Effect of rhBMP-2 dose on bone formation/maturation in a rat critical-size calvarial defect model

Background: Application of recombinant human bone morphogenetic protein-2 (rhBMP-2) has been associated with significant adverse events in craniofacial settings, including swelling and seroma formation. Recent work has demonstrated an inverse relationship between bone formation/maturation and rhBMP-2 dose, frequency/severity of adverse events increasing with rising dose. Objective: The objective of this study was to determine the most effective dose for rhBMP-2 soak-loaded onto an absorbable collagen sponge (ACS) carrier for bone formation/maturation using an established defect model. Methods: One hundred sixty-eight outbred male Sprague-Dawley rats, age 11-13 weeks, weight 325-375 g randomized into seven groups of 24 subdivided into groups of eight, were used to provide radiographic and light microscopy observations of bone formation/maturation and aberrant healing events at 2, 4 and 8 weeks following application of rhBMP-2/ACS into critical-size, (sic)8-mm, through-through, calvarial osteotomy defects for a dose of 1.25, 2.5, 5.0, 10.0 and 20.0 mu g rhBMP-2/defect, or serve as ACS or sham-surgery controls. Results: rhBMP-2 dosages >= 2.5 mu g/defect showed histological defect closure >90% within 2 weeks, and complete resolution within 4 weeks. Adverse healing events including swelling, excessive bone formation or seroma formation could not be determined with certainty in this defect model. Notably ACS control sites showed complete defect closure at the 8-week healing interval. Conclusions: rhBMP-2/ACS accelerates local bone formation in the rat criticalsize through-through calvarial defect model once reaching an osteoinductive dose threshold. This threshold may already be reached at a 1.25-/2.5-mu g dose in this model. No further enhancement to bone formation/maturation may be observed adding rhBMP-2 above the 2.5-mu g dose. The 1.25-20.0 mu g dose range did not invoke appreciable aberrant healing events.