Table 3.

Studies on liquid biopsy in Merkel cell carcinoma.

Study	Title	Bio-Marker	Inclusion Criteria	n	Detection Method	Prognosis Relevance	Ref
Blom et al. (2014)	Clinical utility of a circulating tumor cell assay in Merkel cell carcinoma	CTCs	Stage I–IV	34	CellSearch—Epithelial kit	CTCs associated with survival, prediction of treatment response, shorter OS and reflect disease burden.	[139]
Gaiser et al. (2015)	Evaluating blood levels of neuron specific enolase, chromogranin A, and circulating tumor cells as Merkel cell carcinoma biomarkers	CTCs	Stage I–IV	30	Maintrac	Correlation between CTC detection and disease outcomes.	[140]
Samimi et al. (2016)	Prognostic value of antibodies to Merkel cell polyomavirus T antigens and VP1 protein in patients with Merkel cell carcinoma	Anti-MCPyV-antibody	Stage I–IV	143	ELISA	Basal level of anti-VP1 antibodies used as prognostic marker. Anti-T-antigen antibodies are marker of disease recurrence or progression if detected >12 months after diagnosis	[141]
Fan et al. (2018)	Circulating cell-free miR-375 as surrogate marker of tumor burden in Merkel cell carcinoma	miRNA	Stage I–IV	102	RT-qPCR	Circulating miR-375 is a useful biomarker for tumor burden, therapy monitoring and follow-up of patients with MCC.	[142]
Riethdorf et al. (2019)	Detection and characterization of circulating tumor cells in patients with Merkel cell carcinoma	CTCs	Stage I–IV	51	CellSearch—CXC kit	Correlations between CTC counts and MCC aggressiveness.	[143]
Boyer et al. (2020)	Circulating tumor cell detection and polyomavirus status in Merkel cell carcinoma	CTCs	Stage I –IV	19	•CellSearch—CTC kit •RosetteSep/DEPArray	CTC presence associated with tumor stage and number of organs with metastases.	[144]

CTC: circulating tumor cell, OS: Overall Survival, MCPyV: Merkel Cell Polyomavirus, ctDNA: circulating tumor DNA, miRNA: microRNA, ELISA: Enzyme-linked immunosorbent assay, RT-qPCR: Reverse-transcriptase Polymerase Chain Reaction.