Phase 1/2 dose expansion study evaluating first-in-class eIF4A inhibitor zotatifin in patients with ER+ metastatic breast cancer.

1080 Background: Zotatifin (eFT226) is a first-in-class, potent and sequence selective inhibitor of RNA helicase eIF4A that promotes a stable mRNA:eIF4A:drug ternary complex at specific polypurine motifs within the 5’-UTR, preventing translation of select transcripts. In preclinical models zotatifin treatment simultaneously down-regulated translation of numerous oncogenes including CDK4, ERα, ERBB2, KRAS, and CCND1. Methods: Following a 3+3 dose escalation portion of the protocol (Part 1), Part 2 is a Simon’s two-stage design of cohorts of up to 18 patients (pts) with ER+ metastatic breast cancer enrolled at the recommended phase 2 dose of zotatifin (0.07 mg/kg IV two weeks on/one week off) in combination with fulvestrant (ful) (ECBF: Expansion, Combination, Breast, Ful) or in combination with ful and abemaciclib (abema) (ECBF+A: Expansion, Combination, Breast, Ful + Abema). Key eligibility criteria included at least one line of therapy for advanced/metastatic disease, progression on hormone therapy, and in addition, the ECBF cohort required prior CDK 4/6 inhibitor (CDKI). Primary endpoint was objective response rate (ORR) per RECIST v1.1. Additional endpoints included safety, other efficacy analyses, and for Parts 1 and 2, characterization of pharmacodynamic (PD) markers and pharmacokinetics. Results: As of a data cut-off of Jan 31, 2023, 18 pts were enrolled in each cohort. Median prior metastatic lines of therapy were 3 and 7 in ECBF+A and ECBF, respectively. In 16 evaluable pts in ECBF+A cohort, there were two confirmed partial responses (PR), two additional PRs awaiting confirmatory scans, and 8 pts with best overall response (OR) of stable disease (SD). All pts with PR/unconfirmed PR received prior CDKI and ful, including one pt with disease progression on combination abema and ful as the last therapy prior to enrollment. In 18 evaluable pts in ECBF, there was one confirmed PR and six SD as best OR. There were no dose-limiting toxicities or grade (Gr) 5 adverse events (AEs) in either cohort. In ECBF+A the most common AEs (all Gr 1/2) were diarrhea (44%) and nausea (31%) and 21% of pts had Gr 3/4 AEs. The most common AEs in ECBF (all Gr 1/2) were nausea (39%) and constipation (27%), and 28% of pts had Gr 3/4 AEs. Blood-based PD markers and ctDNA in Part 1 showed dose-dependent evidence of target engagement and anti-tumor activity. Conclusions: In dose expansion cohorts of heavily pre-treated metastatic breast cancer pts, eIF4A inhibitor zotatifin showed evidence of anti-tumor activity in combination with fulvestrant and abemaciclib and had a favorable safety profile consisting of primarily grade 1/2 adverse events. Clinical trial information: NCT04092673 .