MIMIT seminar series - bone regeneration

Regeneration of bone using off-the-shelf injectables: In-vivo homing of stem cells –
Professor Jons Hilborn

Bone conduction requires three main components; cells, matrix and growth factors. Bone is good at repairing itself, however severe injuries require therapy. Using human or animal tissue involves risk of virus transfer and ethical issues, but tissue engineering produces unique polymers with biological properties. The basic problem with foreign materials is the reaction of surrounding tissue i.e. formation of scar tissue/ poor vascularisation of the bone etc, therefore materials must be able to promote cell colonisation, nutrient diffusion and vascularisation.

There are two main approaches; in-vivo techniques that involve bone grafts, signalling molecules or cell/gene therapy or in vitro techniques i.e. culture system with scaffolds and a cell source. Traditionally stem cells are harvested, isolated, differentiated, seeded into a scaffold and then multiplied. Cells need a nutrient supply and can be stimulated to produce more protein and guide growth direction. Previous studies illustrate how bone can be fabricated from patient muscle, where CT scans demonstrate the ectopic bone formation. However poor vascularisation is still a problem, illustrating that formation of bone at the defect site is better than moving it.

Bone regeneration uses protein initiators called bone morphogenetic proteins (BMP’s); which regulate cartilage and bone differentiation. These proteins have specific cell surface receptors that bind to target receptors, initiating a cascade of events in the cell. The nucleic DNA is activated, gene products are produced and the cell transforms into osteoblasts and so forth. Clinical trials commonly use collagenous bone matrix, however other studies used heparin/chitosan injectables with BMP2 induced bone, on a titanium mesh covering the defect area. Side effects such as inflammation and fever indicate that a better carrier matrix is required. The ultimate goal is to produce a non animal, safe, bioresorbable composite of polymer and ECM with biochemical info to trigger tissue regeneration.

Prof Hilborn’s group used hyaluronan as a carrier for BMP2 which is advantageous as it is identical in all vertebrates, water soluble, biocompatible/biodegradable and functional groups can be modified. As a liquid, it is injected into the area to form a hydrogel (98-99% water) at pH 7.4/37c. The gel forms in less than 1 minute, by cross linkage in situ and hardens in 2-3 hours. Rapid gelation is vital in order to retain shape and prevent spreading after the subcutaneous injection. The gel degrades after staying in the muscle for 4 weeks and there is no indication of toxicity to surrounding cells. In vivo experiments were also carried out investigating the homing of stem cells, an osteoconductive nano-sized hydroxyapaptile filler was used as high density bone. The product was tested in mini-pigs, where a defect of 4cm x 2cm was created, no spontaneous healing occurred and then the gel was injected to form fully vascularised bone. There are proposed clinical trials for the use of this product in patients with cleft palates as they show more consistency compared to skull defects.

This hyaluronan based injectable has the necessary biocompatible properties and is much preferred than surgery. Pre clinical data is good and there is also the possibility of introducing ascorbic acid/ vitamin C in clinical trials to promote growth. It also has the potential to be used for other problems such as bone disease, formation of cartilage i.e. rheumatism and possibly repair in the nerves, muscle and brain.