
==== Front
Spine (Phila Pa 1976)
Spine (Phila Pa 1976)
BRS
Spine
0362-2436
1528-1159
Lippincott Williams & Wilkins Hagerstown, MD

38988089
SPINE167403
10.1097/BRS.0000000000005091
00011
3
Basic Science
In vivo Assessment of AMP2, a Novel Ceramic-Binding BMP-2, in Ovine Lumbar Interbody Fusion
Christou Chris BVSc, PhD achris.christou@sahmri.com

Varcoe Tamara PhD atamara.varcoe@sahmri.com

Williams Georgia MMagRes Tech ageorgia.williams@sahmri.com

Heil Todd PhD todd.heil@theradaptive.com
b
Leifeld Sarah BSc sarah.leifeld@theradaptive.com
b
Park Hyeon PhD bhyeon@theradaptive.com

Peckham Steve PhD cpeckham.steve@gmail.com

Stewart David PhD bdavid.stewart@theradaptive.com

Greenbaum John MBA bjohn.greenbaum@theradaptive.com

Wang Tian PhD dt.wang@unsw.edu.au

Pelletier Matthew PhD dm.pelletier@unsw.edu.au

Walsh William PhD dw.walsh@unsw.edu.au

Alvarez Luis PhD bluis@theradaptive.com

a South Australia Health and Medical Research Institute, Preclinical, Imaging & Research Laboratories (SAHMRI-PIRL), Gilles Plains, SA
b Theradaptive, Inc. Frederick, MD
c S M Peckham Consulting LLC, Memphis, TN
d Surgical and Orthopaedic Research Laboratory, University of New South Wales, NSW
Address correspondence and reprint requests to Chris Christou, BVSc, PhD, 101 Blacks Rd, Gilles Plains, South Australia, Australia, 5086; E-mail: Chris.Christou@sahmri.com
1 10 2024
11 7 2024
49 19 13811390
27 3 2024
14 6 2024
Copyright © 2024 The Author(s). Published by Wolters Kluwer Health, Inc.
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ This is an open access article distributed under the terms of the Creative Commons Attribution-Non Commercial-No Derivatives License 4.0 (CCBY-NC-ND), where it is permissible to download and share the work provided it is properly cited. The work cannot be changed in any way or used commercially without permission from the journal. http://creativecommons.org/licenses/by-nc-nd/4.0/

Study Design.

Assessment of bone formation in an ovine interbody fusion study.

Objective.

To compare OsteoAdapt SP, which consists of AMP-2, a modified variant of recombinant human bone morphogenetic protein (rhBMP-2) bound to a tricalcium phosphate-containing carrier, to autologous iliac crest bone graft (ICBG) in a lumbar interbody fusion model.

Summary of Background Data.

Treatment of lumbar disk degeneration often involves spinal fusion to reduce pain and motion at the affected spinal segment by insertion of a cage containing bone graft material. Three graft materials were compared in this study—ICBG and OsteoAdapt SP (low or high dose).

Methods.

The sheep underwent lateral lumbar fusion surgery with PEEK or Titanium interbody cages packed with OsteoAdapt SP (low or high dose) or ICBG. Outcomes were evaluated at 8-, 16- and 26- weeks. Newly formed bone quality, bone mineralization, and fusion were assessed by manual palpation, qualitative and semi-quantitative histopathology, histomorphometry, computed tomography (CT), and micro-CT (mCT) analysis.

Results.

OsteoAdapt SP was implanted into 43 animals and ICBG into 21 animals (L3-L4). No group showed evidence of systemic toxicity by multiple assessments. All levels were fused by manual palpation at 26 weeks. Serial CT scans showed increasing fusion scores over time. Both doses of OsteoAdapt SP resulted in robust new bone formation and progression of fusion in the interbody cage. Range of motion tests for treatment groups was lower compared with ICBG at 8- and 16 weeks. Similarly, histology at eight weeks demonstrated more robust new bone formation for both OsteoAdapt SP groups compared to autograft.

Conclusion.

We have demonstrated the preclinical safety and efficacy of OsteoAdapt SP in a clinically relevant large animal model, supporting faster and more robust new bone formation within the interbody cage, comparable to or better than the gold standard, ICBG, in all measures.

