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e13045 Background: Combination therapeutic strategies including CDK4/6 inhibitors and an endocrine backbone are the standard of care of treatment for patients with estrogen receptor positive (ER+)/HER2- advanced breast cancer (ABC). Endocrine agents mainly include aromatase inhibitors, which target ER-driven transcription, and fulvestrant, which functions as ER antagonist. PI3K-AKT-mTOR is a key point of resistance to endocrine therapy, activated in 40% of these patients by mutations concentrated in critical regions of PIK3CA, coding for the p110 catalytic subunit α of PI3K. Additionally, 30% of patients previously exposed to non-steroidal aromatase inhibitors develop mutations in the ligand binding domain of ESR1, causing endocrine resistance by constitutive activation of the ER. Furthermore, metastasis and primary tumors may show a highly heterogenous mutational landscape. Monitoring the dynamic changes of these mutations in ctDNA may provide a non-invasive, real-time and accessible tool to convey predictive/prognostic information and guide decisions on sequential endocrine therapies. Methods: Pre-planned interim analysis results of an observational, prospective cohort pilot study to assess the predictive and prognostic value of monitoring PIK3CA and ESR1 mutations in ctDNA of patients with endocrine-resistant ER+/HER2- ABC. We have studied longitudinal liquid biopsies from 30 patients using digital PCR to interrogate PIK3CA mutations (H1047R / E545K) and ESR1 mutations (D538G / Y537S). Blood samples were drawn at the time of progression to endocrine therapy, at 8 weeks of subsequent endocrine line and at new progression. This exploratory analysis will provide preliminary data on clinical homogeneity, treatment regimens, median follow-up and progression-free survival, mutation incidence and intraindividual variation of mutant allele frequency and copy number. The percentage of progressors at 24 weeks of follow-up according to the mutational status will be evaluated by using the Fisher’s exact test. The predictive potential of ctDNA biomarkers will be characterized by ROC curve analysis. Tracking ctDNA mutations to predict endocrine resistance in a real-world setting represents a critical step towards precision medicine in oncology.

69 Background: Plasma immune biomarkers such as soluble plasma PD-L1 (sPD-L1) may serve as surrogates of the immune condition of cancer patients and be potential markers of response to different immunotherapy modalities. Very few data exist regarding sPD-L1 in patients with primary brain tumors. Our aim was to study the levels of sPD-L1 in patients with gliomas according to histologic grade and IDH mutation status. Methods: Patients (pts) with grade II to IV gliomas were prospectively enrolled. Single-time-point plasma samples were obtained prior to adjuvant radiotherapy +/- chemotherapy. sPD-L1 determined using ELISA with a rabbit polyclonal anti-PDL1/CD274 antibody as capture reagent. Results: Between February 2017 and August 2019, 44 patients (pts) with gliomas and 12 healthy controls (HC) were enrolled. N=11 grade II, n=9 grade III, n=24 grade IV; n=26 IDHwt, n=18 IDHmut. Higher sPD-L1 levels in glioma pts compared to HC (60.8 vs 47.8 ng/ml, p=0.05). Higher sPD-L1 levels in grade II vs grades III-IV (73.3 vs 60 ng/ml, p=0.09). Higher sPD-L1 in IDHmut grades II-III vs IDHwt grades II-III + IDHmut/wt grade IV (73 vs 59 ng/ml, p=0.07). Non-significantly higher sPD-L1 in grades II-III vs grade IV (73 vs 59 ng/ml, p=0.46). No difference in sPD-L1 in IDHmut vs IDHwt (63.3 vs 59.1 ng/ml, p=0.496), nor in IDHwt vs HC (59.1 vs 47.8 ng/ml, p=0.109). Trend to significance in IDHmut vs HC (63.3 vs 47.8, p=0.062). Conclusions: sPD-L1 levels were significantly higher in glioma pts compared to HC. A trend was seen towards higher sPD-L1 levels in lower-grade (grades II, III) IDHmut gliomas. These findings may indicate a different systemic immune profile for currently defined glioma groups based on histologic grade and IDH status and merit confirmation in a larger sample.

