Fig. 1. Source of liquid biopsy tumor material. Solid tumor
masses may shed circulating tumor cell (CTC), circulating cell-free
nucleic acids (ctDNA), and exosome into the bloodstream.
Several instruments have been developed to achieve optimal LB assessment
(Fig. 2, Table 1). CellSearch (Veridex, USA), the first
instrument approved by the United States Food and Drug Administration as
an instrument for separating and detecting CTCs in LB, has been widely
used for breast, colorectal, and prostate cancer diagnosis. Using this
instrument, CTCs are separated from blood cells by magnetic
nanoparticles coated with anti-epithelial cellular adhesion molecule
(EpCAM) antibodies [16]. Immunofluorescence analysis is then
performed on the separated cells for CTC characterization. This approach
provides a CTC detection specificity of > 99% and
sensitivity of > 97%. As the
detection of circulating cell-free
nucleic acids depends on molecular biology techniques that are complex
to operate and require additional analytical platforms (such as
quantitative polymerase chain reaction [qPCR], BEAMing, Safe-SeqS,
and CAPP-Seq) [9,17], LB-based instruments are commonly used to
extract cell-free nucleic acids. For example, the
KingFisher
Flex instrument has been applied for cfDNA extraction from blood samples
based on the magnetic enrichment technique [18], wherein cfDNA
becomes positively charged by magnetic beads that bind to the negatively
charged phosphate DNA backbone. For exosome analysis, serial
centrifugation or ultracentrifugation steps are performed to eliminate
nanoscale contaminants [19]. However, these methods require
time-consuming sample preparation protocols and cannot achieve high
purity or high-throughput analysis. To address these limitations, an
instrument named EXODUS was developed based on a novel ultrafiltration
strategy [20]. In this instrument, exosomes are filtered through
nanoporous membranes, and double-coupled harmonic oscillations are
introduced to these membranes to generate transverse waves to inhibit
fouling effects, resulting in enhanced processing speed, yield, and
purity.
Nowadays, although many LB instruments have been proposed for cancer
diagnosis, most of them still exist a wide gap between the laboratory
devices and the commercialization of instrument. According to the
differences of working mechanism and application object, the developed
instruments also face various limitations. In this article, the
challenges of the existed LB instrument, as well as the opportunities
for next-generation LB instruments are discussed, aiming at improving
the sensitivity, efficiency, and accuracy of cancer diagnosis.