Basic Studies on Radioimmunotargeting of CD133-Positive HCT116 Cancer Stem Cells

Opportunities in Cancer Therapies: Deciphering the Role of Cancer Stem Cells in Tumour Repopulation

Tumour repopulation during treatment is a well acknowledged yet still challenging aspect of cancer management. The latest research results show clear evidence towards the existence of cancer stem cells (CSCs) that are responsible for tumour repopulation, dissemination, and distant metastases in most solid cancers. Cancer stem cell quiescence and the loss of asymmetrical division are two powerful mechanisms behind repopulation. Another important aspect in the context of cancer stem cells is cell plasticity, which was shown to be triggered during fractionated radiotherapy, leading to cell dedifferentiation and thus reactivation of stem-like properties. Repopulation during treatment is not limited to radiotherapy, as there is clinical proof for repopulation mechanisms to be activated through other conventional treatment techniques, such as chemotherapy. The dynamic nature of stem-like cancer cells often elicits resistance to treatment by escaping drug-induced cell death. The aims of this scoping review are (1) to describe the main mechanisms used by cancer stem cells to initiate tumour repopulation during therapy; (2) to present clinical evidence for tumour repopulation during radio- and chemotherapy; (3) to illustrate current trends in the identification of CSCs using specific imaging techniques; and (4) to highlight novel technologies that show potential in the eradication of CSCs.

Cancer stem cells and their niche in cancer progression and therapy

High recurrence and metastasis rates and poor prognoses are the major challenges of current cancer therapy. Mounting evidence suggests that cancer stem cells (CSCs) play an important role in cancer development, chemoradiotherapy resistance, recurrence, and metastasis. Therefore, targeted CSC therapy has become a new strategy for solving the problems of cancer metastasis and recurrence. Since the properties of CSCs are regulated by the specific tumour microenvironment, the so-called CSC niche, which targets crosstalk between CSCs and their niches, is vital in our pursuit of new therapeutic opportunities to prevent cancer from recurring. In this review, we aim to highlight the factors within the CSC niche that have important roles in regulating CSC properties, including the extracellular matrix (ECM), stromal cells (e.g., associated macrophages (TAMs), cancer-associated fibroblasts (CAFs), and mesenchymal stem cells (MSCs)), and physiological changes (e.g., inflammation, hypoxia, and angiogenesis). We also discuss recent progress regarding therapies targeting CSCs and their niche to elucidate developments of more effective therapeutic strategies to eliminate cancer.

Cannot Target What Cannot Be Seen: Molecular Imaging of Cancer Stem Cells

Cancer stem cells are known to play a key role in tumour development, proliferation, and metastases. Their unique properties confer resistance to therapy, often leading to treatment failure. It is believed that research into the identification, targeting, and eradication of these cells can revolutionise oncological treatment. Based on the principle that what cannot be seen, cannot be targeted, a primary step in cancer management is the identification of these cells. The current review aims to encompass the state-of-the-art functional imaging techniques that enable the identification of cancer stem cells via various pathways and mechanisms. The paper presents in vivo molecular techniques that are currently available or await clinical implementation. Challenges and future prospects are highlighted to open new research avenues in cancer stem cell imaging.

Indium-111-labeled CD166-targeted peptide as a potential nuclear imaging agent for detecting colorectal cancer stem-like cells in a xenograft mouse model

BACKGROUND

Cancer stem cells (CSCs) are involved in drug resistance, metastasis, and relapse of cancers, which can significantly affect tumor therapy. Hence, to develop specifically therapeutic target probe at CSCs for improvement of survival and quality of life of cancer patients is urgently needed. The CD166 protein has been suggested to be involved in colorectal cancer (CRC) tumorigenesis and to be considered a marker for colorectal CSCs (CRCSCs) detection. In this study, therefore, we attend to apply a nuclear imaging agent probe, Glycine₁₈-Cystine-linked CD166-targeted peptides (CD166tp-G₁₈C), to detect the changes of CD166 level in a CRC xenograft mouse model.

RESULTS

We isolated the CD166-positive cells from the HCT15 CRC cell line (CD166⁺HCT15) and evaluated their morphology and ability of clone formation, migration, protein expression, and drug resistance. The CD166-positive HCT15 cells display the CSCs characteristics. We discovered and designed a CD166-targeted peptide (CD166tp-G₁₈C) as a targeted probe of CRC stem-like cell for cell binding assay. The CD166tp-G₁₈C confirmed the CD166 protein targeting ability in CD166⁺HCT15 cells. The diethylenetriaminopentaacetic acid (DTPA)-conjugated CD166tp-G₁₈C further was labeled with indium-111 (¹¹¹In-DTPA-CD166tp-G₁₈C) as nuclear imaging agent for imaging and bio-distribution analysis in vivo. Finally, we observed that the ¹¹¹In-DTPA-CD166tp-G₁₈C was significantly enhanced in tumor tissues of CD166⁺HCT15 xenograft mice as compared to the non-CD166tp-G₁₈C control.

CONCLUSIONS

Our results indicated that the indium-111-labeled CD166tp-G₁₈C may be served as a powerful tool for colorectal CSCs nuclear imaging in the CRC patients.

