Cancer is a disease with the interplay of several genes. We endeavor to understand
these genetic changes and treat it effectively. We have been improving our techniques
in molecular biology towards this very goal. The role of tissue biopsy to establish
the diagnosis is the cornerstone for initiating therapies.
Liquid biopsy is a new tool that has recently been added to the armamentarium of oncologists.
The whole society is abuzz with this “new” tool with articles appearing in newspapers
and magazines. So, let us look at it in the right perceptive.
The NCI defines liquid biopsy as, “a test done on a sample of blood to look for cancer
cells from a tumour that are circulating in blood or for pieces of DNA from tumour
cells that are in the blood.”[1]
It is a fact that as solid tumours grow beyond 1-2 mms, from being 'non-vascular'
need to get 'vascular' for their growth and survival. This is accomplished by the
process of “angiogenic switch,” a process that we understand fairly well today. The
neovasculature formed is leaky and permits tumor cells to enter circulation and thus
aiding the process of metastasis. Furthermore, every day, a lot of other nucleic acids
are shed into the bloodstream. It is estimated that each milliliter of blood contains
25 ng of cell-free DNA; thus, a tumor of 100 g sheds about 3.3% of tumor DNA daily.
This is the basis for the concept of liquid biopsy.
At present, the concept of liquid biopsy has expanded to encompass studying circulating
tumor cells (CTCs), DNA, RNA fragments, and exosomes from blood. The other sources
of genetic material could be urine, saliva, sputum, cerebrospinal fluid, and bronchoalveolar
lavage.
We understand the universal phenomenon of tumor heterogeneity, and this limits our
knowledge of the whole tumor when studied on a tissue biopsy. The concept of liquid
biopsy to a very large extent overcomes this drawback. It also permits serial, real-time
acquisition of information due to the ease of access of genetic material, unlike a
tissue biopsy.
The applications of the liquid biopsy are tremendous, from studying tumor biology
and evolution, monitoring of treatment, assessing minimal residual disease, and detection
of resistance and its mechanisms to screening and drug development.
Cristofanilli et al. studied CTCs in breast cancer and enumerating their numbers correlated with outcomes;[2] studies by Cohen in colorectal cancer[3] and de Bono in prostate cancer[4] showed similar findings. Zhang et al. published a meta-analysis of 49 studies with 6815 breast cancer patients and showed
a correlation of the number of CTCs with progression-free survival and overall survival.[5] However, working with CTCs has a myriad of problems and hence many researchers have
not pursued this path.
Circulating tumor DNA (ctDNA) is easier to work with and hence the efforts have progressed
well with the same. Furthermore, the rapid fall in costing of genomic analysis has
aided in extensive work in this field. Enormous data on several cancers have emerged
from the Genome Atlas project. Several methods have been used to study the ctDNA including
real-time polymerase chain reaction (PCR), digital droplet PCR, BEAMing, and next-generation
sequencing (NGS). Each of these has its share of advantages and disadvantages. One
needs to suit the platform depending on the need of the study. NGS is one of the most
sensitive platforms for looking at several parameters in one go. All the platforms
need adequate amounts of DNA, the lesser the quantity, one needs to use the most sensitive
method.
A lot of work has been done on nonsmall cell lung cancer (NSCLC). In fact, it has
become a well-accepted modality to look at resistance mechanisms in patients on tyrosine
kinase inhibitors (TKIs). This is particularly significant in our populations where
the epidermal growth factor receptor (EGFR) mutation rates are high as 35%.
Goto et al. have revealed that ctDNA could very well be used to study the EGFR mutations, and
it has shown good correlation with outcomes with gefitinib comparable to the tissue-tested
cohort.[6]
We are now able to look for the most common resistance mechanism, the T 790 M mutation,
which occurs in 65%–70% of patients. Liquid biopsy is helpful in many of these patients
as the tumor may be in difficult-to-access areas or the patients may not be medically
fit for a transthoracic biopsy. We are now quite confident to treat patients with
T790M with osimertinib when detected either in plasma or tissue as the outcomes have
been shown to be identical.[7]
Studies are underway to quantitatively assess T790 M and use it as a biomarker to
serially follow up patients.[8]
Carpenter et al., in a cohort of 102 NSCLC patients during a period of February 2015 to March 2016,
showed that ctDNA analysis was helpful in detecting EGFR mutations in 86 samples and
56 of these patients had mutations that could be helped with potential off-label drugs.[9]
The role of ctDNA has been exploited in studying breast cancer as well. O'Leary at
ASCO 2017 presented ctDNA assessment to predict sensitivity to palbociclib and fulvestrant
in breast cancer.
The SOLAR-1 study of P13K inhibitor in breast cancer evaluated P13K mutations in plasma
and showed that it could be used as a biomarker to select patients for alpelisib.[10]
Recently, the researchers of Johns Hopkins University published results about their
blood test “cancerSEEK”. They looked for certain proteins in blood to prove the presence
of cancer. Lichtenfeld, the Deputy Chief Medical Officer of the American Cancer Society,
says that “cancer's early detection by means other than X-rays, colonoscopies, or
Pap smears has been subject of research for at least 2 decades, now we have been learning
about ctDNA to evaluate cancer and about pros and cons of trying to find cancer early.”[11]
The benefits of early detection are several, but we have learned the lessons of lead-bias
from conventional screening procedures. Furthermore, the lessons from detecting elevated
PSA, we know that many people die with prostate cancer rather than because of it.
Similar is the story of elevated CA-125. Thus as in screening, we have a lot to learn
about the optimal use of liquid biopsy.
Venkatesan et al. in the ASCO 2016 Education Book have discussed tumor evolutionary principles and
have shown the role of studying ctDNA for the same.
To conclude, the concept of liquid biopsy to study all the elements that can be obtained
in the least invasive way throws open several opportunities to understand cancer better
and thus helps our patients – after all, that is the objective of science and discovery.