Key Words:

Interbody fusion
ovine
spine
bone growth
SDCT
OPEN-ACCESSTRUE
==== Body
pmcThe spine supports about one-half of the body’s weight and is a highly flexible structure. The disk is prone to degeneration due to age and injury, which alters its mechanical properties. This can influence adjacent structures and result in chronic and debilitating pain. When conservative measures fail to relieve pain and disability, spinal fusion is often performed to eliminate or at least greatly reduce motion at the affected spinal segment. Lumbar spinal fusion typically requires implantation of posterior rod and screw instrumentation for immediate stabilization and an interbody cage containing graft material to maintain the intervertebral space and allow bone fusion.1,2

The gold standard of grafting material has been an autograft from the iliac crest (iliac crest bone graft, ICBG), despite the morbidity associated with pain, increased risk of postoperative haemorrhage and increased surgical time.3 The aim of all ICBG substitutes is to mimic the osteogenic, osteoconductive, and osteoinductive properties of autograft. While many inorganic bone grafting materials have osteoconductive properties, they require the addition of an osteoinductive protein such as recombinant human bone morphogenetic protein-2 (rhBMP-2) to serve as an autograft replacement. The bone-forming capability of the commercially available rhBMP-2 formulation is excellent; however, uncontrolled release kinetics and supraphysiologic doses have led to restricted use due to off-target effects, including bone resorption and heterotopic bone formation.3–5 AMP2 is a novel variant of rhBMP-2, designed to bind with extremely high affinity and specificity to the calcium phosphate component of a carrier matrix. This extremely high-affinity results in the retention of the AMP2 on the carrier such that AMP2 is available on timescales that more closely match the timescales required to induce new bone. This also avoids the bolus release normally observed with rhBMP-2 and limits off-target effects by inducing new bone formation in an anatomically precise manner only where the graft is placed. OsteoAdapt SP bone graft contains AMP2 bound to electrospun fibers of polylactic-co-glycolic acid (PLGA), containing beta-tricalcium phosphate (βTCP) granules.6 The use of a sheep as a lumbar spinal fusion model is well established.7,8 In this study, OsteoAdapt SP at two dose levels was compared with ICBG in an ovine lumbar interbody fusion model at 8-, 16- and 26 weeks postimplantation to identify formulations for use in a human clinical investigation.

MATERIALS AND METHODS

OsteoAdapt SP

The OsteoAdapt SP used in this study comprised 70% by weight β-tricalcium phosphate, 30% by weight PLGA, and AMP2 osteoinductive protein. The low-dose formulation contained 0.8 mg AMP2 per cc of carrier matrix, while the high dose contained 2 mg AMP2 per cc of carrier matrix.

Surgical Method

This study was conducted under a protocol approved by the South Australian Health and Medical Research Institute (SAHMRI) IACUC and Animal Care and Use Review Office (ACURO) of the US Army Medical Research and Development Command. Skeletally mature (minimum two-year-old and 50 kg in bodyweight) female sheep were enrolled in the study following a veterinary exam and normal presurgical blood results. Twenty-four hours before surgery a fentanyl patch (2 ug/kg/hr) was applied to the antebrachium for pre-emptive analgesia and replaced at the time of surgery for 72 hours of postoperative analgesia.9 Anaesthesia was induced with xylazine (0.2 mg/kg IM) followed by ketamine (6 mg/kg IM). The sheep were intubated, and anesthesia was maintained with isoflurane and 100% oxygen at 2 L/minute. Sheep were given penicillin and meloxicam at the time of anesthetic induction. The sheep underwent single-level lateral lumbar interbody fusion surgery. With the sheep in lateral recumbency, two 19 g needles were placed in the epaxial musculature as markers. The L3/4-disk space was identified using fluoroscopy and counting down to the markers from the last rib. The needles were retained following imaging as reference points during the surgical exposure. An incision was made in the left lateral sub-lumbar space, the external and internal oblique muscles were bluntly dissected providing access to the spinal column using a retroperitoneal approach. Self-retaining retractors were then used to maintain access to the L3–L4 disk. A lateral window was made in the annulus fibrosis to allow access to the nucleus pulposus. The nucleus was removed, and the endplates were prepared for the interbody cages. The cages, 17 mm×10 mm×6 mm polyetheretherketone (PEEK), (Theradaptive Inc, Frederick, MD), were packed with low or high dose OsteoAdapt SP or reference item, autologous bone. Just before loading into the cage, 40 mg of OsteoAdapt SP was hydrated with ~0.2 mL of autologous acid citrate dextrose (ACD)-anticoagulated blood collected from the test animal at the time of surgery. The autograft used was harvested from the iliac crest and morselised. After implantation of the interbody device, the sheep was repositioned into sternal recumbency and bilateral posterior rod and pedicle screw instrumentation was implanted through a separate incision (Titanium rods [5.5 mm diameter, 58 mm length] and pedicle screws [4.5 mm diameter, 35 mm length]). Postoperative fluoroscopy and CT images were taken to assess implantation sites. Study end points were at 8-, 16- and 26 weeks (n=6 /treatment/time point) for a total of n=54 sheep.

