Circulating tumor cell clusters and circulating tumor cell-derived explant models as a tool for treatment response.

Abstract

BioTechniquesVol. 69, No. 1 Expert opinionOpen AccessCirculating tumor cell clusters and circulating tumor cell-derived explant models as a tool for treatment responseBiderahalli Kruthika Prasanna, Ajay Balakrishnan & Prashant KumarBiderahalli Kruthika PrasannaInstitute of Bioinformatics, International Technology Park, Bangalore 560066, IndiaSearch for more papers by this author, Ajay BalakrishnanInstitute of Bioinformatics, International Technology Park, Bangalore 560066, IndiaManipal Academy of Higher Education, Manipal 576104, Karnataka, IndiaSearch for more papers by this author & Prashant Kumar*Author for correspondence: E-mail Address: prashant@ibioinformatics.orgInstitute of Bioinformatics, International Technology Park, Bangalore 560066, IndiaManipal Academy of Higher Education, Manipal 576104, Karnataka, IndiaSearch for more papers by this authorPublished Online:18 May 2020https://doi.org/10.2144/btn-2020-0029AboutSectionsPDF/EPUB ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinkedInRedditEmail Keywords: circulating tumor cell-derived explantscirculating tumor cell clusterscirculating tumor cell microembolicirculating tumor cellsCancer cells intravasate from their primary sites into blood vessels. These cancer cells in blood circulation are called circulating tumor cells (CTCs). Additionally, CTCs in the blood extravasate to reach distant organs and become involved in micro or macro metastasis [1]. Liquid biopsy utilizes blood-based biomarkers for the detection of cancer and facilitates real-time monitoring of tumor progression and drug response. This technique involves the detection of ctDNA or CTCs. Although ctDNA would be less challenging compared with CTCs, CTCs present a broader picture of tumor biology. The main challenge in isolating CTCs from whole blood is the large number of normal blood cells (e.g., neutrophils [1.8–7.7 × 106 per ml], basophils [0.25–1 × 106 per ml], eosinophils [0.05–0.4 × 106 per ml], monocytes [0.1–0.8 × 106 per ml], lymphocytes [1–4 × 106 per ml], red blood cells [4–6.2 × 109/ml] and platelets [2.5–4 × 108 per ml]), whereas very few CTCs are present per milliliter of blood [2].Techniques for CTC capture can be broadly classified into two types based on their biological and physical properties [3]. Isolation techniques, which are based on the biological properties, involve an antibody or an antibody cocktail to positively enrich the tumor cells or negatively deplete the white blood cells. Based on physical properties, the difference in the size, shape and deformability of cancer cells are used to isolate CTCs from other blood cells. Despite several technological advances, there is no gold standard method to isolate CTCs. Our recent work showed that CTCs exhibit both epithelial and mesenchymal phenotypes [4]. The dynamic changes from epithelial-to-intermediate phenotype makes it very challenging to isolate these cells based on single markers. Currently, the CellSearch® CTC Test is the only US FDA approved method for CTC enrichment in breast [5], colorectal [6] and prostate cancers [7].It is also worthwhile to note that CTCs not only exist as single cells but also in clusters with white blood cells and other cells in the circulation. CTC clusters, which have three or more distinct nuclei, are called CTC microemboli. CTCs observed in clusters have a better survival advantage in the blood circulation than CTCs that are present as single cells. It is also reported that the metastatic initiating potential of CTC clusters is 23–50-fold higher than single CTCs [8]. In the same study, it was reported that plakoglobin is highly expressed in CTC clusters, and thus its expression can possibly expedite the process of metastasis.Because the CTC population is very scarce, and to capture CTC clusters, ex vivo conditions were used to culture these rare populations of CTCs. Recently, we established an ex vivo culture of CTCs without prior enrichment. The blood from cancer patients was subjected to red blood cell lysis, and the nucleated cell fraction was cultured for 14 days in specially designed ellipsoidal microwells, maintained in hypoxic condition (1% O2) [9]. The clusters formed in the microwells were classified into very tight, tight and loose clusters, depending on the compactness of the cells in clusters. We have previously reported that the tightness of the clusters correlated with poor survival and response to the treatment [10]. The study has demonstrated that the short time culturing of CTCs can also be used to predict the response to the drug by comparing the IC50 values of the drugs on isolated CTCs during pre and post-treatment [11].One of the interesting aspects is the viability of CTCs in ex vivo conditions. It is still challenging for researchers to establish permanent cell lines from CTCs. Yu et al. published a groundbreaking study, in which they demonstrated the possibility of performing drug susceptibility testing on cultured CTC lines [12]. They demonstrated the sensitivity and resistance of the six CTC lines on treatment with several chemotherapeutic drugs and their combinations. However, there is still a need to establish a more robust method to take the CTC-based drug susceptibility for clinical usage.Development of CTC-derived xenograft (CDX) models would provide a better proxy to establish such drug testing in which the isolated CTCs are injected into nude mice to generate tumors. For the first time to our knowledge, the CDX models were derived from small cell lung cancer patients. It has been observed that the response to etoposide and platinum treatment were similar between the CDX models and the patient-derived CTCs [13]. Moreover, Yu et al. have also established three CDX models from the six established CTC lines. They found similar histological features of the primary tumors and the corresponding CDX models [12]. Another study by Cayrefourcq et al. established a CDX model from the CTCs isolated from colon cancer [14]. The CDX models can closely mimic the status of the tumor for a particular patient, as CDX is generated from the CTCs, which would rather give information on the real-time progression of tumors. However, the success rate in generating CDX models are the rate-limiting factors for its clinical implications.The genomics efforts by The Cancer Genome Atlas have given tremendous insight into the evolution of tumors over time. However, real-time monitoring of disease progression and drug response in patients is an unmet need. In the current era of personalized medicine, a real-time liquid biopsy approach using CTCs holds great promise in monitoring response to treatment and predicting survival.AcknowledgmentsThe authors would like to thank the Department of Science and Technology, Ramanujan Fellowship for research support.Financial & competing interests disclosureP Kumar is a recipient of the Ramanujan Fellowship awarded by the Department of Science and Technology, Government of India. A Balakrishnan is a recipient of a Senior Research Fellowship from the Council of Scientific & Industrial Research, Government of India. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.No writing assistance was utilized in the production of this manuscript.Open accessThis work is licensed under the Attribution-NonCommercial-NoDerivatives 4.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/4.0/Papers of special note have been highlighted as: • of interest; •• of considerable interest