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Immunotherapy (IO) has increasingly been demonstrated to provide therapeutic benefit to patients with metastatic colorectal cancer (mCRC). However, only a subset of mCRC tumors respond to IO. Monitoring response with tumor biomarkers like carcinoembryonic antigen (CEA) has been challenging in patients with microsatellite stable (MSS) mCRC due to low expression of CEA (CEA/lo). Noninvasive blood-based biomarkers such as circulating tumor DNA (ctDNA) can inform early treatment response and augment radiographic monitoring. We describe a case study of a patient with chemotherapy-refractory CEA/lo MSS mCRC, with metastatic disease present in a cardiophrenic lymph node. The patient was given 2 cycles of combination IO (ipilimumab/nivolumab). Response was monitored by ctDNA using a multiplex PCR next-generation sequencing assay, CEA, and CT scan. After IO administration, ctDNA levels rapidly declined, becoming undetectable. This was concurrent with radiographic resolution of the lymph node metastasis. Serial monitoring of CEA during this same period was uninformative, with no significant changes observed. Significant decline in ctDNA identified metastatic response to IO in a patient with CEA/lo, MSS mCRC and was concurrently validated by CT scan. This case study provides evidence that ctDNA can be used as a prospective surrogate for radiographic tumor response.

IntroductionImmunotherapy (IO) has transformed the treatment paradigm for a wide variety of solid tumours. However, assessment of response can be challenging with conventional radiological imaging (eg, iRECIST), which do not precisely capture the unique response patterns of tumours treated with IO. Emerging data suggest that circulating tumour DNA (ctDNA) can aid in response assessment in patients with solid tumours receiving IO. The short half-life of ctDNA puts it in a unique position for early treatment response monitoring. The BESPOKE IO study is designed to investigate the clinical utility of serial ctDNA testing to assess treatment response using a tumour-informed, bespoke ctDNA assay (Signatera) and to determine its impact on clinical decision-making with respect to continuation/discontinuation, or escalation/de-escalation of immunotherapy in patients with advanced solid tumours.Methods and analysisThe BESPOKE IO is a multicentre, prospective, observational study with a goal to enroll over 1500 patients with solid tumours receiving IO in up to 100 US sites. Patients will be followed for up to 2 years with serial ctDNA analysis, timed with every other treatment cycle. The primary endpoint is to determine the percentage of patients who will have their treatment regimen changed as guided by post-treatment bespoke ctDNA results along with standard response assessment tools. The major secondary endpoints include progression-free survival, overall survival and overall response rate based on the ctDNA dynamics.Ethics and disseminationThe BESPOKE IO study was approved by the WCG Institutional Review Board (Natera-20–043-NCP BESPOKE Study of ctDNA Guided Immunotherapy (BESPOKE IO)) on 22 February 2021. Data protection and privacy regulations will be strictly observed in the capturing, forwarding, processing and storing patients’ data. Natera will approve the publication of any study results in accordance with the site-specific contract.Trial registration numberNCT04761783.

Immune checkpoint inhibitors have shown great promise in treating patients with mismatch repair deficient/microsatellite instability high (dMMR/MSI-H) colorectal cancer (CRC). Although single-agent pembrolizumab has been approved for first-line treatment of dMMR/MSI-H metastatic CRC, combination therapy with cytotoxic T-lymphocyte-associated protein-4 (CTLA-4) inhibition (ipilimumab/nivolumab) has reported higher response rates. It is unclear whether patients who progress on PD-1 inhibition will respond to CTLA-4 blockade. Here, we report a case series of three patients with dMMR/MSI-H mCRC, where a durable and ongoing response to nivolumab with ipilimumab was achieved after initial progression with pembrolizumab monotherapy. Blood-based biomarkers such as carcinoembryonic antigen and CA 19-9 were employed to assess treatment response and monitor disease progression along with circulating tumor DNA (ctDNA). Our findings indicate ctDNA’s potential to accurately monitor response to therapy and detect disease progression, as validated by standard imaging. This case series demonstrates that CTLA-4 rescue is worthy of additional investigation as a treatment strategy after progression on PD-1 blockade in patients with dMMR/MSI-high mCRC. Our data support the utilization and expansion of clinical studies with combination therapies and using ctDNA kinetics as early dynamic marker for therapy response assessment.

