Monitoring the effect of first-line treatment in RAS/RAF mutated metastatic colorectal cancer by serial analysis of tumor specific DNA in plasma.

Abstract

3593 Background: Personalized medicine calls for an early indicator of treatment failure. Circulating tumor DNA (ctDNA) is a promising marker in this setting and our prospective study explored the association between disease control and change of ctDNA during first line chemotherapy in patients with RAS/RAF mutated metastatic colorectal cancer (mCRC). Methods: The present study included 138 mCRC patients receiving standard first line combination chemotherapy. In patients with a RAS/RAF mutated tumor the same mutation was quantified in the plasma using droplet digital PCR (ddPCR). The fractional abundance of ctDNA (ctDNA level) was assessed in plasma before treatment start and at every treatment cycle until radiologically defined progressive disease (PD). Results: RAS/RAF mutations were detected in the plasma from 77 patients (94% of patients with a tumor mutation). Twenty patients progressed on treatment and 57 stopped treatment without progression. The presence of a RAS/RAF mutation in plasma correlated to overall survival (OS) with a median of 24.2 months for patients with a wild-type tumor compared to 12.7 months for patients with a mution in plasma. A substantial increase in ctDNA level was highly associated with progression on treatment (risk ratio = 4.58, 95%CI = 1.99-10.51, p < 0.0001). Furthermore, with a stable ctDNA level the chance of non-progression was 88.2% (range 76.1-95.6%). The first substantial increase in ctDNA level occurred at a median of 51 days (range 14-133 days) before radiologically confirmed PD. Conclusions: The results indicate that ctDNA level may be predictive of treatment effect in patients with mCRC. An increase was observed to correlate with high risk of progression with a relevant lead time, whereas an unchanging ctDNA level related to stable disease.