P2.04-53 Serial Ultra-Deep Sequencing of ctDNA Reveals the Clonal Evolution in Non-Small Cell Lung Cancer Patients Treated with Pembrolizumab

Abstract

Immune-therapy with anti-PD1 inhibitors, such as pembrolizumab, is revolutionizing the treatment of non-small cell lung cancers (NSCLC). However, identifying patients for the potential therapeutic response and predicting therapy resistance and early relapse remains a challenge. Between 2016 and 2018, 60 patients were treated with pembrolizumab, among who 12 NSCLC patients had both baseline (before treatment) and serial (on treatment) periodical circulating tumor DNA (ctDNA) samples. Those samples were sequenced on a 329 pan cancer-related gene panel. Analyses of tumor burden, blood tumor mutational burden (bTMB), maximum somatic allele frequency (MSAF), and tumor clonal structure were performed in association with clinical response. Resistance mutations involved in relapse and metastases were further investigated. ctDNA was detected and mutational profiling was performed for each patient. Those with a high baseline bTMBlevel showed significantly improved progression-freesurvival (PFS) after pembrolizumab treatment. Tumor burden and therapeutic response significantly correlated with the MSAF instead of the bTMB. Clone analysis detected tumor progression about 2-4 months ahead of computedtomography (CT) scan. One mutation in gene PTCH1 (Protein patched homolog 1) and two acquired anti-PD1 resistance mutations of gene B2M (β2 microglobulin) were identified in association with distant metastasis. The evolutionary tree of a representative patient was also described. This pilot study showed thatMSAF could be another good indicator of therapeutic response, and clonal analysis could be clinically useful in monitoring clonal dynamics, and detecting resistance and early relapse.