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Purpose To determine the potential for detection of incidental germline cancer predisposition mutations through cell-free DNA (cfDNA) analyses in patients who underwent solid tumor somatic mutation evaluation. Patients and Methods Data were evaluated from 10,888 unselected patients with advanced (stage III/IV) cancer who underwent Guardant360 testing between November 2015 and December 2016. The main outcome was prevalence of putative germline mutations identified among 16 actionable hereditary cancer predisposition genes. Results More than 50 cancer types were studied, including lung (41%), breast (19%), colorectal (8%), prostate (6%), pancreatic (3%), and ovarian (2%). Average patient age was 63.5 years (range, 18 to 95 years); 43% were male. One hundred and fifty-six individuals (1.4%) had suspected hereditary cancer mutations in 11 genes. Putative germline mutations were more frequent in individuals younger than 50 years versus those 50 years and older (3.0% v 1.2%, respectively; P < .001). Highest yields of putative germline findings were in patients with ovarian (8.13%), prostate (3.46%), pancreatic (3.34%), and breast (2.2%) cancer. Putative germline mutation identification was consistent among 12 individuals with multiple samples. Patients with circulating tumor DNA copy number variation and/or reversion mutations suggestive of functional loss of the wild-type allele in the tumor DNA also are described. Conclusion Detection of putative germline mutations from cfDNA is feasible across multiple genes and cancer types without prior mutation knowledge. Many mutations were found in cancers without clear guidelines for hereditary cancer genetic counseling/testing. Given the clinical significance of identifying hereditary cancer predisposition for patients and their families as well as targetable germline alterations such as in BRCA1 or BRCA2, research on the best way to validate and return potential germline results from cfDNA analysis to clinicians and patients is needed.

PARP1/2 inhibitors are effective against BRCA2-deficient tumors. The PARP inhibitor (PARPi) olaparib received FDA breakthrough designation for treatment of metastatic castration-resistant prostate cancers (CRPC) carrying mutations in BRCA1/2 or ATM genes. Emergent resistance to PARPi has been associated with tumor-specific BRCA2 mutations that revert the normal open reading frame rescuing homologous recombination. We describe a case of metastatic CRPC with germline BRCA2 mutation with acquired resistance to olaparib related to biallelic BRCA2 reversion mutations of both the germline and somatic loss of function alleles detected by circulating tumor DNA testing. We also summarize a retrospective analysis of 1,534 prostate cancer cases with ctDNA analysis showing a 1.6% incidence of germline BRCA2 mutations. Within the germline BRCA2-positive cases exposed to platinum chemotherapy or PARP inhibition, the prevalence of reversion mutations was 40%. This report documents the frequency of reversion mutations in a large cohort of prostate cancer patients carrying of BRCA mutations. It also shows the potential utility of ctDNA analyses for early detection of reversion mutation driving tumor resistance.

