Susan O'Brien

Susan O'Brien, MD is Associate Director for Clinical Science at the Chao Family Comprehensive Cancer Center at the University of California Irvine and Professor of Medicine at UC Irvine School of Medicine. Previously a longstanding faculty member at MD Anderson Cancer Center's Department of Leukemia, Dr. O'Brien has over three decades of experience as one of the most prominent CLL clinical trialists in the United States. She was the overall lead principal investigator of the pivotal PCYC-1102 phase Ib/II trial of ibrutinib in CLL (JCO 2013), one of the foundational studies demonstrating ibrutinib's dramatic efficacy and tolerability across multiple CLL patient populations — including treatment-naive, relapsed/refractory, and high-risk del(17p) patients — that provided the critical evidence base supporting the FDA's breakthrough designation and accelerated approval of ibrutinib for CLL in 2014. Her work on PCYC-1102 established the activity, dosing, and toxicity profile of ibrutinib and reported the characteristic ibrutinib-induced lymphocytosis that distinguished BTK inhibitor responses from traditional response criteria. Dr. O'Brien has also been a leading investigator in CLL chemoimmunotherapy trials, contributed to the development of ofatumumab in refractory CLL (phase III COMPLEMENT trials), and has extensively studied the cardiovascular toxicities — particularly atrial fibrillation — associated with BTK inhibitors, influencing clinical management guidelines. With more than 400 publications, she is one of the most published CLL investigators globally.