Circulating Tumor Cell Adaptations Shape Cancer Spread and Next-Generation Diagnostic Technologies

A comprehensive scientific review published in Frontiers in Cell and Developmental Biology synthesizes recent progress in understanding the biology, isolation technologies, and clinical evaluation of circulating tumor cells across solid malignancies. Authored by Wentao Zhang and colleagues from The First Affiliated Hospital and The First Clinical College of Henan Medical University, the analysis examines circulating tumor cells, or CTCs, not simply as passive cellular shed from primary or secondary lesions, but as active biological intermediates in the hematogenous metastatic cascade. While traditional tissue biopsy remains the cornerstone of cancer diagnosis and genomic analysis, its clinical deployment is constrained by physical invasiveness, limited anatomical accessibility, and the inability of localized tissue sampling to capture spatial and temporal tumor heterogeneity during systemic disease progression. Liquid biopsy has therefore emerged as an essential, minimally invasive approach to track disease kinetics longitudinally. Unlike circulating tumor DNA, which circulates predominantly as fragmented genetic material, CTCs represent intact cellular units capable of providing genomic, transcriptomic, proteomic, and functional information. As tumor cells traverse the bloodstream, they encounter intense oxidative, mechanical, and immune pressures, forcing dynamic biological adjustments that govern metastatic colonization.

The review emphasizes that circulating tumor cells exhibit pronounced phenotypic and organizational heterogeneity that directly influences their survival and dissemination capacity. In patients with breast cancer, investigators have documented diverse epithelial, mesenchymal, and hybrid intermediate states, with shifting proportional distributions observed during disease progression and therapeutic intervention. While shifts in circulating cell populations do not definitively track the exact state transitions of individual cells, they reflect substantial phenotypic plasticity. Beyond single cells, multicellular circulating structures known as CTC clusters demonstrate significantly greater metastatic efficiency than solitary counterparts. Experimental models show that intercellular adhesion within clusters is functionally mediated by junctional proteins such as plakoglobin. In preclinical breast cancer studies, plakoglobin depletion disrupted cell-to-cell adhesion, impaired the formation of circulating clusters, and markedly reduced metastatic colonization in the lungs, proving that cohesive intercellular architecture provides a distinct biological advantage during vascular transit.

Survival in the circulating microenvironment is further fortified by direct physical and molecular interactions between circulating tumor cells and host blood elements. Platelets and leukocytes actively engage with disseminating cancer cells, providing protective shields and biochemical support. Platelet-associated surface markers are frequently identified on clinical CTCs, and laboratory experiments indicate that platelets can transfer major histocompatibility complex class I molecules directly onto tumor surfaces, suppressing surveillance and lysis by natural killer cells. Similarly, neutrophils form stable heterotypic clusters with circulating tumor cells in patients with breast cancer. Preclinical investigations show that these neutrophil associations drive cell-cycle progression and accelerate metastatic seeding through sustained interleukin-6 and interleukin-1-beta cytokine signaling. Once cells navigate these cellular interactions, local hemodynamic parameters dictate their eventual fate. Detailed experimental imaging reveals that physical blood-flow forces govern vascular arrest, endothelial adhesion, and subsequent extravasation into distant organs, demonstrating that intravascular fluid mechanics actively shape the anatomical distribution of metastatic lesions.

Metabolic reprogramming serves as another fundamental pillar supporting circulating tumor cell viability during detached vascular transit. Cells enter the circulation conditioned by metabolic stress experienced within primary tumors, including microenvironmental hypoxia, which promotes intercellular cohesion and stimulates the release of highly metastatic clusters. Once detached, tumor cells must adapt to prevent detachment-induced apoptosis, termed anoikis. Breast cancer studies demonstrate that circulating cells elevate peroxisome proliferator-activated receptor gamma coactivator 1-alpha, or PGC-1-alpha, expression, thereby enhancing mitochondrial biogenesis and oxidative phosphorylation; experimental inhibition of PGC-1-alpha significantly dampens dissemination. Concurrently, fatty acid oxidation provides essential energy and redox maintenance. Detached breast cancer cells activate focal adhesion kinase and signal transducer and activator of transcription 3 signaling, inducing nicotinamide N-methyltransferase, or NNMT, expression. This upregulation drives carnitine palmitoyltransferase 1A-dependent fatty acid oxidation, lowering oxidative stress and fostering anoikis resistance, although whether this pathway mediates human immune evasion remains unproven. In models of PD-1-resistant non-small cell lung cancer, CD36-positive circulating tumor cells display pronounced lipid accumulation, heightened mitochondrial respiration, and fatty acid oxidation dependence, which suppress T-cell activation while driving cellular migration.