e15151 Background: RAS mutations predict a lack of response to anti-EGFR therapies in metastatic colorectal cancer (mCRC). BEAMing technology is useful for detecting hot-spot mutations in ctDNA in mCRC. Analysis of these mutations in DNA from Circulating Tumor Cells (CTC) may increase the predictive value in mCRC patients (pts). Our aim was to explore the feasibility of studying RAS status using BEAMing in DNA from CTC. Methods: First, spiking experiments (SE) using wild-type (WT) and KRAS-mutated (MUT) cell lines were performed to establish the limit of detection (LOD) for RAS analysis with BEAMing. Second, SE were performed with CTC collected by CellCelector (removes non-CTC background achieving 100% purity of CTC). Finally, BEAMing was used for RAS analysis in ctDNA and in DNA from CTC isolated either with IsoFlux or with CellCelector in 9 mCRC pts with confirmed RAS mutation in primary tumor. Total DNA from CTC was preamplified using RepliG. Results: In SE, 10 and 5 KRAS MUT-cells using different backgrounds of WT-cells (10-0.2% MUT-cells) were detected using BEAMing. However, 3 and 1 MUT-cells (0.009-0%) were not detected. In SE of CTCs collected with CellCelector, BEAMing detected KRAS mutations with 50, 20, 10, 6, 4, 2 and 1 cell (MAF: 23.8%±3.8). A mutation (codon 13) was detected in CTC from one patient positive in tissue and ctDNA (CellCelector; 15 CTCs; MAF: 11.4%). Discordant results were found in 8 patients when CTCs were isolated using Isoflux (min: 0, max: 9 CTCs). CTC from another patient were possibly mutated but WT in ctDNA. Conclusions: This pilot study indicates that RAS mutations can be detected in CTCs using BEAMing. Reducing the non-CTC cellular background may be needed in cases with low CTC number. Molecular information provided by CTC and ctDNA may prove complementary and useful for taking therapeutic decisions in mCRC. These results merit confirmation in larger, prospective studies.

e17551 Background: Few studies have addressed the carcinogenic process of laryngeal cancer from its premalignant phases. Our aim was to compare mutational status of laryngeal dysplasias (LD) evolving to laryngeal cancer (ELC) and not evolving to LC (NELC). Methods: Retrospective study of LD diagnosed between 2007 and 2011. A customized 15-gene NGS panel (DICER, IRF6, NOTCH1, NOTCH2, NOTCH3, NOTCH2NLA, PIK3CA, PTEN, RB1, RIPK4, SYNE1, SYNE2, TP53, TP63, CASP8) was used for mutational analysis with Truseq Custom Amplicon (Illumina) performed in FFPE LD samples and results compared between LD-ELC and LD-NELC. Results: Sixty-four patients (pts) with LD were identified. LD-ELC (N = 23) and LD-NELC (N = 41) were scored as mild (N = 40; NELC: 32 / ELC:8), moderate (N = 8; NELC: 4/ ELC: 4) and severe (N = 16; NELC:5 / ELC: 11). Prior or current moderate-to-heavy tobacco smoking ( > 10 pack-year): 53/64 (83%). Males: N = 50 (ELC: 21; NELC: 29); Female: N = 14 (ELC: 2; NELC: 12). Median time to cancer among 23 LD-ELC: 8 m (range: 0-46). Forty-seven gene variants were detected in ELC not found in NELC, of which 27 were pathogenic/likely pathogenic: Frameshift: 8 (1 in NOTCH2, 1 in PTEN, 2 in RB1, 2 in SYNE1 y 2 in SYNE2); Stop-gained: 3 (1 in NOTCH2NL, 2 in SYNE2); Splicing: 3 (1 in RB1, 1 in RIPK4 y 1 in TP53); Missense: 12 (1 in IRF6, 2 in NOTCH1, 1 in RB1, 2 in RIPK4, 6 in SYNE1). Conclusions: LD from pts ELC showed a different mutational profile than LD-NELC. These results show promise for identifying pts at higher risk for developing LC and should be validated in a larger, prospective study.

