibiotic susceptibility; bone graft infection; bone regeneration; drug delivery; local antibiotic delivery; rabbit tibia.

Figure 1

In-vivo release-kinetics for clindamycin. MSSA: Methicillin-sensitive Staphylococcus aureus; S. epidermidis: Staphylococcus epidermidis; MIC90: minimal inhibitory concentration required to inhibit 90% of isolates. Data are presented as mean±standard deviations.

Figure 2

In-vivo release-kinetics for gentamycin. MSSA: Methicillin-sensitive Staphylococcus aureus; S. epidermidis: Staphylococcus epidermidis; MIC90: minimal inhibitory concentration required to inhibit 90% of isolates. Data are presented as mean±standard deviations.

Figure 3

In vivo-release kinetics of vancomycin from the combinational coating. MSSA: Methicillin-sensitive Staphylococcus aureus; S. epidermidis: Staphylococcus epidermidis; MIC90: minimal inhibitory concentration required to inhibit 90% of isolates. Data are presented as mean±standard deviations.

Figure 4

In vivo-release kinetics of rifampicin from the combinational coating. MSSA: Methicillin-sensitive Staphylococcus aureus; S. epidermidis: Staphylococcus epidermidis; MIC90: minimal inhibitory concentration required to inhibit 90% of isolates. Data are presented as mean±standard deviations.

Figure 5

Representative histological images at 120 days post implantation in rabbit tibia of allogeneic bone blocks impregnated with clindamycin; hematoxylin and eosin staining. (A) Overview of the implantation bed of the allogeneic bone block (dashed line). (B) - (D) Photomicrographs of the tissue reactions to the allogenic bone graft (AB) and the poly-L-lactic acid coating (asterisks) surrounded by connective tissue (CT), with associated multinucleated giant cells (black arrowheads), lymphocytes (blue arrow), and macrophages (black arrows) attached to the poly-L-lactic acid coating structures. B: Bone tissue; FT: fatty tissue; MT: muscle tissue. Original magnifications: (A) 100×, scale bar: 5 mm; (B) 10×, scale bar: 200 μm; and (C, D) 40×, scale bar: 20 μm.