Tóth, J

3594 Background: The efficacy of checkpoint inhibitor immunotherapy in MSI-H mCRC has not been replicated in MSS mCRC. Therefore, additional interventions are needed to convert immunologically “cold” MSS CRC to “hot” tumors resembling MSI-H tumors. PolyPEPI1018 is an off-the-shelf, multi-peptide vaccine containing 12 immunogenic epitopes derived from 7 shared tumor antigens which demonstrated early evidence of clinical activity in first-line MSS mCRC. Methods: Patients with MSS mCRC who have progressed on 2-3 lines of prior chemotherapy regimen received PolyPEPI1018 (1.2 mg, sc) and atezolizumab (1,200 mg, iv) Q3W until disease progression or unacceptable toxicity. The primary endpoint was safety. A Simon 2-stage design was applied. Data on objective response rate (ORR), disease control rate (DCR), progression-free survival (PFS), overall survival (OS) and correlation studies will be presented. Results: 18 patients (66 % female) were enrolled for stage 1 of the study. Median age was 54 years (range 38–79), 50% had liver metastases and 67% received at least 3 lines of prior therapies. PD-L1 expression was <1% for 85% of patients. The combination was well-tolerated; most common side effect related to treatment was Grade (Gr) 1-2 local skin reaction (n=22). Gr 2 events (n=2) at least possibly related to treatment were nausea, pyrexia and increased blood alkaline phosphatase. There were no Gr 3-5 events or study discontinuation due to treatment AE. The ORR was 0% and the DCR was 61%. The mPFS was 2.7 months (95%CI 1.4-4.0) and the mOS was not reached (95%CI 8.75 – NE months) at the data cut-off date. Ex-vivo FluoroSpot identified 13/16 subjects with robust, vaccine-specific CD8+ and/or CD4+ T cell responses detected against multiple vaccine antigens. For patients with available biopsy pairs (n=8), both the average PD-L1 expression (IC%, p=0.029) and the density of CD8+ tumor-infiltrating lymphocytes (TILs, p=0.019) were significantly increased, post-treatment. Patients with increased PFS (> 12 weeks) had both higher levels of post-treatment PD-L1 (p=0.007) and TILs (p=0.016) than patients with PFS ≤ 12 weeks. Conclusions: PolyPEPI1018 in combination with atezolizumab was well-tolerated with no Grade 3 AEs observed. Despite no objective tumor responses could be detected, correlative data suggest contribution of PolyPEPI1018-induced immune responses to the modulation of tumor microenvironment and to improved disease control. The study did not proceed to stage 2; it is on-going for the collection of survival data. Clinical trial information: NCT05243862 .

3595 Background: PolyPEPI1018 is an off-the-shelf, multi-peptide vaccine containing 12 immunogenic epitopes derived from 7 tumor-specific antigens frequently expressed in patients with mCRC. Following early evidence of clinical activity of PolyPEPI1018 in first-line MSS mCRC, here we report the results of a phase Ib study of PolyPEPI1018 vaccine plus trifluridine/tipiracil (TAS-102) in late-stage mCRC patients. Methods: Patients with MSS mCRC who have progressed on ≤2 lines of prior chemotherapy regimen for mCRC received PolyPEPI1018 subcutaneously on days 1 and 15 and TAS-102 orally twice daily on days 1-5 and 8-15 of a 28-day cycle. Treatment continued for up to 7 cycles, until disease progression or unacceptable toxicity. Immunomonitoring was performed at both blood and tumor levels prior to and on study treatment. The primary endpoint of the study was safety and tolerability. Data on objective response rate (ORR), disease control rate (DCR), progression-free survival (PFS), overall survival (OS) and correlation studies will be presented. Results: 15 patients started treatment. Median age was 55 years (range 31–71), 73% had liver metastases (mets), 73% had KRAS mutant tumors. The most common side effect related to PolyPEPI1018 was Grade (Gr) 1-2 local skin reactions in 93% of patients. Gr 3 events (n = 15) at least possibly related to treatment were fatigue, decreased white blood cell, lymphocyte and neutrophil counts, nausea, diarrhea, myalgia, and maculo-papular rash. There were no Gr 4 or 5 events. The ORR was 0%. The DCR was 53.3% in the overall population and 36% and 100% for the patients with liver mets and without liver mets, respectively. The mPFS was 4 months (95%CI 2.2-6.1) in the overall population and 2.3 months (95%CI 2.1-NE) and 7.6 months (95%CI 4.4-NE) for the patients with liver mets and without liver mets, respectively (HR = 5.73 (95%CI 1.21, 27.06); p = 0.015). At the data cut-off date (Feb 10, 2023), the mOS was 8.7 months (95%CI 6.9-NE; not mature) in the overall population and it was not reached in patients without liver mets. Patients with increased PFS (≥ 24 weeks) had more robust vaccine-specific humoral and T cell responses induced by higher number of vaccine peptides than patients with PFS < 24 weeks. Conclusions: To our knowledge, this is the first study investigating combination of a cancer vaccine with TAS-102 chemotherapy in advanced MSS mCRC. Our results show that PolyPEPI1018 plus TAS-102 was well-tolerated with few grade 3 AEs beyond what is expected with TAS-102 monotherapy. Despite no objective tumor responses could be detected, correlative data suggest contribution of PolyPEPI1018-induced immunological responses to the improved clinical benefit and the combination warrants further testing. Clinical trial information: NCT05130060 .