Background High microsatellite instability (MSI-H) in metastatic colorectal cancer (mCRC) is associated with a beneficial response to immunotherapy. Additionally, within MSI-H cancers, tumor mutational burden (TMB) is independently predictive of immunotherapy responsiveness.1 Durable responses to therapy have been demonstrated in patients with MSI-H mCRC treated with Nivolumab and Ipilimumab.2 However, less is known about treatment responsiveness in patients with high mutational burden mCRC that demonstrates microsatellite stability (MSI-L). Methods We report on a 55-year-old female with a PALB-2 germline mutation who presented with a right-sided colonic adenocarcinoma with the involvement of the omentum and liver. The patient received 6 cycles of neoadjuvant FOLFOX, followed by an extended right hemicolectomy, omentectomy, and partial liver resection. The surgical specimen revealed a moderately differentiated adenocarcinoma in the cecum demonstrating a poor response to chemotherapy, 0/23 lymph nodes positive, one focus of adenocarcinoma in the liver with clear margins, and focal omental involvement with adenocarcinoma. The patient subsequently underwent 6 cycles of ‘adjuvant’ FOLFOX, with Oxaliplatin omitted after 3 cycles secondary to peripheral neuropathy. Soon after the patient experienced a recurrence that involved the anterior abdominal wall, between the peritoneum, and stomach, which was subsequently resected with negative margins. Molecular profiling of this metastatic focus revealed a TMB of 15.4 mutations per megabase, proficient Mismatch Repair (pMMR), a PDL1 CPS score of 26, and microsatellite stable (MSS) status. First, ctDNA analysis was performed at the time of recurrence and was found to be positive. Based on the TMB score of 15.4 and an elevated PDL1 score, the patient was initiated on Nivolumab and Ipilimumab. ctDNA measurements were obtained at the patient‘s request. Results DNA assessment performed after surgery and prior to initiation of immunotherapy revealed an approximate doubling of ctDNA levels, measured in mean tumor molecules (MTM) per mL of plasma, every month. During this period of time and correlating with the rise in ctDNA levels, the patient developed a new and enlarging FDG avid cardiophrenic lymph node. Following 2 cycles of Nivolumab and Ipilimumab, the FDG avid lymph node completely resolved and ctDNA clearance was observed (figure 1). Conclusions Here we present a case of ctDNA clearance correlating with a radiographic resolution of metastatic disease in a patient with MSS mCRC. The data is provocative and suggests a possible contributory role of ctDNA-based testing as an additional monitoring parameter to measure disease-responsiveness to immunotherapy. Further investigation is warranted. Ethics Approval N/A Consent N/A References Schrock AB, Ouyang C, Sandhu J, Sokol E, Jin D, Ross J8, Miller VA, Lim D, Amanam l, Chao J, Catenacci D, Cho M, Braiteh 7, Klempner SJ, Ali 8M, Fakih M. Tumor mutational burden is predictive of response to immune checkpoint inhibitors in MSl-high metastatic colorectal cancer. Ann Oncol 2019;30(7):1096–1103 Overman MJ, et al. Durable Clinical/Benefit With Nivolumab Plus lpilimumab in DNA Mismatch Repair-Deficient/Microsatellite Instability-High Metastatic Colorectal Cancer. Clin Oncol 2018;36(8):773–779.

TPS235 Background: Circulating tumor DNA (ctDNA) has emerged as a biomarker for non-invasive longitudinal monitoring of tumor progression in cancer management. Through the recent advances in next generation sequencing (NGS) technologies and personalized assays, ctDNA has been heralded as a promising tool to detect residual disease, relapse, and monitor treatment response in hematologic malignancies and solid tumors. Our study aims to determine whether treatment related ctDNA dynamics can be used as a reliable indicator to predict pathologic complete response (pCR) in patients with rectal cancer receiving neoadjuvant treatment. Methods: This is a prospective observational cohort study. The primary aim is to estimate the sensitivity and specificity of ctDNA clearance in predicting pCR in patients undergoing neoadjuvant therapy. The secondary aim is to evaluate the feasibility of using ctDNA as a surveillance method to detect progression of rectal cancer during neoadjuvant therapy and relapse in the subsequent follow up period. ctDNA levels are collected from newly diagnosed rectal cancer patients at 7 discrete time points: at diagnosis or screening, during neoadjuvant therapy, after completion of neoadjuvant therapy and 1 month, 2 months, 4 months, 6 months after surgery. This will be followed by every 3 months ctDNA testing for surveillance for up to 2 years. We expect to enroll approximately 30 patients at our institution. The subjects will be sorted into two groups: responders and non-responders based on whether they achieve pCR. ctDNA level between two groups will subsequently be compared. The use of ctDNA to predict pCR in rectal cancer patients may allow for many of these patients to safely avoid surgery if undetectable ctDNA at the end of neoadjuvant therapy strongly correlates with pCR. We are actively enrolling patients into this prospective observational study and expect to report the data in the near future. The data we obtain will be combined with those from several institutions around the US doing similar studies with the same test. Data analysis will subsequently be conducted. [Table: see text]

