eFFECTOR Therapeutics has reported positive interim data updates from a Phase II expansion cohort of zotatifin along with fulvestrant and abemaciclib (ZFA triplet) for the treatment of ER+ metastatic breast cancer (mBC).

The expansion cohort enrolled 20 patients who received the ZFA triplet with zotatifin 0.07mg/kg on days 1 and 8 of 21-day cycles.

Out of 19 patients, five RECIST-evaluable patients achieved a partial response including four confirmed and one unconfirmed.

The ZFA triplet was found to be well tolerated and three patients discontinued due to adverse events of any cause. The majority of AEs were Grade 1 or 2.

eFFECTOR Therapeutics president and CEO Steve Worland said: “The activity observed with the ZFA triplet, with partial responses seen in 26% of patients, is substantially higher than we would expect for treating such heavily pretreated patients with just fulvestrant and abemaciclib.

“We believe this activity level reflects in part zotatifin’s mechanism of down-regulating cyclins D and E, and the synergy this provides with CDK4/6 inhibition.

“The partial response observed at a higher dose of zotatifin in combination with only fulvestrant is encouraging and we look forward to reporting data from the completed dose escalation in the second half of 2023.”

Partial response was also observed in one of three patients who received zotatifin 0.1mg/kg and fulvestrant (ZF doublet).