Background: Triple-negative breast cancer (TNBC) is the subtype of breast cancer with the worst prognosis and is the subtype most often associated with germline mutations of BRCA1 and certain other genes. Alpha-lactalbumin (aLA) is a milk protein that is expressed in lactating breasts but not at other times or in other normal tissues. Expression of aLA is found in approximately 70% of TNBC (Cancers PMID: 27322324) so is an attractive immunologic target for TNBC based on the “retired protein hypothesis” (Semin Immunol PMID: 31926646). Pre-clinical studies have shown that vaccination with aLA provides treatment of established and, more potently, protection from development of autochthonous tumors in transgenic murine models of breast cancer and against 4T1 transplantable breast cancer in BALB/c mice (Nat Med PMID: 20512124). We are conducting a Phase I trial in patients with early stage TNBC to demonstrate the safety of this approach and to document the ability to produce a meaningful immunologic response to aLA. Methods: Patients with ER-negative, PR-negative, HER2-negative breast cancer of pathologic stage I-III or who had residual disease after standard pre-operative systemic therapy are being entered into a Phase I trial of alpha-lactalbumin with a GMP-grade zymosan adjuvant in Montanide ISA 51 VG vehicle. Participants must be within 3 years of initiation of treatment and have no evidence of recurrence. Patients receive a total of 3 vaccinations administered once every 2 weeks with doses escalated using a 3+3 trial design. Toxic events of Common Terminology Criteria for Adverse Events (CTCAE) grade 2 or greater are considered dose-limiting. Dose levels being tested are alpha-lactalbumin/zymosan 0.01 mg/0.01 mg, 0.1 mg/0.1 mg, and 1 mg/1 mg. Patients are being monitored for toxicity until 84 days after the first vaccination or resolution of toxicity, whichever is later. Blood is being drawn prior to therapy and 14, 28, and 56 days after the first vaccination to assess cellular response using enzyme-linked immunosorbent spot (ELISpot) assays of interferon-gamma and interleukin-17 production in response to aLA. Humoral response to aLA vaccination is being assessed by enzyme-linked immunosorbent assay (ELISA). After identification of the Maximum Tolerated Dose we will expand the dose levels associated with effective tumor immunity and enroll a cohort of patients without cancer planning to undergo prophylactic bilateral mastectomy. Funding Source: Department of Defense (W81XWH-17-1-0592 and W81XWH-17-1-0593). Clinical trial information: NCT04674306.