26 ctDNA clearance and radiographic resolution of lymph node metastasis in a patient with metastatic microsatellite stable colorectal cancer on immunotherapy

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).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.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).Abstract 26 Figure 1ctDNA time-course demonstrating ctDNA kineticsTime-point A represents the initial ctDNA assay, performed at the time of resection of peritoneal metastasis. An additional time-point (B) drawn a month later reveals a further increase in ctDNA. Time-point C represents a peak in ctDNA levels, concomitant with the new emergence of a PET avid cardiophrenic lymph node. Combination Immunotherapy (IO) was begun shortly after time-point C. Time-point D represents ctDNA clearance and radiographic resolution of lymph node metastasis after two cycles of IO. MTM/mL - mean tumor molecules/milliliter of plasmaHere 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.N/AN/ASchrock 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):1096–1103Overman 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.