When do colorectal tumors die following therapy?

Introduction

Chemotherapy and radiotherapy are still the mainstay cancer therapies in multiple tumor types. The main tool for evaluation of treatment response is imaging, performed at intervals of several months. A rising promising approach that may revolutionize oncology is the analysis of blood for circulating tumor cell-free DNA (cfDNA). Its level represents nearly `real-time` tumor turnover and has the potential to improve decision making regarding treatment efficacy.

Here, we present the preliminary results of our study aimed to predict treatment response based on changes in cfDNA within short time frames, to facilitate faster response evaluation in colorectal cancer patients.

Methods

We have recruited 2 cohorts of newly diagnosed patients: metastatic colorectal cancer (MCRC) or locally advanced rectal carcinoma (LARC). All patients gave informed consent and were treated with standard protocols (systemic chemotherapy, pre-operative chemo-radiation lasting 5-6 weeks, respectively). Plasma of MCRC patients was drawn at baseline and then daily for 5 days. In the LARC cohort, plasma was collected at baseline, at first and last day of each radiation week. Detection of unique intestinal methylation patterns in cfDNA was performed, enabling the quantification of intestinal-derived cfDNA level. Patients` outcomes were determined at first imaging scan results (MCRC group) or at surgical outcomes (tumor viability status for LARC cohort).

Results

Ten patients were recruited to MCRC group (n=54 samples) and twenty patients into the LARC cohort (n=150 samples). In the MCRC cohort, no correlation between treatment response and variation in intestine-derived cfDNA was detected. Clear cfDNA peak representing tumor death was not seen. Yet, shallow whole-genome-sequencing (x0.5) of cfDNA revealed obvious effects of chemotherapy on cancer-derived cfDNA.

Nonetheless, in the LARC group, 20% of patients achieved maximal response to therapy, defined as no viable tumor cells on surgical specimen following therapy. We have detected a significant difference between intestine-derived cfDNA levels in maximal responders vs. poor responders. The significant difference was evident as early as second week of treatment (lower levels of cfDNA in max. responders, p=0.007). Interestingly, in many patients cfDNA levels were higher at end of treatment weeks, in comparison to Sundays, probably representing therapy-induced tumor death.

Conclusions

Intense liquid biopsy approach using colorectal–specific methylation markers is feasible and enables to monitor treatment and predict outcomes in patients with LARC. Additional research may promote this tool to select patients in which rectal surgery may be omitted. In the metastatic setting, we have not clearly identified specific time of treatment-induced tumor death. We assume that predictive tumor response will be evident at later stages of therapy. Further research is undoubtedly required.