In vitro cytocompatibility evaluation of poly(octamethylene citrate) monomers toward their use in orthopedic regenerative engineering

Citrate based polymer poly(octamethylene citrate) (POC) has shown promise when formulated into composite material containing up to 65 wt% hydroxylapatite (HA) for orthopedic applications. Despite significant research into POC, insufficient information about the biocompatibility of the monomers 1,8-Octanediol and Citrate used in its synthesis is available. Herein, we investigated the acute cytotoxicity, immune response, and long-term functionality of both monomers. Our results showed a cell-type dependent cytotoxicity of the two monomers: 1,8-Octanediol induced less acute toxicity to 3T3 fibroblasts than Citrate while presenting comparable cytotoxicity to MG63 osteoblast-like cells; however, Citrate demonstrated enhanced compatibility with hMSCs compared to 1,8-Octanediol. The critical cytotoxic concentration values EC30 and EC50, standard for comparing cytotoxicity of chemicals, were also provided. Additionally, Citrate showed slower and less inhibitory effects on long-term hMSC cell proliferation compared with 1,8-Octanediol. Furthermore, osteogenic differentiation of hMSCs exposure to Citrate resulted in less inhibitory effect on alkaline phosphatase (ALP) production. Neither monomer triggered undesired pro-inflammatory responses. In combination with diffusion model analysis of monomer release from cylindrical implants, based on which the maximum concentration of monomers in contact with bone tissue was estimated to be 2.2 × 10⁻⁴ mmol/L, far lower than the critical cytotoxic concentrations as well as the 1,8-Octanediol concentration (0.4 mg/mL or 2.7 mmol/L) affecting hMSCs differentiation, we provide strong evidence for the cytocompatibility of the two monomers degraded from citrate-based composites in the orthopedic setting.