Evaluations

Blood was collected from each animal before implantation and at 8-, 16- and 26 weeks postimplantation to assess health status through routine diagnostic hematological (Roche Sysmex XN Vet 1000 Sysmex Corporation, Kobe, Japan) and biochemical parameters (Advia 1800 Siemens Healthcare Diagnostics Inc, Flanders NJ, USA) (Appendix, Supplemental Digital Content 1, http://links.lww.com/BRS/C476). In-life computed tomography (CT) scans (0.75 mm slices) were taken postoperatively for each animal every four weeks to the 20-week time point, and then at the 26-week end point (Siemens Biograph mCT, Siemens Healthcare Germany). At end point, animals underwent a comprehensive necropsy, and tissue samples were collected for histopathology. Spinal fixation was removed and the fusion mass was assessed by manual palpation by the veterinary surgeon (CC) who was blinded to the treatment groups with each segment scored as “fused” or “not fused” based on the absence or presence of motion.10 Local tissue effects were evaluated through macroscopic observations and qualitative and semi-quantitative histopathologic analysis consistent with ISO 10993:6. The quality of the newly formed bone, bone mineralization, as well as fusion assessment of the implant site, were made using CT and mCT images and mechanical range of motion testing. Systemic safety and toxicity were evaluated in the distant tissues as per ISO 10993:11. The evaluation was conducted by a Board-Certified Veterinary Pathologist.

Radiography

The in vivo CT images were graded from one to four by two independent assessors (C.C. and G.W.), blinded to the treatment group and time, according to published fusion criteria for bone regeneration within the fusion cage, as shown in Table 1.11

TABLE 1 Fusion Grading System for In Vivo CT Analysis

Score	Description	
1	Nonfusion: significant radiolucent lines or gaps surrounding the cage with no bone ingrowth or bridging, 0%-25% of the graft area showing no lucency between the implant and vertebral bone	
2	Partial fusion: some lucency or gaps, with 25%-50% of the graft area showing no lucency between the implant and vertebral bone, as well as bone ingrowth in the implant and partial bone bridging through the implant	
3	Partial fusion: some lucency or gaps, with 50%-75% of the graft area showing no lucency between the implant and vertebral bone, as well as bone ingrowth in the implant and partial bone bridging through the implant	
4	Complete fusion: 75%-100% of the graft area showing no lucency between implant and vertebral bone, with bone ingrowth and complete bone bridging	

Micro-CT was performed on the spines using an Inveon Scanner (Siemens, USA). Resolution was set to 38 microns for all scans. Three-dimensional (3D) models were reconstructed from the µCT images and examined for fusion in the axial, sagittal, and coronal planes. Images were graded by two trained and experienced observers blinded to the treatment group and time point. Each sample was graded for bone with a scale of 1 to 4, representing the amount of bone qualitatively at the level shown in Table 2 below. 3D reconstructions were also reviewed to evaluate the overall fusion status as per Tables 1 and 2.

