Liquid Biopsy in Melanoma: Significance in Diagnostics, Prediction and Treatment Monitoring

Abstract

Liquid biopsy is a common term referring to circulating tumor cells and other biomarkers, such as circulating tumor DNA (ctDNA) or extracellular vesicles. Liquid biopsy presents a range of clinical advantages, such as the low invasiveness of the blood sample collection and continuous control of the tumor progression. In addition, this approach enables the mechanisms of drug resistance to be determined in various methods of cancer treatment, including immunotherapy. However, in the case of melanoma, the application of liquid biopsy in patient stratification and therapy needs further investigation. This review attempts to collect all of the relevant and recent information about circulating melanoma cells (CMCs) related to the context of malignant melanoma and immunotherapy. Furthermore, the biology of liquid biopsy analytes, including CMCs, ctDNA, mRNA and exosomes, as well as techniques for their detection and isolation, are also described. The available data support the notion that thoughtful selection of biomarkers and technologies for their detection can contribute to the development of precision medicine by increasing the efficacy of cancer diagnostics and treatment.

Keywords: circulating melanoma cells (CMCs); circulating tumor cells (CTCs); immunotherapy; liquid biopsy; malignant melanoma; metastasis; targeted treatment.

Figure 1

Immunotherapy of PD-1–PD-L1/L2 axis blockade—mechanism of action. Anti-PD-1 monoclonal antibodies nivolumab and pembrolizumab enhance the anti-cancer response by blocking the binding of PD-1 to PD-L1/L2, thus activating T-cells to eradicate cancer cells. Abbreviations: PD-1, programmed cell death 1; PD-L1/L2, programmed cell death ligand 1/2; T-cell R, T-cell receptor; Ab, antibody.

Figure 2

A schematic of liquid biopsy in a patient with a solid tumor. Tumor cells circulate in blood, with those that are more aggressive (metastasis-initiator cells) settling in targeted distant organs and initiating metastasis. Blood collected from cancer patients contains circulating tumor cells (CTCs), clusters or circulating tumor microemboli (CTM), proteins, circulating cell-free tumor DNA (ctDNA), extracellular vesicles (EVs), such as exosomes, and tumor educated platelets (TEPs). These complementary circulating biomarkers can be detected and provide real-time information on tumor progression, prognosis, and treatment response. Abbreviations: CTCs, circulating tumor cells; ctDNA, circulating tumor DNA; CTM, circulating tumor microemboli.

Figure 3

CellSearch^® strategy to enrich and detect CMCs in patients with a melanoma. The positive CMC enrichment is based on the expression of CD146 by melanoma cells. Subsequently, CMC detection is based on the expression of the high molecular weight melanoma-associated antigen, as well as the presence of a nucleus (visualized using DAPI). The exclusion markers are (i) CD45, specific for leukocytes and some endothelial cells and (ii) CD34, expressed by endothelial cells. As endothelial cells also express CD146, CD34 is important to consider to discriminate these cells from CMCs that are CD34⁽⁻⁾. Abbreviation: MCAM, melanoma cell adhesion molecule; HMW-MAA, high molecular weight; PE, phycoerythrin; APC, allophycocyanin.