Isolating these cellular units from peripheral blood represents an immense engineering hurdle because circulating tumor cells are extraordinarily rare, frequently numbering in the single digits per milliliter against billions of erythrocytes and millions of leukocytes. Early isolation platforms relied strictly on physical distinctions, capitalizing on the observation that malignant epithelial cells typically possess diameters exceeding 15 micrometers, making them larger and less deformable than standard blood elements. Label-free approaches, including microporous filtration and spiral microfluidic devices, leverage inertial lift forces and Dean flow to separate circulating cells based entirely on size and hydrodynamic properties. Because spiral microfluidics operates without reliance on surface antigens, it circumvents a major failure mode of antibody-based capture: the downregulation of epithelial cell adhesion molecule, or EpCAM, during epithelial-mesenchymal transition. However, size-based separation cannot completely eliminate background contamination from larger white blood cells, necessitating refined purification strategies.

To overcome purity limitations, affinity-based isolation platforms have achieved regulatory and clinical milestones, led by the CellSearch system, an immunomagnetic platform cleared by the United States Food and Drug Administration that targets EpCAM-positive cells. While CellSearch offers standardized and highly reproducible workflows, its dependence on epithelial markers leaves it vulnerable to false-negative results when malignant cells undergo mesenchymal transition. Consequently, negative-enrichment methods have gained prominence by selectively removing non-malignant background cells rather than targeting tumor-specific antigens. By employing magnetic beads directed against CD45 and CD66b, platforms can simultaneously deplete white blood cells and neutrophils, leaving viable, unperturbed circulating tumor cells suitable for downstream ex vivo culture and multi-omic analysis. In parallel, functional detection systems such as EPISPOT and droplet microfluidics identify viable cells by capturing specific secreted proteins released following short-term culture, though these platforms remain technically demanding and cannot capture non-secretory malignant cells.

Technological refinement has rapidly expanded circulating tumor cell evaluation from basic enumeration to high-resolution molecular and phenotypic characterization. Immunofluorescence staining and single-cell RNA sequencing now yield granular insights into cellular identity, transcriptional heterogeneity, and actionable therapeutic targets. In lung cancer, sensitive molecular assays successfully detect epidermal growth factor receptor, or EGFR, mutations within isolated circulating cells, while in prostate cancer, immunofluorescence staining assesses the presence and nuclear localization of androgen receptor splice variant 7, or AR-V7. Concurrently, artificial intelligence and deep-learning image processing algorithms have entered diagnostic workflows, minimizing observer subjectivity and improving the identification of complex structures such as circulating tumor microemboli. This technological progression from quantitative cell counting to qualitative functional profiling establishes an objective framework for translating liquid biopsy findings into oncology practice.

Clinical evaluations of circulating tumor cells in breast and lung cancers illustrate significant prognostic strength alongside important diagnostic constraints. In early-stage breast cancer, circulating cells are detected in only 15 to 25 percent of patients, demonstrating that current isolation rates remain insufficient for stand-alone screening or established adjunctive diagnosis. Baseline presence prior to neoadjuvant therapy correlates with poorer distant disease-free and overall survival, and dynamic declines during chemotherapy track therapeutic response, though standardized cutoffs and sampling intervals require prospective validation. Molecular signatures offer additional stratification; elevated STAT3 and PRKCB paired with diminished CDK6 expression in circulating cells yielded an area under the curve of 0.841 in a 16-patient validation cohort assessing non-response to CDK4/6 inhibitors. Conversely, circulating PD-L1 expression remains exploratory and unvalidated for predicting immunotherapy response. In lung cancer, an investigational CTC RNA score distinguished early non-small cell lung cancer from benign nodules with an area under the curve of 0.93, while the microfluidic Labyrinth platform demonstrated an area under the curve of 0.84 in 113 surgical patients, but screening utility remains unproven. In advanced non-small cell lung cancer, baseline counts of five or more cells per 7.5 milliliters by CellSearch, or the presence of sub-five-micrometer circulating cells detected via SE-iFISH, predicted shorter progression-free survival, whereas a 44.4 percent decline in cell count during EGFR tyrosine kinase inhibitor therapy correlated with prolonged progression-free survival.

