STEMVAC in Patients With Early Stage Triple Negative Breast Cancer

Inclusion Criteria:

• Participants with triple negative breast cancer, stages IB, II or III. Estrogenreceptor (ER)-negative and progesterone receptor (PR)-negative is defined as breastcancer with less than 10% of ER or PR expression. HER2 negative is defined as:

• 0-1+ HER2 expression by immunohistochemistry (IHC) OR

• Fluorescence in situ hybridization (FISH) negative OR

• HER2 2+ and FISH negative

• Note: Participants with low ER positivity (≤10%) who are already on adjuvanthormonal therapy will be allowed on study and can continue their adjuvanthormonal treatment during study participation.

• Participants must have completed all standard of care (or investigational) systemictherapy (including immune modulating agents) and radiotherapy if used between 28 and 365 days prior to enrollment

• Note: Treatment with a bisphosphonate or denosumab to prevent bone loss is notconsidered to be systemic therapy for breast cancer and its use within the 28 daypre-enrollment period or while on study is not exclusionary. Treatment with adjuvantolaparib within the 28 day pre-enrollment period or while on study is notexclusionary.

• Participants must agree to avoid systemic steroids for the duration of the treatmentperiod and until completion of the 1 month post 2nd booster vaccine visit (end oftreatment)

• Participants must be at least 18 years of age

• Note: Because no dosing or adverse event (AE) data are currently available on theuse of STEMVAC in participants < 18 years of age, children and adolescents areexcluded from this study but will be eligible for future pediatric trials, ifapplicable.

• Participants must have Eastern Cooperative Oncology Group (ECOG) performance statusscore of ≤ 1

• White blood cell (WBC) >= 3000/mm^3 (within 60 days of enrollment and at least 28days post standard of care [SOC] treatment)

• Lymphocyte count >= 800/mm^3 (within 60 days of enrollment and at least 28 days poststandard of care [SOC] treatment)

• Platelet count >= 100,000/mm^3 (within 60 days of enrollment and at least 28 dayspost standard of care [SOC] treatment)

• Hemoglobin (Hgb) >= 10 g/dl (within 60 days of enrollment and at least 28 days poststandard of care [SOC] treatment)

• Serum creatinine =< 1.2 mg/dl OR creatinine clearance > 60 ml/min (within 60 days ofenrollment and at least 28 days post standard of care [SOC] treatment)

• Total bilirubin =< 1.5 X upper limit of institutional normal (ULN) (within 60 daysof enrollment and at least 28 days post standard of care [SOC] treatment)

• AST (aspartate aminotransferase)/serum glutamic-oxaloacetic transaminase (SGOT) =< 1.5 X upper limit of institutional normal (ULN) (within 60 days of enrollment and atleast 28 days post standard of care [SOC] treatment)

• Must have recovered from major infections and/or surgical procedures, and in theopinion of the investigator, not have any significant active concurrent medicalillnesses or condition precluding protocol treatment

• The effects of STEMVAC on the developing human fetus are unknown. For this reason,

• Female participant agrees to use adequate contraception (examples include:estrogen and/or progestogen containing hormonal contraception, barrier method (condom, cervical cap) or abstinence) while on the study and until 1 monthafter the 2nd booster vaccination when/if engaging in sex that could lead topregnancy. Exceptions: Females who have had a hysterectomy, tubal ligation orbilateral oophorectomy OR meet one of the following criteria forpostmenopausal: Age > 60 or age < 60 with >= 12 months amenorrhea andfollicle-stimulating hormone (FSH) within the testing facility's postmenopausalrange

• Female participant agrees to inform her study physician immediately should shebecome pregnant or suspect she is pregnant while participating in this study

• Male participants who are having sex that can lead to pregnancy must use anacceptable form of contraception (vasectomy with the absence of sperm, sexualabstinence, condoms) throughout the course of the study

• Must be 14 days between a vaccine (i.e. COVID19, Flu, shingrix, Tdap, etc.) andany STEMVAC vaccination. **Note: This does not apply to the Td vaccine.

• Patients must be willing to not undergo major elective surgical procedures withgeneral anesthesia or conscious sedation through the end of treatment visit. (Note:port removal is allowable)

• Ability to understand and the willingness to sign a written informed consentdocument

Exclusion Criteria:

• Contraindication or known hypersensitivity to receiving sargramostim (rhuGM-CSF) orother products

• History of allergic reactions attributed to compounds of similar chemical orbiologic composition to STEMVAC

• Participants receiving any other investigational agents

• Chronic use of nonsteroidal anti-inflammatory drugs (NSAIDs) is prohibited duringthe treatment period of the study, except when taken as low-dose (81 mg) aspirintherapy. Prohibited chronic use is defined as daily use for more than 7 days

• Participants with any clinically significant autoimmune disease uncontrolled withtreatment

• Uncontrolled intercurrent illness or psychiatric illness/social situations thatwould limit compliance with study requirements

• Pregnant and breastfeeding women are excluded from this study

• Known history of human immunodeficiency virus (HIV) infection, hepatitis B, orhepatitis C

• Note: These individuals are excluded in order to avoid confounding an existingcondition with an immune response to STEMVAC

• Chronic usage of immunosuppressants and glucocorticoids (methotrexate for RA, etc.)

• History of invasive breast cancer prior to TNBC diagnosis * Note: Prior DCIS isallowable