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Murray Primary circulating prostate cells

cancer cells will determine their ability to disseminate with established performance characteristics for
into the circulation and may not reflect the general which there is scientific evidence that elucidates
characteristics of the primary tumor due to the the clinical significance of the results obtained. The
heterogeneous nature of individual cancer cells within stability, accuracy, and reproducibility of the assay
the general tumor cell population. are fundamental. Pre-analytical, analytical, and post-
analytical variables all have to be controlled during the
In order to implant in distant sites CPCs must survive assay process. Parkinson et al. have extensively
[59]
in the circulation. Only a few of the millions of tumor reviewed this topic as have Panteleakou et al.
[60]
cells that are shed into the circulation are able to reach Pre-analytical factors include the type of collection
a distant site, implant, survive, evade the immune tube (including anticoagulant, storage, and transport
system, and eventually form a metastasis. It has been conditions of the analytical variables), the type
suggested that only 0.01% of circulating tumor cells of enrichment and enumeration methods used,
can produce a single bony metastasis. [51,52] CPCs the sensitivity and specificity of the assay, the
obtained from men with castrate-resistant prostate reproducibility of the assay between laboratories,
cancer failed to produce metastasis when implanted in and assay-specific controls. Other factors include the
immune-compromised mice. [53] disease characteristics, how often the target cells are
detectable in the study population or in other diseases
Firstly, circulating tumor cells have to resist anchorage or normal people, the positive and negative predictive
dependent cell death; over-expression of anti-apoptotic values, and establishing cutoff values for a positive or
proteins such as Bcl-2 overexpression or activation negative test.
[54]
of specific pathways such as tropomyosin-related
[55]
kinase B (TrkB) have been reported. Secondly, they Enrichment of circulating tumor cells from
have to evade the host’s immune systems. Circulating blood
tumor cells from patients with colorectal cancer CD47 Methods for circulating tumor cell enrichment fall into
expression were increased. This marker is considered three basic categories: density gradient centrifugation,
to be an anti-phagocytic signal expressed on cancer cell filtration based on size or microfluidics, and immune-
cells to prevent macrophages and dendritic cells magnetic isolation, often anti-EpCAM antibodies; or a
from attacking them. The counterpart of this anti- combination of methods.
phagocytotic mechanism, the expression of pro-
phagocytic calreticulin, was significantly decreased. [56] Density gradient centrifugation is a simple, fast,
and cheap process, separating cells based on their
Circulating tumor cells escape immune surveillance differing densities. Circulating tumor cells separate
by shielding themselves from the immune cell with the mononuclear blood cells (density < 1.077 g/mL),
population. It has been proposed that myeloid-derived forming an opaque layer which can be removed and
suppressor cells facilitate the survival of cancer cells further analyzed. Red blood cells and granulocytes
by creating a defensive shield. These myeloid-derived (density > 1.077 g/mL), being denser, settle towards
suppressors adhere to some of the circulating cancer the bottom of the tube. The method has poor sensitivity,
cells, conferring a survival advantage. [57] Circulating as tumor cells may be lost when cells sediment to the
tumors cells are rapidly coated by platelets. This may granulocyte layer, or, if present as cell clusters, when
cause transfer of major histocompatibility complex they aggregate to the bottom of the tube. This may be
(MHC) class I antigens on the tumor cell surface important because circulating tumor cell clusters have
resulting in a high level of platelet-derived normal been reported in patients with metastatic prostate
MHC class I. This coating of phenotypic normality cancer and have been correlated with a worse
[61]
disrupts the normal recognition of tumor cells by outcome in breast cancer. [62]
natural killer cells and T cell mediated immunity, thus
permitting tumor cell survival. [58] Furthermore, if the centrifugation is performed
immediately, whole blood may be mixed with the
METHODS TO DETECT AND CHARACTERIZE gradient solution, causing contamination. The
®
CIRCULATING TUMOR CELLS OncoQuick system uses a porous barrier to prevent
such contamination. It has been reported that this
All methods of detecting circulating tumor cells system improves the depletion of mononuclear cells
are based first on enrichment of circulating tumor resulting in higher relative tumor cell enrichment as
cells from venous blood and then on detection. compared with standard gel separation. However,
The Food and Drug Administration (FDA) defines a using cell-spiked blood samples there was a similar
validated biomarker assay as a system of analysis tumor cell recovery rate of between 70% and 90%. [63,64]
456 Journal of Cancer Metastasis and Treatment ¦ Volume 2 ¦ December 16, 2016