Abstract LB106: Ultra-short ctDNA: A new paradigm for ctDNA liquid biopsy

Abstract Mononucleosomal ctDNA (mnctDNA, 157-bp, double-stranded) spearheads liquid biopsy (LB) for cancer detection, treatment and clinical care. The bottleneck of mnctDNA LB are low copy number of ctDNA and limit of detection of platform technologies. Recent discovery of ultra-short ctDNA (usctDNA, 52-54 nt, single-stranded) that harbors somatic mutations prompted us to explore the quantification and stoichiometry of usctDNA to mnctDNA in plasma and saliva of NSCLC patients. Paired plasma and saliva from 20 advanced NSCLC patients with tissue genotyped positive for L858R were processed for cfDNA extraction and quantification with Qubit and Tape Station. Digital PCR assays targeting L858R somatic mutation to generate amplicons of 57-bp (usctDNA) and 100-bp (mnctDNA) were performed by UCLA and independently by NIST. Results in Table 1 revealed that three findings of usctDNA that will have potential to impact ctDNA liquid biopsy. First, the somatic mutation (L858R) is present in usctDNA. Second, The stoichiometric ratio of usctDNA to mnctDNA is similar in plasma and saliva at 1.6X. For the wild-type counterpart, the ratio is 1.1X for plasma and 0.6X for saliva. Third, the copy number of usctDNA and mnctDNA in saliva to plasma is 7.8X and 7.7X. Conclusion: Detection of tumor-associated/specific EGFR L858R somatic mutations in uscfDNA is translationally impactful, adding 1.6X of the ctDNA targets for molecular detection, effectively cumulating 2.6X of total ctDNA targets, permitting earlier cancer detection, treatment monitoring and therapy prognostication. The finding that saliva harbors 7X of the us- and mnctDNA further raises the prospect of advancing saliva ctDNA liquid biopsy for NSCLC detection as it may effectively allow 15.5X (7.8X+7.7X) ability to detect malignancy-associated ctDNA. Table 1. Quantification of usctDNA and mnctDNA in Plasma and Saliva Mut usctDNA (L858R) WT cfDNA usctDNA mnctDNA U/M uscfDNA mncfDNA U/M Plasma 1555 ± 326 964 ± 209 1.6X 15201 ± 1509 13261 ± 1424 Saliva 12158 ± 1731 7442 ± 1020 1.6X 340896 ± 27170 601733 ± 71573 1.1X Saliva/Plasma (fold X) 7.8 7.7 1.6X 22.4 45.3 0.6X Citation Format: Feng Li, Fang Wei, Hua-Jun He, Chien-Chung Lin, Szu-Chun Yang, Allen Wei-Lun Huang, Cheuk Ying Tang, Yong Kim, David T. Wong. Ultra-short ctDNA: A new paradigm for ctDNA liquid biopsy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(7_Suppl):Abstract nr LB106.