Abstract 2300: Non-invasive liquid biopsy assay for the detection of ASCL1 expression in CTCs of patients with lung cancer

• Published in: Cancer research (Chicago, Ill.) Ill.), 2023-04, Vol.83 (7_Supplement), p.2300-2300
• Main Authors: Fernandez, Luisa, Kunihiro, Andrew, Frilles, Mark, Tubbs, Alisa, Christiansen, Jason, Bourdon, David
• Format: Article
• Language: English
• ISSN: 1538-7445; 1538-7445
• DOI: 10.1158/1538-7445.AM2023-2300

Background: Lung cancer (LC) is the deadliest cancer worldwide, with small cell LC (SCLC) and neuroendocrine (NE) LC being the most aggressive and lethal forms. The transcription factor, achaete-scute homolog 1 (ASCL1), regulates neuronal and NE cell development, and is a hallmark of the ASCL1(+) SCLC subtype. Further, investigating subtype has become increasingly important as new drug candidates emerge for SCLC that may be efficacious in only some patients. ASCL1 by IHC tissue biopsy is highly specific, detecting patients who have transformed from NSCLC to SCLC and have acquired resistance to EGFR tyrosine kinase inhibitor therapy in NSCLC. Quantification of ASCL1 on circulating tumor cells (CTCs) is a blood-based, non-invasive alternative to tissue biopsy that provides functional protein-level expression data to monitor lung cancer patients across the disease continuum, informing patient treatment. Methods: A 4-color immunofluorescence (IF) assay was developed to detect ASCL1 expression in CTCs from routine blood samples, comprising detection of nucleated cells (DAPI), cytokeratin positive CTCs, CD45(+) white blood cells (WBC), and ASCL1(+) cells. During validation, cancer cell lines (SHP-77 [ASCL(+)] and U2-OS [ASCL1(-)]) were spiked into normal blood at a ratio of 1:10,000 (CLC/WBC) and plated onto coated glass slides. Slides were stained, imaged, and CTC candidates were identified with a proprietary imaging algorithm. The threshold of ASCL1 positivity (316 MFI) included 99% of all ASCL1(-), U2-OS cells. Assay specificity, sensitivity, overall accuracy, and subcellular localization of biomarker expression were assessed. The ability of the assay to quantify ASCL1 expression in CTCs was confirmed using 31 samples from 11 Extensive Stage SCLC patients. Results: The ASCL1 CTC assay achieved an overall accuracy of 0.992 (95% CI 0.081, 0.993), and an analytical specificity and sensitivity of 0.990 and 0.995, respectively. The assay displayed a positive predictive value and a negative predictive value of 0.979 and 0.998, respectively, determined by evaluating ASCL1 expression in SHP-77 (N=4062) and U2-OS (N=8645). ASCL1 expression was detected in the nucleus of all SHP-77 and majority of SCLC patient CTCs. Of the 11 Extensive Stage SCLC patients tested, 9 (82%) showed at least one conventional CTC (0.02 - 9.63 CTCs/mL), and all 9 (100%) had at least 1 ASCL1(+) CTC (0.06 - 5.5 CTCs/mL). Conclusion: This blood based ASCL1 assay is a highly specific and sensitiv