TABLE 2 Semiquantitative Microradiographic Fusion and Bone Fill Scoring

Fusion criteria	Score	Bone fill criteria, %	Score	
No new bone - no new bone formation visible	0	0-25	1	
Visible new bone - new bone formation visible, but no continuous bone	1	26-50	2	
Possible fusion - continuous bridging of new bone with visible lucency	2	51-75	3	
Probable fusion - continuous bridging of new bone formation	3	76-100	4	

Range of Motion (ROM) Testing

Following the explant, each spinal segment was vacuum sealed and placed on ice for 12 hours before mechanical testing. All soft tissues were removed from the spinal segments, and range of motion testing was performed at room temperature, as previously reported with the rods removed.1,12 The segments were carefully potted in a resin for ROM evaluation. Range of motion in flexion-extension (FE), lateral bending (LB), and axial rotation (AR) was determined using a robotic 6˚ of freedom musculoskeletal simulator, simVITRO (Simulation Solutions and Cleveland Clinic, Ohio) nondestructively to ±7.5 Nm. Each loading profile was repeated 4.5 times and the mean value for ROM of the last three cycles was used for statistical comparison. ROM data were analyzed using an in-house custom MatLab script. A 7.5 Nm pure moment was applied to the spines in FE, LB, and AR, and the resulting angular deformation was recorded with the testing equipment. Each loading profile was repeated 3 times, and a mean value for FE, LB, and AR was obtained for each treated level.

Spine Histology

Immediately following mechanical testing, the spine segments and spinal cords were fixed in 10% phosphate-buffered formalin at room temperature with gentle rotation on a Labtech rotating shaker for a minimum of 96 hours. The spinal cords were then paraffin-embedded for routine hematoxylin and eosin staining. The spine segments were processed for polymethylmethacrylate (PMMA) histology stained with methylene blue and basic fuschin (minimum 3 sections cut).1,2 Stained sections of the interbody fusion were carefully examined at low magnification (4× objective) as well as under high power fields (10×, 20×, 40× objective) for general tissue response and for the presence of inflammatory cells or tissue necrosis as per ISO 10993:6.

Histomorphometry

Histomorphometry was performed on the images from three PMMA slides using a validated custom MATLAB program (Version 3.3) at the Surgical and Orthopaedic Research Laboratory (SORL) to determine the area percentage bone tissue (new bone and marrow), and other soft tissues present in sections. Low magnification images (1.25×, 1 mm scale bar) were taken to evaluate the entire graft space. Regions of interest (ROI) for each image were determined and outlined using a polygon technique. Graft and bone tissue (mineralized bone and bone marrow elements) were identified by pixel color and morphology, and the area was determined as a percentage of the ROI.

Statistical Analysis

Statistical analyses on in vivo clinical pathology data were conducted using GraphPad Prism, version 9 (Dotmatics, MA, USA). One-way ANOVA was used to assess the effect of treatment on clinical pathology (blood results) at baseline, 8-, 16- and 26 weeks postsurgery. Fusion grading scores from in vivo CT images were analyzed by two-factor ANOVA with treatment and time as factors.

Range of motion comparisons were made using SPSS to perform multivariate ANOVA with post hoc tests where relevant. A mean value was obtained for each animal based on the PMMA slides for the histomorphometry, and ANOVA followed by post hoc tests were performed on data using SPSS for Windows.

Data are reported as the mean ± SD unless indicated otherwise. P<0.05 was considered statistically significant.

RESULTS

Clinical Outcomes

All animals recovered well from surgery. At ~three weeks post-implant, two animals in the high-dose OsteoAdapt SP group were showing signs of partial paresis. In both, an in vivo CT scan before euthanasia confirmed good placement of the interbody cage at L3/4. There was bony lysis and remodelling at the level of L3/4-disk space, affecting the endplates of L3 and L4, and several small dense foci encroaching slightly into the vertebral canal. This suggested possible very mild cord compression or irritation from bone fragments responsible for the clinical symptoms. One of the animals was replaced. At the week-4 scans, two other animals (one in low-dose OsteoAdapt SP and one ICBG group) showed similar bony lysis visible on CT with no ill effects.

There were no differences between any groups in any hematological or biochemical parameters at any of the monthly collection time points. Nor was there any evidence of systemic toxicity in any group, as per clinical pathology (blood biochemistry and hematology) and gross morphology of the tissues at necropsy (data not shown).

