Background: Engineered toxin bodies (ETBs) are proprietarily engineered from a Shiga-like Toxin A subunit fused to antibody-like binding domains. ETBs can force receptor internalization, self-route to the cytosol, and induce cell-kill via inactivation of ribosomes. MT-5111 is a 55 kDa de-immunized ETB targeting HER2, and may not be subject to resistance mechanisms that exist for TKI, ADC, or antibodies. It binds a HER2 epitope distinct from trastuzumab or pertuzumab, could be combined with other HER2 targeting agents, and may have improved tumor penetration. Methods: MT-5111 is evaluated as monotherapy in subj with confirmed HER2+ locally advanced or metastatic cancers. The primary objective is to determine the maximum tolerated dose in subjects (subj) with advanced HER2-positive tumors. Secondary endpoints are PK, tumor response and immunogenicity. Part 1 will escalate doses to identify MTD in up to 42 subj. Part 2 will further evaluate MT-5111 at the MTD in up to 98 subj. All subj will receive MT-5111 on Days 1, 8, and 15 of each 21-day cycle until disease progression, unacceptable toxicity, death, withdrawal of consent or another reason for withdrawal. Part 1 will include subj with any HER2+ solid cancers. Part 2 will enroll 3 expansion cohorts: HER2+ breast (BC), HER2+ gastroesophageal cancer (GEA), and other HER2+ solid cancers. HER2+ must be demonstrated on metastatic lesions in case of metastases. Tumors tested by immunohistochemistry (IHC) must have IHC status of 2+ or 3+, regardless of in-situ hybridization (ISH) results; for BC and GEA, if no IHC is available, ISH per ASCO-CAP guidelines is used. Subj with HER2+ BC should have had at least 2 lines of HER2-directed therapy; subj with HER2+ gastric cancer should have received trastuzumab or have been intolerant to trastuzumab. Subj with evaluable disease may be included in Part 1; in Part 2, all subj must have at least 1 measurable lesion per RECIST 1.1. ECOG should be 0-1, and bone marrow, hepatic, renal, cardiac function should be adequate. Further details can be found on clinicaltrials.gov (NCT04029922). Enrollment has begun in September 2019. Clinical trial information: NCT04029922