Phase 1 Study of a Self-amplifying RNA Vaccine (ITI-5000) in Stage 2-3 Triple Negative Breast Cancer

Inclusion Criteria: 1. Applicable to Part A only. Participants must have completed adjuvant pembrolizumab if they were receiving it before enrolling in the study. 2. Applicable to Cohort 3A and Part B only. Participant must have received neoadjuvant chemotherapy with pembrolizumab, then undergone definitive surgery. At the time of surgery, participant must not have achieved pCR. 3. Applicable to Part B only. Participant is currently receiving or is planned to receive standard of care adjuvant therapy, including pembrolizumab in combination with capecitabine or olaparib in accordance with the FDA-approved label, with ≥3 cycles remaining if receiving 200mg Q3W or ≥2 cycles remaining if receiving 400 mg Q6W at the time of first ITI-5000 dose. 4. Adults aged 18 years or over. 5. Participants provided a signed and dated ICF. 6. Participant agrees not to receive any routine vaccinations until at least 30 days after receiving the last study vaccine. 7. TNBC diagnosed as pathologic stage 2-3 according to American Joint Committee on Cancer (AJCC) and confirmed by histological examination, e.g., negative for HER2 as defined by ASCO CAP 2023 guidelines. ER and PgR receptor negative by immunohistochemistry. Participants with BRCA mutations will be allowed. a. Concurrent endocrine therapy (e.g., tamoxifen, aromatase inhibitors, ovarian suppression) is not permitted during study participation. Concurrent CDK4/6 inhibitors (ribociclib/abemaciclib) are not allowed. 8. Applicable to Part A only. Participants with more than 4 weeks since last active therapy (chemotherapy, radiation therapy, or surgery) and 36 months or less following definitive surgery, based on the period of highest risk for recurrence in participants with stages 2-3 TNBC. 9. Applicable to Part A only. Participants completed all planned previous cancer treatment (e.g., chemotherapy, radiation, therapy, surgery). 10. Participant's ECOG performance status is 0 or 1. 11. Participant had no significant ischemic heart disease or myocardial infarction within 3 months before vaccination #1 and has adequate cardiac function during eligibility evaluation, as evidenced by QTc of ≤470 msec for females or ≤450 msec for males assessed by the Fridericia method (QTcF) and evidenced by the average of measurements from triplicate ECGs at the screening visit. a. The eligibility of participants with ventricular pacemakers for whom the QT interval may not be accurately measurable will be determined on a case-by-case basis by the sponsor in consultation with the medical monitor. 12. Participant has an adequate organ function, as evidenced by the following tests conducted within 7 days before enrollment: i. Hematology examination (excluding blood transfusion or use of hematopoietic stimulating agents for correction): 1\. Hemoglobin ≥8.0 g/L 2. Absolute neutrophil count (ANC) ≥1.0 × 109/L 3. Platelet count ≥100 × 109/L ii. Serum biochemistry examination (excluding recent blood transfusion or albumin administration): 1\. Alanine aminotransferase and AST ≤1.5 times the ULN 2. Alkaline phosphatase ≤2.5 ULN 3. Total bilirubin ≤ 1.5 ULN 4. Serum creatinine ≤1.5 ULN, with creatinine clearance ≥ 50 mL/min (calculated using the Cockcroft-Gault formula) 13. Women of childbearing potential have a negative serum pregnancy test within 3 days before vaccination #1, and they and their partners agree to use highly effective methods of contraception during the study and for 6 months after the last administration of the study drug. a. NOTE: A woman is considered of non-childbearing potential if she has had a documented bilateral oophorectomy, tubal ligation, or hysterectomy, or if she is postmenopausal, defined as ≥12 months of spontaneous amenorrhea without an alternative medical cause (with serum FSH confirmation if \<55 years old). 14. Participant is able to attend the required study visits and follow-up as required by this protocol. 15\. Participant is able to understand and provide a signed informed consent that fulfills the relevant IRB or IEC guidelines prior to study registration. 16\. Participant agrees not to use alternative therapies from the time of informed consent through 30 days following vaccination #3. Participants may be asked to complete a "wash out" period before vaccination #1 at the principal investigator's discretion to ensure the absence of all alternative therapies. NOTE: "Alternative therapies" refer to non-prescription or non-standard medical interventions used with the intent to treat or prevent cancer or its symptoms, including but not limited to herbal remedies, high-dose dietary supplements marketed for therapeutic benefit, homeopathic preparations, or naturopathic treatments. Routine vitamins, minerals, or supportive care measures not expected to affect immune function may be continued at the investigator's discretion. Exclusion Criteria: 1. Applicable to Part B only. Participant discontinued prior treatment with an ICI due to irAEs. 