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Heart & blood

Balixafortide

EvidenceLow research

ClassCXCR4 antagonist peptide

Balixafortide is an investigational CXCR4-blocking peptide studied with chemotherapy for breast cancer. See how it works and why Phase 3 showed no benefit.

Reviewed by: William Maish, MD MBA MPH, Clinical Product Lead
Published: October 5, 2026
Last updated: October 5, 2026

Key takeaway

Balixafortide is an investigational peptide that blocks the CXCR4 receptor, studied by Polyphor in combination with chemotherapy for metastatic breast cancer. An early single-arm trial suggested activity, but the pivotal Phase 3 trial did not show a benefit over chemotherapy alone, and development has effectively stopped. As of 2026 it is not approved.

At a glance

FDA status	Investigational peptide, not approved by the FDA
What it is	CXCR4 antagonist studied with the chemotherapy eribulin in metastatic breast cancer
Research	A Phase 1 trial suggested activity, but the Phase 3 trial (FORTRESS) failed to show benefit
Development	Effectively ceased
Evidence strength	Low research

What is balixafortide?

Balixafortide is an experimental peptide that blocks CXCR4, a receptor involved in how cells move through the body and interact with their surroundings. It was developed by Polyphor (later Spexis) and studied intravenously together with the chemotherapy drug eribulin for HER2-negative metastatic breast cancer. It is investigational and not approved, and its development has effectively ended. It is also known as POL6326.

How does it work?

Balixafortide blocks the CXCR4 receptor¹ and interrupts its signaling with a partner molecule called CXCL12 (SDF-1α). This pathway influences the trafficking of tumor cells, the tumor's surrounding environment, and immune cell behavior. In laboratory work, blocking CXCR4 was proposed to make tumors more responsive to chemotherapy, which is the rationale for combining it with eribulin. This describes the proposed mechanism, separate from the patient outcomes below.

Is it FDA-approved?

No. Balixafortide is investigational and not approved, and the program is effectively discontinued after its pivotal trial.

What does the research show?

The decisive evidence is negative. Two trials tested balixafortide plus eribulin in metastatic breast cancer:

Trial	Design	Result
Phase 1 trial¹, according to PubMed	Early single-arm study in heavily pretreated metastatic breast cancer	Partial responses in about 30% of evaluable patients, encouraging for an uncontrolled study
Phase 3 FORTRESS trial², reported by the company in 2021	Randomized, compared with eribulin alone	Did not meet its primary endpoint; response and clinical-benefit rates were similar to eribulin alone, showing no meaningful improvement from adding balixafortide

The controlled evidence does not support a benefit.

More for heart & blood

Sources: the Phase 1 dose-escalation trial (Pernas et al., The Lancet Oncology, 2018, DOI: 10.1016/S1470-2045(18)30147-5¹), indexed on PubMed; and company reporting of the Phase 3 FORTRESS result. This document is educational, is not medical advice or a health claim, and is not an offer to sell any product.