A first in human study of the new oral selective estrogen receptor degrader AZD9496 for HR+/HER2- advanced breast cancer

Purpose: AZD9496 is an oral non-steroidal, small-molecule inhibitor of estrogen receptor alpha (ERα), and a potent and selective antagonist and degrader of ERα. This first in human Phase 1 study determined the safety and tolerability of ascending doses of oral AZD9496 in women with estrogen receptor (ER)+/HER2- advanced breast cancer, characterized its pharmacokinetic (PK) profile, and made preliminary assessment of antitumor activity. Experimental Design: Forty-five patients received AZD9496 (20 mg once daily to 600 mg twice daily) in a dose-escalation, dose- expansion 'rolling 6' design. Safety, tolerability, and PK activity in each cohort was reviewed before escalating to the next dose. PK was determined by mass spectrometry. Adverse events (AEs) were graded according to the Common Terminology Criteria for Adverse Events (CTCAE) v4.0. Objective tumor response was evaluated by Response Evaluation Criteria in Solid Tumors (RECIST) v1.1. Results: Most common causally related AEs were diarrhea (35.6%), fatigue (31.1%), and nausea (22.2%), and seven patients had grade ≥3 AEs. Three patients experienced a dose-limiting toxicity (DLT): one each at 150 mg BID (abnormal hepatic function), 400 mg BID (diarrhea and elevated liver function tests) and 600 mg BID (diarrhea), and all were reversible. The maximum tolerated dose was not reached. Partial response was confirmed in one patient, who also had decreased tumor marker Ca15.3. Four patients had stable disease at 12 months' follow up. Conclusions: AZD9496 is well tolerated with an acceptable safety profile, showing evidence of prolonged disease stabilization in heavily pre-treated patients with ER+/HER2- advanced breast cancer.

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