Monitoring the Outcomes of Systemic Chemotherapy Including Immune Checkpoint Inhibitor for HER2-Positive Metastatic Gastric Cancer by Liquid Biopsy

Abstract

Purpose: For precision medicine, exploration and monitoring of molecular biomarkers are essential. However, in advanced gastric cancer (GC) with visceral lesions, an invasive procedure cannot be performed repeatedly for the follow-up of molecular biomarkers.

Materials and methods: To verify the clinical implication of serial liquid biopsies targeting circulating tumor DNA (ctDNA) on treatment response, we conducted targeted deep sequencing for serially collected ctDNA of 15 HER2-positive metastatic GC patients treated with anti-PD-1 inhibitor in combination with standard systemic treatment.

Results: In the baseline ctDNAs, 14 patients (93%) harbored more than one genetic alteration. A number of mutations in well-known cancer-related genes, such as KRAS and PIK3CA, were identified. Copy number alterations were identified in eight GCs (53.3%), and amplification of the ERBB2 gene (6/15, 40.0%) was the most recurrent. When we calculated the mean variant allele frequency (VAF) of mutations in each ctDNA as the molecular tumor burden index (mTBI), the mTBI trend was largely consistent with the VAF profiles in both responder and non-responder groups. Notably, in the longitudinal analysis of ctDNA, mTBI provided 2-42 weeks (mean 13.4 weeks) lead time in the detection of disease progression compared to conventional follow-up with CT imaging.

Conclusion: Our data indicate that the serial genetic alteration profiling of ctDNA is feasible to predict treatment response in HER2-positive GC patients in a minimally invasive manner. Practically, ctDNA profiles are useful not only for the molecular diagnosis of GC but also for the selection of GC patients with poor prognosis for systemic treatment (ClinicalTrials.gov identifier: NCT02901301).

Keywords: Gastric cancer; circulating tumor DNA; immune checkpoint inhibitor; liquid biopsy; molecular tumor burden index.

Fig. 1. Swimmer plot of the treatment and blood sampling time points. Each lane represents a single patient’s treatment history of first-line therapy. X-axis represents treatment duration. Yellow triangles represent the time point of blood sampling for ctDNA extraction. GC, gastric cancer; ctDNA, circulating tumor DNA.

Fig. 2. Genetic alterations identified in baseline ctDNA from 15 GC patients. Seventeen genes with 26 mutations and 11 copy number alterations are shown. Upper panel represents the frequency of genetic alterations for each patient. Right-hand panel represents the number of genetic alterations for each gene. Asterisks represent the reported variants in the COSMIC database. ctDNA, circulating tumor DNA; GC, gastric cancer.

Fig. 3. Serial comparison of tumor burden and mTBI from ctDNA. mTBI at baseline were compared serially with tumor burden for each patient, except one patient GC04 who did not harbor any genetic alterations. Green line and black dotted-line represent the ΔmTBI and linear trend in ΔmTBI, respectively. The ΔmTBI was calculated based on the mTBI of the ctDNA sample at baseline. Longitudinal analyses of ctDNA were performed and analyzed among two groups according to their response pattern to the treatment: (A) responder group (GC01–GC07) and (B) non-responder group (GC08–GC15). mTBI, molecular tumor burden index; AU, arbitrary unit; ctDNA, circulating tumor DNA.

Fig. 4. Representative clinical cases for responders and non-responders. Representative case of responder group (A) and non-responder group (B) from ctDNA in comparison with tumor burden and CT imaging. (C) An interesting case of primary tumor progression in the stomach (inlet) without increasing genetic alterations in ctDNA is shown. ctDNA, circulating tumor DNA; GC, gastric cancer; VAF, variant allele frequency; CN, copy number; AU, arbitrary unit; RECIST, Response Evaluation Criteria in Solid Tumor.