Supporting bone regeneration: Evaluation of endocrine peptides and delivery systems for local application of lactoferrin

Abstract

Introduction Fractures, especially in elderly people, have devastating outcomes and can drastically reduce the quality of life. Targeted delivery of growth factors directly to the site of fracture or other skeletal defects has been recognised as a strategy that could improve healing and lead to better clinical outcomes. Delivery systems that can enhance bone formation, accelerate regeneration and healing, and ultimately restore bone integrity and strength, are still largely missing. Aim We aimed to evaluate the myokine irisin and the neuropeptide orexin as potential bone anabolic factors and to develop a clinically suitable drug delivery system for the optimal delivery of the bone anabolic factor lactoferrin (LF). Methods We developed an ex vivo osteocyte culture system, and investigated the effects of irisin and orexin on bone cells at different stages of development in vitro: on bone marrow stromal cells, osteoblasts, osteocytes, and osteoclasts. In vitro studies were conducted with three different delivery systems to assess their ability to deliver anabolic factor LF and their cytocompatibility with osteoblasts. Drug delivery systems demonstrating potential were tested in vivo using a rat calvarial defect model. Results Firstly, we validated our osteocyte-rich ex vivo bone tube system. The osteocytes in bone tubes were responsive to rhPTH(1-34) and decreased expression of Sost. Both irisin and orexin enhanced bone marrow stromal cell differentiation into adipocytes, and orexin also increased osteoblast proliferation. Irisin and orexin did not affect matrix mineralisation by osteoblasts, osteocyte function or osteoclastogenesis. Secondly, we found LF delivered in the commercial INFUSE® ACS and the current formulation of LF/poloxamers did not increase bone regeneration in a rat calvarial defect model. We developed a novel self-assembling peptide that is cytocompatible with osteoblasts and has potential to be used for sustained delivery of LF. Conclusion In conclusion, the present study suggests there is not enough evidence to demonstrate that irisin and orexin have potential as anabolic factors for local bone regeneration. While this study has not yet found a suitable delivery system for LF, we demonstrated that synthetic self-assembling peptide hydrogels have the potential to be further examined for local delivery of LF in vivo.