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IMPORTANCE: Pathologic complete response (pCR) is a known prognostic biomarker for long-term outcomes. The I-SPY2 trial evaluated if the strength of this clinical association persists in the context of a phase 2 neoadjuvant platform trial. OBJECTIVE: To evaluate the association of pCR with event-free survival (EFS) and pCR with distant recurrence–free survival (DRFS) in subpopulations of women with high-risk operable breast cancer treated with standard therapy or one of several novel agents. DESIGN, SETTING, AND PARTICIPANTS: Multicenter platform trial of women with operable clinical stage 2 or 3 breast cancer with no prior surgery or systemic therapy for breast cancer; primary tumors were 2.5 cm or larger. Women with tumors that were ERBB2 negative/hormone receptor (HR) positive with low 70-gene assay score were excluded. Participants were adaptively randomized to one of several different investigational regimens or control therapy within molecular subtypes from March 2010 through 2016. The analysis included participants with follow-up data available as of February 26, 2019. INTERVENTIONS: Standard-of-care neoadjuvant therapy consisting of taxane treatment with or without (as control) one of several investigational agents or combinations followed by doxorubicin and cyclophosphamide. MAIN OUTCOMES AND MEASURES: Pathologic complete response and 3-year EFS and DRFS. RESULTS: Of the 950 participants (median [range] age, 49 [23-77] years), 330 (34.7%) achieved pCR. Three-year EFS and DRFS for patients who achieved pCR were both 95%. Hazard ratios for pCR vs non-pCR were 0.19 for EFS (95% CI, 0.12-0.31) and 0.21 for DRFS (95% CI, 0.13-0.34) and were similar across molecular subtypes, varying from 0.14 to 0.18 for EFS and 0.10 to 0.20 for DRFS. CONCLUSIONS AND RELEVANCE: The 3-year outcomes from the I-SPY2 trial show that, regardless of subtype and/or treatment regimen, including 9 novel therapeutic combinations, achieving pCR after neoadjuvant therapy implies approximately an 80% reduction in recurrence rate. The goal of the I-SPY2 trial is to rapidly identify investigational therapies that may improve pCR when validated in a phase 3 confirmatory trial. Whether pCR is a validated surrogate in the sense that a therapy that improves pCR rate can be assumed to also improve long-term outcome requires further study. TRIAL REGISTRATION: ClinicalTrials.gov Identifier: NCT01042379

Immunohistochemistry (IHC) is often critical for distinction between metastatic carcinomas of Mullerian organ and breast origin. Paired box family protein 8 (PAX8) has been described as a transcription factor highly specific to neoplasms derived from Mullerian organs, thyroid, and kidney. PAX8 IHC with polyclonal and monoclonal antibody reagents was performed on 27 primary and 22 metastatic breast carcinomas. Eight of 27 primary breast carcinomas (30%) were positive for PAX8 with the monoclonal antibody reagent only; 0 of 22 were polyclonal anti-PAX8 immunoreactive. Substantial numbers of metastases had positive immunoreactivity for polyclonal anti-PAX8 (23%). Each of these metastases and additional cases (45% total) also had positive immunoreactivity for monoclonal anti-PAX8, including 5 of 7 brain metastases. IHC with monoclonal anti-PAX8 was positive on 6 of 7 primary breast carcinomas corresponding to PAX8-positive metastases. Together, these results indicate a significant fraction of breast carcinoma metastases and corresponding primary neoplasms have immunoreactivity for PAX8, and positivity rates depend on the antibody used. Diagnoses of metastatic breast carcinoma were achieved with the aid of clinical history and additional IHC in cases of PAX8 immunoreactivity. Contextual interpretation is imperative for PAX8 IHC, particularly when the differential diagnosis includes metastatic breast carcinoma with limited diagnostic material available.