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Galletti et al. Using CTCs in prostate cancer

of personalized medicine. Importantly, CTCs provide Evaluation of androgen receptor expression
the ideal tool to analyze and longitudinally monitor and signaling in CTCs
the expression and/or the dysregulation of several AR is a transcription factor, which plays a key role in
tumor related biomarkers, associated with response prostate homeostasis and drives growth and progression
or resistance to investigational antitumor agents and of PC cells. AR-targeting drugs (ADT, abiraterone
to the already FDA-approved drugs currently used in acetate, enzalutamide) are current standard of care for
clinical practice. PC treatment and exert their anti-tumor activity mostly
by inhibiting AR activation and its consequent nuclear
In the next paragraphs, we will describe the most translocation, ultimately impeding AR mediated tumor
recent results regarding the CTC-based analysis and growth [1,2] . Although most patients initially respond
validation of the most clinically relevant biomarkers in to these therapies, treatment-related resistance
PC [Table 2]. often occurs through a variety of mechanisms . AR
[58]
dysregulation (i.e. amplification, mutations, alternative
CTCs as tool to unveil tumor biology in the splicing) represents the key molecular alteration in
single patient PC, leading to aberrant activation of the receptor
By filling the gap of chronic lack of tumor tissue, and its downstream pathways with consequent tumor
CTCs provide the opportunity to perform broad- progression. Clinical assessment of these molecular
spectrum genomic analyses to unveil the molecular alterations has been challenging and currently there
characteristics of the disease in the individual patient, is no way to clinically predict which patients are more
a key point for a precision medicine-based approach. likely to respond to AR-targeting therapies.
To investigate the use of CTCs to overcome the AR expression and nuclear localization, considered
inaccessibility of metastatic tissue for high throughput readouts of AR activation, have been subject of
genomic testing, Lohr et al. investigated the role translational studies as biomarkers of response to
[12]
of CTCs in comprehensive analyses of cancer AR-targeting drugs in PC. Many research groups
genomes by developing a census-based whole exome have used CTCs as a liquid biopsy to longitudinally
sequencing method to analyze single EpCAM-positive assess these markers and correlate them with
CTCs isolated from CRPC patients. The CTC-data treatment outcome.
were then compared to the genetic profiles of the
primary tumor and one metastatic site. The authors Reyes et al. [59] evaluated AR expression levels and
found that CTCs harbored up to 73% of the somatic subcellular localization in CTCs isolated from a
single nucleotide variants identified in the primary cohort of 20 CRPC patients to develop a platform
tumor and in the metastatic site. Gupta et al. also for CTC protein interrogation. In this study, cells
[56]
conducted a whole genomic copy number analysis of were isolated by gradient medium-based separation
CTCs and matched leukocytes from 16 men with CRPC and were fluorescently stained for EpCAM, CD45
using array-based comparative genomic hybridization and AR. AR expression and nuclear accumulation
(aCGH) and observed CTC-specific genomic gains were then analyzed in confirmed CTCs by
®
(i.e. AR, FOXA1, ERG) and genomic losses (i.e. PTEN, ImageStream X. Results revealed a significant
RAF1, GATA1) in key regulators of PC progression. increase in AR expression levels in CTCs of patients
who progressed on abiraterone compared to patients
Interestingly, Miyamoto et al. [57] performed RNA who were abiraterone-naïve. The authors also looked
sequencing profiles of single CTCs isolated from at AR nuclear localization in the isolated CTCs and
13 metastatic PC patients; they recognized CTC- observed high inter- and intra-patient heterogeneity
specific up-regulation of molecular pathways linked to without a clear correlation with treatment response.
cell growth and adhesion, found AR point mutations Similarly, Crespo et al. [60] quantified AR nuclear
associated with AR inhibitors resistance and expression in CTCs isolated from 48 subjects
identified high intrapatient CTC heterogeneity for AR grouped by absence of prior exposure or resistance to
alterations, showing that more than half of the patients abiraterone or enzalutamide. After CellSearch -based
®
had multiple AR splice variants within different CTCs. enrichment, CTCs were analyzed for the detection
of AR by arranging a user-defined assay based on
These studies provide proof of the feasibility of CTC- the CellTracks Autoprep system. No difference was
based high throughput sequencing in individual observed in nuclear AR expression between treatment
cancer patients using a high-throughput analysis, naïve and resistant patients; however, when analyzing
thus, offering a minimally invasive look into the eight patient-matched CTC samples collected before
mutational landscape of metastatic PC. and after treatment, the authors found an increase in
196 Journal of Cancer Metastasis and Treatment ¦ Volume 3 ¦ September 27, 2017