Manual Palpation

Explanted spinal segments tested for fusion by manual palpation showed that at eight weeks, two animals from the ICBG group and one each from the low and high OsteoAdapt SP groups were not fused. At 16 weeks, one animal in the ICBG group was not fused, while all animals treated with OsteoAdapt SP were assessed as fused. At 26 weeks, all animals in all groups were assessed as fused (Fig. 1 A).

Figure 1 In vivo and ex vivo fusion scores for ICBG (clear box), low-dose OsteoAdapt SP (grey box), and high dose OsteoAdapt (black box) at 8-, 16- and 26 weeks posttreatment. *=P<0.05. A (manual palpation) and B, in vivo fusion scored from CT images as a percent (A) and score (B). Panels C-D show mCT evaluations of fusion (C) and bone grade (D), and the bone in the available void (Panel E) and bone and marrow in the available void (Panel F) from the PMMA histology slides. n=6 per group.

CT Assessment

Assessment based on serial CT scans showed increasing fusion scores over time. The only significant difference was increased fusion score in animals treated with the low OsteoAdapt SP compared to ICBG at the 8-week time point. These results are shown graphically in Fig. 1 (B).

Figure 2 shows coronal CT images of progressive fusion in ICBG and low-dose OsteoAdapt SP.

Figure 2 Coronal CT images showing progressive fusion in ICBG (top panel) and low-dose OsteoAdapt SP (bottom panel).

mCT Fusion and Bone Grading

Figure 1 above (C-D) shows the fusion and bone grades by time point and group. Bone grade was higher for the OsteoAdapt treated levels compared to ICBG at 8 weeks. Fusion grade and bone grade increased between 8 weeks and later time points in the ICBG group. Figure 3 below shows representative mCT images showing the progression of fusion from 8- to 26 weeks for all groups. ICBG and low-dose OsteoAdapt SP. Robust new bone formation is evident in all samples to varying degrees.

Figure 3 Representative mCT images in the sagittal plane at 8 weeks (top panel) and 26 weeks postimplantation (bottom panel) of (A) Low OsteoAdapt SP, (B) High OsteoAdapt SP, and (C) ICBG. At 8 weeks, robust new bone formation is evident to varying degrees in low and high OsteoAdapt animals. In the ICBG animals, initial graft is visible and new bone formation is present, while there are areas of lucency. At 26 weeks, robust new bone formation is evident in all samples.

Range of Motion

The degree of movement decreased with time as the spinal fusion progressed in all groups (Fig. 4). There was a significant treatment effect on the degree of lateral bending and axial rotation (P<0.05), whereby the high OsteoAdapt SP group showed reduced movement compared with the ICBG group at 8 weeks postsurgery. No differences were detected between the low and high-dose treatment groups. Figure 4 shows the mean (± SD) results for each group over time.

Figure 4 Mean (+SD) degrees of bending for lateral bending (A), axial rotation (B), and flexion/extension (C) for ICBG (open boxes), low-dose OsteoAdapt SP (grey boxes), and high-dose OsteoAdapt (black boxes) over time. *=P<0.05 n=6/group/time point.

Spine Histology

Figure 5 shows representative examples of stereo zoom microscopy images as well as higher magnification examples from each group at each time point at 8 weeks postimplantation. Bridging new bone is evident with residual graft material present in the OsteoAdapt animals. New bone formation is observed in the ICBG-treated level without bridging. Residual autografts can be seen surrounded by new bone.

Figure 5 High magnification histology examples at 8 weeks postimplantation for (A) OsteoAdapt SP low-dose (B) OsteoAdapt SP high dose, and (C) IBCG at (i) ×40, (ii) ×200 and (iii) ×400 magnification. In the low and high OsteoAdapt samples, extensive new bone formation is evident, completely bridging the space. Residual graft material is evident at this time point, with some multinucleated cells where the graft was exposed to developing marrow spaces but not when surrounded by bone. ICBG had less bone formation, incomplete bridging, and the presence of developing marrow spaces but also some fibrous tissue.

