Purpose: We aimed to improve efficacy and reduce toxicity of high-risk human epidermal growth factor receptor 2 (HER2)-positive early breast cancer (EBC) treatment by replacing taxanes and trastuzumab with trastuzumab emtansine (T-DM1)., Methods: The phase III KAITLIN study (NCT01966471) included adults with excised HER2-positive EBC (node-positive or node-negative, hormone receptor-negative, and tumor > 2.0 cm). Postsurgery, patients were randomly assigned 1:1 to anthracycline-based chemotherapy (three-four cycles) and then 18 cycles of T-DM1 plus pertuzumab (AC-KP) or taxane (three-four cycles) plus trastuzumab plus pertuzumab (AC-THP). Adjuvant radiotherapy/endocrine therapy was permitted. Coprimary end points were invasive disease-free survival (IDFS) in the intention-to-treat node-positive and overall populations with hierarchical testing., Results: The median follow-up was 57.1 months (interquartile range, 52.1-60.1 months) for AC-THP (n = 918) and 57.0 months (interquartile range, 52.1-59.8 months) for AC-KP (n = 928). There was no significant IDFS difference between arms in the node-positive (n = 1,658; stratified hazard ratio [HR], 0.97; 95% CI, 0.71 to 1.32) or overall population (n = 1846; stratified HR, 0.98; 95% CI, 0.72 to 1.32). In the overall population, the three-year IDFS was 94.2% (95% CI, 92.7 to 95.8) for AC-THP and 93.1% (95% CI, 91.4 to 94.7) for AC-KP. Treatment completion rates (ie, 18 cycles) were 88.4% for AC-THP and 65.0% for AC-KP (difference driven by T-DM1 discontinuation because of laboratory abnormalities [12.5%]). Similar rates of grade ≥ 3 (55.4% v 51.8%) and serious adverse events (23.3% v 21.4%) occurred with AC-THP and AC-KP, respectively. KP decreased clinically meaningful deterioration in global health status versus THP (stratified HR, 0.71; 95% CI, 0.62 to 0.80)., Conclusion: The primary end point was not met. Both arms achieved favorable IDFS. Trastuzumab plus pertuzumab plus chemotherapy remains the standard of care for high-risk HER2-positive EBC., Competing Interests: Ian E. KropEmployment: Freeline Therapeutics (I), PureTech (I)Leadership: AMAG Pharmaceuticals (I), Freeline Therapeutics (I), PureTech (I)Stock and Other Ownership Interests: AMAG Pharmaceuticals (I), Freeline Therapeutics (I), PureTech (I)Honoraria: Genentech/Roche, AstraZeneca¸ CelltrionConsulting or Advisory Role: Genentech/Roche, Seattle Genetics, Daiichi Sankyo, MacroGenics, Novartis, Merck, Bristol Myers Squibb, AstraZenecaResearch Funding: Genentech (Inst), Pfizer (Inst) Seock-Ah ImConsulting or Advisory Role: AstraZeneca, Novartis, Roche/Genentech, Eisai, Pfizer, Amgen, Hanmi, Lilly, GlaxoSmithKline, MSD, Daiichi SankyoResearch Funding: AstraZeneca (Inst), Pfizer (Inst), Roche/Genentech (Inst), Daewoong Pharmaceutical (Inst), Eisai (Inst)Other Relationship: Roche Carlos BarriosStock and Other Ownership Interests: Biomarker, MedSIR, TummiHonoraria: Novartis, Roche/Genentech, Pfizer, GlaxoSmithKline, Sanofi, Boehringer Ingelheim, Eisai¸ MSD, Bayer, Lilly, AstraZeneca, Zodiac PharmaConsulting or