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Cell-free DNA, KRAS, and BRAF mutations in plasma from patients with metastatic colorectal cancer treated with third-line cetuximab and irinotecan.

3599 Background: The possible clinical importance of circulating cell-free DNA (cfDNA) in metastatic colorectal cancer (mCRC) remains to be elucidated. This especially applies to tumor specific mutations. The aim of the present study was to investigate the importance of cfDNA in relation to third line treatment. Furthermore, we wanted to investigate the quantitative relationship of cell free DNA with specific mutated DNA in the plasma.


METHODS
Inclusion criteria were; histopathologically verified chemotherapy resistant mCRC, adequate PS and organ function. Treatment consisted of irinotecan 350 mg/m2 q3w and weekly cetuximab 250 mg/m2 until progression or unacceptable toxicity. Mutational status in tumor was assessed by a commercial kit. A quantitative PCR method was developed and used to asses the number of cfDNA alleles and KRAS and BRAF mutation alleles in plasma at baseline and prior to each cycle.


RESULTS
The study included 108 patients. The majority of KRAS mutations detected in the tumor was also found in the plasma (32/41 [78%]). Two of the 3 BRAF mutations in tumor were detected in the plasma. Plasma cfDNA and KRAS levels (pKRAS) were strongly correlated (r = 0.968, p<10-4). The disease control (DC) rate was 77% in patients with low cfDNA (<25 percentile) and 30% in patients with high (>75% percentile [p=0.009]). Patients with pKRAS levels higher than the 75% percentile had a DC rate of 0% compared to 42% in patients with pKRAS below (p=0.048). Cox analysis confirmed the prognostic importance of both cfDNA and pKRAS and high levels were clear indicators of a poor outcome. Off note, at least one patient harboured a different mutation in the primary tumor compared to the metastasis and plasma samples, and in 4 patients without mutation in the tumor and baseline plasma samples, a KRAS (n=4) or BRAF (n=1) mutation appeared at time of progression.


CONCLUSIONS
KRAS and BRAF analysis in plasma is an alternative to tissue analysis. Quantitative levels of cfDNA and pKRAS were strongly correlated to clinical outcome of third-line treatment of mCRC, and mutational status may change during the course of treatment. Quantification of plasma cfDNA holds promise of clinical application.