The study is composed of a 3 -week Screening Period, the administration of multiple doses vaccine (Day 1, Week 0 ; Day 92, Week 13; Day 183, week 26), a 13-week Follow -up Period between the first and second vaccination and a 12-week Follow -up Period after the last vaccination . The study will be conducted on an outpatient basis. Screening should be performed in parallel with the subject’s completion of the standard -of-care first-line treatment and initiation of the standard -of-care maintenance treatment. Treos Bio ZRT OBERTO -101 Amendment 3 Product: PolyPEPI 1018 Colorectal Cancer Vaccine Page 5 of 81 CONFIDENTIAL The first dose of PolyPEPI 1018 will be administered after the subject initiates the maintenance regimen, and within 3 weeks after the eligibility CT scan was performed (should be performed at the completion of the first -line treatment ). Subjects will be monitored every 3 weeks for 12 weeks after first administration of PolyPEPI1018 (i.e. Weeks 3, 6, 9 and 12) , then vaccinated again at week 13 and monitored every 3 weeks for 12 weeks after second administration of PolyPEPI1018 (i.e. Weeks 16, 19, 22 and 25), then vaccinated again at week 26 and monitored every 3 weeks for 12 weeks after third administration of PolyPEPI1018 (i.e. Weeks 29, 32, 35 and 38) Number of Investigational Sites: Two sites , 1 in the United States (US) and 1 in Europe (Italy) Planned Number of Subjects: Approximately 10 subjects in the USA, to be vaccinated 3 times (USA), in addition to 5 subjects to be vaccinate only 1 time (Italy) Planned D uration of Subject Participation : Approximately 41 weeks, including a 3-w eek Screening Period and 12- week Follow-up Period after third and last PolyPEPI 1018 dose is administered Study Population: Patients with mCRC who have achieved partial response (PR) or stable disease during first -line treatment with chemotherapy plus a biologic drug will be enrolled Test Product, Dose, and Mode of A dministration: The final dosage form of PolyPEPI 1018 is an emulsified solution prepared onsite, by mixing 2 separate solutions of multiple peptides (PolyPEPI1018-Mix1 and PolyPEPI 1018-Mix2) and Montanide ™ as an adjuvant. The sponsor will provide the following: • PolyPEPI 1018-Mix1 - a solution of 2 × 30-mer peptides at 0.2 mg/peptide/ mL in 1.5 mL solution (20% DMSO, 80% water) and stored at -20ºC • PolyPEPI 1018-Mix2 - a solution of 4 × 30-mer peptides at 0.2 mg/peptide/ mL in 1.5 mL solution (20% DMSO, 80% water) and stored at -20ºC The site staff will follow the written vaccine preparation procedure and mix 1.2 mL of PolyPEPI 1018-Mix1, 1.2 mL of PolyPEPI 1018-Mix2, and 2.4 mL of Montanide ™ to produce the emulsified vaccine solution. After preparing the mixture , 1 mL of the mixture must be injected SC into each of 4 selected injection site s (1 mL SC injection per site). After the injection s are administered , the subject should be observed for at least 1 hour for any local and/or systemic allergic reactions . Dosage Form: Emulsified solution consisti ng of PolyPEPI 1018-Mix1, PolyPEPI 1018-Mix2, and Montanide ™ (adjuvant), prepared onsite and used immediately post preparation . Dose Level: 0.2 mg/ peptide, 6 peptides total Mode of Administration: SC injection Reference Therapy, Dose , and Mode of Administration: No reference therapy is used in this study Inclusion C riteria: 1. Male or female subjects, 18-75 years of age at time of S creening who provide written informed consent prior to initiation of any study procedure 2. Histologically confirmed metastatic adenocarcinoma originating from the colon or the rectum Treos Bio ZRT OBERTO -101 Amendment 3 Product: PolyPEPI 1018 Colorectal Cancer Vaccine Page 6 of 81 CONFIDENTIAL 3. Presence of at least 1 measurable referenc e lesion according to the Response Evaluation Criteria in Solid Tumors ( RECIST) version 1.1 criteria 4. Experienced PR or stable disease during first -line treatment with a systemic chemotherapy regimen and 1 biological therapy regimen 5. Maintenance therapy with a fluoropyrimidine (5- fluorouracil or capecitabine) plus the same biologic agent (bevacizumab, cetuximab or panitumumab) used during induction, scheduled to initiate prior to the first day of treatment with the study dr ug 6. No more t han 1 line of chemotherapy regimen for mCRC (adjuvant therapy for non -metastasized disease is allowed if terminated more than 6 months before Screening and without recurrence within 6 months after the end of adjuvant treatment) 7. Last CT scan at 3 weeks or less before the first day of treatment 8. Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 9. Women of childbearing potential must agree to appropriately use an effective form of contraception (failure rate of <1% per year) for 3 months from the day of the treatment. An effective form of contraception is defined as using hormonal contraceptives or an intrauterine device combined with at least 1 of the following forms of contraception: a diaphragm, cervical cap or condom 10. Men must agree to use an effective form of contraception (as defined above), and not donate sperm for 3 months from the day of the treatment 11. White blood cell count ≥3.0 × 10 9/L with neutrophils ≥1.5 × 109/L 12. Platelets ≥100 × 109/L, hemoglobin ≥5.6 mmol/L (corresponding to 9 g/dL) 13. Serum bilirubin ≤1.5 × upper limit of normal (ULN) set by the site 14. Alanine amino transferase ( ALAT) and aspartate amino transferase (ASAT) ≤2.5 × ULN in the absence of liver metastases. ALAT and ASAT ≤5 × ULN set by the site in the presence of liv er metastases 15. Serum creatinine ≤1.5 × ULN set by the site and creatinine clearance >30 mL/min using Cockroft formula 16. Relevant toxicities of prior therapies must have resolved to ≤Grade 1 , except for oxaliplatin -related neuropathy or alopecia 17. Anticipated li fe expectancy ≥6 month s 18. Subject is willing and able to comply with the requirements of the protocol Exclusion C riteria: 1. Received chronic systemic immune therapy or immunosuppressant medication other than steroids within the last 6 weeks prior to start of study treatment 2. Received continuous systemic steroid treatment within the last 2 weeks prior to start of study treatment 3. Colorectal cancer with documented high microsatellite instability (MSI-H) 4. Colorectal cancer with documented BRAF mutations 5. Pre-existing systemic autoimmune or antibody -mediated diseases or immune deficiency diseases 6. Central nervous system (CNS) metastases 7. Active or uncontrolled severe infections or undiagnosed febrile condition >38ºC Treos Bio ZRT OBERTO -101 Amendment 3 Product: PolyPEPI 1018 Colorectal Cancer Vaccine Page 7 of 81 CONFIDENTIAL 8. Acute or subacute intestinal obstruction or history of chronic intestinal inflammatory diseases 9.