Advisory Role: Boehringer Ingelheim, Roche/Genentech, Novartis, GlaxoSmithKline, Eisai, Pfizer, AstraZeneca, Libbs, MSD Oncology, United Medical, LillyResearch Funding: Pfizer (Inst), Novartis (Inst), Amgen (Inst), AstraZeneca (Inst), Boehringer Ingelheim (Inst), GlaxoSmithKline (Inst), Roche/Genentech (Inst), Lilly (Inst), Sanofi (Inst), Taiho Pharmaceutical (Inst), Mylan (Inst), Merrimack (Inst), Merck (Inst), AbbVie (Inst), Astellas Pharma (Inst), BioMarin (Inst), Bristol Myers Squibb (Inst), Daiichi Sankyo (Inst), Abraxis BioScience (Inst), AB Science (Inst), Asana Biosciences (Inst), Medivation (Inst), Exelixis (Inst), ImClone Systems (Inst), LEO Pharma (Inst), Millennium (Inst), Janssen (Inst), Clinica Atlantis (Inst), INC Research (Inst), Halozyme (Inst), Covance (Inst), Celgene (Inst), inVentiv Health (Inst), Merck KGaA (Inst), Shanghai Henlius Biotech (Inst), Polyphor (Inst), PharmaMar (Inst)Travel, Accommodations, Expenses: Roche/Genentech, Novartis, Pfizer, BMS Brazil, AstraZeneca, MSD Oncology, Lilly Hervé BonnefoiTravel, Accommodations, Expenses: Roche, Pfizer Julie GralowConsulting or Advisory Role: Genentech, AstraZeneca, Hexal, Puma Biotechnology, Roche, Novartis, Seattle Genetics, Genomic Health Masakazu ToiHonoraria: Novartis, Takeda, Eisai, AstraZeneca, Chugai Pharma, Taiho Pharmaceutical, Yakult Pharmaceutical, Shimadzu, Pfizer, Lilly, Kyowa Kirin, Devicor Medical Products, Exact Sciences, Nippon KayakuConsulting or Advisory Role: Daiichi Sankyo, Kyowa Kirin, Bertis, Athenex, BMS, Kansai Medical, Terumo, Lux Biosciences, AstraZeneca¸ LillySpeakers' Bureau: Pfizer, AstraZeneca, Lilly, Daiichi SankyoResearch Funding: Taiho Pharmaceutical, Chugai Pharma, Shimadzu, Astellas Pharma, AFI technology, Japan Breast Cancer Research Group, Pfizer, Eisai, Daiichi Sankyo, AstraZeneca, Nippon Kayaku, Kyoto Breast Cancer Research Network, Shionogi, Lilly, Yakult Pharmaceutical, Lux Biosciences (Inst), GL Science (Inst)Patents, Royalties, Other Intellectual Property: JP 2017-143763 WO2017/131162A1, PCT/JP2016/004374Travel, Accommodations, Expenses: EisaiOther Relationship: Japan Breast Cancer Research Group, Kyoto Breast Cancer Research Network, Organization for Oncology and Translational Research, Breast Cancer research and Treatment, Frontier in Women's Cancer, Scientific Reports, International Journal of Clinical Oncology, Cancer Science, Asian Journal of Surgery, Asian Journal of Breast Surgery, British Journal of Cancer Luca GianniConsulting or Advisory Role: Roche, Pfizer, AstraZeneca, Genomic Health, Merck Sharp & Dohme, Synaffix, Celgene, Lilly, ADC Therapeutics, Odonate Therapeutics, Oncolytics, Genenta Science, Sandoz, METIS Precision Medicine, Novartis, Revolution Medicines, Zymeworks, G1 Therapeutics, Genentech, Onkaido Therapeutics, Taiho Pharmaceutical, Seattle Genetics, Synthon, Forty Seven, Sanofi, Amgen, Menarini, BioMedical Insights, Artemida Pharma LtdSpeakers' Bureau: Roche Product, Zymeworks, Revolution MedicinesPatents, Royalties, Other Intellectual Property: RocheTravel, Accommodations, Expenses: