Zymeworks is pleased to announce, along with our partners Jazz Pharmaceuticals, updated data from the Phase 2a trial of investigational zanidatamab, a HER2-targeted bispecific antibody, in combination with palbociclib, a CDK4/6 inhibitor, and fulvestrant, a selective estrogen receptor antagonist, in patients with HER2-positive (HER2+)/HR-positive (HR+) metastatic breast cancer (mBC) as part of a late-breaking oral presentation at the 2023 San Antonio Breast Cancer Symposium (SABCS).

Data from 51 patients with heavily pretreated HER2+/HR+ mBC (median of 4 prior regimens in the metastatic setting) who were treated with zanidatamab plus palbociclib and fulvestrant demonstrated a progression-free survival at six months (PFS6) of 67% (n=34) [95% confidence interval: 52, 79]. Secondary endpoint findings included a median progression-free survival (mPFS) of 12 months [95% CI: 8, 15] and a confirmed objective response rate (cORR) of 35% [95% CI: 21, 50] with a median duration of response (DOR) of 15 months. The combination regimen was well tolerated with a manageable safety profile.

Trial Results

Results of the Phase 2a trial (NCT04224272) presented at SABCS indicate that zanidatamab in combination with palbociclib and fulvestrant, demonstrated meaningful PFS outcomes with a well tolerated safety profile in patients with heavily pretreated HER2+/HR+ mBC.

The single-arm trial evaluated zanidatamab plus palbociclib and fulvestrant in 51 patients with HER2+/HR+ unresectable, locally advanced or metastatic breast cancer who had received prior treatment with at least trastuzumab, pertuzumab, and T-DM1, and no prior treatment with a CDK4/6 inhibitor. Patients treated with the combination regimen received a median of four prior systemic regimens in the metastatic setting (range, 1-12).

Recommended doses of the zanidatamab plus palbociclib and fulvestrant combination therapy were determined in Part 1 of the study. The primary endpoint of Part 2 was PFS6. Other endpoints included mPFS, cORR per RECIST v1.1, DCR and DOR.

At the time of data cutoff (August 3, 2023), treatment with zanidatamab in combination with palbociclib and fulvestrant resulted in a PFS6 of 67% (n=34) and mPFS of 12 months [95% CI: 8, 15]. Median duration of follow-up was 16 months (range, 2-32). Patients treated with the combination regimen achieved a cORR of 35% and DCR of 91%.

Zanidatamab plus palbociclib and fulvestrant was well tolerated with a manageable safety profile. One serious treatment-related AE (transaminases increased) was reported (which resolved). No treatment-related deaths were reported. The most common treatment-related AEs (>20% of patients) were diarrhea, neutrophil count decrease/neutropenia, nausea, stomatitis, anemia, vomiting and asthenia. One patient discontinued the combination treatment due to an AE; three patients discontinued palbociclib due to an AE.

The abstract is available to conference registrants on the SABCS conference website here. (Abstract Number LBO1-04).

Additional data being presented at SABCS for zanidatamab include a spotlight poster presentation highlighting positive results of an investigator-sponsored Phase 1 trial evaluating neoadjuvant single-agent zanidatamab in patients with stage 1 node-negative HER2+ breast cancer (Abstract Number PS09-03).