Circulating Tumor DNA (ctDNA) as a Prognostic Tool in Patients With Advanced Lung Adenocarcinoma

Although EGFR- and ALK-TKIs can achieve a response rate as high as 70%, all patients treated with TKIs invariably develop resistance to the therapy. The median progression-free survival is 10-16 months. The most common mechanism of acquired resistance to TKIs is the therapy-induced clonal selection of a minor subpopulation of resistant cancer cells that were present in the original tumor. Emergence of the EGFR mutation T790M occurs in about 50-70% of patients with acquired resistance to EGFR-TKIs. Other EGFR mutations and mutations in phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha (PIK3CA) and B-Raf Proto-Oncogene (BRAF) are also associated with EGFR-TKI resistance, but they occur at low frequencies. Resistance to ALK-TKI is more complex and involves various resistant mutations.

TKI resistance remains a major problem in clinical management of NSCLC. Patients with acquired resistance can be treated with second generation TKIs, though none are FDA approved yet, or by combination therapy strategies. Therefore, molecular characterization of tumor throughout the course of disease is helpful to match new drugs to the tumor's evolving genomic profile and guide effective personalized therapies. However, serial tissue sampling to monitor molecular signatures of tumor is invasive, impractical, and not a routine clinical practice. Obtaining sufficient tissue materials for genotyping is also a major hurdle in tissue sampling. There is a need to develop a technology that permits non-invasive serial analysis of the tumor genomic profiles.

Cell-free circulating DNA is fragmented DNA found in circulation that is not associated with cells or cell fragments. When tumor cells die, they release tumor DNA into the bloodstream. The cell-free circulating DNA derived from tumors, known as circulating tumor DNA (ctDNA), carries mutations present in the tumor and hence can be distinguished from cell-free circulating DNA derived from normal cells. It has been shown that the detection of ctDNA and its concentration correlate with tumor stage and cancer survival. Moreover, ctDNA in plasma can be used to detect genomic alterations in solid cancers, and that there is a high concordance in detected mutations between paired formalin-fixed paraffin-embedded (FFPE) and plasma DNA samples.

In a study of acquired resistance to EGFR blockade in colorectal cancer patients, repeated serum samples were collected at 4-week intervals until disease progression. Using mathematical modeling, this study had the following important findings: resistant mutations were present in a clonal subpopulation within the tumors prior to the initiation of treatment, it took a fairly consistent period of time (about 5-6 months) for the subclone to expand and repopulate the lesion, and circulating resistant mutations could be detected several months before radiographic evidence of disease progression. This seminal study demonstrated the potential of using a ctDNA test to track genomic evolution and selection in tumors in a non-invasive manner in order to facilitate individualized therapies and hence to prolong remission.

The investigators have demonstrated plasma detection of EGFR mutations in patients with advanced stage lung adenocarcinoma bearing EGFR mutations, correlating with prognosis of subjects on EGFR-TKI. One prospective study had used real-time polymerase chain reaction (RT-PCR) to detect EGFR mutations in ctDNA from patients with advanced NSCLC. Among patients who were EGFR mutation + at baseline (pre-treatment), those who lost the EGFR mutation at cycle 3 of treatment (chemotherapy +/- erlotinib) had better progression free survival; median survivals were 7.2 vs. 12.0 months in patients who were EGFR mutation (+,+) and (+,-) at baseline and cycle 3, respectively.