P97: Bioresorbable composites based on hydroxyapatite dispersed in poly-L-lactide matrix

Abstract

Orthopedic oncologic prostheses are essential in postoperative therapy and improvement of life quality of oncological patients. However, construction and composition of implants has to be improved for better biocompatibility and reduction of inflammatory responses. The materials based on polylactic acid (PLA) and hydroxyapatite (HA) have been suggested as promising solution for implant coating in order to improve reconstruction of bone defects, to reduce risk of inflammation and implant rejection. Objective To obtain the bioresorbable composites using biocompatible poly-L-lactide (PLA) with HA and to evaluate its effect on bone remodeling rate. Materials and methods After dissolution of PLA∗ in chloroform the powder-like HA (d = 20–40 nm) was added under vigorous stirring (PLA: HA = 75:25). The PLA/HA suspension was sonicated (40 kHz) and precipitated in ethanol. Fine fibers (d = 0.1– 0.5 μm, of more than 2 mm length) were formed. The PLA/HA composite fibers were air-dried; they consisted of HA nanoparticles homogeneously distributed in the PLA matrix. The composites ability to participate in bone remodeling was estimated by determination of the polymer matrix dissolution rate and by formation of the CPL on the surface of the composites. Results The degradation rate of the polymer matrix was calculated by measuring concentration of lactic acid (by HPLC method), being released from PLA macromolecules as the result of the hydrolysis in the physiologic solution (pH 7, ω(NaCl) = 0.9%). The PLA degradation rate during the first 4 days of soaking of the substrates (d = 10 mm, S = 190 mm2, m = 0.20 ± 0.01 g) in the solution is rather high (Clactic acid = 10 wt%), while on day 5 it decreases and stays constant during the rest of the time. The HA content increase in the composite results in the enhanced hydrolysis of PLA, caused both by the partial acid-base interaction between PLA and HA, and due to presence of the phase boundaries. Interesting to note that pure PLA (without HA) is hardly hydrolyzed in the physiologic solution even on the 30-th day of the experiment. Formation of calcium phosphate layer (CPL) was done in the SBF media, simulating the mineral constituents of the blood plasma at 37 °C, over 28 days period. The total concentration of Ca2+ and Mg2+ ions in the SBF-solution was determined by daily trilonometry in the ammonia buffer with eriochrome black T as indicator. This investigation clearly shows that the substrates both of HA and PLA/HA promote the CLP formation on their surfaces. On day 14 of the substrates soaking in the SBF solution the increment of Ca2+, Mg2+ ions adsorbed on the HA substrate rose to the value of m (Ca2+ + Mg2+)/VSBF = 0.11 mg/l, while the same value on the composite PLA/HA substrate was 0.06 mg/l. Thus the rate of adsorption of Ca2+ ions on the surface of HA is higher than that on the surface of the PLA/HA substrates. It is the result of the high HA content in the material. Pure PLA does not promote adsorption of Ca2+ and Mg2+ ions from the SBF media. Thus the HA particles incorporated in the PLA matrix may cause fast the CLP formation improving the ossification in the bone reconstruction processes. Biocompatibility and anti-inflammatory activity of PLA, HA and PLA/HA composite was studied in a pilot experiment in the cell-mediated immune response of individual donors in vitro using CD14+ human monocytes. Insignificant pro-inflammatory cytokine secretion of the M1 type (TNFα) is observed in the presence of PLA, while HA and PLA/HA composites don’t contribute to their release. The PLA promotes the release of M2 cytokine CCL18 on day 6 of macrophage culturing suggesting potential anti-inflammatory properties of the material. Conclusion The composite PLA/HA supports active bone-like layer formation on its surface improving ossification in the bone reconstruction process. The pilot experiment suggested that PLA/HA does not cause inflammatory reactions, while PLA can potentially regulate the balance between M1/M2 reaction to suppress unnecessary inflammation. ∗Mol. mass ca 60 kDa, prepared in the Laboratory of Catalytic Research of Tomsk State University. This Research is supported by Tomsk State University Competitiveness Improvement Program . The work was partially supported by the project Russian Foundation for Basic Research RFBR # 15-08-05496_a and EU IMMODGEL project, Grant No. (602694). Work was conducted with the application of the Tomsk regional common use center technical equipment acquired thanks to a grant of the Russian Ministry of the Agreement No. 14.594.21.0001 (RFMEFI59414X0001).