3555 Background: HER2 amp occurs in 1-4% of mCRC pts. Two single arm phase 2 studies, HERACLES and MyPathway, showed efficacy for dual HER2-targeted therapy in pts with RAS wild type ( RAS wt) mCRC with HER2 amp detected in tumor tissue; however, efficacy for pts prospectively enrolled with HER2 amp identified in ctDNA is unknown. Furthermore, the efficacy of real-world non-HER2-targeted SOC for HER2 amplified RASwt mCRC pts is not clear. Methods: We conducted a phase 2 trial to evaluate the efficacy of pertuzumab (P) plus trastuzumab (T) in RASwt mCRC pts with HER2 amp centrally confirmed by tissue (IHC and/or FISH) and/or ctDNA (Guardant360) who had progressed on SOC including EGFR blockade. Pts received intravenous P (840 mg loading dose followed by 420 mg) and T (8 mg/kg loading dose followed by 6 mg/kg) every 3 weeks. The primary endpoint was confirmed objective response rate (ORR) by investigator assessment, analyzed for two primary populations: pts with HER2 amp in tissue (tissue+) or in ctDNA (ctDNA+). Efficacy of real-world non-HER2-targeted SOC for HER2 amplified RASwt mCRC pts was prospectively assessed in a concurrent registry: the SCRUM-Japan registry. Results: Among 75 pts screened, concordance of HER2 amp between tissue and ctDNA was 83%. The primary endpoint was met in each cohort of TRIUMPH, with confirmed ORR of 30% (95% CI 14-50%) in 27 tissue+ pts and 28% (12-49%) in 25 ctDNA+ pts. In contrast, ORR in first salvage SOC after EGFR blockade was 0% (0.0-24.7%) in the real-world cohort. Median progression free and overall survival were 4.0 months (1.4-5.6) and 10.1 months (4.5-16.5) in the tissue+ pts and 3.1 months (1.4-5.6) and 8.8 months (4.3-12.9) in the ctDNA+ pts. One pt withdrew due to an adverse event (grade 3 decreased ejection fraction), but no treatment related deaths occurred. In exploratory analyses, pts without ctDNA mutations of RAS/ BRAFV600/ PIK3CA/ HER2 were more likely to respond to P+T than those with a ctDNA mutation in at least one of these genes (ORR 44% vs. 0% in tissue+ and 37% vs. 0% in ctDNA+). Decreased ctDNA fraction and HER2 plasma copy number at 3 weeks after treatment initiation corresponded to P+T response. At least one actionable alteration emerged after progression in 16 (62%) of 26 pts with ctDNA results at both baseline and progression. Among 5 pts who achieved response and had ctDNA results at both time points, 4 pts acquired actionable alteration at progression. Conclusions: We demonstrate promising efficacy and safety of P+T for RASwt mCRC pts with HER2 amp in either tumor tissue or ctDNA. Our results show that complete ctDNA genotyping identifies pts most likely to benefit from dual HER2 blockade and can be used to monitor response and detect actionable resistance biomarkers. Clinical trial information: UMIN000027887 and UMIN000028058.

9027 Background: Somatic genomic testing is recommended by numerous expert guidelines to inform targeted therapy treatment for patients with advanced lung adenocarcinoma (aLUAD). The NILE study was a prospective observational study that demonstrated non-inferiority of cell-free circulating tumor DNA (cfDNA)-based tumor genotyping compared to tissue-based genotyping to find targetable genomic alterations in patients with newly diagnosed aLUAD. As the cohort has matured, clinical outcomes data can now be reported. Methods: This prospective, multicenter North American study (NCT03615443) enrolled patients with previously untreated aLUAD who had standard of care (SOC) tissue genotyping performed and concurrent comprehensive cfDNA analysis using the commercially available Guardant360 assay (Guardant Health, Redwood City, CA). After 12 months of study enrollment, objective response rates, disease control rate, and time to treatment data were collected for patients with targetable genomic alterations, as defined by NCCN guidelines, who were treated with physician’s choice of therapy. Results: Among 282 patients on the study, 89 (31.6%) had an actionable biomarker detected by tissue (21.3%) and/or cfDNA (27.3%) analysis. Sixty-one (68.5%) of these patients were treated with an FDA-approved targeted therapy guided by somatic genotyping results ( EGFR, ALK, ROS1). Thirty-three patients were eligible for clinical response evaluation and demonstrated an objective response rate of 58% and disease control rate of 94%. Twenty-five (76%) achieved a durable response > 6 months; 17 (52%) achieved a durable response > 12 months. Patients responded to targeted therapy regardless of the variant allele frequency of the target alteration. The time to treatment (TtT) was significantly faster for cfDNA-informed biomarker detection as compared to tissue genotyping (median 18 vs 31 days, respectively; p = 0.0008). Conclusions: This is the first prospective community-based study to find that cfDNA detects guideline-recommended biomarkers at a rate similar to tissue genotyping, and therapeutic outcomes based on plasma-based comprehensive genomic profiling are comparable to published tissue-based targeted therapy clinical outcomes. The NILE study complements and confirms findings in the prospective FLAURA and SLLIP studies, which exclusively enrolled at academic sites. Clinical trial information: NCT03615443.