Figure 6 shows a representative image from each group at 26 weeks postimplantation. Both OsteoAdapt SP dose levels supported the new bone formation and normal remodelling with time, facilitating fusions between the treated levels. OsteoAdapt SP facilitated new bone formation directly on the graft material within the implanted cages. The graft resorbed with time but was still present at the latest time point. New bone is evident, bridging the disk space and integrating with the host vertebral bodies. Some residual fibers were present in the OsetoAdapt SP treated levels, generally in direct opposition to new bone. The fusion masses are remodelled by this phase, with marrow spaces evident and osteoblasts surrounding developing marrow regions.

Figure 6 High magnification histology examples at 26 weeks postimplantation for (A) OsteoAdapt SP low-dose (B) OsteoAdapt SP high dose, and (C) IBCG at (i) ×40, (ii) ×200, and (iii) ×400 magnification. In the low and high OsteoAdapt samples, extensive new bone formation is evident, completely bridging the space with greater remodeling than 8- and 12 weeks. The residual graft material is still present, but less than earlier time points, with some multinucleated cells where the graft was exposed to developing marrow spaces but less than earlier time points. ICBG had significantly increased bone formation and bridging, and less fibrous tissue at 26 weeks compared with the earlier time points.

ICBG was resorbed and remodelled in a normal manner consistent with the expected response autograft. The spinal cords showed no changes in all groups. Neuronal cell bodies and myelinated nerve fibers were unaffected by the surgical procedure.

To determine OsteoAdapt SP tissue reactivity, histology slides were scored according to ISO 10993:6. The ICBG score was subtracted from the test treatment averages. Under the conditions of the study, all test samples of each dose group at all time points had an average score (test sample minus control) of less than 2.21. As per 10993:6, this demonstrates that the test material has none or minimal reaction upon implantation.

Bone Histomorphology

OsteoAdapt SP supported the new bone formation and normal remodelling at both dose levels. At 8 weeks postoperatively, low-dose OsteoAdapt SP animals were found to have significantly increased bone within the available void compared with the ICBG group (Fig. 1, E). Furthermore, animals from the low-dose and high-dose OsteoAdapt SP groups had significantly increased bone and bone marrow within the available void compared to the ICBG group at 8 weeks postimplantation (Fig. 1, F). At this time point, the ICBG group also had more soft, fibrous, or collagenous tissue compared with the OsteoAdapt SP groups. At 16 weeks postimplantation, the low and high OsteoAdapt SP groups had a lower percentage of marrow compared with the ICBG group. There were no other differences between the groups at this time point.

For analyses of the 26 weeks postimplantation, there were no differences between the groups for the bone, marrow, or combination in the available void. As expected, all groups showed an increase in bone in the available void from 8- to 26 weeks postimplantation.

DISCUSSION

This study demonstrates the preclinical safety and efficacy of OsteoAdapt SP at two concentrations in a clinically relevant large animal model. Performance and safety of OsteoAdapt SP was comparable to, or better than, the gold standard, ICBG, in all measures.

Terminal time points (8-, 16- and 26 weeks following implantation) were chosen to evaluate early, medium-term, and long-term safety and performance of the test and reference items and are consistent with the range of end points from similar studies.1,13–18 Fusion with the autograft was expected to occur well before 26 weeks, and therefore, this time point represents a steady state of bone fusion and cellular response to the test item, compared to the reference item.

Both dose levels of OsteoAdapt SP facilitated bone fusion with a normal bone formation response. An early, mild transient osteolytic response was noted in some animals, which has also been reported elsewhere when using BMP.4 However, as this was seen across all three treatment groups, it suggests that the cause was surgical procedure-related rather than due to graft material effects. The nature of the calcium-bound AMP2 and its retention on the scaffold appears to have controlled the severe osteolysis previously reported with BMP2, depending on the dose. The aim of this study was to evaluate the safety and performance of the AMP2/carrier medical device. Histologically, the graft material resorbed with time and was still present at the last time point. Regions of residual graft material were incorporated directly in bone, as well as some exposed material resorbing in a macrophage-mediated response. Fusions progressed with normal bone healing and remodelling, providing a continuous bony bridge at the treated levels. Local cell and tissue responses, based on ISO 10993:6, did not reveal any adverse reactions to the graft material and local release of OsteoAdapt SP using the current delivery system. Local spinal cord analysis did not show any adverse reaction.