Celgene, Roche, Chugai Pharma, Pfizer Sandra M. SwainConsulting or Advisory Role: Inivata, Genentech/Roche, Daiichi Sankyo, Molecular Templates, Athenex, Silverback Therapeutics, AstraZeneca, Exact Sciences, Natera, Beijing Medical Award Foundation, Lilly, MerckResearch Funding: Genentech (Inst), Kailos Genetics (Inst)Travel, Accommodations, Expenses: Caris Life Sciences, Daiichi Sankyo, Genentech/RocheOther Relationship: AstraZeneca, RocheOpen Payments Link: https://openpaymentsdata.cms.gov/physician/801195/associated-research-funding Michelino De LaurentiisHonoraria: Roche, Novartis, Pfizer, Lilly, Amgen, Pierre Fabre, AstraZeneca, MSD, Seattle Genetics, Gilead Sciences, Takeda, IpsenConsulting or Advisory Role: Roche, Novartis, Pfizer, Lilly, Amgen, AstraZeneca, MSD, Pierre Fabre, Seattle Genetics, Gilead Sciences, Ipsen, TakedaSpeakers' Bureau: NovartisResearch Funding: Novartis, Roche, Lilly, Puma Biotechnology, Pfizer, Daiichi Sankyo, MSD, MacroGenics, Bristol Myers Squibb Zbigniew NoweckiTravel, Accommodations, Expenses: Roche/Genentech Chiun-Sheng HuangHonoraria: Roche, Novartis, Pfizer, AstraZenecaConsulting or Advisory Role: Amgen, Pfizer, Roche, Lilly, AstraZeneca, NovartisSpeakers' Bureau: Amgen, Pfizer, Roche, Novartis, AstraZenecaResearch Funding: AstraZeneca (Inst), Lilly (Inst), MSD (Inst), Novartis (Inst)¸ Pfizer (Inst), Roche (Inst), OBI Pharma (Inst), EirGenix (Inst), Daiichi Sankyo (Inst)Travel, Accommodations, Expenses: Pfizer, Roche, AstraZeneca Yoshinori ItoResearch Funding: Daiichi Sankyo (Inst), Chugai Pharma (Inst), Novartis (Inst), Parexel (Inst), EPS Associates Co, Ltd (Inst), MSD (Inst), AstraZeneca (Inst), Lilly (Inst) Jigna ShahEmployment: GenentechStock and Other Ownership Interests: GenentechTravel, Accommodations, Expenses: GenentechOther Relationship: Genentech Thomas BouletEmployment: Roche/Genentech Haiying LiuEmployment: GenentechStock and Other Ownership Interests: Roche/GenentechTravel, Accommodations, Expenses: Roche/Genentech Harrison MachariaEmployment: Roche/Genentech Peter TraskEmployment: Roche/GenentechStock and Other Ownership Interests: Genentech, Pfizer Chunyan SongEmployment: Roche/GenentechStock and Other Ownership Interests: Roche/Genentech Eric P. WinerHonoraria: Genentech/Roche, Genomic HealthConsulting or Advisory Role: Leap Therapeutics, Seattle Genetics, Jounce Therapeutics, GlaxoSmithKline, Carrick Therapeutics, Lilly, G1 Therapeutics, Syros Pharmaceuticals, Genentech/Roche, Gilead Sciences, Zymeworks, AthenexResearch Funding: Genentech (Inst)Other Relationship: InfiniteMD Nadia HarbeckStock and Other Ownership Interests: West German Study GroupHonoraria: Roche, Novartis, Amgen, Pfizer, AstraZeneca¸ Pierre Fabre, Daiichi-Sankyo, Exact Sciences, MSD, Seattle GeneticsConsulting or Advisory Role: Roche/Genentech, Novartis, Pfizer, Lilly, Sandoz, Daiichi Sankyo, AstraZeneca, West German Study Group (Inst), Pierre Fabre, Seattle Genetics, MSDResearch Funding: Roche/Genentech, Lilly, MSD¸ AstraZenecaNo other potential conflicts of interest were reported.