A Phase 1, Multicenter, Open-label, First-in-Human Study of ITI-5000 (Self-Amplifying RNA Vaccine) Alone and in Combination With Pembrolizumab in Participants With Stage II-III Triple-Negative Breast Cancer (TNBC) Who Have Completed Standard Curative Intent Therapy (VITAL-TNBC Study)

Inclusion Criteria: * Age: Adults aged 18 years or over. * Consent: Provided a signed and dated informed consent form (ICF). * Diagnosis: Histologically confirmed stage 2-3 triple-negative breast cancer (TNBC), defined as HER2-negative, ER-negative, and PgR-negative by immunohistochemistry. BRCA mutations are allowed. * Prior Treatment: Completed all planned standard therapy (surgery, chemotherapy, radiation, and/or pembrolizumab as applicable) and be within 36 months of definitive surgery. * Performance Status: ECOG performance status of 0 or 1. * Organ Function: Adequate organ function at baseline (hematology, biochemistry, etc.). * Cardiac Function: No significant ischemic heart disease or myocardial infarction within 3 months before vaccination #1; QTc ≤470 msec for females or ≤450 msec for males. * Pregnancy: Women of childbearing potential must have a negative serum pregnancy test within 3 days before vaccination #1 and agree to use highly effective contraception during the study and for 123 (Part A) or 137 (Part B) days after last study drug. * Compliance: Able to attend required study visits and follow-up. * Understanding: Able to understand and provide signed informed consent per IRB/IEC guidelines. * Vaccinations: Agrees not to receive routine vaccinations until at least 30 days after the last study vaccine. * Alternative Therapies: Agrees not to use alternative therapies from the time of informed consent through 30 days following vaccination #3. Exclusion Criteria: * Part B only: Discontinued prior treatment with an immune checkpoint inhibitor (ICI) due to immune-related adverse events (irAEs). * Recent Surgery/Therapy: Major surgery within 4 weeks before vaccination #1 or received cancer-directed therapy or investigational drug/device within 4 weeks or 5 half-lives before vaccination #1. * Part B only: Received other PD-1/PD-L1 inhibitors (besides pembrolizumab) without proper washout. * Toxicities: Unresolved toxicities from prior immunotherapy or chemotherapy (must be ≤ Grade 1 or baseline, or deemed irreversible and not worsened by immunotherapy). * Medical Illness: Significant medical illness, underlying health condition, or abnormal laboratory finding increasing risk. * Autoimmune Disease: Active autoimmune disease requiring immunosuppressive treatment within the last year. * Pregnancy/Lactation: Female participants trying to conceive, pregnant, or lactating. * Positive Pregnancy Test: Positive serum pregnancy test at screening or positive urine test at baseline. * Other Trials: Concurrent participation in any other interventional clinical trial. * Allergies: Known allergies to any components of the study vaccine. * Anaphylaxis: History of anaphylaxis requiring medical intervention (including severe reactions to other mRNA vaccines). * Cardiac History: History of stroke, transient ischemic attack, unstable angina, or myocardial infarction within 3 months prior to first dose. * Myocarditis/Pericarditis: History of myocarditis or pericarditis. * Heart Failure: Symptomatic congestive heart failure (NYHA Class III or IV), significant arrhythmia, or LVEF \<45%. * QT Risk: History of risk factors for torsade de pointes or use of QT-prolonging medications. * Vaccines: Received mRNA or live virus vaccine within 28 days of planned vaccination #1 (flu and COVID boosters prohibited in that window). * Prior Malignancy: Prior malignancy except adequately treated basal/squamous cell skin cancer, in situ cervical cancer, or disease-free for ≥3 years. * Organ Transplant: History of organ transplant requiring immunosuppression; unstable HIV/AIDS. * Hepatitis: Known active hepatitis B or C. * Compliance: Unable or unwilling to comply with study schedule/procedures. * Injection/Blood Draw: Contraindication to IM injections or blood draws.