Analysis of circulating tumor cells (CTC) has received enormous attentions for its potential to obtain diverse information of tumors dynamically. Nowadays fast-developing detection technologies facilitate its biological research of tumor dissemination and clinical implications. This review is going to discuss clinical applications of CTC including prognosis and prediction of metastatic progression, surveillance of therapeutic response and identification of therapeutic targets and resistance mechanisms and to further introduce present detection technologies of CTC.
Currently, circulating tumor cell (CTC), a crucial intermediate of metastasis from primary tumor to distant organ sites [1], has arisen enormous attention for its obvious diagnostic, prognostic and predictive potential for personalized medicine. Through dissemination of tumor cells to distant organ sites in bloodstream, it has been demonstrated that CTC can gain key characteristics required for metastasis such as recruitment of immune cells, inflammatory chemokines and cytokines, while still facing significant barriers including physical stress, oxidative stress and anoikis, resulting in its short half-life [2]. It is noted that not only CTC numbers can be quantified, but also changes in DNA, RNA, and protein levels describe multiple heterogeneities compared with nonmalignant cells. And present studies have proposed that both single CTC and CTC clusters contribute to metastasis and CTC clusters with epithelial and mesenchymal phenotypes may be more metastatic than single cells [3].
We are going to further discuss the clinical applications and detection technologies concerning CTC which attracts tremendous interests in the medical frontier from three aspects including prognosis and prediction of metastatic relapse and mortality, surveillance of therapeutic response and identification of therapeutic targets and resistance mechanisms.
In this review, we collected published original research articles (impact factor, IF>10) in Pubmed in the recent three years and classified researches in each application subset (Figure 1) [2,4-57]. In consistent with its metastatic characteristics, we found that CTC might be more applicable in prognosis and surveillance of metastatic cancer rather than early cancer screening.
Figure 1: CTC-related publications counting via PubMed in the recent there years (IF>10).
A. Comparison of published paper amount on early cancer screening (0), prognosis and prediction of
metastatic progression(27) [4,32-57], surveillance of therapeutic responses(17) [15-31], and identification of
therapeutic targets and resistance mechanisms(12) [2, 4-14], respectively.
B. Comparison of published paper amount on different CTC-enrichment methodologies: size(5)[2,4,13,42,48],
other physical properties(5)[10, 36, 64-66], positive selection(anti-EpCAM, 6)[19,22,34,43, 50,55], positive
selection(anti-other molecules with or without anti-EpCAM, 10)[7,19,24,28,45,46,69,72], negative selection(8)
[6,8,10,20,23,29,41,65], combined positive and negative selection expect CellSearh(3)[12,21,67], CellSearch(23)
[5,9,11,15,17,25 27,32,35,37,38,44,47,49,51-54,56,77,78], and aptamers(1) [68].
Prognosis and prediction of metastatic relapses and progression
There are a number of studies presenting the compelling correlation between CTC and prognosis in patients with various tumor types, especially for assessment of survival in clinical trials. Netterberg’s team demonstrated that CTC counts could be useful for early predicting Overall Survival in patients with metastatic colorectal cancer [58]. Also in non-small cell lung cancer (NSCLC), one recent study said higher pretreatment CTC and persistence of CTC posttreatment were significantly associated with elevated risk of recurrence outside the targeted treatment site in patients with early-stage NSCLC treated with stereotactic body radiotherapy [39], while another retrospective assessment showed that cerebrospinal fluid CTC was correlated with risk of death (Hazard Ratio: 3.39, 95% confidence interval(CI): 1.01–11.37; P=0.048) in patients with leptomeningeal metastases from NSCLC [55], while . Not only CTC quantification are associated with prognosis, but also the compounds in CTC can be prognostic biomarkers. It was reported that out of 47 patients with aggressive variant prostate cancer from whom 257 individual CTC were sequenced (1-22 CTC/patient), twenty (42.6%) had concurrent 2+ tumor suppressor genes losses in at least one CTC in association with poor survival and increased genomic instability, inferred by high large-scale transitions scores [14].
Surveillance of therapeutic responses
At present, a large number of researches stated that CTC counts, and its molecular analysis could predict and monitor drug responses in metastatic cancer [4,27,29]. For example, in castration-resistant prostate cancer (CRPC), CTC increases were associated with worse prognosis, suggesting alternative therapies after three cycles of chemotherapy [59], and another Phase III clinical trial reported CTC number could serve as a response measure of Prolonged Survival for metastatic CRPC [16].
