On May 25, 2023, eFFECTOR Therapeutics, Inc. (the “Company”) announced positive interim data updates from a Phase 2 expansion cohort evaluating zotatifin combined with fulvestrant and abemaciclib (“ZFA triplet”) in patients with ER+ metastatic breast cancer (“mBC”). These data, as well as initial data from further dose escalation, will be presented as part of a poster presentation at the American Society of Clinical Oncology Annual Meeting in Chicago, Illinois.
New interim data were presented on the fully enrolled expansion cohort of patients (n=20) who received the ZFA triplet with zotatifin dosed at 0.07 mg/kg on Days 1 and 8 of 21-day cycles. Patients were heavily pre-treated, having received a median of four prior lines of therapy for metastatic disease. Five out of 19 (26%) RECIST-evaluable patients achieved a partial response (“PR”), including four confirmed and one unconfirmed. All five patients who achieved a PR had previously progressed on prior CDK4/6 and fulvestrant treatments, and all 5 had received one or more prior lines of chemotherapy. The disease control rate (“DCR”), reflecting patients with at least one on-treatment scan showing PR or stable disease (“SD”), was 14 of 19 (74%). PRs were seen in patients with and without mutations in the ESR1 and PIK3CA genes. As of the data cutoff date, 4 patients remained on therapy and progression free survival (“PFS”) as well as clinical benefit rate (“CBR”) data are not yet mature. The ZFA triplet was generally well tolerated, with 3 patients discontinuing due to adverse events (“AEs”) of any cause, and the large majority of AEs being Grade 1 or 2. The most frequent Grade 3 AEs were diarrhea in 3 of 20 (15%) patients, similar to the 13% frequency reported in the registrational Monarch 2 trial for abemaciclib and fulvestrant, and dyspnea in 2 of 20 (10%) patients.
New data were also reported for patients in resumed dose escalation cohorts utilizing zotatifin combined with fulvestrant (“ZF doublet”). In the one fully enrolled cohort, wherein patients received zotatifin dosed at 0.1 mg/kg every other week (“Q2W”) combined with fulvestrant, 1 of 3 (33%) patients achieved a PR on the first on-treatment scan, which was confirmed on the second on-treatment scan that occurred after the data cutoff, and remained on study. This patient had received four lines of prior therapy for metastatic disease, with progressive disease (“PD”) being the best overall response (“BOR”) to all four prior lines. Dose escalation is continuing at 0.14 mg/kg Q2W and 0.07 mg/kg once weekly (“QW”). Zotatifin has been generally well tolerated in dose escalation cohorts, with no dose limiting toxicities or serious adverse events observed.
Data for the ZF doublet with zotatifin at 0.07 mg/kg dosed on Days 1 and 8 of 21-day cycles, originally reported on January 5, 2023, were also updated. The patient with a confirmed PR showed a duration of response (“DOR”) of approximately 13 months. The patient with prolonged SD remained on study at 12 months. The Company expects to report topline data from completed dose escalation cohorts evaluating ZF in the second half of 2023.
Data on changes in circulating tumor DNA (“ctDNA”) were also updated from January 5, 2023, when the Company reported a dose-dependent decrease in ctDNA in patients receiving zotatifin-based regimens. In the new data set, 9 patients who received the ZFA triplet at 0.07 mg/kg zotatifin had both pre- and on-treatment samples available for ctDNA analysis. Four additional patients came off study prior to the protocol-specified time to collect on-treatment ctDNA (Day 43). Eight patients had decreases in ctDNA of greater than 50%, representing 89% of the patients with on-treatment samples available for analysis, and 62% of patients who either had available on-treatment samples or who had discontinued prior to collection of on-treatment samples. The specific alleles observed to be decreased or eliminated in on-treatment ctDNA, including mutations in ESR1 and PIK3CA genes, reflect common mechanisms of resistance to endocrine therapy in ER+ BC and are consistent with zotatifin’s mechanism of action maintaining sensitivity after resistance to endocrine therapy has emerged. The patient who achieved a confirmed PR in the ZF dose escalation group also eliminated detectable ctDNA at Day 29, including mutant alleles of ESR1 and ERBB2 genes, after having received only two doses of zotatifin.
All data was presented as of a data cut-off date of May 3, 2023.