In prostate and colorectal cancers, circulating tumor cell monitoring provides robust prognostic insights while encountering platform-specific boundaries. In metastatic hormone-sensitive prostate cancer, the SWOG S1216 trial established that baseline CellSearch counts of five or more cells per 7.5 milliliters conferred significantly shorter overall survival compared to undetectable counts, yielding an adjusted hazard ratio of 3.22. In metastatic castration-resistant prostate cancer, nuclear AR-V7 protein localized within circulating cells correlated with resistance to abiraterone and enzalutamide, yet an analysis of 29 patients found no association between AR-V7 mRNA and cabazitaxel outcomes, proving that AR-V7 status does not imply general chemotherapy resistance. In colorectal cancer, combining fecal immunochemical testing with circulating cell enumeration yielded an area under the curve of 0.837 for neoplasia detection, and an RNA scoring model demonstrated 91.0 percent accuracy across combined cohorts. For metastatic colorectal cancer, baseline CellSearch counts of three or more cells per 7.5 milliliters indicated inferior progression-free and overall survival, while sequencing revealed an exploratory association between non-silent BCL9L mutations and reduced overall survival based on four positive patients in a 29-patient cohort. Pharmacometric modeling from the CAIRO2 trial simulated survival gains from treatment switching guided by cell and extracellular vesicle kinetics, but prospective clinical benefit remains unproven.

Investigations in head and neck mucosal squamous cell carcinoma further underscore the clinical relevance and current limits of circulating cell detection. Using the AccuCyte-CyteFinder platform, researchers detected circulating cells in 46 of 57 patients, representing an 81 percent detection rate in established cancer rather than a screening parameter, with counts of five or more cells per 7.5 milliliters correlating with disease relapse. In an independent cohort analyzed via OncoDiscover, patients with more than two cells per 1.5 milliliters exhibited a median overall survival of 22 months compared to 40 months for lower counts, emphasizing that cutoffs derived from differing blood volumes and platforms cannot be treated interchangeably. Elevated Sonic hedgehog and Nrf2 expression in whole blood and tumor tissue correlated with cell positivity, but because expression was not isolated to circulating cells, causal pathways remain unconfirmed. Postoperative increases predicted two-year relapse or mortality in 85 percent of evaluable surgical patients, and in a phase II neoadjuvant trial of paclitaxel, carboplatin, and tislelizumab, five of six patients with post-treatment cell reductions achieved major pathological response, though this exploratory finding cannot isolate specific immunotherapy benefits.

The review concludes that while circulating tumor cell enumeration reliably stratifies survival risk in defined metastatic settings, translating dynamic monitoring into actionable clinical decisions requires rigorous prospective trial design. Historical trials highlight these complexities: in SWOG S0500, rapidly switching chemotherapy regimens after one cycle based solely on persistently elevated cell counts failed to improve overall survival in metastatic breast cancer. In the STIC CTC trial, guiding treatment choice between chemotherapy and endocrine therapy using circulating cell counts demonstrated non-inferior progression-free survival, but the study was conducted prior to routine first-line CDK4/6 inhibitor use, restricting its modern application. Variations in enrichment mechanisms, captured cell subpopulations, blood collection volumes, and analytical cutoffs continue to hinder cross-study comparisons. Ultimately, the authors emphasize that clinical adoption depends on conducting randomized, prospective trials that validate standardized thresholds, incorporate single-cell multi-omic profiling, and definitively demonstrate that altering patient management based on circulating tumor cell parameters directly improves overall survival.

Subject of Research: Biological adaptations, detection platforms, and clinical applications of circulating tumor cells in solid tumors

Article Title: Circulating tumor cells in solid tumors: biological adaptation, detection technologies, and clinical implications

Article References: Zhang, W., Xu, K., Niu, J., Cui, Y., Roy, M., Xu, K., Liu, Y., Lu, P., Lu, X., & Ji, Y. (2026). Circulating tumor cells in solid tumors: biological adaptation, detection technologies, and clinical implications. Frontiers in Cell and Developmental Biology, 14, Article 1975581. https://doi.org/10.3389/fcell.2026.1975581

Image Credits: AI Generated

DOI: 10.3389/fcell.2026.1975581

Keywords: circulating tumor cells, liquid biopsy, metastatic dissemination, phenotypic plasticity, epithelial-mesenchymal transition, CTC clusters, immunometabolism, fatty acid oxidation, microfluidics, CellSearch, negative enrichment, single-cell sequencing, breast cancer, lung cancer, prostate cancer, colorectal cancer, head and neck cancer, precision oncology