Creating in vitro 3D tumour models: a guide for the biofabrication of a primary osteosarcoma model.

Osteosarcoma (OS) is a highly aggressive primary bone tumour. The mainstay for its treatment is multi-agent chemotherapy and surgical resection, with a 50-70% 5-year survival rate. Despite the huge effort made by clinicians and researchers in the last 30 years, limited progress has been made to improve patient outcomes. As novel therapeutic approaches for OS become available, such as monoclonal antibodies, small molecules, and immunotherapies, the need for OS preclinical model development becomes equally pressing. Three-dimensional OS models represent an alternative system to study this tumour: in contrast to two-dimensional monolayers, three-dimensional matrices can recapitulate key elements of the tumour microenvironment, such as the cellular interaction with the bone mineralized matrix. The advancement of tissue engineering and biofabrication techniques enables the incorporation of specific tumour microenvironmental aspects into three-dimensional models, to investigate the contribution of individual components to tumour progression and enhance understanding of basic OS biology. The use of biomaterials that mimic the extracellular matrix could also facilitate the testing of drugs targeting the tumour microenvironment itself, allowing a larger range of therapeutics to be tested, while averting the ethical implications and high cost associated with in vivo preclinical models. This review aims to serve as a practical guide by delineating the OS tumour microenvironment ("what it is like"), and in turn propose various biofabrication strategies to create a three-dimensional model ("how to recreate it"), to improve the in vitro representation of the OS tumour and ultimately generate more accurate drug response profiles.