9553 Background: Genotyping is required to identify cancer patients (pts) eligible for targeted therapy; however, many do not receive biomarker testing, in part due to limitations associated with tissue-only genotyping practices and the growing list of biomarkers recommended to be tested. Liquid biopsy overcomes many of these limitations but is not yet fully adopted. We report here the clinical performance of a comprehensive liquid biopsy test based on next generation sequencing (NGS) of circulating tumor DNA (ctDNA) for the identification of NSCLC patients with EGFR exon 19 deletions (ex19del) or L858R mutations ( EGFRm) or EGFR T790M, eligible for treatment with osimertinib. Methods: 441 (79%) of 556 pts randomized in FLAURA (NCT02296125; first-line osimertinib vs comparator EGFR TKI in EGFRm NSCLC) and 300 (72%) of 419 pts from AURA3 (NCT012151981; osimertinib vs chemotherapy in NSCLC pts with T790M at progression on EGFR TKI) were retrospectively tested with Guardant360 (G360), a 74-gene ctDNA NGS assay assessing single nucleotide variants, insertion-deletions, amplifications, and fusions in genes relevant to targeted therapy selection as well as microsatellite instability. Progression-free survival (PFS) of pts with EGFRm or T790M detected by G360 was compared to pts detected by the cobas EGFR Mutation Test (cobas) using tissue or plasma with an unadjusted cox model. Results: Treatment with osimertinib was associated with a significant PFS benefit relative to control therapy in NSCLC pts with EGFRm (FLAURA) and T790M (AURA3) detected using G360 (Table). Observed clinical benefit for pts with EGFRm or T790M detected by G360 was similar to that for pts with EGFRm or T790M identified by cobas using tissue or plasma specimens. Conclusions: This analysis demonstrates that G360 accurately identifies pts for osimertinib therapy while simultaneously providing comprehensive genotyping for other therapeutic molecular targets. The application of NGS liquid biopsy has the potential to increase rates of pts genotyped and access to precision medicine. [Table: see text]

10050 Background: ctDNA detected by ddPCR predicts ICI response in MM, although its utility is limited to pts with known recurring mutations eg. BRAF, NRAS, KIT. We sought to overcome this limitation by using a next generation sequencing approach in BRAF V600 wild type (WT) MM pts. Methods: Plasma was collected at baseline and Week (wk) 6 in 35 BRAF V600 WT MM pts treated with ICI. Cell free (cf)DNA was analyzed using Guardant360 and only somatic non-synonymous and promoter variants were considered. Pts who failed cfDNA extraction at baseline were excluded (n = 3). Favorable ctDNA was defined as undetectable ctDNA at wk 6 and unfavorable ctDNA defined as detectable ctDNA at wk6. Response was according to RECIST at first restaging. Results: Of the evaluable 32 pts (64 plasma samples), median baseline cfDNA quantity was 33ng (range 4-657ng) and ctDNA was detected in 29/32 pts (91%). All 3 pts with undetectable baseline ctDNA had less than 10ng cfDNA compared to only 1/29 pts with detectable baseline ctDNA. Number of mutations identified in the 29 ctDNA-positive pts was 4 per pt (range 1-22). Response assessment was performed on 30 evaluable pts. Candidate driver mutation(s) in BRAF, NF1, or N/K/HRAS were identified in 26/30 pts. These mutations were often detected with other established mutations involved in tumorigenesis (eg. TERT promoter), or passenger mutations (eg. clonal hematopoiesis). Analysis of driver mutations revealed a sensitivity and specificity in predicting treatment failure of 92% and 93%, respectively (table). When all mutations identified were evaluated for treatment response, 9/18 responding pts retained some ctDNA at wk 6, although this never included TERT variants. The resulting sensitivity and specificity in predicting treatment failure changed to 100% and 50%, respectively, when all cfDNA variants were included. Conclusions: The extensive coverage of Guardant360 improves ctDNA detection in BRAF V600 WT MM pts, allowing non-invasive, rapid, and longitudinal assessment of response in a broader population. The expanded coverage also identifies passenger variants of potential non-MM origin, eg. clonal hematopoiesis, and with significant overlap with ctDNA, it is not possible to distinguish between the two in the circulation. We therefore recommend identification and monitoring of known cancer driver mutations only. [Table: see text]

