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The tunability of SELEX procedure is an essential feature to supply bioaffinity receptors (aptamers) almost on demand for analytical and therapeutic purposes. This longstanding ambition is, however, not straightforward. Non-invasive cancer diagnosis, so called liquid biopsy, requires collection of body fluids with minimal or no sample pretreatment. In those raw matrices, aptamers must recognize minute amounts of biomarkers that are not unique entities but large sets of variants evolving with the disease stage. The susceptibility of aptasensors to assay conditions has driven the selection of aptamers to natural environments to ensure their optimum performance in clinical samples. We present herein a compilation of the SELEX procedures in natural milieus. By revising the electrochemical aptasensors applied to clinical samples for cancer diagnosis and tracing back to the original SELEX we analyze whether aptamers raised using these SELEX strategies are being incorporated to the diagnostic devices and how aptasensors are finding their way to a market dominated by antibody-based assays. Image 1 • Electrochemical aptasensors tested in clinical samples for cancer biomarkers. • Critical revision of aptasensors to detect CTCs, exosomes and proteins. • Relevance of natural environment during SELEX on aptamer performance. • Prospects of in-vitro diagnostic devices based on electrochemical aptasensors. [ABSTRACT FROM AUTHOR]

The extracellular matrix (ECM) plays an essential role in tumor progression and invasion through its continuous remodeling. The growth of most carcinomas is associated with an excessive collagen deposition that provides the proper environment for tumor development and chemoresistance. The α1 chain of a minor human collagen, type XI, is overexpressed in some tumor stroma, but not found in normal stroma. To test the clinical utility of this collagen as a cancer biomarker, specific receptors are needed. Available antibodies do not show enough selectivity or are directed toward the propeptide region that is cleaved when the protein is released to the ECM. Here we show the selection of an aptamer for the specific C-telopeptide region using a 16-mer peptide as the target for the SELEX. The aptamer selected with a K d of ∼25 nM was able to capture the collagen XI from cell lysates. It was also used for target detection in a mixed antibody-aptamer sandwich assay showing it can be useful for diagnostic purposes in biological fluids. [Display omitted] • Extracellular matrix components are an understudied source of cancer biomarkers. • Detection of collagen XI, overexpressed in some cancers, needs suitable probes. • A 16mer peptide was the target for the first aptamer for α chain of human collagen XI. • Mass spectrometry verifies collagen XI as the target of D1 aptamer from cell lysates. • Antibody-aptamer sandwich assay detects colXIα1 in cell lysates. [ABSTRACT FROM AUTHOR]