Contribution of tumor-derived extracellular vesicles in the establishment of the pre-metastatic niche: lessons learned from past experimentations and future directions.

Tumor-derived extracellular vesicles (TEVs) have been shown to actively contribute to the establishment of the pre-metastatic niche (PMN) through multiple mechanisms of action and the transfer of cargo material to host cells. Here, we report a review of 120 manuscripts published between January 2010 and April 2025 describing observations from in vivo experiments aimed at the examination of the contribution of TEVs to the PMN and the metastatic niche (MN). Whereas most of these publications reported observations made with models of exogenous administration of TEVs prepared in vitro, five publications used endogenous models of TEVs released by implanted tumors to track their fate and examine their role in the PMN. Breast and colon cancers and melanoma are the most common types studied, and lung, liver, bone, and lymph nodes are the most common sites of metastasis examined. Methods to isolate, characterize and label TEVs in exogenous models vary significantly with differential ultracentrifugation (DUC) being the most common method of isolation used, electron microscopy, nano tracking analysis (NTA) and western blotting being used for their characterization, and lipophilic dyes used for labeling. Immunodeficient and immunocompetent mice were used in the majority (94%) with some studies done in zebrafish. Single and multiple administrations of TEVs with doses ranging from 5 to 200 µg/mouse were reported. A review of the cells targeted by TEVs, the organs involved and the effects observed in the PMN and MN, led us to identify seven functional hallmarks: (1) Immune reprogramming, (2) Stromal cell activation, (3) Vascular permeability and angiogenesis, (4) Lymphatic remodeling and lymphangiogenesis, (5) Bone remodeling, (6) Extracellular matrix (ECM) remodeling and (7) Direct tumorigenicity. Altogether, these effects create an inflammatory and immunosuppressive microenvironment that favors tumor growth. Exogenous and endogenous models have both advantages and limitations and provide important complementary information. In both models, loss of function experiments aimed at determining whether TEVs are necessary for the formation of the PMN and MN represent an important challenge. Such experiments, however, will be critical to provide evidence that targeting TEVs could be a viable approach to inhibit or prevent metastasis in patients affected by cancer.