Elizabeth L. Hedberg

rhBMP-2 release from injectable poly(DL-lactic-co-glycolic acid)/calcium-phosphate cement composites.

Background: In bone tissue engineering, poly(DL-lactic-co-glycolic acid) (PLGA) microparticles are frequently used as a delivery vehicle for bioactive molecules. Calcium phosphate cement is an injectable, osteoconductive, and degradable bone cement that sets in situ. The objective of this study was to create an injectable composite based on calcium phosphate cement embedded with PLGA microparticles for sustained delivery of recombinant human bone morphogenetic protein-2 (rhBMP-2).Methods: 125 I-labeled rhBMP-2 was incorporated in PLGA microparticles. PLGA microparticle/calcium-phosphate cement composites were prepared in a ratio of 30:70 by weight. Material properties were evaluated by scanning electron microscopy, microcomputed tomography, and mechanical testing. Release kinetics of rhBMP-2 from PLGA/calcium-phosphate cement disks and PLGA microparticles alone were determined in vitro in two buffer solutions (pH 7.4 and pH 4.0) for up to twenty-eight days.Results: The entrapment yield of rhBMP-2 in PLGA microparticles was a mean (and standard deviation) of 79% ± 8%. Analysis showed spherical PLGA microparticles (average size, 17.2 ±1.3 &mgr;m) distributed homogeneously throughout the nanoporous disks. The average compressive strength was significantly lower (p < 0.001) for PLGA and calcium-phosphate cement composite scaffolds than for calcium-phosphate cement scaffolds alone (6.4 ± 0.6 MPa compared with 38.6 ± 2.6 MPa, respectively). Average rhBMP-2 loading was 5.0 ± 0.4 &mgr;g per 75-mm 3 disk. Release of rhBMP-2 was limited for all formulations. At pH 7.4, 3.1% ± 0.1% of the rhBMP-2 was released from the PLGA/calcium-phosphate cement disks and 18.0% ± 1.9% was released from the PLGA microparticles alone after twenty-eight days. At pH 4.0, PLGA/calcium-phosphate cement disks revealed more release of rhBMP-2 than did PLGA microparticles alone (14.5% ± 6.3% compared with 5.4% ± 0.7%) by day 28.Conclusions: These results indicate that preparation of a PLGA/calcium-phosphate cement composite for the delivery of rhBMP-2 is feasible and that the release of rhBMP-2 is dependent on the composite composition and nanostructure as well as the pH of the release medium.Clinical Relevance: An osteoconductive and osteoinductive rhBMP-2-loaded PLGA/calcium-phosphate cement composite may potentially result in an injectable bone-graft substitute for the regeneration of bone in ectopic or orthotopic sites.

In vivo degradation of porous poly(propylene fumarate)/poly(DL-lactic-co-glycolic acid) composite scaffolds

This study investigated the in vivo degradation of poly(propylene fumarate) (PPF)/poly(DL-lactic-co-glycolic acid) (PLGA) composite scaffolds designed for controlled release of osteogenic factors. PPF/PLGA composites were implanted into 15.0mm segmental defects in the rabbit radius, harvested after 12 and 18 weeks, and analyzed using histological techniques to assess the extent of polymer degradation as well as the tissue response within the pores of the scaffolds. Polymer degradation was limited to micro-fragmentation of the scaffold at the ends and edges of the implant at both 12 and 18 weeks. The tissue within the pores of the scaffold consisted of fibrous tissue, blood vessels and some inflammatory cells. In areas where polymer breakdown was evident, an increased inflammatory response was observed. In contrast, areas of bone ingrowth into the polymer scaffold were characterized by minimal inflammatory response and polymer degradation. Our results show that minimal degradation of porous PPF occurs within 18 weeks of implantation in a rabbit model. Further, the in vivo degradation data of porous PPF/PLGA scaffolds are comparable with earlier obtained in vitro data.

Controlled release of an osteogenic peptide from injectable biodegradable polymeric composites.

