Synthetic hydrogel scaffold is an effective vehicle for delivery of INFUSE (rhBMP2) to critical-sized calvaria bone defects in rats

Abstract

Medtronic's INFUSE Bone Graft provides surgeons with a potent tool for stimulating bone formation. Current delivery vehicles that rely on Absorbable Collagen Sponges (ACS) require excessive quantities of the active ingredient in INFUSE, recombinant human Bone Morphogenic Protein-2 (rhBMP2), to achieve physiologically relevant concentrations of the growth factor, driving up the cost of the product and increasing the likelihood of undesirable side effects in neighboring tissues. We demonstrate that a simple light-mediated, thiol-ene chemistry can be used to create an effective polymer delivery vehicle for rhBMP2, eliminating the use of xenographic materials and reducing the dose of rhBMP2 required to achieve therapeutic effects. Comprised entirely of synthetic components, this system entraps rhBMP2 within a biocompatible hydrogel scaffold that is degraded by naturally occurring remodeling enzymes, clearing the way for new tissue formation. When tested side-by-side with ACS in a critical-sized bone defect model in rats, this polymeric delivery system significantly increased bone formation over ACS controls.

Figure 1

Synthetic hydrogel delivery system. (A) Monomer solutions containing three basic polymer components (4-arm PEG backbone functionalized with norbornene groups, MMP-degradable peptide containing, and short cell adhesion peptides) are polymerized by a photoinitiated reaction to create a synthetic hydrogel matrix. For the studies described in this work, rhBMP2 was added to the monomer solution before polymerization. (B) Hydrogel discs implanted within the calvaria defects were created by polymerizing the monomer solutions in molds created with an 8mm biopsy punch. Monomer solutions with different concentrations of rhBMP2 were applied as a liquid to the molds; 385nm light was used to polymerize the discs, which were then removed from the molds and placed within the defect site.

Figure 2

Calvaria defect closure. (A) Representative images from each experimental group are presented to illustrate defect closure over the 6 week study. Areas that do not meet threshold levels of optical density consistent with bone are represented in red. (B) Week 0 defect area measurement was considered the original defect size, providing a reference for percent defect area closed. Statistical comparisons were made with the untreated controls, and error bars represent 95% Confidence Intervals; # p ≤ 0.05, * p ≤ 0.01.

Figure 3

Skull cap bone volume. (A) Representative 3D rendering at Week 6 from each experimental group are presented to illustrate bone formation at the defect site. Extraneous bone generation is observed in animals treated with hydrogels containing 2.0µg rhBMP2. (B) Quantification of new bone volume within skull cap sections. Week 0 skull cap bone volume is subtracted from the Week 2 and Week 6 measurements to represent new bone formation. Statistical comparisons were made with the untreated controls, and error bars represent 95% Confidence Intervals; * p ≤ 0.01.

Figure 4

Histological evaluation of bone regeneration. Coronal tissue sections through bone defect sites were prepared from de-calcified skulls taken upon completion of the study. Representative sections stained with Masson’s Trichrome are shown with arrows pointing to original defect edge. Additional histology images are presented in Supplemental Figure 3. Scale bars represent 1mm.