Identification of therapeutic targets and resistance mechanisms
In the past few years, characterizing of rare, heterogeneous CTC greatly provides thorough insight into metastasis and helps develop novel targeted therapies particularly by high throughput sequencing. Franses and their colleagues identified stemness gene LIN28B expression in CTC is prognostic for pancreatic ductal adenocarcinoma by RNA-seq and investigated LIN28B molecular mechanism on metastasis [60]. Recently, single cell sequencing enables to monitor mutation status for therapeutic resistance [4] and identify DNA methylation remodeling from CTC cluster to single CTC which was proved to enhance metastasis [61]. According to these excellent findings, molecular characterization of CTC provides a unique opportunity to determinate drivers of dissemination and guide prospective treatments targeting the “seeds” of metastasis. In addition, CTC-derived eXplant (CDX) and a CDX-derived cell line established by CTC are promising tools for exploring new strategies on metastasis or drug-resistance [61,62].
In practice, both single CTC and CTC clusters are extremely
rare in blood, detection technology which directly determines its
abundance and purity is the most challenging part for thorough
biological studies and clinical applications as well of CTC. In order to
increase the concentration of CTC by several log units, a great many
enrichment methods have developed rapidly which can be divided
into two main groups: label-free methods based on invasive capacity
or physical properties such as size, deformity and density, or labeldependent
ones including positive or negative selection depending
on the immunoaffinity [63]. Particularly, CellSearch approved by
FDA have been widely applied in dozens of clinical trials (Figure
1) [2,4,6-8,10,12,13,19-24,28,29,34,36,41-43,45,46,48,50,55,64-
72]. Nowadays, current strategies are prone to develop devices
combining antibody cocktails with advanced techniques like
immunomagnetic beads, nanoparticles or microfluidics to isolate
and identify CTC for higher specificity and sensitivity [3,73,74].
CTC displaying similar molecular properties of primary tumor
tissues and additional metastatic-associated changes can reveal
the biology of the tumor dissemination that has not been clearly
elucidated yet and guide the diagnosis, prognosis, monitoring
and treatment of metastatic diseases. Due to its noninvasive
and repeatable features, it has been validated as diagnostic and
prognostic surrogates of tissue sampling and predictive indicators
of recurrence and resistance for efficient therapeutic interventions
[4,75]. Cancer screening at an asymptomatic stage always starts
with determining the regulation in retrospective case-control trails
and then setting cohort trails containing large populations requires
long follow-up time to validate it. Practically, only few studies
evaluated CTC as a biomarker for early cancer detection because it
takes time for tumor cells to progress from primary site into blood
and CTC might be zero or extremely low at an early stage. However,
one group reported that CTC allowed early diagnosis of lung cancer
in patients with chronic obstructive pulmonary disease [75], and
another preliminary study has estimated that their CTC detection
method based on RNA signature could enable a noninvasive early
diagnosis of hepatocellular carcinoma in populations where viral
hepatitis and cirrhosis are prevalent [76]. Though focusing on a
population with high risk of developing cancer could speed up the
long validation process, these studies need large further cohort to
provide more evidences.
The biggest obstacle of CTC is its detection technologies.
Basically, the heterogeneity in phenotypes and genotypes has
made it challenging to directly capture CTC and raised the question
whether the panel that we use to identify CTC is sensitive and
specific enough for a certain tumor type. Label-free and aptamersdependent
methods are novel isolations means that can overcome
this shortage to some extent, but the potential applications of some
highly sensitive detection methods need clinical trials with larger
population to validate. More importantly, it is essential to evaluate
and establish criteria for the clinical utility of CTC to assure its
efficacy [77,78].
In spite of these challenges, these exciting findings on CTC
highlight its crucial role during metastatic process as well as its
significant application value of predicting prognosis or drugresponses
and developing novel therapeutic strategies. We believe
that CTC with these outstanding advantages promise to have a
bright future.
The authors acknowledge grants from Shanghai Municipal
Population and Family Planning Commission (201540251, SW) and
National Natural Science Foundation of China (81972803, SW).
Professor, Chief Doctor, Director of Department of Pediatric Surgery, Associate Director of Department of Surgery, Doctoral Supervisor Tongji hospital, Tongji medical college, Huazhong University of Science and Technology
Senior Research Engineer and Professor, Center for Refining and Petrochemicals, Research Institute, King Fahd University of Petroleum and Minerals (KFUPM), Dhahran, Saudi Arabia
Interim Dean, College of Education and Health Sciences, Director of Biomechanics Laboratory, Sport Science Innovation Program, Bridgewater State University