Poly(D,L-lactic-co-glycolic acid)/poly(ethylene glycol) (PLGA/PEG) blend microparticles loaded with the osteogenic peptide TP508 were added to a mixture of poly(propylene fumarate) (PPF), poly(propylene fumarate)-diacrylate (PPF-DA), and sodium chloride (NaCl) for the fabrication of PPF composite scaffolds that could allow for tissue ingrowth as well as for the controlled release of TP508 when implanted in an orthopedic defect site. In this study, PPF composites were fabricated and the in vitro release kinetics of TP508 were determined. TP508 loading within the PLGA/PEG microparticles, PEG content within the PLGA/PEG microparticles, the microparticle content of the PPF composite polymer component, and the leachable porogen initial mass percent of the PPF composites were varied according to a fractional factorial design and the effect of each variable on the release kinetics was determined for up to 28 days. Each composite formulation released TP508 with a unique release profile. The initial release (release through day 1) of the PLGA/PEG microparticles was reduced upon inclusion in the PPF composite formulations. Day 1 normalized cumulative mass release from PPF composites ranged from 0.14+/-0.01 to 0.41+/-0.01, whereas the release from PLGA/PEG microparticles ranged from 0.31+/-0.02 to 0.58+/-0.01. After 28 days, PPF composites released 53+/-4% to 86+/-2% of the entrapped peptide resulting in cumulative mass releases ranging from 0.14+/-0.01 microg TP508/mm(3) scaffold to 2.46+/-0.05 microg TP508/mm(3) scaffold. The results presented here demonstrate that PPF composites can be used for the controlled release of TP508 and that alterations in the composites composition can lead to modulation of the TP508 release kinetics. These composites can be used to explore the effects varied release kinetics and dosages on the formation of bone in vivo.

Preparation of macroporous poly(2-hydroxyethyl methacrylate) hydrogels by enhanced phase separation.

Macroporous poly(2-hydroxyethyl methacrylate) (p(HEMA)) hydrogels were prepared in the presence of a 0.3-0.7 M NaCl solution. The pore morphology of the p(HEMA) hydrogels was dependent on the concentration of NaCl for a constant monomer solution to aqueous solution ratio. Swelling studies showed an increase in equilibrium water content and hydrogel porosity as the NaCl concentration in the polymerization medium increased from 0 to 0.7 M. The equilibrium water content, however, decreased as the NaCl concentration in the swelling medium increased. The frozen water content increased and non-frozen water decreased with an increase in the NaCl concentration in the polymerization medium. Mechanical testing indicated that the elastic modulus of the hydrogels was not affected by the increased porosity until the pores became interconnected. These data suggest that the addition of NaCl to the polymerization medium results in a multi-phase separation during fabrication that produces macroporous hydrogels of controlled morphology.

Controlled release of a tissue inducing peptide from oligo(poly(ethylene glycol) fumarate) hydrogels for orthopedic tissue engineering

The objective of this research is to develop injectable, in situ polymerizable polymer scaffolds for the controlled release of inductive factors for bone and cartilage tissue engineering. To that end, the novel polymer oligo(poly(ethylene glycol) fumarate) (OPF) was synthesized with varying poly(ethylene glycol) (PEG) chain lengths to create crosslinked networks of varying mesh size. A 23 amino acid peptide, TP508, was incorporated into OPF networks either directly or embedded in poly(DL-lactic-co-glycolicacid) (PLGA) microparticle carriers and the release kinetics of the TP508 was examined. After 30 hours, hydrogels of PEG chains of molecular weight 10,000 (PF10K) had released greater percentages of the total TP508 (53/spl plusmn/3 wt% of directly loaded TP508) than those fabricated with PEG chain of molecular weight 1,000 (PF1K) (31/spl plusmn/7 wt%). This effect was also observed upon the inclusion of PLGA microparticle carriers. For hydrogels of either PEG chain length, release of TP508 was greatly reduced with the use of microparticle carriers (6/spl plusmn/1 and 2/spl plusmn/1 wt% for PF10K and PF1K, respectively). Our results demonstrate that TP508 can be incorporated into OPF hydrogels and that the release kinetics of the peptide can be modulated through alterations in the scaffold mesh size and the use of a microparticle carrier.