592 Background: Accurate detection of RAS mutations in metastatic colorectal cancer (mCRC) patients is of high clinical importance for therapy selection as RAS detection methods lacking sensitivity may lead to poor patient outcomes. Liquid biopsy has emerged as a viable alternative to individualize the management and treatment of mCRC patients. However, there is a current need to cross-compare the performance of liquid biopsy platforms. The OncoBEAM RAS dPCR assay offers highly sensitive RAS mutation detection in clinical practice, achieving > 90% concordance when compared to results obtained by tumor mutation testing. The Idylla platform offers a highly-automated qPCR platform for RAS mutation testing of tissue samples, and has shown potential for liquid biopsy. The objective of this study was to provide a head-to-head comparison of the sensitivity of OncoBEAM and Idylla for KRAS mutation detection in plasma from mCRC patients. Methods: Plasma samples from 92 mCRC patients determined to be KRAS-positive using OncoBEAM were re-tested using Idylla. Samples with mutant allelic fractions (MAF) below 5% were selected for analysis. The positive percent agreement (PPA) of KRAS mutation results was compared for replicate samples analyzed by OncoBEAM and Idylla. Results: So far, Idylla detected KRAS mutations in 63 out of 92 (68.4%) OncoBEAM KRAS-positive plasma samples. Categorization of results based on MAF% revealed distinct differences in sensitivity between the two technologies. Conclusions: OncoBEAM demonstrated significantly greater sensitivity for plasma detection of RAS mutations than Idylla. Moreover, these data identify a “gray zone” below 1% MAF where Idylla fails to identify RAS-positivity in patient plasma samples. These findings serve as a reminder that liquid biopsy assays with diminished sensitivity may lack the dynamic range to provide accurate and timely RAS mutational status information to properly guide highly individualized anti-EGFR therapy and chemotherapy treatment decisions that may benefit patient outcomes. [Table: see text]

e20008 Background: Epidermal growth factor receptor tyrosine kinase inhibitors (EGFR-TKIs) are current treatments for advanced non-small cell lung cancer (NSCLC) with activating EGFR gene mutations. Histological samples are the standard tumor materials for EGFR mutation analysis. However, the accessibility of tumor samples is not always possible in advanced NSCLC patients. Moreover, a high percentage of EGFR mutated NSCLC patients will develop resistance to EGFR-TKIs such as T790M mutation. CLART CMA EGFR LB is a novel diagnostic assay able to detect 39 high-prevalence mutations associated with sensitivity or resistance to tyrosine kinase inhibitor treatment. Methods: A highly sensitive and specific method was developed for detection of EGFR mutations (G719X, T790M, L858R, L861Q, insertions in exon 20, and deletions in exon 19) in plasma samples (cfDNA). CLART CMA EGFR LB is based on a preamplification step with a multiplex ARMS-PCR and microarray detection system. Clinical testing was performed using 51 clinical plasma samples: 27 contained L858R, T790M, and deletions in exon 19; 24 contained wild type alleles. 20 samples were cross checked by next generation sequencing performed on the PGM platform (Ion Ampliseq™ Custom, deep-coverage 15000x). Results: Analytical sensitivity was assessed using recombinant plasmids, results ranged between 10-1000 copies/5µl for all mutations. cfDNA from cell lines with the mutations L858R, T790M and deletions in exon 19 at different frequencies (cfDNA Reference Standard, Horizon) were assessed. The system was able to detect the mutations present in a frequency of 2%. The analysis of 51 samples allowed establishing the diagnostic sensitivity and specificity in 93.33% and 100% respectively. CLART CMA EGFR LB showed similar analytical and diagnostic sensitivity than NGS for detecting L858R, T790M and deletions in exon 19. Conclusions: Our data support the use of CLART CMA EGFR LB for clinical testing prior to the selection of the appropriate treatment in NSCLC, monitoring the patient evolution and the emergence of resistance mutations such as T790M in plasma samples.