Controlled bone formation using ultrasound-triggered release of BMP-2 from liposomes

Recombinant human bone morphogenetic protein 2 (rhBMP-2) is used clinically to enhance implant-mediated bone regeneration. However, there are risks associated with the high rhBMP-2 dose that is required in the implant to mitigate diffusional loss over the therapeutic timespan. On-demand, localized control over delivery of rhBMP-2, days after implantation, would therefore be an attractive solution in the area of bone repair and reconstruction, yet this has posed a significant challenge, with little data to support in vivo efficacy to date. To address this, we have developed novel liposome-rhBMP-2 nanocomplexes that release rhBMP-2 in response to non-thermogenic, clinical diagnostic ultrasound exposure. In vitro validation shows that rhBMP-2 release is in proportion to applied ultrasound pressure and duration of exposure. Moreover, here we show in vivo validation of this ultrasound-triggered rhBMP-2 delivery system in a standard mouse bone regeneration model. Implanted into hindleg muscles, the liposome-rhBMP-2 nanocomplexes induced local bone formation only after ultrasound exposure. Such post-implantation control of delivery has potential to improve the safety, efficacy and cost of rhBMP-2 use in bone reconstruction. Furthermore, this first proof-of-concept demonstration of in vivo efficacy for ultrasound-triggered liposomal delivery of rhBMP-2 has broader implications for tunable delivery of a variety of drugs and biologics in medicine and tissue engineering.