TPS277 Background: Identifying non-responders to expensive salvage therapies with modest benefits and substantial treatment related adverse events (TRAEs) (e.g. regorafenib/TAS102 in mCRC) is necessary to maximize benefits and limit toxicities. Serial ctDNA sequencing is reliable for tracking tumor dynamics and appears to predict resistance to therapy earlier than radiographic progression. Methods: TACT-D is a randomized study (N = 100) to validate the ability of changes in ctDNA (ΔctDNA) to predict resistance early and in limiting toxicities. We hypothesize that increase in ctDNA (measured by variant allele fraction) at 2 weeks (wk) into treatment can predict resistance earlier than standard radiographic means [at 8-12 wk] and detecting resistance early can enable prompt change in therapy resulting in reduction of TRAEs. Pts with mCRC eligible for either regorafenib/TAS102 are randomized 2:1 to either standard of care (SOC) or ctDNA arm. On SOC arm, treatment is given as per current paradigm i.e. for 8 wk and then restaging. On ctDNA arm, decision to continue therapy is based on ctDNA change between baseline and 2 weeks [ΔctDNA = ctDNA (C1D15 – C1D1)]. Increase in ctDNA triggers early radiographic staging (4 wk). Treatment is continued for disease stability/regression and discontinued for progression. Study has 2 co-primary endpoints: 1) Association of Δ ctDNA and radiographic progression [62 pts on SOC arm, have 94% power (2-sided α 0.05) to detect difference of 95% vs. 58% in progressive disease between pts with increase vs decrease in ctDNA] and 2) Compare proportion of pts experiencing TRAEs within 4 months between study arms [67 in SOC arm and 33 in ctDNA arm have 82% power (2-sided α 0.05) to detect a 30% decrease in toxicity]. Key secondary endpoints include: patient-reported outcomes (MD Anderson Symptom Inventory and PRO-CTCAE), OS, clinical events of special interest (hospitalizations/ER visits/medical interventions such as blood transfusions/IV hydration), clinical trial referral and cost effectiveness. Study is now actively accruing pts (NCT03844620). Funding: MD Anderson Cancer Center, Houston, TX & Guardant Health Inc., Redwood City, CA. Clinical trial information: NCT03844620.

15 Background: Accurate genotyping is mandatory for the management of mCRC pts. Tissue-based testing is still the SoC; however, is not available for all pts. and may be exhausted by serial testing, resulting in incomplete genotyping. We aimed to establish the validity of comprehensive non-invasive ctDNA testing in 1L mCRC pts for whom SoC tissue genotyping was available. Methods: 1L mCRC pts were tested with a comprehensive ctDNA test (Guardant360), a RAS ctDNA test (OncoBeam), and SoC tissue testing at the time of diagnosis. The primary endpoint was NCCN guideline biomarker discovery rate ( KRAS, NRAS, and BRAF mutations, ERBB2 amplification, and microsatellite instability). Results: In 91 evaluable pts, the biomarker discovery rate was 54.9% (50/91) for SoC tissue testing, 59.3% (54/91) for comprehensive ctDNA testing ( p= 0.0318 for non-inferiority vs. SoC), and 42.9% (39/91) for RAS ctDNA testing (inferiority not rejected vs. SoC). Both comprehensive and RAS ctDNA testing showed high positive agreement (85%, 44/52, and 86%, 31/36) and negative agreement (96%, 268/279, and 93%, 93/100) relative to SoC tissue testing at the gene level. Expanding genotyping beyond KRAS codon 12/13 mutations increased biomarker discovery rate by 56% for tissue testing (50/91 vs. 32/91, McNemar’s p< 0.0001) and by 64% for comprehensive ctDNA (54/91 vs. 33/91, McNemar’s p< 0.0001). Turnaround time was significantly shorter for comprehensive ctDNA testing vs. SoC tissue testing (mean 11.7 days vs. 23.0 days, paired T-test p= 0.0002). On retrospective analysis, 92% of biomarker-positive pts would have been identified at 2 weeks using the comprehensive ctDNA test for initial genotyping with reflex to tissue for biomarker-negative pts, whereas initial use of SoC tissue testing would have identified only 85% of positive pts at 4 weeks (Fisher’s exact p< 0.0001). Conclusions: As previously reported for lung cancer, comprehensive ctDNA testing in 1L mCRC identifies at least as many biomarker-positive pts as SoC tissue genotyping with high concordance to tissue and in half the turnaround time.

