The bone morphogenetic protein-2 (BMP-2) is one of the most potent growth factors for bone repair. In the clinic, BMP-2 is widely used for spinal fusion, particularly in the product called InFUSE Bone Graft® (Medtronic). Despite its strong efficacy, the safety of BMP-2 remains questionable as some treated patients suffer from serious side-effects, such as ectopic bone formation, nerve damage, severe inflammation and cancer. In this project, we engineered a bridge protein that effectively slowed the release of BMP-2 from collagen sponges, the carrier material used in InFUSE Bone Graft®, thus allowing significant dose reduction of BMP-2 for bone repair.
The bridge protein was designed for dual affinity to collagen I and to BMP-2. Specifically, it was made by the fusion of the fragment antigen-binding (Fab) of an anti-collagen antibody to the growth factor-binding domain of laminin, which displays high affinity to BMP-2. The bridge protein was produced in Human Embryonic Kidney (HEK) 293 mammalian cells and purified by affinity-mediated chromatography. In all experiments, the bridge protein was simply admixed to BMP-2 prior to incorporation into collagen sponges. We first tested the efficacy of the bridge protein in slowing BMP-2 release in vitro. Then, we assessed the therapeutic efficacy of BMP-2 ± bridge protein, delivered in collagen sponges, in two in vivo mouse models of bone regeneration, a critical-size calvarial defect model and an intervertebral defect model newly developed by us to mimic spinal fusion in mice. Bone regeneration was monitored via in vivo CT scan imaging.
In vitro, the bridge protein strongly enhanced the retention of BMP-2 into collagen sponges. Indeed, BMP-2 was released over more than 7 days versus about 3 days in presence or absence of the bridge protein respectively. Upon single implantation in vivo, the bridge protein permitted significant improvement of bone regeneration at very low doses of BMP-2, as measured by the volume of newly formed bone, the defect coverage and the rate of spinal fusion. Positive results were consistently observed in both the calvarial and intervertebral defect models in mice . In addition to retention in collagen sponges, we demonstrated that the bridge protein allowed local retention of BMP-2 in the endogenous collagenous extracellular matrix (ECM) of tissues.
We engineered a bridge protein that substantially improved the delivery of BMP-2 from collagen-based materials. Combining the bridge protein to BMP-2 significantly enhanced bone regeneration in vivo, therefore allowing good therapeutic efficacy at very low dose of BMP-2. Such protein engineering approach for growth factor delivery may be generalizable to many other applications of tissue engineering, considering the broad use of collagen-based materials in regenerative medicine (e.g., collagen sponges, collagen hydrogels, decellularized matrices).
- Briquez, P. S. et al., Sci. Adv. 7:eabh4302 (2021)"