Imaging of Tumor Characteristics and Molecular Pathways With PET: Developments Over the Last Decade Toward Personalized Cancer Therapy

PURPOSE

Improvements in personalized therapy are made possible by the advances in molecular biology that led to developments in molecular imaging, allowing highly specific in vivo imaging of biological processes. Positron emission tomography (PET) is the most specific and sensitive imaging technique for in vivo molecular targets and pathways, offering quantification and evaluation of functional properties of the targeted anatomy.

MATERIALS AND METHODS

This work is an integrative research review that summarizes and evaluates the accumulated current status of knowledge of recent advances in PET imaging for cancer diagnosis and treatment, concentrating on novel radiotracers and evaluating their advantages and disadvantages in cancer characterization. Medline search was conducted, limited to English publications from 2007 onward. Identified manuscripts were evaluated for most recent developments in PET imaging of cancer hypoxia, angiogenesis, proliferation, and clonogenic cancer stem cells (CSC).

RESULTS

There is an expansion observed from purely metabolic-based PET imaging toward antibody-based PET to achieve more information on cancer characteristics to identify hypoxia, proangiogenic factors, CSC, and others. ⁶⁴Cu-ATSM, for example, can be used both as a hypoxia and a CSC marker.

CONCLUSIONS

Progress in the field of functional imaging will possibly lead to more specific tumor targeting and personalized treatment, increasing tumor control and improving quality of life.

Targeting cancer stem cells with an 131I-labeled anti-AC133 monoclonal antibody in human colorectal cancer xenografts

INTRODUCTION

Cancer stem cells (CSCs) are a subpopulation within a tumor, which possesses the characteristics of self-renewal, differentiation, tumorigenicity, and drug resistance. The aim of this study was to target the colorectal CSC marker CD133 with an(131)I-labeled specific monoclonal antibody (AC133 mAb) in a nude mouse xenograft model.

METHODS

Colorectal adenocarcinoma cells (LoVo cell line) were separated into CD133(+) and CD133(-) cells by magnetic activated cell sorting. CD133(+), CD133(-), and unsorted LoVo cells were cultured and then implanted subcutaneously into the lower limbs of nude mice (n = 5). AC133 mAb was labeled with (131)I by the iodogen method.

RESULTS

The radiolabeled compound, (131)I-AC133 mAb, showed high stability, specificity, and immunoactivity in vitro. Obvious accumulation of (131)I-AC133 mAb was seen in nude mice bearing xenografts of CD133(+) and unsorted LoVo cells, but no uptake was found in mice bearing CD133(-) xenografts or specifically blocked xenografts. Biodistribution analysis showed that the tumor uptake of (131)I-AC133 mAb was 6.97 ± 1.40, 1.35 ± 0.48, 6.12 ± 1.91, and 1.61 ± 0.44% ID/g (n = 4) at day 7 after injection of (131)I-AC133 mAb in CD133(+), CD133(-), unsorted LoVo cell and specifically blocked xenografts, respectively. The results of immunofluorescence, autoradiography, and western blotting further verified the specific binding of (131)I-AC133 mAb to CD133(+) tumors.

CONCLUSIONS

This study demonstrates the possibility of targeting CSCs with a radiolabeled AC133 mAb in colorectal cancer xenografts based on in vitro, ex vivo, and in vivo experiments. Our findings suggest a new method for imaging CSCs non-invasively.

Isolation, identification and expression of specific human CD133 antibodies

CD133, a 120 KDa glycoprotein is a transmembrane glycoprotein which has been recently used as a cancer stem cell (CSCs) marker in a variety of carcinomas. CD133⁺ cells possess strong tumorigenicity, responsible for tumor initiation and maintenance. Therefore, the goal of our study was to develop a novel CD133 humanized antibody as a promising target for cancer therapy. CD133 purified proteins were used for panning the naive human-semi-synthetic Tomlinson I + J phagemid library. The second extracellular domain (loop1) and the third extracellular domain (loop2) of CD133 were expressed in E. coli. In this study, we adopted a novel five-round selection strategy based on moderate stringent selection during the first rounds. This unique strategy was aimed at avoiding the loss of rare phages with high affinity to target proteins. After the five rounds of specific panning, six phage-antibody clones which specifically recognized recombinant human CD133 protein were obtained. The desirable phage clone named CD133-scFv-1 was cloned into the expression vector, then induced and purified. We show that CD133-scFv-1 and commercial murine antibody 293C3 could compete with each other in the indirect competitive immunoassay. Our work may lay the groundwork for future studies involving biological functions and applications of the CD133 humanized antibody.

Emerging trends for radioimmunotherapy in solid tumors

Due to its ability to target both known and occult lesions, radioimmunotherapy (RIT) is an attractive therapeutic modality for solid tumors. Poor tumor uptake and undesirable pharmacokinetics, however, have precluded the administration of radioimmunoconjugates at therapeutically relevant doses thereby limiting the clinical utility of RIT. In solid tumors, efficacy of RIT is further compromised by heterogeneities in blood flow, tumor stroma, expression of target antigens and radioresistance. As a result significant efforts have been invested toward developing strategies to overcome these impediments. Further, there is an emerging interest in exploiting short-range, high energy α-particle emitting radionuclides for the eradication of minimal residual and micrometastatic disease. As a result several modalities for localized therapy and models of minimal disease have been developed for preclinical evaluation. This review provides a brief update on the recent efforts toward improving the efficacy of RIT for solid tumors, and development of RIT strategies for minimal disease associated with solid tumors. Further, some of promising approaches to improve tumor targeting, which showed promise in the past, but have now been ignored are also discussed.