Circulating cancer-derived small extracellular vesicles (EVs) are nanoscale membranous vesicles shed from cancer cells that are released into surrounding body fluids. Small EVs contain biomolecules associated with cancer such as DNA and proteins for cell-to-cell communication. Therefore, small EVs have been regarded as important cancer biomarkers for liquid biopsy-based cancer diagnosis and drug treatment monitoring. However, because of the high heterogeneity and low level of small EVs in body fluids, there is a high demand for sensitive detection and characterization of such vesicles at a molecular level. In this study, we have developed a sensitive and effective approach to simultaneously profile multiple protein biomarkers expressed on cancer-derived small EVs using surface-enhanced Raman spectroscopy (SERS) nanotags in a single test, without complex isolation steps. Rapid and multiplexed phenotypic profiling of small EVs is achieved by mixing specific detection antibody-coated SERS nanotags, filtered conditioned EV-suspended medium (conditioned EVs), and capture antibody (CD63)-conjugated magnetic beads to form a sandwich immunoassay. As a proof-of-concept demonstration, we applied this approach to characterize pancreatic cancer-derived EVs by simultaneously detecting three specific EV surface receptors including Glypican-1, epithelial cell adhesion molecules (EpCAMs), and CD44 variant isoform 6 (CD44V6). The sensitivity of this method was measured down to 2.3 × 106 particles/mL, which is more sensitive and shows higher multiplexing capability than most other reported EV profiling techniques, such as western blot, enzyme-linked immunosorbent assay, and flow cytometry. Furthermore, phenotypic profiling of small EVs from colorectal cancer and bladder cancer cell lines (SW480 and C3) was conducted and compared to those derived from pancreatic cancer (Panc-1), highlighting the significant difference in EV phenotypes for various cancer cell types suspended in both phosphate-buffered saline and plasma. Thus, we believe that this technology enables a comprehensive evaluation of small secreted EV heterogeneity with high sensitivity, offering strong potential for accurate noninvasive cancer diagnosis and monitoring of drug treatment. In addition, this assay provides point-of-care use because of the easy sample preparation and portable nature of the Raman spectrometer.
Keywords: SERS nanotags; extracellular vesicles; immunoassay; multiplex detection; phenotype.