TPS3156 Background: Approximately 7% of advanced solid malignancies have FGFR gene alterations. However, standard treatment for FGFR-altered malignancies has not been established. Moreover, circulating tumor DNA (ctDNA) analysis has a potential to accurately identify FGFR alterations by assessing spatial and temporal intratumoral heterogeneity, which have shown to be associated with a poor prognosis and resistance to anti-cancer therapy. Methods: We are conducting an investigator-initiated multicenter phase II basket-type trial to investigate efficacy and safety of TAS-120, a highly selective covalent pan-FGFR inhibitor, for the patients with advanced solid malignancies with FGFR alterations identified by ctDNA analysis as a part of the Nationwide Cancer Genome Screening Project (GOZILA study, UMIN000029315). Eligibility criteria include histologically confirmed unresectable advanced or recurrent solid tumors regardless of histology of origin; ECOG PS of 0 or 1; refractory or intolerant to the standard therapies; and clonal FGFR alterations ( FGFR1-3 gain-of-function mutations, FGFR1,2 amplifications and FGFR2,3 fusions) identified by a 73-gene sequencing ctDNA panel (Guardant360). Enrolled patients will receive TAS-120 20 mg once daily, orally, in a 21 day-cycle. The primary endpoint is to clarify objective response rate (ORR) assessed by investigators per RECIST v1.1. The secondary endpoints are to evaluate progression-free survival, duration of response, time to treatment failure, disease control rate, overall survival, ORR by central determination, and incidence of adverse events. Target sample size is determined as 26 to test the null hypothesis of ORR as 5% with one-sided alpha level of 2.5% and power of 80% to detect an expected value of ORR as 25%. Furthermore, tumor tissue and ctDNA will be serially collected and analyzed to investigate the resistance mechanisms and provide clinically meaningful biomarker which may be used for identifying and implementing treatment changes. Clinical trial information: 194624.

3528 Background: Anti-EGFR therapy (tx) in RAS wild-type (wt) metastatic colorectal cancer (mCRC) induces resistance through acquired genomic alterations. We aimed to define such alterations in Nationwide Cancer Genome Screening Project tissue and blood specimens, using next generation sequencing (NGS). Methods: Tumor specimens in patients (pts) with RAS wt mCRC were obtained from SCRUM-Japan GI-SCREEN (tissue) and GOZILA (circulating tumor DNA [ctDNA] from blood). Genomic alterations were compared using the Oncomine Comprehensive Assay (SCRUM) and Guardant360 (GOZILA), before anti-EGFR tx and after progression. Results: 373 total actionable alterations were identified in 71 pts with available matched tissue and ctDNA; 255 (68%) were acquired after anti-EGFR tx progression. Frequently seen acquired oncogenic alterations included KRAS mutations (27%) and amplifications (amps) of EGFR (41%), CDK6 (24%), BRAF (20%), MYC (17%), MET (14%), PIK3CA (11%), FGFR1 (11%), and KRAS (10%). Fusions of RET, ALK, and FGFR3 were newly acquired in 1-4%. Acquired alterations co-arose in multiple pathways, including the cell cycle, PI3K-AKT, and MAPK, although 29% of pts had none. Acquired mutations were less frequently clonal versus primary mutations (p7) adjusted plasma copy numbers (ApCN). Acquired ERBB2 amps were identified in 3 pts (4%) with a median ApCN of 4, one of whom (ApCN=4.2), treated with dual HER2 blockade, progressed after 2 cycles. Conclusions: Our integrated analysis revealed that anti-EGFR tx of pts with RAS WT mCRC led to acquired genomic alterations in multiple oncogenic pathways. Although most acquired alterations were subclonal, a subset of oncogenic alterations had relatively high clonality and ApCN, suggesting potential targets for overcoming acquired resistance to anti-EGFR tx. Early progression in a pt with an ApCN of 4.2 suggests low-level/subclonal acquired alterations may not be effective treatment targets.

