Massively parallel sequencing (MPS) of circulating DNA in patients with metastatic colorectal cancer (mCRC): Prognostic significance and early changes during chemotherapy (CT).

Abstract

11015 Background: Prognostic and predictive biomarkers in mCRC are urgently needed. Circulating tumor cells are a promising blood biomarker, but are detectable in only a minority of pts. Recently, analysis of circulating tumor DNA (ctDNA) has shown promise as a liquid biopsy, reflecting the evolving mutational status of the tumor. Here we explored baseline ctDNA as a prognostic marker, and early changes in ctDNA as a marker of CT response. Methods: Serial plasma samples and CEA were collected at baseline (D1), day 3 (D3) and cycle 2 day 1 (C2D1) from 40 mCRC pts receiving standard combination CT. Restaging scans performed at 8 weeks were centrally assessed using RECIST criteria. Samples were analyzed at Johns Hopkins Kimmel Cancer Center. Initially tumor tissue was analyzed for hotspot mutations in TP53, APC, KRAS, BRAF, PIK3CA and FBXW7. The same mutation was queried and quantified in plasma using a MPS platform (Safe-SeqS). Log-rank test was used to compare survival curves and Wilcoxon matched pairs test was used to compare paired plasma samples. Results: Preliminary data is available on 19 pts in this ongoing study. Using our initial panel at least 1 mutation was found in 16 of 19 (84.2%) tumors (7 KRAS, 3 TP53, 3 BRAF, 2 APC and 1 PIK3CA), with matching ctDNA found for each pt. For the remaining 3 cases a further panel of mutations is being analyzed. Median D1 cell free DNA (cfDNA) and ctDNA levels were 1.98 ng/ul (0.15 – 57.18) and 523 mutant fragments (frag)/ml (0.4 – 109,876), respectively. Pts with D1 cfDNA of ≥ 2.5 compared with < 2.5 had shorter median overall survival (OS; 6.9 v 12.2 months, p = 0.0086), with a trend for shorter progression-free survival (PFS; 3.6 v 8.2 months, p = 0.3477). A surge of ctDNA level from D1 to D3 was typically observed (median increase: 91.2 frag/ml, p = 0.0313), followed by a drop (median decrease from D1 to C2D1: 208.8 frag/ml, p = 0.0098). All pts with a decrease in ctDNA at C2D1 had a reduction in tumor size at 8 weeks. Conclusions: In all cases of mCRC where tumor mutation was identified, matching ctDNA was detected in plasma. Circulating DNA is a promising marker of prognosis. Early changes in DNA levels may be a useful marker of tumor response.