Tyrosine-derived polycarbonate scaffolds for bone regeneration in a rabbit radius critical-size defect model.

The aim of the study was to determine bone regeneration in a rabbit radius critical-size defect (CSD) model using a specific polymer composition (E1001(1k)) from a library of tyrosine-derived polycarbonate scaffolds coated with a calcium phosphate (CP) formulation (E1001(1k) + CP) supplemented with recombinant human bone morphogenetic protein-2 (rhBMP-2). Specific doses of rhBMP-2 (0, 17, and 35 µg/scaffold) were used. E1001(1k) + CP scaffolds were implanted in unilateral segmental defects (15 mm length) in the radial diaphyses of New Zealand White rabbits. At 4 and 8 weeks post-implantation, bone regeneration was determined using micro-computed tomography (µCT), histology, and histomorphometry. The quantitative outcome data suggest that E1001(1k) + CP scaffolds with rhBMP-2 were biocompatible and promoted bone regeneration in segmental bone defects. Histological examination of the implant sites showed that scaffolds made of E1001(1k) + CP did not elicit adverse cellular or tissue responses throughout test periods up to 8 weeks. Noteworthy is that the incorporation of a very small amount of rhBMP-2 into the scaffolds (as low as 17 µg/defect site) promoted significant bone regeneration compared to scaffolds consisting of E1001(1k) + CP alone. This finding indicates that E1001(1k) + CP may be an effective platform for bone regeneration in a critical size rabbit radius segmental defect model, requiring only a minimal dose of rhBMP-2.

Next-generation resorbable polymer scaffolds with surface-precipitated calcium phosphate coatings.

Next-generation synthetic bone graft therapies will most likely be composed of resorbable polymers in combination with bioactive components. In this article, we continue our exploration of E1001(1k), a tyrosine-derived polycarbonate, as an orthopedic implant material. Specifically, we use E1001(1k), which is degradable, nontoxic, and osteoconductive, to fabricate porous bone regeneration scaffolds that were enhanced by two different types of calcium phosphate (CP) coatings: in one case, pure dicalcium phosphate dihydrate was precipitated on the scaffold surface and throughout its porous structure (E1001(1k) + CP). In the other case, bone matrix minerals (BMM) such as zinc, manganese and fluoride were co-precipitated within the dicalcium phosphate dihydrate coating (E1001(1k) + BMM). These scaffold compositions were compared against each other and against ChronOS (Synthes USA, West Chester, PA, USA), a clinically used bone graft substitute (BGS), which served as the positive control in our experimental design. This BGS is composed of poly(lactide co-ε-caprolactone) and beta-tricalcium phosphate. We used the established rabbit calvaria critical-sized defect model to determine bone regeneration within the defect for each of the three scaffold compositions. New bone formation was determined after 2, 4, 6, 8 and 12 weeks by micro-computerized tomography (µCT) and histology. The experimental tyrosine-derived polycarbonate, enhanced with dicalcium phosphate dihydrate, E1001(1k) + CP, supported significant bone formation within the defects and was superior to the same scaffold containing a mix of BMM, E1001(1k) + BMM. The comparison with the commercially available BGS was complicated by the large variability in bone formation observed for the laboratory preparations of E1001(1k) scaffolds. At all time points, there was a trend for E1001(1k) + CP to be superior to the commercial BGS. However, only at the 6-week time point did this trend reach statistical significance. Detailed analysis of the µCT data suggested an increase in bone formation from 2 through 12 weeks in implant sites treated with E1001(1k) + CP. At 2 and 4 weeks post-implantation, bone formation occurred at the interface where the E1001(1k) + CP scaffold was in contact with the bone borders of the implant site. Thereafter, during weeks 6, 8 and 12 bone formation progressed throughout the E1001(1k) + CP test implants. This trend was not observed with E1001(1k) + BMM scaffolds or the clinically used BGS. Our results suggest that E1001(1k) + CP should be tested further for osteoregenerative applications.

Bone regeneration in a rabbit critical-sized calvarial model using tyrosine-derived polycarbonate scaffolds.

Porous three-dimensional tyrosine-derived polycarbonate (TyrPC) scaffolds with a bimodal pore distribution were fabricated to mimic bone architecture using a combination of salt-leaching and phase separation techniques. TyrPC scaffolds degraded in register with bone regeneration during the 6-week study period and compressive moduli of the scaffolds were maintained >0.5 MPa at 6 weeks of incubation in PBS at 37 °C. The TyrPC scaffolds either unsupplemented or supplemented with recombinant human bone morphogenetic protein-2 (rhBMP-2) were implanted in a rabbit calvarial critical-sized defect (CSD) model and the TyrPC scaffolds treated with rhBMP-2 or TyrPC coated with calcium phosphate scaffold alone promoted bone regeneration in a rabbit calvarial CSD at 6 weeks postimplantation. A synthetic TyrPC polymeric scaffold either without a biological supplement or with a minimal dose of rhBMP-2 induced bone regeneration comparable to a commercially available bone graft substitute in a nonrodent CSD animal model.

Osteogenic differentiation of pre-osteoblasts on biomimetic tyrosine-derived polycarbonate scaffolds.

The osteogenic potential of biomimetic tyrosine-derived polycarbonate (TyrPC) scaffolds containing either an ethyl ester or a methyl ester group combined with recombinant human bone morphogenetic protein-2 (rhBMP-2) was assessed using the preosteoblast cell line MC3T3-E1. Each composition of TyrPC was fabricated into 3D porous scaffolds with a bimodal pore distribution of micropores <20 µm and macropores between 200 and 400 µm. Scanning electron microscopy (SEM) characterization suggested MC3T3-E1 cell attachment on the TyrPC scaffold surface. Moreover, the 3D TyrPC-containing ethyl ester side chains supported osteogenic lineage progression, alkaline phosphatase (ALP), and osteocalcin (OCN) expression as well as an increase in calcium content compared with the scaffolds containing the methyl ester group. The release profiles of rhBMP-2 from the 3D TyrPC scaffolds by 15 days suggested a biphasic rhBMP-2 release. There was no significant difference in bioactivity between rhBMP-2 releasate from the scaffolds and exogenous rhBMP-2. Lastly, the TyrPC containing rhBMP-2 promoted more ALP activity and mineralization of MC3T3-E1 cells compared with TyrPC without rhBMP-2. Consequently, the data strongly suggest that TyrPC scaffolds will provide a highly useful platform for bone tissue engineering.