e14557 Background: Circulating tumor DNA (ctDNA) analysis has been successfully applied to therapy selection and treatment monitoring in advanced cancer patients. However, it is not yet established whether ctDNA can be used clinically for early cancer detection or predicting tumor recurrence in early stage lung cancer patients. Methods: We analyzed pre-operative plasma samples from 55 early stage NSCLC patients (stages I-IIIA) using next-generation sequencing to detect somatic mutations and differential epigenomics patterns, including methylation signatures. Results: Using somatic mutation analysis alone, ctDNA was detected in 42% (23/55) of patients, whereas combined mutational and epigenomic analysis detected ctDNA in 71%. ctDNA detection rate also varied markedly between lung squamous cell carcinoma (SCC) and adenocarcinoma (ADC);using combined analysis of somatic mutations and epigenomic patterns, ctDNA was detected in all SCC patients, while only 55% of ADC (12/22) were ctDNA-positive (p= 0.006). Within the ADC subgroup, ctDNA detection rates using the combined approach were dependent on disease stage: 47% (8/17) in stage I, 100% (2/2) in stage II, and 100% (2/2) in stage IIIA. Importantly, pre-operative ctDNA status was correlated with tumor recurrence post-resection; three of eight (38%) ctDNA-positive stage I ADC patients recurred within 2 years of resection, while only one of nine (11%) ctDNA-negative stage I ADC patients recurred (p= 0.29). Conclusions: Taken together, we show that the combination of somatic mutation detection and epigenomic analysis outperforms each individual biomarker in the detection of ctDNA in early stage lung cancer. Importantly, we also demonstrate that pre-operative ctDNA detection may identify a high-risk population of early stage lung cancer patients that may benefit from (neo)adjuvant therapy.

65 Background: Trastuzumab (T) + chemo is standard first-line therapy (tx) for HER2+ gastroesophageal adenocarcinoma (GEA) pts, though progression ensues in 6-8 months. The approved second-line tx is ramucirumab +/- paclitaxel (R+PAC). Pts with GC are less responsive to R+PAC than gastroesophageal junction (GEJ) pts, in particular HER2+ GC, and no anti-HER2 agents are approved in post-T setting. We report results of combination M+P in HER2+ GC pts and describe a biological rationale for this population. M is an anti-HER2 mAb Fc optimized for enhanced binding to activating FcgRIIIa (CD16A) and decreased binding to inhibitory FcgRIIb (CD32B). M demonstrated an enhanced Fc-dependent MoA, including enhanced ADCC. Methods: HER2+, PD-L1-unselected, second-line GEA pts post T progression received M (15 mg/kg) + P (200 mg) Q3wk. Safety, objective response rate (ORR), median overall & progression-free survival (mOS, mPFS), disease control rate (DCR), circulating tumor DNA, & tumor PD-L1 expression were assessed. Results: To date, 66 GEA pts were dosed; 35 (53%) GC and 31 (47%) GEJ. Overall, 12/66 (18.2%) had tx-related adverse events ≥ grade 3; 5 had drug-related SAEs: dehydration, diabetic ketoacidosis, hypotension and pneumonitis, each a single event, and 2 events of autoimmune hepatitis. Eligibility was based on archival HER2 expression; an exploratory endpoint measured retention of HER2 expression post-T by ERBB2 ctDNA. HER2 expression was lost in 23/56 (41.1%) of pts tested post T. HER2 retention was higher in pts with GC versus GEJ (65.8% vs. 44.8%) and in GEA pts with IHC 3+ vs 2+ archival tumors (61.7% vs 47.4%, respectively). Furthermore, GC had higher PD-L1 expression than GEJ, 53.3 vs. 33.3%, respectively. This coincided with more responses in IHC3+ GC pts, ORR 12/29 (41.4%; 95% CI 23.5-61.1), DCR 21/29 (72.4%; 95% CI 52.8-87.3), mPFS 5.5 months (95% CI 2.3-7.6), mOS not reached, with lower bound of 9.1 months for 95% CI. Enrollment of an additional 25 pts enriched for IHC3+ GC is ongoing. Conclusions: Results suggest that M+P, a chemo-free regimen, demonstrates acceptable tolerability and has encouraging preliminary activity in second-line HER2+ GEA, specifically in GC pts who retain ERBB2 amp prior to second-line tx. Clinical trial information: NCT02689284.

