On June 9, 2026 Zetagen Therapeutics, a clinical-stage biopharmaceutical company developing novel therapies for primary and metastatic breast cancer, reported preliminary topline results from its Phase 2a clinical study evaluating ZetaMet (Zeta BC 003) in subjects with metastatic breast cancer (MBC) with lytic bone lesions.

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In this open-label study, no skeletal-related events (SREs) or fractures were observed, no treatment-emergent adverse events (AEs) or serious adverse events (SAEs) were reported, and cessation of tumor activity was noted in treated vertebral bodies. Zeta BC 003 is an investigational product that has received Breakthrough Designation from the U.S. Food and Drug Administration (FDA) and has not been approved by the FDA or any regulatory authority.

"We are encouraged by the preliminary findings from this Phase 2a study, which provide important clinical observations on the investigational use of ZetaMet in patients with metastatic breast cancer involving bone," said Joe C. Loy, President & CEO of Zetagen Therapeutics. "These results also highlight a major achievement for our team, overcoming longstanding industry challenges in intratumoral administration by developing proprietary carriers which deliver compounds that demonstrate solubility and localized bio-adhesion."

The preliminary findings were presented by Dr. Bryan Margulies, Chief Scientific Officer, and Joe C. Loy, President & CEO, at the American Society of Clinical Oncology (ASCO) (Free ASCO Whitepaper) Annual Meeting on Monday, June 2, 2026. Abstract: View Source

Study Overview
The open-label Phase 2a study (ZGMBC; NCT05280067), conducted at the University of British Columbia, enrolled 10 subjects with a mean age of 52, representing multiple MBC subtypes:

HR+ (n=6)
HR+/HER2+ (n=2)
HER2+/HR– (n=1)
TNBC (n=1)
Five subjects had breakthrough lytic lesions despite prior bisphosphonate therapy. One subject died due to pleural edema, unrelated to study treatment. Across the 10 subjects, 11 target lesions received a single intratumoral injection of Zeta BC 003 under sedation. Four adjacent, non-injected lesions were also evaluated.

Study Context
Historical literature reports SRE rates of approximately 53% (Parke et al., Journal Oncologist, 2018) in metastatic breast cancer with bone involvement under conventional therapy, with SREs, particularly fractures, associated with a reduction in overall survival of approximately 4.8 months (Saad et al., Journal of the National Cancer Institute, 2004).

Observed Study Findings
No SREs or fractures reported
Cessation of tumor activity within treated vertebral bodies
Therapeutic spread observed to adjacent, untreated lesions within the same vertebral body
No treatment-emergent AEs or SAEs
Mean bone defect volume decreased 65.4% at Day 84 (±20.5%; p=0.0003)
Mean bone defect volume decreased 84.1% at Day 180 (±13.1%; p<0.0001)
Pain scores (NRS) decreased by 4.16 points (p<0.05)
Opioid use (MED) decreased 33–67% among opioid-treated subjects
Spinal stability (SINS) improved 18.5% (p<0.05)
Quality of life improved:
PCS increased 24.2%
MCS increased 12.1%
While cross-study comparisons have inherent limitations, the absence of AEs and SREs in this Phase 2a study provides supportive information for continued investigation.

These observations are also consistent with two published Expanded Access case reports (seven lesions, 2-year follow-up; Palma et al., Pain Management, 2023), which similarly demonstrated absence of SREs, cessation of tumor activity, neo-trabecular bone formation, and therapeutic spread within treated vertebral bodies.

(Press release, Zetagen Therapeutics, JUN 9, 2026, View Source [SID1234666509])