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Key Points Organs of future metastasis are selectively and actively modified by the primary tumour before metastatic spread has occurred. Tumours induce the formation of microenvironments in distant organs that are conducive to the survival and outgrowth of tumour cells before their arrival at these sites. These microenvironments are termed pre-metastatic niches (PMNs). PMN formation is a stepwise process resulting from the combined systemic effects of tumour-secreted factors and tumour-shed extracellular vesicles. PMN formation is initiated with local changes such as the induction of vascular leakiness, remodelling of stroma and extracellular matrix, followed by systemic effects on the immune system. The development of new technologies and approaches to identify PMNs in distant organ sites in patients could revolutionize cancer treatment and lead to pre-emptive treatments to hinder metastasis. The PMN is a new paradigm for the initiation of metastasis. Our ability to fight metastasis would benefit greatly from understanding the pathological processes occurring before the development of macrometastases. This Review summarizes the natural progression of pre-metastatic niche formation and evolution, highlighting recent advances and future hurdles. It is well established that organs of future metastasis are not passive receivers of circulating tumour cells, but are instead selectively and actively modified by the primary tumour before metastatic spread has even occurred. Sowing the 'seeds' of metastasis requires the action of tumour-secreted factors and tumour-shed extracellular vesicles that enable the 'soil' at distant metastatic sites to encourage the outgrowth of incoming cancer cells. In this Review, we summarize the main processes and new mechanisms involved in the formation of the pre-metastatic niche.

Metastasis is the main cause of death in people with cancer. To colonize distant organs, circulating tumour cells must overcome many obstacles through mechanisms that we are only now starting to understand. These include infiltrating distant tissue, evading immune defences, adapting to supportive niches, surviving as latent tumour-initiating seeds and eventually breaking out to replace the host tissue. They make metastasis a highly inefficient process. However, once metastases have been established, current treatments frequently fail to provide durable responses. An improved understanding of the mechanistic determinants of such colonization is needed to better prevent and treat metastatic cancer.

Background Circulating tumor cells (CTCs) are prognostic in patients with advanced breast cancer (ABC). However, no data exist about their use in patients treated with palbociclib. We analyzed the prognostic role of CTC counts in patients enrolled in the cTREnd study, a pre-planned translational sub-study of TREnd (NCT02549430), that randomized patients with ABC to palbociclib alone or palbociclib plus the endocrine therapy received in the prior line of treatment. Moreover, we evaluated RB1 gene expression on CTCs and explored its prognostic role within the cTREnd subpopulation. Methods Forty-six patients with ER-positive, HER2-negative ABC were analyzed. Blood samples were collected before starting palbociclib treatment (timepoint T0), after the first cycle of treatment (timepoint T1), and at disease progression (timepoint T2). CTCs were isolated and counted by CellSearch® System using the CellSearch™Epithelial Cell kit. Progression-free survival (PFS), clinical benefit (CB) during study treatment, and time to treatment failure (TTF) after study treatment were correlated with CTC counts. Samples with ≥ 5 CTCs were sorted by DEPArray system® (DA). RB1 and GAPDH gene expression levels were measured by ddPCR. Results All 46 patients were suitable for CTCs analysis. CTC count at T0 did not show significant prognostic value in terms of PFS and CB. Patients with at least one detectable CTC at T1 ( n  = 26) had a worse PFS than those with 0 CTCs ( n  = 16) ( p  = 0.02). At T1, patients with an increase of at least three CTCs showed reduced PFS compared to those with no increase (mPFS = 3 versus 9 months, ( p  = 0.004). Finally, patients with ≥ 5 CTCs at T2 ( n  = 6/23) who received chemotherapy as post-study treatment had a shorter TTF ( p  = 0.02). Gene expression data for RB1 were obtained from 19 patients. CTCs showed heterogeneous RB1 expression. Patients with detectable expression of RB1 at any timepoint showed better, but not statistically significant, outcomes than those with undetectable levels. Conclusions CTC count seems to be a promising modality in monitoring palbociclib response. Moreover, CTC count at the time of progression could predict clinical outcome post-palbociclib. RB1 expression analysis on CTCs is feasible and may provide additional prognostic information. Results should be interpreted with caution given the small studied sample size.

Tumor cells frequently disseminate through the lymphatic system during metastatic spread of breast cancer and many other types of cancer. Yet it is not clear how tumor cells make their way into the lymphatic system and how they choose between lymphatic and blood vessels for migration. Here we report that mammary tumor cells undergoing epithelial–mesenchymal transition (EMT) in response to transforming growth factor-β (TGF-β1) become activated for targeted migration through the lymphatic system, similar to dendritic cells (DCs) during inflammation. EMT cells preferentially migrated toward lymphatic vessels compared with blood vessels, both in vivo and in 3D cultures. A mechanism of this targeted migration was traced to the capacity of TGF-β1 to promote CCR7/CCL21-mediated crosstalk between tumor cells and lymphatic endothelial cells. On one hand, TGF-β1 promoted CCR7 expression in EMT cells through p38 MAP kinase-mediated activation of the JunB transcription factor. Blockade of CCR7, or treatment with a p38 MAP kinase inhibitor, reduced lymphatic dissemination of EMT cells in syngeneic mice. On the other hand, TGF-β1 promoted CCL21 expression in lymphatic endothelial cells. CCL21 acted in a paracrine fashion to mediate chemotactic migration of EMT cells toward lymphatic endothelial cells. The results identify TGF-β1-induced EMT as a mechanism, which activates tumor cells for targeted, DC-like migration through the lymphatic system. Furthermore, it suggests that p38 MAP kinase inhibition may be a useful strategy to inhibit EMT and lymphogenic spread of tumor cells.

