In metastatic colorectal cancer (mCRC) recent studies have shown the importance to accurately quantify low-abundance mutations of RAS pathway because response to anti-EGFR therapy may depend on certain mutation thresholds. We designed a clinical trial to compare clinical outcomes of patients selected with different analytical sensitivity thresholds for RAS/BRAF mutated alleles using a highly sensitive and quantitative technique of digital PCR (dPCR).
Hotspots including RAS (KRAS and NRAS exons 2/3/4) and BRAF (exon 15) were prospectively analyzed in tumour FFPE samples from 61 patients with mCRC included in the ULTRA trial. Patients had received one or two previous chemotherapy lines and were deemed resistant to irinotecan. Response rate (RR), progression-free survival (PFS) and overall survival (OS) were correlated with the mutational status based on three different cut-off points (0.1%, 1% and 5%).
The overall RR was 51.7% and comparative analysis of clinical outcomes translated into a differential progression free survival (PFS), response rate (RR) and progression disease (PD) in the different cohorts defined by the 3 selected analytical sensitivity cut-off points (Table). PFS prediction was higher when we considered a threshold of 5% in RAS/BRAF scenario (HR mut vs wt = 3.85; CI95% [1.16-12.82], p = 0.018).
Optimal sensitivity RAS/BRAF mutational analysis cut-off for clinical outcome prediction lies between 1 and 5% (closer to 5%). Increasing analytical sensitivity worsens patient’s selection. Further sensitivity threshold comparative analysis will define an optimal cut-off.Table:
|Highly-sensitive digital PCR|
|cut-off 0.1%||cut-off 1%||cut-off 5%|
|RAS + BRAF||mut/wt (n/n)||14/47||8/53||3/58|
|RR % (mut/wt)||46.2/52.1||37.5/52.8||33.3/51.7|
|SD % (mut/wt)||46.2/33.3||50.0/34.0||33.3/36.2|
|PD % (mut/wt)||7.7/12.5||12.5/11.3||33.3/10.3|
|PFS months||median (mut/wt)||9.3/7.6||7.4/7.6||4.0/8.8|
|HR (mut vs wt)||0.82||0.77||3.85|
|OS months||median (mut/wt)||17.4/12.5||26,22/13.88||16.05/16.18|
|HR (mut vs wt)||0.552||0.542||1.57|
Clinical trial identification
Legal entity responsible for the study
Spanish Cooperative Group for the Treatment of Digestive Tumors (TTD)
Public funding from the Spanish Ministry of Health, Social Policy and Equality. (Trial Ref: EC11-050)
J.M. Viéitez: Consultant or advisory relationship and research funding: Amgen. M. Valladares-Ayerbes: Consultant or advisory relationship and honoraria: Roche, Amgen, Merck Serono. E. Aranda Aguilar: Honoraria for advisory role from Amgen, Bayer, Celgene, Merck, Roche, Sanofi. All other authors have declared no conflicts of interest.