MagSense® Human Epidermal Growth Factor Receptor 2 (HER2) Imaging Agent (MSH2IA)

This Phase 1b/2 study is designed as an open-label, multicenter, 3 group, interventional study in which Phase 1b will evaluate the pharmacokinetics (PK) of MSH2IA and Phase 2 will confirm lowest effective dose as well as evaluate image timing, and diagnostic performance of a selected dose of MSH2IA when administered by subareolar or peritumoral injection. No hypothesis testing is planned. Comparative statistics will be used to generate hypotheses to be tested in subsequent studies.

Inclusion Criteria: 1. Female participants ≥18 years of age and are willing to provide written informed consent. 2. In the opinion of the Investigator, willing and able to comply with required study procedures. 3. Histologically confirmed primary HER2+ breast cancer; participants with a second primary tumor in the contralateral breast are eligible, but only 1 side will be evaluated. 4. Primary tumor must be HER2+, defined according to ASCO/CAP 2018 guidelines as either IHC 3+ or IHC 2+ with confirmatory ISH-positive. 5. Phase 1b and Phase 2 (Imaging Schedule) participants must be node suspicious by SOC assessment. 6. No previous treatment of the primary tumor or ipsilateral axilla that includes surgery (other than biopsy), radiotherapy, or a biologic such as trastuzumab. In Phase 2 (Diagnostic Performance) only, participants who have received NCT are eligible if treatment has not included trastuzumab. 7. Female participants of nonchildbearing potential: defined as either surgically sterilized (eg, hysterectomy, bilateral salpingectomy, bilateral oophorectomy ≥6 weeks before screening) or postmenopausal (ie, no menses for 12 months without an alternative medical cause. 8. Female participants of childbearing potential are eligible to participate if not pregnant, not breastfeeding, and using birth control. Exclusion Criteria: Participants will be excluded from the study if ANY of the following criteria are applicable: 1. Contraindications to the use of the Magtrace™ and Sentimag® Magnetic Localization System (Magtrace 2018), including use before NCT when MRI will be the primary imaging method for evaluation of breast cancer 2. Previous or current diseases of Fe metabolism or history of adverse reactions to intravenous Fe products 3. Known hypersensitivity to trastuzumab, iron oxide, or polyethylene glycol compounds 4. Treatment by Herceptin® or biosimilar within prior six months 5. Planned or prior use of Magtrace™ and Sentimag as a diagnostic agent, as it might interfere with MagSense® evaluations 6. Known clinical or radiological evidence of distant metastases (Stage IV disease) 7. Inflammatory breast cancer 8. Prior surgical axillary procedure, including SLNB or ALND, or previous radiation on the ipsilateral side of the breast cancer primary 9. Prior history of breast cancer in the ipsilateral breast 10. Metal devices or fragments other than post-breast biopsy clips in the ipsilateral axilla or in the ipsilateral chest 11. Any contraindications to MRI imaging