Abstract 5602: Detection of circulating cell-free tumor DNA (ctDNA) in patients with small cell lung cancer (SCLC)
Authors: Mohan Sumitra , Foy Victoria , Leong Hui Sun , Schofield Pietà G. , Ayub Mahmood , Smith Nigel K. , Sahoo Sudhakar , Sik-Kim Chang , Priest Lynsey , Carter Mathew , Carr Hedley T. , Miller Crispin , Finn-Faivre Corinne , Blackhall Fiona , Rothwell Dominic G. , Dive Caroline , Brady Gerard 
Published: August 2018

Introduction: Tumor genomes can be reconstructed from the molecular information obtained from circulating cell-free DNA (cfDNA) and circulating tumor cells (CTCs) obtained from the peripheral blood of patients with cancer. The analysis of cfDNA and CTCs is a minimally invasive approach and represents a powerful research tool, with potential as a companion diagnostic for both patient stratification and monitoring. Here, we use cfDNA next-generation sequencing (NGS) analysis to assess and compare ctDNA profiles in patients with SCLC.

Methods: Whole genome sequencing (WGS) libraries were prepared from cfDNA isolated from the pre-treatment peripheral blood samples from 69 patients and 32 cancer-free controls. Libraries were subjected to WGS to establish genome wide copy number aberrations (CNA) as well as targeted mutation analysis of 110 SCLC associated genes. Quantitative metrics were calculated from CNA such as Percent Genome Altered (PGA; percentage of genomic regions altered), Z-score (measure of standard deviation) and Moran's I (measure of spatial autocorrelation). In addition CellSearch®, an epitope dependent enrichment platform was used to enumerate CTCs from a parallel blood sample.

Results: An examination of CNA patterns revealed SCLC associated changes such as losses on chromosomes 3p, 5q and 17p and gains on chromosome 3q and 5p as well as amplification of MYC in 21/69 (30%) and SOX2 in 36/69 (52%) as well losses on FHIT in 40/69 (58%), RASSFI in 38/69(55%) and RB1 in 24/69 (35%) patients. A combination of three CNA metrics enabled detection of tumor associated changes in 58/64 (84%) patients, with Moran's I emerging as the most sensitive CNA metric for detecting ctDNA. Targeted NGS detected tumor associated mutations in 60/64 (94%) with TP53 mutations detected in 50 patients (83% of patients with any detectable ctDNA). Somatic mutations and CNA were detected in both limited stage SCLC (LS-SCLC, confined to 1 hemithorax) and extensive stage (ES-SCLC, with distant metastases) with statistically significant differences seen for CNA metrics and variant allele frequencies of mutations consistent with higher levels of ctDNA in ES-SCLC. However, no significant differences between ES and LS were observed in the mutation patterns with respect to DNA damage repair, RAS and PI3K and transcriptional regulation pathways between the two stages of SCLC. There was considerable overlap between the detection of ctDNA and CTC counts, with cfDNA NGS readouts detecting tumor related changes in 96% of patients and CTCs were detected only in 76% of patients.

Conclusion: We have established sensitive methods for detecting ctDNA in cfDNA and combined with CTC enumeration we have an effective liquid biopsy for 98% of patients (96% in LS and 100% in ES) in this cohort. Future work will involve utilising this optimised NGS approach in an independent cohort of patients to correlate cfDNA metrics with clinical outcome.

Citation Format: Sumitra Mohan, Victoria Foy, Hui Sun Leong, Pietà G. Schofield, Mahmood Ayub, Nigel K. Smith, Sudhakar Sahoo, Chang Sik-Kim, Lynsey Priest, Mathew Carter, Hedley T. Carr, Crispin Miller, Corinne Finn-Faivre, Fiona Blackhall, Dominic G. Rothwell, Caroline Dive, Gerard Brady. Detection of circulating cell-free tumor DNA (ctDNA) in patients with small cell lung cancer (SCLC) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5602.