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Tumor Discovery CTC characterization for EGFR mutations

improved clinical outcomes, particularly progression-free As a result of the benefits experienced by NSCLC patients
survival (PFS) and overall survival (OS), in patients with with EGFR mutations, such as longer time to progression
exon 18 – 21 mutations in the epidermal growth factor and/or survival, through tailored therapies that suit their
receptor (EGFR) gene who were treated with tyrosine molecular pathology along with cost reduction associated
kinase inhibitors (TKIs). These TKIs are agents that with prudent use of TKIs, means that molecular testing
2,3
specifically bind in, or close to, the ATP cleft of EGFR. has been recommended by several regulatory bodies as
4
Such mutations stabilize the binding of TKIs to ATP, the standard of care. 23-25 The effectiveness of such patient
thereby inhibiting constitutive autophosphorylation and stratification is highly dependent on detailed and efficient
blocking the amplified catalytic activation of the tyrosine capture of the genomic environment of the malignancy in
kinase domain after ligand binding. 5 real time. 26
The response of NSCLC patients with mutations in In the clinic, the tumor biopsy used for detecting the
6
exon 18 – 21 of the EGFR gene to TKIs is highly varied. mutational profile of patients with malignancies, currently
Common mutations, such as deletions in exon 19 (LREA the gold standard may not be adequate for effective
regions), which removes codons 746 – 750, and the codon molecular testing. First, around 40% of patients with
27
858 mutation on exon 21 where leucine replaces arginine, NSCLC may not be eligible for surgery, because their
account for around 85% of EGFR mutations in NSCLC malignancies are at an advanced stage, and they are too
and have been reported to be associated with better tumor weak to undergo surgery. Furthermore, a single lesion
28
responses (longer time to progression/survival) to TKIs or segment of a tissue biopsy may not provide sufficient
when compared to other EGFR mutations. Similarly, information on tumor heterogeneity. Re-sampling to
29
7,8
several clinical studies have reported that tumors with monitor mutational changes that have some influence
mutations in exon 18 are responsive to TKIs, whereas on progression and resistance is not practicable for the
deletions and insertions in exon 18 have been associated patient; thus, alternatives are being actively explored.
30
with short-lived responses to TKIs. 9-11 Exon 19 deletions Cell-free DNA (cfDNA) is one alternative that has been
around the non-LREA regions are less responsive to TKIs utilized successfully for mutational analysis. However, its
12
when compared to deletions in these regions. Mutations widespread use has been limited because of issues with low
in exon 19 are rare and account for approximately 0.5% sensitivity particularly with regard to identifying resistant
of all EGFR mutations. Tumors with exon 19 mutations clones with specific mutations. 31,32
13
have heterogeneous responses to TKIs, ranging from Circulating tumor cells (CTCs) have been explored as a
moderately responsive (L747F, E746G, and P733L) to fully prognostic indicator for PFS and OS 33,34 and as a diagnostic
resistant (D761Y and L747S). 14,15 tool for the detection of mutations with varying degrees
Around 15% of NSCLC patients with an EGFR mutation of success. Some researchers indicate that the CTC sample
have either an exon 20 mutation and/or an insertion. matrix may be superior to tumor biopsy as the number
Aberrations in exon 20 have been associated with tumors of mutations detected increase, with the hypothesis
that are mostly non-responsive to TKIs. The T790M being that CTCs reflect cells derived from the primary
16
mutation on codon 790 of exon 20 is the most clinically malignancy and metastatic sites. 35,36 However, other
relevant mutation on this exon, with around 10% of studies have reported equal or fewer numbers of mutations
17
18
patients with advanced lung cancer having this mutation. obtained from CTCs when compared to tissue biopsy and/
Interestingly, more than 50% of patients with an exon 19 or cfDNA. 37,38 The implementation of CTC sampling as
deletion or L858R point mutation undergoing treatment a diagnostic tool has been limited by their relatively low
with TKIs acquire the T790M mutation and thus develop numbers in the blood, especially in the early stages of
19
resistance to first- and second-generation TKIs. However, tumor development, relatively cumbersome work flows,
patients with the T790M mutation are responsive to third- and the high costs of the techniques. In addition, some
generation TKIs (osimertinib). Tumors with the C797S of the approaches for isolating CTCs from the blood have
20
mutation and exon 20 insertions (D770_N771insNPG at reported moderate purity values of ≤60% and yield of CTCs
residues 762 – 775) are also generally non-responsive to at ≤70%. 39,40 Most approaches for isolating CTCs from the
TKIs. Exon 21 mutations are rare and have been associated blood of patients with NSCLC for subsequent downstream
21
with lower sensitivity to TKIs (L861Q, L862V, A859X, and detection of EGFR mutations have employed the principles
V851X) when compared to L858R mutations. Because of of immune isolation and/or size disparity between CTCs
22
their scarcity, the sensitivity of tumors with many exon 21 and other blood cells for isolation and polymerase chain
mutations (e.g., E866K, H870Y, H825L, H870R, G863S, reaction (PCR) techniques to detect mutations in exons
and P848L) to TKIs is yet to be determined. 22 18 – 21. 41,42 The translation of these devices to routine use

Volume 3 Issue 4 (2024) 2 doi: 10.36922/td.3987