Cell-free tumor DNA (ctDNA) has the potential to enable non-invasive diagnostic tests for personalized medicine in providing similar molecular information as that derived from invasive tumor biopsies. The histology-independent phase II SHIVA trial matches patients with targeted therapeutics based on previous screening of multiple somatic mutations using metastatic biopsies. To evaluate the utility of ctDNA in this trial, as an ancillary study we performed de novo detection of somatic mutations using plasma DNA compared to metastasis biopsies in 34 patients covering 18 different tumor types, scanning 46 genes and more than 6800 COSMIC mutations with a multiplexed next-generation sequencing panel. In 27 patients, 28 of 29 mutations identified in metastasis biopsies (97%) were detected in matched ctDNA. Among these 27 patients, one additional mutation was found in ctDNA only. In the seven other patients, mutation detection from metastasis biopsy failed due to inadequate biopsy material, but was successful in all plasma DNA samples providing three more potential actionable mutations. These results suggest that ctDNA analysis is a potential alternative and/or replacement to analyses using costly, harmful and lengthy tissue biopsies of metastasis, irrespective of cancer type and metastatic site, for multiplexed mutation detection in selecting personalized therapies based on the patient's tumor genetic content. •We report de novo multiplexed detection of targetable ctDNA mutations.•We report ctDNA mutations detection from multiple tumor types (SHIVA trial).•There is no correlation between ctDNA and total plasma DNA quantity.•ctDNA corresponds to ∼25% of total cfDNA fraction for detectable plasma mutations.•ctDNA is as remarkable surrogate of tumor biopsy for de novo mutation calling.

Background Tumor-associated macrophages (TAMs) are major components of tumor microenvironment that frequently associated with tumor metastasis in human cancers. Circulating tumor cell (CTC), originating from primary tumor sites, is considered to be the precursors of tumor metastasis. However, the regulatory mechanism of TAMs in CTC-mediated tumor metastasis still remains unclear. Methods Immunohistochemical staining was used to detect the macrophages infiltration (CD68 and CD163), epithelial–mesenchymal transition (EMT) markers (E-cadherin and Vimentin) expression in serial sections of human colorectal cancer (CRC) specimens. Then, the correlations between macrophages infiltration and clinicopathologic features, mesenchymal CTC ratio, and patients’ prognosis were analyzed. A co-culture assay in vitro was used to evaluate the role of TAMs on CRC EMT, migration and invasion, and ELISA, luciferase reporter assay and CHIP were performed to uncover the underlying mechanism. Furthermore, an in vivo model was carried out to confirm the effect of TAMs on mesenchymal CTC-mediated metastasis. Results Clinically, CD163 + TAMs infiltrated in invasive front was associated with EMT, mesenchymal CTC ratio, and poor prognosis in patients with CRC. CRC–conditioned macrophages regulated EMT program to enhance CRC cells migration and invasion by secreting IL6. TAMs-derived IL6 activated the JAK2/STAT3 pathway, and activated STAT3 transcriptionally inhibited the tumor suppressor miR-506-3p in CRC cells. miR-506-3p, a key miRNA regulating FoxQ1, was downregulated in CRC cells, resulting in increased FoxQ1 expression, which in turn led to the production of CCL2 that promoted macrophage recruitment. Inhibition of CCL2 or IL6 broke this loop and reduced macrophage migration and mesenchymal CTC-mediated metastasis, respectively. Conclusions Our data indicates that TAMs induce EMT program to enhance CRC migration, invasion, and CTC-mediated metastasis by regulating the JAK2/STAT3/miR-506-3p/FoxQ1 axis, which in turn leads to the production of CCL2 that promote macrophage recruitment, revealing a new cross-talk between immune cells and tumor cells in CRC microenvironment.

Metastasis is a dynamic succession of events involving the dissemination of tumour cells to distant sites within the body, ultimately reducing the survival of patients with cancer. To colonize distant organs and, therefore, systemically disseminate within the organism, cancer cells and associated factors exploit several bodily fluid systems, which provide a natural transportation route. Indeed, the flow mechanics of the blood and lymphatic circulatory systems can be co-opted to improve the efficiency of cancer cell transit from the primary tumour, extravasation and metastatic seeding. Flow rates, vessel size and shear stress can all influence the survival of cancer cells in the circulation and control organotropic seeding patterns. Thus, in addition to using these fluids as a means to travel throughout the body, cancer cells exploit the underlying physical forces within these fluids to successfully seed distant metastases. In this Review, we describe how circulating tumour cells and tumour-associated factors leverage bodily fluids, their underlying forces and imposed stresses during metastasis. As the contribution of bodily fluids and their mechanics raises interesting questions about the biology of the metastatic cascade, an improved understanding of this process might provide a new avenue for targeting cancer cells in transit.This Review discusses the role of bodily fluids and their underlying forces and imposed stresses in metastasis, highlighting the contributions of fluid mechanics to tumour cell intravasation, intravascular arrest and extravasation as well as to dissemination of tumour-derived factors.

