A First-in-human, Clinical Trial Assessing the Safety of ES2B-C001-S01 With or Without [Adjuvant] in Patients With HER2-expressing Metastatic Breast Cancer.

This first-in-human study tests the safety of a new vaccine-type treatment called ES2B-C001-S01 (given alone or with an immune booster called an adjuvant) in people with HER2-positive metastatic or locally advanced breast cancer that cannot be surgically removed. **You may be eligible if...** - You are 18 or older - You have HER2-positive metastatic or locally advanced inoperable breast cancer - Your life expectancy is at least 3 months - You are in reasonably good condition (ECOG 0–2) - Your organ function (bone marrow, kidney, liver, heart) is adequate - If female, you are not pregnant and are using reliable contraception **You may NOT be eligible if...** - You are currently receiving intravenous chemotherapy or checkpoint immunotherapy (or received them in the past month) - Your brain metastases are symptomatic and require high-dose steroids - You have severe uncontrolled heart disease or heart failure (NYHA Class > II) or an LVEF below 55% - You have active autoimmune disease (except well-controlled thyroid or diabetes) - You have a systemic infection requiring IV antibiotics - You are pregnant or breastfeeding - You received a live or live-attenuated vaccine within the past 30 days Talk to your doctor to see if this trial is right for you.