Histopathology of the harvested organs, as per ISO 10993:11, did not reveal any adverse reaction or systemic off-target effects of the AMP2, further demonstrating the precise delivery of AMP2 and only the local effects of the new formulation.

OsteoAdapt SP at both dose levels supported an initial, more rapid new bone response within the interbody cage. However, at the latter time points, it proved equivalent to ICBG. The earlier time points presented in this sheep model allowed the detection of differences that might be meaningful in more challenging conditions such as spinal fusion in people with co-morbidities. Measures of spinal fusion, including in vivo CT, range of motion testing and micro-CT, demonstrated the OsteoAdapt SP performed better than ICBG at early time points, demonstrating faster bone growth following surgery. Clinically, faster fusion may translate into an earlier return to normal activities, which leads to a lower burden on the healthcare system. It also reduces the opportunity for failure of the spinal implants (screw/rod systems). With the AMP2 osteoinductive protein specifically bound to the carrier matrix, OsteoAdapt SP represents an advancement over current technologies by allowing for an anatomically precise prolonged delivery of osteoinductive AMP2 to the cells in the defect site, thereby avoiding the rapid supraphysiologic release of BMP2 from the carrier matrix and subsequent risk of exuberant bone formation. In turn, this limits the risk of pain and adverse effects from off-target responses. In addition, as an off-the-shelf pre-bound unitary preparation, OsteoAdapt SP simplifies product preparation to a single step and reduces surgical time.

In conclusion, this study has demonstrated the safety and efficacy of AMP2 and the utility of OsteoAdapt SP as a better substitute for ICBG.

Limitations to this study include the slightly oversized cages which required some loss of endplate morphology to allow implantation, this therefore does not fully replicate the clinical situation. Evidence is present in some histological sections indicating growth plates that have not fully closed, potentially impacting the fusion process.

Key Points

A large animal model spinal fusion model was used to evaluate a novel BMP2 formulation

The use of a modified calcium-bound BMP2 showed healing equivalent to or better than ICBG.

The controlled release kinetics of the modified BMP2 assisted healing and reduced the risk of off-target effects.

The study shows good safety and efficacy data.

Supplementary Material

SUPPLEMENTARY MATERIAL

Acknowledgments

The authors thank the facilities and scientific and technical assistance of the National Imaging Facility, a National Collaborative Research Infrastructure Strategy (NCRIS) capability, at the Large Animal Research and Imaging Facility (LARIF), of the Preclinical, Imaging and Research Laboratories (PIRL), South Australian Health and Medical Research Institute (SAHMRI).

The device(s)/drug(s) that is/are the subject of this manuscript is/are being evaluated as part of an ongoing FDA-approved investigational protocol (IDE) or corresponding national protocol to replace the reliance on iliac crest bone graft in spinal fusion surgeries.

C.C. all surgery and Veterinary care, scoring of CT; T.V., in-life phase of the study; G.W. all in vivo radiography and CT; W.W., M.P. all micro CT, ROM, spine histology, scoring of fusion and bone growth; T.H., S.L., H.P., S.P., D.S., J.G., and L.A. product development of AMP2 and OsteoAdapt.

Supplemental Digital Content is available for this article. Direct URL citations are provided in the HTML and PDF versions of this article on the journal's website, www.spinejournal.com.

Opinions, interpretations, conclusions, and recommendations are those of the author and are not necessarily endorsed by the Department of Defense. In conducting research using animals, the investigator(s) adheres to the laws of the United States and regulations of the Department of Agriculture. The U.S. Army Medical Research Acquisition Activity, 820 Chandler Street, Fort Detrick, MD 21702-5014, is the awarding and administering acquisition office.

This study was sponsored by Theradaptive Inc. This work was supported by the office of the Assistant Secretary of Defense for Health Affairs endorsed by the Department of Defense through the Peer Reviewed Medical Research Program under Award Nos. W81XWH-20-1-0305 and W81XWH-22-1-0875.

The authors report no conflicts of interest.
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