Liquid Biopsy

The molecular landscapes of solid tumors are presently established from surgical or bioptic tissue samples. During tumor progression and treatment, multiple clonal populations compete with one another — and selections leads to the emergence of those that replicate and spread faster and are the least susceptible to treatments. Tissue-based tumor profiles are subjected to sampling bias, provide only a snapshot of the tumor heterogeneity and cannot be obtained repeatedly. 
Recently, genomic profiles of circulating, cell-free tumor DNA (ctDNA) were shown to closely match those of the corresponding tumors. 
These findings have major implications in molecular pathology and clinical oncology. Many studies have shown the potential of ctDNA and circulating tumor cells (CTCs) analyses (commonly referred to as “liquid biopsies”) to determine the genomic profile of cancer patients, monitor response to treatment, assess the emergence of resistance and quantify minimal residual disease.  
The molecular profiles gathered from ctDNA can be complemented with those obtained analyzing CTCs and RNA-containing vesicles such as exosomes. Various approaches, including protein, DNA and RNA analyses, can be applied to explore CTCs content.

The development of highly sensitive technologies such as next generation sequencing (NGS) and digital PCR-based techniques have enabled clinicians to repeatedly and non-invasively interrogate the evolution of human cancers. 

Recently, ctDNA was used to genotype colorectal tumors (CRCs) and track clonal evolution during treatment with anti-EGFR antibodies, identifying alterations underlying primary and acquired resistance in KRAS, NRAS, MET, ERBB2, EGFR and MAP2K1 genes. 

Additionally, it was found that mutant KRAS clones that emerged during EGFR blockade declined upon withdrawal of anti-EGFR antibodies, indicating that clonal evolution continues beyond clinical progression. ctDNA profiles of individuals who benefit from multiple challenges with anti-EGFR antibodies exhibit pulsatile levels of mutant KRAS, providing a molecular explanation for the efficacy of rechallenge therapies based on EGFR blockade. 
Remarkably, ctDNA analyses were instrumental to show that responses to targeted therapies can be driven by distinct resistance mechanisms arising within separate tumor lesions in the same patient. 

As more trials evaluating targeted therapy strategies designed to overcome specific acquired resistance mechanisms enter the clinic, genomic results from single-tumor biopsies should be interpreted with caution. By contrast, liquid biopsy approaches have the potential to detect the presence of simultaneous resistance mechanisms residing in separate metastases in a single patient and to monitor the effects of subsequent targeted therapies.