Preparation of polymer-based multimodal imaging agent to visualize the process of bone regeneration

The objective of this study is to design a new multimodal imaging system for the evaluation of bone regeneration process. Pamidronate (PA) of bisphosphonates with a high affinity for hydroxyapatite, was introduced to pullulan with different molecular weights (PA-pullulan). Then, two probes for fluorescence and magnetic resonance (MR) imagings were introduced into the PA-pullulan to prepare the PA-pullulan conjugates containing both the imaging probes (PA-pullulan-F/M). The PA-pullulan-F/M conjugates had an affinity for hydroxyapatite. A gelatin hydrogel incorporating bone morphogentic protein (BMP)-2 was prepared and implanted subcutaneously into mice to obtain an animal model of bone regeneration. When intravenously injected into mice with the bone tissue ectopically formed by the BMP-2-incorporated hydrogel to fluorescently evaluate their body distribution, the PA-pullulan-F/M conjugates were accumulated in the bone tissue regenerated. The time profile of fluorescent intensity well corresponded with that of calcium amount in the bone tissue newly formed. In addition, the PA-pullulan-F/M conjugates showed an MR ability similar to Gd-DTPA (gadopentetate dimeglumine) of a MR imaging agent clinically used. The MR signal around the bone tissue newly formed was enhanced for mice injected with PA-pullulan-F/M prepared from pullulan with the molecular weight of 6000. It is concluded that the PA-pullulan-F/M conjugate is a useful multimodal agent of polymeric delivery system to evaluate the process of bone regeneration.