2. Participant underwent major surgery within 4 weeks before the planned day of vaccination #1 or received any other investigational drug or device within 4 weeks or 5 half-lives of that agent (whichever is shorter) before the planned day of Vaccination #1. i. Applicable to Part A only. Participant received cancer-directed therapy (chemotherapy, radiotherapy, biologic or immunotherapy, etc.) within 4 weeks or 5 half-lives of that agent (whichever is shorter) before the planned day of Vaccination #1. ii. Applicable to Part B only. Participants who received any PD-1 or PD-L1 inhibitor other than pembrolizumab will be excluded unless they have completed a washout period of ≥ 4 weeks or 5 half-lives of that agent (whichever is shorter) before Vaccination #1. 3. Participant has toxicities due to prior immunotherapy. For the participant to be eligible, these toxicities must either have returned to ≤ Grade 1 or baseline or been deemed irreversible and in the opinion of the investigator not worsened by immunotherapy (e.g., ICI-endocrinopathies). Participants with any cardiac toxicities (regardless of the grade, etc.) will be excluded. 4. Participant has toxicities due to prior chemotherapy that have not been resolved or considered stable and clinically manageable (e.g., neuropathies). Participants with any cardiac toxicities will be excluded. 5. Participant has a significant medical illness, underlying health condition, or abnormal laboratory finding that, in the investigator's opinion, would increase the risk of participating in the study. 6. Participants with an active autoimmune disease requiring immunosuppressive treatment within the last year (excluding irAEs), such as chronic prolonged systemic corticosteroid use (defined as corticosteroid use lasting one month or more). 7. Female participants who are trying to conceive, are pregnant, or lactating. 8. A positive serum pregnancy test at screening and/ or a positive human chorionic gonadotropin (hCG) urine test at baseline in women of childbearing potential. 9. Participant concurrently participates in any other interventional clinical trial. 10. Participant has known allergies to any of the components of the study vaccine. 11. Participant has a history of anaphylaxis requiring medical intervention (including severe reactions to other mRNA vaccines, such as those against SARS-COV-2, etc.). 12. Participant has a history of stroke, transient ischemic attack, unstable angina, or myocardial infarction within 3 months prior to the first dose of study treatment. 13. Participant has a history of myocarditis or pericarditis. 14. Participant has symptomatic congestive heart failure according to New York Heart Association (NYHA) classification, Class III or IV (per NYHA Classification), clinically significant cardiac arrhythmia, or a known left ventricular ejection fraction \<45%. 15. Participant has a history of risk factors for torsade de pointes (e.g., heart failure, hypokalemia, family history of long QT syndrome) or requires the use during study participation of concomitant medications known or suspected to prolong the QT/QTc interval, with the exception of drugs with low risk of QT/QTc prolongation that are used as standard premedication (e.g., diphenhydramine, famotidine, ondansetron). 16. Participant received an mRNA or a live virus vaccine within 28 days of the planned vaccination #1. Flu and COVID vaccinations/boosters are also prohibited within 28 days before the planned day of vaccination #1. Vaccines that do not contain live virus are permitted. 17. Participant has prior malignancy, except for the following: i. adequately treated basal-cell or squamous-cell skin cancer, ii. in situ cervical cancer, iii. any other cancer from which the participant has been disease-free for at least 3 years. 18. Participant has a history of organ transplant requiring immunosuppression. Participants with unstable human immunodeficiency virus (HIV) or acquired immunodeficiency syndrome (AIDS) are not eligible. a. Stable and well controlled HIV/AIDS participants on retroviral therapy, defined as those with no dose change within 4 weeks before the planned day of vaccination #1 and no anticipated dose change, are eligible. 19. Participant with known active hepatitis B or C are not eligible. Active hepatitis B is defined as a known positive hepatitis B surface antigen (HBsAg) result. Active hepatitis C is defined by a known positive hepatitis C antibody result and known quantitative hepatitis C virus RNA results greater than the lower limits of detection of the assay. a. Participants with a history of infection with hepatitis B virus or HCV may enroll if the viral load is undetectable per quantitative polymerase chain reaction (PCR) and/or nucleic acid testing. 20. Participant was assessed by the investigator as being unable or unwilling to comply with the requirements of the treatment schedule and study procedures for any reason. 21. Participant has a contraindication to IM injections or blood draws.