MARGetuximab Or Trastuzumab (MARGOT): A Phase II Study Comparing Neoadjuvant Paclitaxel/Margetuximab/Pertuzumab to Paclitaxel/Trastuzumab/Pertuzumab in Patients With Stage II-III HER2-positive Breast Cancer

• Stage II or III (according to AJCC cancer staging manual anatomic staging table, 8th edition) histologically confirmed invasive carcinoma of the breast. A minimum tumor size of 1.5 cm (in breast mass or axillary lymph node) determined by physical exam or imaging (whichever is larger) is required. Patients with inflammatory breast carcinoma (T4d) are NOT eligible.

• Centrally confirmed to have a low affinity CD16 germline genotype (FF or FV)

• HER-2 positive by 2018 American Society of Clinical Oncology/College of American Pathologists criteria, as assessed by standard institutional guidelines (central testing is not required).

• ER/PR determination is required. ER- and PR-assays should be performed by immunohistochemical methods according to standard institutional guidelines

• Bilateral breast cancers are allowed as long as both cancers are HER2-positive (as defined in 3.1.2), or the contralateral cancer is a \<1 cm, ER+ tumor.

• Patients with multifocal or multicentric disease are eligible if the treating investigator hasdetermined the patient should be treated as HER2-positive.

• Breast imaging should include dedicated ultrasound of the ipsilateral axilla. For subjects with a clinically positive axilla based on exam or imaging, a fine needle aspiration or core biopsy procedure will be performed to determine the presence of metastatic disease in the lymph nodes (though lymph node sampling procedure need not be resulted prior to patient's registration on trial, as long as all other eligibility are met).

• Men and women (with any menopausal status) ≥18 years of age are eligible.

• ECOG performance status 0 or 1

• Required laboratory values demonstrating adequate organ function:

‣ ANC ≥ 1000/mm3

⁃ Hemoglobin ≥ 9 g/dl

⁃ Platelets ≥ 100,000/mm3

⁃ Serum creatinine \< 1.5 x ULN (institutional) OR calculated GFR ≥ 60mL/min

⁃ Total bilirubin ≤ 1.5 x ULN (institutional). For patients with Gilbert Syndrome, the direct bilirubin should be within the institutional normal range OR total bilirubin ≤ 2.0 mg/dL.

⁃ AST and ALT ≤ 2.5x ULN (institutional) Left ventricular ejection fraction (LVEF) ≥ 50%.

• Women of childbearing potential must have a negative serum pregnancy test within 14 days of treatment start. Childbearing potential is defined as: those who have not been surgically sterilized and/or have had a menstrual period in the past 12 months

• Women of childbearing potential and men with partners of childbearing potential must be willing to use one highly effective form of non-hormonal contraception or two effective forms of non-hormonal contraception by the patient and/or partner and continue its use for the duration of the study treatment and for 7 months after the last dose of study treatment.

• Patients with a history of ipsilateral or contralateral DCIS or LCIS are eligible.

• Patients undergoing breast conservation therapy (i.e. lumpectomy) must not have any contraindications to radiation therapy.

• Non-English-speaking patients are eligible but will be exempt from patient-completed questionnaires.

• Willing and able to sign informed consent.

• Willing to undergo breast biopsy for research purposes.