Rochester, Minnesota Clinical Profile


Ciara O'Sullivan, M.B., B.Ch., is an assistant professor of oncology at Mayo Clinic who specializes in breast cancer and developmental therapeutics. Her principal research activities consist of designing and conducting clinical trials that overcome drug resistance and test new drugs in the treatment of breast cancer. Her research is clinical and translational in nature, and represents novel investigator-initiated research.

Focus areas

Dr. O'Sullivan's research primarily focuses on three areas:

  • Determining the mechanisms of drug resistance in people with advanced or metastatic endocrine-resistant breast cancer. Dr. O'Sullivan and her colleague, Matthew P. Goetz, M.D., are co-chairs of the PROMISE Study, which aims to identify novel genomic variants and pathways associated with endocrine resistance in women with advanced hormone receptor positive breast cancer treated with the cyclin-dependent kinase 4/6 inhibitor palbociclib and endocrine therapy.
  • Investigating novel therapeutic combinations in metastatic, human epidermal growth factor receptor 2 (HER2)-positive, breast cancer. Dr. O'Sullivan has combined her interest in HER2-positive breast cancer and CDK4/6 inhibitors by initiating a phase II study of the CDK4/6 inhibitor, abemaciclib, combined with trastuzumab emtansine (T-DM1) for women with metastatic HER2-positive breast cancer who progressed on taxane and dual HER2-targeted treatment. This trial opened at several centers throughout the U.S. via the Academic and Community Cancer Research United (ACCRU) in late 2020.
  • Tailoring treatments for patients with HER2-positive breast cancer treated with curative intent. Dr. O'Sullivan is chair of a large Food and Drug Administration registration trial A011801 called T-DM1 and Tucatinib Compared With T-DM1 Alone in Preventing Relapses in People With High Risk HER2-Positive Breast Cancer. This Alliance for Clinical Trials in Oncology-sponsored trial is scheduled to open in November 2020.

Significance to patient care

Dr. O'Sullivan investigates the mechanisms of resistance to currently available breast cancer drugs with a goal to personalize treatments for patients, and develop new and more-effective therapies. Her work also focuses on tailoring treatment options for patients with early-stage breast cancer to maximize efficacy and minimize treatment-related toxicities.

Professional highlights

  • Recipient, Career Development Award, Conquer Cancer Foundation, American Society of Clinical Oncology (ASCO), 2018
  • Recipient, Merit Award, Conquer Cancer Foundation, ASCO, 2014
  • Recipient, Clinical Scholars Award, San Antonio Breast Cancer Symposium, 2013

Clinical Studies

See my clinical studies


See my publications


Primary Appointment

  1. Consultant, Division of Medical Oncology, Department of Oncology

Academic Rank

  1. Assistant Professor of Oncology


  1. MB BCh/BAO University College Dublin
  2. Diploma - Completed Leaving Certificate (580/600 points) The Institute of Education Secondary School

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