Characterizing the host response to rhPDGF-BB in a rat spinal arthrodesis model

Abstract

Background: Due to the constraints surrounding autograft bone, surgeons have turned to osteoinductive agents to augment spinal fusion. Reports of complications and questionable efficacy slowed the adoption of these alternatives. Recombinant human platelet-derived growth factor B homodimer (rhPDGF-BB) has been Food and Drug Administration (FDA)-approved (Augment) to promote fusion in other areas of orthopedics, but its characterization in spine fusion has not yet been tested. The purpose of this study is to characterize the host response to PDGF-BB in vivo.

Methods: Eighty female Fischer rats underwent L4-5 posterolateral fusion using one of four implant types: (a) iliac crest syngeneic allograft harvested from syngeneic donors, (b) β-TCP/bovine collagen matrix (β-TCP/Col) with sodium acetate buffer, (c) β-TCP/Col with 0.3 mg/mL "low dose," or (d) β-TCP/Col with 3.0 mg/mL "high dose" of rhPDGF-BB. Animals underwent magnetic resonance imaging (MRI) and serum cytokine quantification at 4, 7, 10, and 21 days, postoperatively. Tissues were processed for immunofluorescence staining for Ki67 and von Willebrand factor (vWF) to assess neovascularization.

Results: MRI demonstrated no differences in fluid accumulation among the four treatment groups at any of the time points. Serum cytokine analysis showed no clinically significant differences between treatment groups in 20 of the 27 cytokines. Inflammatory cytokines IFN-γ, IL-1β, IL-18, MCP-1, MIP-1α, TNF-α were not induced by rhPDGF-BB. Histology showed no differences in cell infiltration, and Ki67 and vWF immunofluorescence staining was similar among groups.

Conclusions: rhPDGF-BB delivered with a β-TCP/Col matrix exerts no exaggerated systemic or local host inflammatory response when compared to iliac crest syngeneic allograft bone or the control carrier. rhPDGF-BB mixed with a β-TCP/Col matrix could be a viable and safe biologic alternative to syngeneic allograft in spine fusion. Further studies need to be performed to evaluate efficacy in this setting.

Keywords: MRI; PDGF; arthrodesis; biologic; cytokine; platelet‐derived growth factor; spine surgery.

FIGURE 1

Modified rat arthrodesis model with resected tissue demonstrating (A) the exposure of the lumbar transverse process and fusion bed and (B) the implanted test article with resected paraspinal muscles. The implanted test article is outlined by the yellow dashed lines

FIGURE 2

MRI analysis data with qualitative axial slices (A) and quantitative volumes of hyperintensity over time (B). (A) Representative T2‐weighted axial MRI slices of the lumbar spine at L4‐L5 disc space with overlays from ITK‐SNAP. The light red overlay indicates the muscle tissue that was selected. The aqua overlay indicates the autosegmented areas of hyperintensity, representing areas of inflammation. (B) The mean volume of hyperintense areas on T2‐weighted axial MRI slices over time. No statistically significant differences were seen among any of the groups at any of the timepoints

FIGURE 3

Inflammatory cytokine analysis comparing the fold‐change of the cytokines isolated in the rats with the high‐dose PDGF + matrix test implant compared to those with allograft showing no significant differences over time. IL‐18 at Day 4 and MCP‐1 at Day 7 in the rats with the implanted test articles were not statistically different. Low‐dose PDGF + matrix similarly showed no statistical differences over time

FIGURE 4

Representative H&E‐stained histology images at 1.25× magnification of each treatment group at each time point. (A‐D) Allograft. (E‐H) Matrix alone. (I‐L) Low‐dose (0.3 mg/mL) rhPDGF‐BB + matrix. (M‐P) High‐dose (3.0 mg/mL) rhPDGF‐BB + matrix. Scale bar represents 1000 μm. AG, allograft bone; IVB, intervertebral bone; IVD, intervertebral disc; TCP, β‐TCP particle

FIGURE 5

Representative H&E‐stained histology images at 40× magnification of each treatment group at each time point. (A‐D) Allograft. (E‐H) Matrix alone. (I‐L) Low‐dose (0.3 mg/mL) rhPDGF‐BB + matrix. (M‐P) High‐dose (3.0 mg/mL) rhPDGF‐BB + matrix. Scale bar represents 50 μm. AG, allograft bone; IVB, intervertebral bone; IVD, intervertebral disc; TCP, β‐TCP particle

FIGURE 6

Representative immunofluorescence images of (A) Ki67‐ and (B) vWF‐stained tissue slices. Ki67 and vWF signal are shown in green. White dotted circles outline the implant region, thin dotted line indicate the vertebral bodies. Quantified Ki67 (C) and vWF (D) signal, 4, 7, 10, and 21 days postoperatively, reported as binary area (μm²). No statistically significant differences seen