To evaluate the efficacy of lapatinib, a dual EGFR and HER2 tyrosine kinase inhibitor, in therapy-resistant HER2-positive CTCs in metastatic breast cancer (MBC). Patients with MBC and HER2-positive CTCs despite disease stabilization or response to prior therapy, received lapatinib 1500 mg daily in monthly cycles, till disease progression or CTC increase. CTC monitoring was performed by immunofluorescent microscopy using cytospins of peripheral blood mononuclear cells (PBMCs) double stained for HER2 or EGFR and cytokeratin. A total of 120 cycles were administered in 22 patients; median age was 62.5 years, 15 (68.2%) patients were post-menopausal and 20 (90.1%) had HER2-negative primary tumors. At the end of the second course, HER2-positive CTC counts decreased in 76.2% of patients; the median number of HER2-positive CTCs/patient also declined significantly (p = 0.013), however the decrease was significant only among patients presenting disease stabilization (p = 0.018) but not among those with disease progression during lapatinib treatment. No objective responses were observed. All CTC-positive patients harbored EGFR-positive CTCs on progression compared to 62.5% at baseline (p = 0.054). The ratio of EGFR-positive CTCs/total CTCs detected in all patients increased from 17.1% at baseline to 37.6% on progression, whereas the mean percentage of HER2-negative CTCs/patient increased from 2.4% to 30.6% (p = 0.03). The above results indicate that lapatinib is effective in decreasing HER2-positive CTCs in patients with MBC irrespectively of the HER2 status of the primary tumor and imply the feasibility of monitoring the molecular changes on CTCs during treatment with targeted agents. Clinical trial.gov NCT00694252.

There is a growing body of evidence that HER2 status can change during disease recurrence or progression in breast cancer patients. In this context, re-evaluation of HER2 status by assessment of HER2 expression on circulating tumor cells (CTCs) is a strategy with potential clinical application. The aim of this trial was to determine the HER2 status of CTCs in metastatic breast cancer patients comparing two CTC assays. A total of 254 patients with metastatic breast cancer from nine German university breast cancer centers were enrolled in this prospective study. HER2 status of CTCs was assessed using both the FDA-approved CellSearch® assay and AdnaTest BreastCancer™. Using the CellSearch assay, 122 of 245 (50%) patients had ≥5 CTCs, and HER2-positive CTCs were observed in 50 (41%) of these patients. Ninety of 229 (39%) patients were CTC positive using AdnaTest BreastCancer, and HER2 positivity rate was 47% (42 of 90). The rate of breast cancer patients with HER2-negative primary tumors but HER2-positive CTCs was 32% (25 of 78) and 49% (28 of 57) using the CellSearch assay and AdnaTest BreastCancer, respectively. Considering only those patients who had CTCs on both tests (n = 62), concordant results regarding HER2 positivity were obtained in 50% of the patients (31/62) (P = 0.96, κ = −0.006). HER2-positive CTCs can be detected in a relevant number of patients with HER2 negative primary tumors. Therefore, it will be mandatory to correlate the assay-dependent HER2 status of CTCs to the clinical response on HER2-targeted therapies.

Immune checkpoint inhibitors (ICIs) have been introduced in breast cancer (BC) treatment and better biomarkers are needed to predict benefit. Circulating tumor cells (CTCs) are prognostic in BC, but knowledge is limited on CTCs in the context of ICI therapy. In this study, serial sampling of CTCs (CellSearch system) was evaluated in 82 patients with metastatic BC enrolled in two randomized trials investigating ICI plus chemotherapy. Programmed death‐ligand 1 (PD‐L1) expression on CTCs was also measured. Patients with ≥ 2 CTCs per 7.5 mL at baseline had gene expression profiles in tumor suggestive of increased T‐cell activity, including increased tumor inflammation signature (TIS) in both triple‐negative (P = 0.010) and hormone receptor‐positive (P = 0.024) disease. Patients with luminal A BC had higher CTC levels. The association between CTC status and outcome was most apparent 4 weeks into therapy. PD‐L1 expression in CTCs was observed in 6/17 CTC‐positive patients and was associated with inferior survival. In conclusion, our study indicates that CTC numbers may inform on tumor immune composition, as well as prognosis. These findings suggest a potential of using CTCs as an accessible biomarker source in BC patients treated with immunotherapy. In this explorative biomarker analysis, we assessed serial sampling of circulating tumor cells (CTCs) with CellSearch in two randomized trials testing immune checkpoint inhibitors (ICIs) in metastatic breast cancer. Our data demonstrate a prognostic potential of CTCs, most apparent 4 weeks into ICI therapy. Furthermore, higher CTC counts were associated with increased markers of tumor immune activity.