4103 Background: Hepatocellular carcinoma (HCC) and biliary tract cancers [BTC - including cholangiocarcinoma (CCA) and gall bladder cancers (GBC)] represent a heterogeneous group of diseases. Glycoprotein-based tumor markers like alpha-fetoprotein (AFP) or carbohydrate antigen 19-9 (CA-19-9) though part of standard-of-care (SOC), lack sensitivity, and specificity. A fair proportion of these cancers do not produce these glycoproteins. Circulating tumor DNA (ctDNA) testing can fill this void and be used for the assessment of molecular residual disease (MRD), as well as for surveillance purposes in patients with HCC or BTC. Prospective evaluation of this methodology in clinical practice has been limited to date. Methods: A personalized and tumor-informed multiplex PCR assay (Signatera bespoke mPCR NGS assay) was used for the detection and quantification of ctDNA. Serial time points were collected on a subset of patients to monitor their ctDNA levels in response to treatment. Results: Here we analyzed 200 plasma samples from a total of 90 patients with HCC (n=27) and BTC (n=63), comprising 46 patients with CCA and 17 patients with GBC. Sample-level ctDNA positivity rates, determined using a tumor-informed assay are presented in table. ctDNA detection was significantly associated with the stage of disease (Wilcoxon rank-sum test, p

Lung branching morphogenesis relies on a number of factors, including proper epithelial cell proliferation and differentiation, cell polarity, and migration. Rac1, a small Rho GTPase, orchestrates a number of these cellular processes, including cell proliferation and differentiation, cellular alignment, and polarization. Furthermore, Rac1 modulates both noncanonical and canonical Wnt signaling, important pathways in lung branching morphogenesis. Culture of embryonic mouse lung explants in the presence of the Rac1 inhibitor (NSC23766) resulted in a dose-dependent decrease in branching. Increased cell death and BrdU uptake were notably seen in the mesenchyme, while no direct effect on the epithelium was observed. Moreover, vasculogenesis was impaired following Rac1 inhibition as shown by decreased Vegfa expression and impaired LacZ staining in Flk1-Lacz reporter mice. Rac1 inhibition decreased Fgf10 expression in conjunction with many of its associated factors. Moreover, using the reporter lines TOPGAL and Axin2-LacZ, there was an evident decrease in canonical Wnt signaling in the explants treated with the Rac1 inhibitor. Activation of canonical Wnt pathway using WNT3a or WNT7b only partially rescued the branching inhibition. Moreover, these results were validated on human explants, where Rac1 inhibition resulted in impaired branching and decreased AXIN2 and FGFR2b expression. We therefore conclude that Rac1 regulates lung branching morphogenesis, in part through canonical Wnt signaling. However, the exact mechanisms by which Rac1 interacts with canonical Wnt in human and mouse lung requires further investigation.

Enhancing the endogenous regenerative capacity of the mammalian heart is a promising strategy that can lead to potential treatment of injured cardiac tissues. Studies on heart regeneration in zebrafish and neonatal mice have shown that cardiomyocyte proliferation is essential for replenishing myocardium. We will review recent advancements that have demonstrated the importance of Neuregulin 1/ErbB2 and innervation in regulating cardiomyocyte proliferation using both adult zebrafish and neonatal mouse heart regeneration models. Emerging findings suggest that different populations of macrophages and inflammation might contribute to regenerative versus fibrotic responses. Finally, we will discuss variation in the severity of the cardiac injury and size of the wound, which may explain the range of outcomes observed in different injury models.