Antitumor necrotic factor agent promotes BMP-2-induced ectopic bone formation

Etanercept (ETN), which is a recombinant human soluble tumor necrosis factor (TNF) receptor that inhibits TNF activity, is effective in the treatment of rheumatoid arthritis. We investigated the effect of ETN on recombinant human bone morphogenetic protein-2 (rhBMP-2)-induced ectopic bone formation in vivo. A block copolymer composed of poly-d,l-lactic acid with random insertion of p-dioxanone and polyethylene glycol (PLA–DX–PEG polymer) was used as the delivery system. Polymer discs (6 mm, 30 mg) containing 5 μg rhBMP-2 were implanted into the left dorsal muscle pouch of mice (n = 50). In the systemic administration groups (n = 5 per group), ETN was subcutaneously injected (25 mg/human = 12.5 μg/mouse) twice per week in a dose-dependent manner (placebo, 12.5 × 10⁻³, 12.5 × 10⁻¹, 12.5, 125 μg), whereas a single dose of ETN (placebo, 12.5 × 10⁻³, 12.5 × 10⁻¹, 12.5, 125 μg) was embedded in each rhBMP-2 polymer disc in the local administration groups (n = 5 per group). Three weeks after implantation, the mice were killed and the implants were analyzed. Implants in the optimally dosed groups had increased radiodensity, which was consistent with a significant increase in bone mineral content of the ossicles. Bone histomorphology revealed a significant increase in bone volume/total volume, number of osteoblasts, osteoblast surface/bone surface, and a significant decrease in the number of osteoclasts, osteoclast surface/bone surface in the optimal dosed systemic and locally administered groups. These data suggest that the optimal dose of ETN, administered either systemically or locally, enhanced the bone-inducing capacity of BMP with no apparent adverse systemic effects.