P1235: GENOTYPING ON CIRCULATING TUMOR DNA IMPROVES MUTATION DETECTION RATE IN HIGH-RISK DIFFUSE LARGE B-CELL LYMPHOMA

Abstract

Background: Diffuse large B cell lymphoma (DLBCL) is one of the most common lymphomas with heterogeneous clinical outcome. The circulating tumor DNA (ctDNA) represents a novel non-invasive source of tumor genetic material that overcomes anatomical biases of tissue biopsy, and thus has been widely used in aggressive lymphomas. Aims: In this study, we collected a total of 170 samples from 104 DLBCL patients with different disease status, including 62 with paired ctDNA and tissue biopsy genomic DNA (gDNA) samples, to determine the clinical relevance of ctDNA genotyping as well as its concordance with tissue biopsy gDNA genotyping. Methods: A targeted sequencing gene panel including all exons and critical introns of 112 genes that are recurrently mutated in DLBCL was used for this study (Burning Rock Biotech Co. Ltd, Guangzhou, China). The samples were divided into 4 groups according to the disease status at enrollment: ① treatment-sensitive (TS), representing samples from 39 treatment-naïve DLBCLs who responded to frontline treatment; ② primary refractory (Pri-R), representing samples from 10 treatment naïve DLBCLs who failed to achieve partial remission (PR) at the interim evaluation or complete remission (CR) at the end of frontline treatment; ③ early-relapse/refractory (ERR), representing samples collected at the time of relapse from 45 patients who relapsed within 12 months after diagnosis or refractory to prior treatment; ④ late-relapse (LR), representing samples collected at the time of relapse from 13 patients who relapsed more than 12 months after diagnosis. Results: With a mean sequencing depth of 16140× (range: 8377×−23356×) in ctDNA and 1868× (range: 1009×−2881×) in gDNA, variants were identified in 88.2% (90/102) ctDNAs and profiles were similar to that of gDNA genotyping. The mean VAF of ctDNA mutations showed a good correlation with elevated lactate dehydrogenase (LDH, P=0.0008) and β2-microglobulin (β2-MG, P=0.039), as well as the metabolism tumor volume (MTV, R 0.38; P=0.0024) and the total lesion glycolysis (TLG, R 0.40; P=0.0012) assessed by PET/CT. Besides, patients with a mean VAF of ≥1.5% (determined by X-tile) had significantly shorter PFS (P=0.0004) and OS (P=0.0024). In the 62 patients who had paired ctDNA and gDNA samples, we found ctDNA genotyping outperformed gDNA genotyping in ERR DLBCLs. The proportion of ctDNA-specific variants in ERR DLBCLs was significantly higher than that of the other three groups (TS: 7.3%, 29/398; Pri-R: 10.3%, 4/39; ERR: 25.9%, 225/870; LR: 3.2%, 4/126; P<0.001). We further investigated the profile of ctDNA-specific mutations in ERR DLBCLs and found that a considerable amount of unfavorable gene mutations, including 33.3% (4/12) of TP53 mutations, 66% (4/6) of PRDM1 mutations, and 62.5% (5/8) of EP300 mutations.On the other hand, the detection rate of genetic subtypes was 51.6% (32/62) and 50% (31/62) in ctDNA and gDNA in the 62 patients with paired samples.When the two sample types were combined together, the detection rate of genetic subtypes could reach 58.1% (36/62). Interestingly, it was found that all but one (6/7) ctDNA-specific subtype belonged to the MCD subtype. In fact, 35.3% (6/17) MCD DLBCLs were only identified by ctDNA but not by gDNA, suggesting the addition of ctDNA genotyping substantially improved the detection rate of MCD subtype (figure 1). Image:Summary/Conclusion: Our study highlights the clinical application of ctDNA, in particular in patients with high-risk and early relapsed or refractory diseases, where ctDNA analysis represents a complementary approach to tissue biopsy gDNA genotyping.