Background α-Lactalbumin (aLA) is expressed in lactating breasts and 70% of triple-negative breast cancer (TNBC) but not at other times or in other tissues (PMID: 27322324). Based on the ‘retired protein hypothesis’ (PMID: 31926646) vaccination with aLA provided protection from development of autochthonous tumors in transgenic murine models of breast cancer and inhibited growth of established 4T1 transplantable breast tumors in BALB/c mice (PMID: 20512124).

Methods We are performing a Phase I trial of recombinant human aLA with GMP-grade zymosan adjuvant in Montanide ISA 51 VG vehicle in 3 cohorts of subjects: 1A) patients with high-risk TNBC who have completed all standard treatment; 1C) patients with TNBC who have residual cancer after primary chemo-immunotherapy and are receiving post-operative treatment with pembrolizumab +/- capecitabine; and 1B) patients with BRCA1, BRCA2, or PALB2 mutations who are undergoing risk-reducing mastectomies. Three vaccinations are given once every 2 weeks. Events of Common Terminology Criteria for Adverse Events (CTCAE) Grade ≥ 2 are considered dose-limiting toxicities (DLTs).

Results We have vaccinated 21 patients in Group 1A, 3 in Group 1C, and 2 in Group 1B (NCT04674306). CTCAE toxicity by dose level (DL) is summarized below (table 1) by grade for each study cohort. All DLTs were injection site reactions, with ulceration and need for incisional drainage representing the Grade 3 events. 12 of 16 patients assayed to date in Group 1A met protocol specified definitions of an immune response based on ELISpot assays to determine frequencies of T cells producing IFNγ and/or IL-17 in response to recombinant aLA, including 4 of 6 subjects at DL1. ELISpot data for additional subjects in all groups will be available in August 2024.

Conclusions DL1 is the maximum tolerated dose (MTD) to date and produces an immune response in most patients. The aLA vaccine given at DL1 has been tolerable to date when given alone or concurrently with pembrolizumab in patients treated for TNBC, and when given to healthy patients undergoing elective prophylactic mastectomy. Immune data in these new cohorts will be reported. Data from this trial will be insufficient to assess clinical efficacy and will be explored in Phase II.