TPS179 Background: Tumor mutational burden (TMB) is an emerging biomarker for immune checkpoint inhibitors (ICIs) in non-small cell lung cancer (NSCLC). Analysis of circulating tumor DNA (ctDNA) has been reported to effectively identify patients likely to respond to ICIs by effectively and non-invasively evaluating the TMB in the tumor in NSCLC and gastric cancer. Methods: We are conducting an investigator-initiated multicenter phase II basket trial to investigate efficacy and safety of nivolumab monotherapy in patients with advanced gastrointestinal (GI) cancers with high bTMB identified by ctDNA analysis as part of the Nationwide Cancer Genome Screening Project (SCRUM-Japan GI-SCREEN). Eligibility criteria include histologically confirmed unresectable or recurrent GI malignancies; ECOG PS of 0 or 1; refractory or intolerant to the standard therapies; and high bTMB identified by a 73-gene sequencing ctDNA panel (Guardant360) regardless of microsatellite instability status. Patients will be enrolled into one of four disease-specific cohorts (colorectal, gastric, esophageal, and other GI cancer cohort), and receive intravenous nivolumab monotherapy of 360 mg every 3 weeks. The bTMB score is calculated by adjustment of mutation count by tumor fraction, and tentative bTMB level cut-offs were determined according to objective response rate (ORR) reported for ICI treatment for each tumor subtype in previous trials. The trial will utilize a two-stage design with a Bayesian hierarchical model, and tentative bTMB level cut-off will be re-assessed in the first stage. Primary endpoint in each stage is the disease control rate at 6 week and the ORR assessed by investigators per RECIST v1.1, respectively. Target sample size is determined as 70 in total so that the statistical power in each disease-specific cohort calculated based on a Bayesian posterior distribution attains 70 to 80% with one-sided alpha level in each cohort of approximately 10%. For biomarker analysis, tumor tissue and ctDNA will be serially collected and analyzed by whole-exome, transcriptome, and T cell receptor sequencing. This trial was initiated since September 2018. Clinical trial information: UMIN000033182.

3506Background: The rate of HER2 (ERBB2) copy number amplification (CNA) in metastatic colorectal cancer (mCRC) ranges from 2-13%. HERACLES, a phase II trial of trastuzumab and lapatinib (T+L) in H...

12044Background: ctDNA in post-op CRC pts correlates with molecular residual disease and may be useful to guide adjuvant therapy. However, initial studies employed clinically impractical assays ind...

1514 Background: No studies have yet described incidental detection of germline cancer predisposition mutations using circulating cell-free DNA (cfDNA). Methods: Deidentified cfDNA sequencing data from 10288 advanced cancer patients (pts) undergoing clinical circulating tumor DNA testing (Guardant360, 73 genes) were included in this study. CfDNA was extracted from plasma and quantified. A DNA library was prepared and sequenced to 15,000X average read depth. Using Ingenuity Variant Analysis, point mutations and small indels suspicious for germline origin (allele fraction 40-60%) were classified following American College of Medical Genetics and Genomics guidelines. Results: More than 50 cancer types were studied, including lung (40%), breast (20%), CRC (8%), prostate (6%), and pancreas (3%). Average age was 63.6 years (range:18-95), 42% were male. Of 34,873 putative germline variants identified, 520 (1.5%) were pathogenic or likely pathogenic (PV), 16,939 (49%) were of uncertain significance, and 17,414 (50%) were benign or likely benign. Of the 250 pts (2.4%) with hereditary cancer syndrome gene PVs, 83 were excluded due to high level of somatic tumor burden leaving 167 (1.6%) with putative germline PVs; rates were higher in pts 50 overall (3.3% vs 1.4%, p=0.02) and in breast cancer pts (4.3% vs 1.5%, p=0.03). Conclusions: The observed frequency of incidentally identified putative germline PVs is expectedly lower than the true germline rate; however, these findings illustrate that detection from cfDNA is clinically feasible. Importantly, incidental germline findings could impact oncology treatment planning (e.g. PARP inhibitors for BRCA1/2 mutations) and could benefit families via increased surveillance/primary prevention. Further research is needed to explore how to report potential germline results to clinicians using a systems-based approach. [Table: see text]