BMP-2 and BMP-2/7 Heterodimers Conjugated to a Fibrin/Hyaluronic Acid Hydrogel in a Large Animal Model of Mild Intervertebral Disc Degeneration

Abstract

Intervertebral disc (IVD) degeneration is etiologically associated with low back pain and is currently only treated in severe cases with spinal fusion. Regenerative medicine attempts to restore degenerated tissue by means of cells, hydrogels, and/or growth factors and can therefore be used to slow, halt, or reverse the degeneration of the IVD in a minimally invasive manner. Previously, the growth factors bone morphogenetic proteins 2 and 7 (BMP-2, -7) were shown to enhance disc regeneration, in vitro and in vivo. Since BMPs have only a short in vivo half-life, and to prevent heterotopic ossification, we evaluated the use of a slow release system for BMP-2 homodimers and BMP-2/7 heterodimers for IVD regeneration. BMP growth factors were conjugated to a fibrin/hyaluronic acid (FB/HA) hydrogel and intradiscally injected in a goat model of mild IVD degeneration to study safety and efficacy. Mild degeneration was induced in five lumbar discs of seven adult Dutch milk goats, by injections with the enzyme chondroitinase ABC. After 12 weeks, discs were treated with either FB/HA-hydrogel only or supplemented with 1 or 5 μg/mL of BMP-2 or BMP-2/7. BMPs were linked to the FB/HA hydrogels using a transglutaminase moiety, to be released through an incorporated plasmin cleavage site. After another 12 weeks, goats were sacrificed and discs were assessed using radiography, MRI T2* mapping, and biochemical and histological analyses. All animals maintained weight throughout the study and no heterotopic bone formation or other adverse effects were noted during follow-up. Radiographs showed significant disc height loss upon induction of mild degeneration. MRI T2* mapping showed strong and significant correlations with biochemistry and histology as shown before. Surprisingly, no differences could be demonstrated in any parameter between intervention groups. To our knowledge, this is the first large animal study evaluating BMPs conjugated to an FB/HA-hydrogel for the treatment of mild IVD degeneration. The conjugated BMP-2 and BMP-2/7 appeared safe, but no disc regeneration was observed. Possible explanations include too low dosages, short follow-up time, and/or insufficient release of the conjugated BMPs. These aspects should be addressed in future studies.

A large animal model that recapitulates the spectrum of human intervertebral disc degeneration

“…Previous studies of low dose intradiscal injection of ChABC (approximately 0.04U per disc) reported non-significant changes in disc height index at 12-weeks, but significant alterations to histologic scores and MRI. 18,46 Increases in disc ROM in lateral bending and axial rotation, and reduced stiffness in axial rotation have also been reported in this model, consistent with the alterations in mechanical properties observed in the current study. 47 Increases in catabolic gene expression have also been illustrated in this model with increasing severity of degeneration.…”

Hyaluronic Acid Microgels Modulate Inflammation and Key Matrix Molecules toward a Regenerative Signature in the Injured Annulus Fibrosus

“…Approaches involving the use of different natural ECM‐based scaffolds, such as hyaluronic acid (HA), collagen, and fibrin, gene therapy, growth factor injection, cell‐based therapies, and tissue engineering approaches, are being developed to induce regeneration of the IVD . For this reason, several in vivo studies have been undertaken to shed light on the biomechanical properties, and the biology of the intervertebral disc regeneration due to distinct treatments utilizing different degeneration models …”

Implant Composed of Demineralized Bone and Mesenchymal Stem Cells Genetically Modified with AdBMP2/AdBMP7 for the Regeneration of Bone Fractures in Ovis aries

“…Recently, a slow release method for rhBMP2 and rhBMP7 was tested in a fibrin hydrogel of hyaluronic acid for regeneration of spinal disc in goats in a period of 12 weeks; but no disc regeneration was observed in any of the study groups. The authors postulate that this could occur due to the low doses of BMPs, a short followup time, and/or insufficient release of the heterodimers rhBMP2/rhBMP7 [20]. …”