Background Metastatic breast cancer (BCa) remains the major cause of mortality, with bone tissue being the primary site of occurrence. To date, there is no clinical intervention available to treat bone metastasis. Currently, 2D and animal models are vastly employed for the investigation of BCa metastasis and the screening of drugs as potential treatments. However, in vitro and in vivo models lack complexity and remain far from ideal, failing to resemble metastasis as in human tissue. Hypothesis Bone marrow stromal cells (BMSCs) are key contributors to the tumour microenvironment and are known to promote cancer progression, but how they become activated is not fully understood. A 3D functional model of BCa metastasising to bone tissue can be printed for the study of cancer migration and metastasis activation, supporting the replacement of animal models. I hypothesize that BMSCs and BCa cells can be 3D bioprinted to generate a functional three-dimensional model able to recapitulate the pathophysiological characteristics of metastasising BCa, resulting in functional as a novel biomimetic platform. Aims Harnessing the ability of a unique 3D microfluidic bioprinter, I aim to fabricate a 3D printed platform designed to house a connection between the printed breast and bone tissue. I aim to fully characterise the metastasising 3D model investigating the molecular and physiological growth and spreading of BCa towards the printed bone tissue. I aim to use immune checkpoint inhibitor (ICI) agents to investigate the immunomodulation of metastatic migration and homing in bone tissue. Ultimately, an ex vivo model will be used to test antiangiogenic micro RNAs (miRNAs) therapeutics on the 3D breast-to-bone model. Experimental Design I will be using a custom-made microfluidic 3D bioprinter to deposit, independently, BCa and BMSCs. Cells will be embedded in biomatrices (biomaterial inks) able to sustain proliferation and tissue maturation. Viability and proliferation of BCa and BMSCs will be performed, investigating the model functionality using molecular and histological protocols. Crucially, patientspecific BCa cells will be used for the printing of a functional platform, ultimately used for combination immunotherapy (cIT) and miRNA against BCa metastasis. Expected Results The programme envisions the manufacturing of a functional 3D platform for the modelling of metastasising events from BCa to, specifically, bone tissue. I anticipate the successful generation of a 3D printed model, providing paramount data on the BCa metastasising mechanism to aid pre-clinical investigation on the tumour spreading and incidence. The proposed 3D breast-tobone model will accelerate the testing of drugs and their translation to the clinics, replacing animal models, often observed to be unable to fully recapitulate metastasis complexity in preclinical test evaluation. Impact On Cancer There is a critical and significant unmet need for the finding of a functional treatment of bone metastasis. The breast-to-bone model will resemble, ex vivo, the BCa pathophysiology with a skeletal primary metastatic site, contributing to the replacement of animal models in BCa research. Crucially, the breast-to-bone 3D platform holds the unparalleled potential to be versatile and, most importantly, adaptable to model other metastasising tumours of interest (e.g lung, prostate).