Cancer is a major public health problem and it remains the leading cause of death worldwide. There were an estimated 18.1 million cancer and 9.9 million cancer deaths cases around the world in 2020 (Sung et al., 2021), and it is expected that by 2040 these figures would reach 28.0 million new cancer cases and 16.2 million cancer deaths (Ferlay et al., 2020). It is estimated that cancer accounts for about 1 in every 6 deaths worldwide (2020), mainly due to the presence of metastatic disease that in most cases is not a curable disease and patients cannot achieve long-term survival. These figures show the need to overcome our current limitations on the knowledge about cancer, particularly on the molecular mechanisms that drive cancer metastasis.

Cancer metastasis is a complex multi-step process in which tumour cells undergo structural and functional changes that allow them to move away from the primary tumour and disseminate to distant organs and tissues. The metastatic cascade involves: (i) detachment of tumour cells, migration and infiltration of tumour cells into adjacent tissue; (ii) Intravasation, mainly by transendothelial migration of tumour cells, into nearby vessels; (iii) Transient travelling and survival of tumour cells in blood circulation; (iv) Arrest of CTCs at secondary sites and extravasation; (v) Proliferation in distant tissues allowing colonization and growth of metastases (Melzer, Von Der Ohe, & Hass, 2017).

The way that metastatic disease is treated has not significantly changed in the past few decades, and in a large proportion of cases, treatments do not take into account the inter-patient and intra-tumour heterogeneity. Such heterogeneity is responsible for the ineffective detection and is closely related to cancer progression, resistance to therapy, and recurrences (Hanahan and Weinberg, 2011, Jamal-Hanjani et al., 2015, Stanta and Bonin, 2018). Indeed, most current treatment strategies are based on molecular information derived from the primary tumour, which may have limited efficacy over the metastases due to the constant evolution of tumour cells during the disease. In this context, the technological development within the last two decades has allowed us to isolate and study the population of circulating tumour cells (CTCs) through the alternative tumour interrogation method known as “Liquid biopsy”. CTCs are tumour cells shed by the primary tumour or metastasis foci found in the peripheral blood circulation system of cancer patients. CTCs are regarded as the main drivers of metastases formation (Dianat-Moghadam et al., 2020) and can potentially be used as noninvasive biomarkers predicting therapeutic response (Kilgour et al., 2020, Liu et al., 2021). Therefore, studying the functional and molecular characteristics of CTCs may provide in-depth knowledge regarding the metastatic process, and could help to overcome the problem represented by tumour heterogeneity, guiding treatment selection. However, CTCs are a rare population of cells, found at a ratio of one CTC to 106–107 nucleated blood cells (Pantel, Brakenhoff, & Brandt, 2008), and they are often not viable, which represents a hurdle to molecular and specifically functional studies. This means that in order to gather valuable information about the biology of CTCs and the metastatic process cancer metastasis models, representing the complex biology of CTCs, as well as the expansion of viable CTCs would be valuable tools.

In this chapter, we will provide an up to date view of the diverse models used to study the biology of CTCs as well as the methods developed for CTC culture and expansion, in vitro, in vivo and ex vivo.