Retention of ¹²⁵I‐labeled recombinant human bone morphogenetic protein‐2 by biphasic calcium phosphate or a composite sponge in a rabbit posterolateral spine arthrodesis model

Abstract

The purpose of this study was to characterize the retention kinetics of recombinant human bone morphogenetic protein-2 (rhBMP-2) applied to two calcium-based delivery matrices. Biphasic calcium phosphate (BCP) and a composite containing BCP in an absorbable collagen sponge (BCP/ACS) were evaluated using a spinal fusion model in rabbits. rhBMP-2 labeled with radioactive iodine ('"I) was used as a tracer to assess in vivo retention of rhBMP-2 in the presence of these materials (nine animals per material studied). Over a 36 day study period, animals were assessed for the following: percent administered dose retained at the implant site as measured by scintigraphic imaging (counting) with a gamma camera (all animals), radiography of the implant site (all animals), radioactivity in blood and plasma (all animals), and radioactivity in the urine and feces (three animals for each material). Radioactivity data were corrected for the decay of "' I and the attenuation between the implant in vivo and the gamma camera.Differences observed between the two materials for the area under the retention vs. time profile (AUC; 988Ybday for BCP vs. 1070%*day for BCP/ACS, p = 0.57) and the mean residence time (MRT; 10.2 days for BCP vs. 7.6 days for BCP/ACS, p = 0.06) were not statistically significant. Initial retention/incorporation of rhBMP-2 was slightly higher for rhBMP-2/BCP/ACS than for rhBMP-2/BCP (96.8% vs. 86.O%, p < 0.05). Animals receiving rhBMP-2/BCP showed a longer terminal retention half-life ( t i / * ) than did those receiving rhBMP-2/BCP/ACS (7.5 vs. 4.5 days, p < 0.05). The urinary radioactivity recovery data supported the data obtained by scintigraphy. Over the 36 day collection period, essentially complete recovery of radioactivity (dose) in urine was observed for rhBMP-2/BCP and rhBMP-21BCPIACS and the majority of the radioactivity (approximately 95%)) was soluble in trichloroacetic acid, suggesting extensive catabolism of rhBMP-2 before renal excretion. Fecal recovery of radioactivity was low, approximately 2-3%.In conclusion, rhBMP-2 was retained at the implant site when delivered with either BCP or BCPiACS based on mean residence time and area under the retention curve vs. time profile. Use of these matrices resulted in detectable rhBMP-2 levels at the surgical site for over a week in contrast to data reported with several other matrices that lasted less time. Systemic catabolism and elimination of the rhBMP-2 was extensive and systemic presence of the protein was negligible.

125I-labeled OP-1 is Locally Retained in a Rabbit Lumbar Fusion Model

“…Carrier biomaterials act as frameworks for osteogenic cell colonization and improve localization of implanted protein [12,16]. It is unclear whether fusion enhancement depends on the action of carrier-bound BMP or of freely diffusible BMP slowly released from the carrier [18].…”

“…It is unclear whether fusion enhancement depends on the action of carrier-bound BMP or of freely diffusible BMP slowly released from the carrier [18]. Nevertheless, elucidation of retention kinetics may aid evaluation and development of BMP delivery systems to optimize clinical outcomes [12,18].…”

“…Delivery of 125 Ilabeled rhBMP-2 on an absorbable collagen sponge (ACS) in the rat ectopic model yielded MRTs of approximately 3 days [18]. In a rabbit posterolateral lumbar fusion model, implantation of radiolabeled rhBMP-2 on a biphasic calcium phosphate-absorbable collagen sponge (BCP/ACS) combination or on BCP alone produced MRTs of 7.6 and 10.2 days, respectively, although this difference was not statistically significant [12].…”

“…Although local treatment with the BMPs is not associated with a specific peripheral effect in toxicology studies, these proteins have been shown to have transforming effects in such diverse organs as the kidneys [21], heart [4], and brain [9]. Using scintigraphy to measure blood sample radioactivity, systemic exposure to rhBMP-2 in a rabbit spine fusion model has been judged to be negligible [12]. The actual delivery of implanted rhBMPs to tissues throughout the body, however, has yet to be systematically analyzed in a lumbar fusion model.…”

Spine fusion by gene therapy

“…Another issue is that BMP molecules are soluble and can diffuse away or become inactivated in vivo. 30 Therefore, a single administration dose strategy may have a very limited duration of action and may not be ideal in certain spine fusion applications. Perhaps, because of one or more of these factors, not all clinical trials have yielded equally successful results.…”