Also known as: POL6326 · POL-6326
Balixafortide (POL6326) is a 16-residue cyclic peptide that blocks the CXCR4 receptor, developed by Polyphor (later Spexis). It mobilized stem cells in early trials, but its only Phase 3 trial, in metastatic breast cancer, failed in 2021, and no active development has been reported since.
Balixafortide was built on Polyphor's protein epitope mimetic platform, a beta-hairpin cyclic peptide shaped to block CXCR4. It was studied in two directions: mobilizing blood stem cells, and making breast cancer cells more sensitive to chemotherapy.
In a healthy-volunteer dose-escalation study (Karpova et al., 2017), a 1–2 hour infusion of 500–2,500 µg/kg mobilized stem cells at every dose, plateauing at 1,500–2,500 µg/kg, at levels predicted to yield a standard transplant dose from one apheresis. Volunteers found it easier to tolerate than G-CSF. Exploratory Phase 2a work in stem cell mobilization followed.
The pivotal Phase 3 FORTRESS trial (432 patients, HER2-negative metastatic breast cancer) tested balixafortide plus eribulin against eribulin alone and missed its response-rate endpoint in June 2021. The trial was terminated and a follow-on study was withdrawn. Spexis reported a positive renal-impairment safety study in 2022, but no new trials have been reported since. For an approved drug with the same mechanism, see Motixafortide.
Balixafortide binds CXCR4 with high potency (IC₅₀ under 10 nM in β-arrestin and calcium-flux assays), blocking the CXCL12 signal that keeps stem cells, and some tumor cells, anchored in the bone marrow.
Its backbone-cyclized, disulfide-bridged hairpin with two D-prolines holds the binding loop rigid, giving receptor selectivity and protease stability that linear peptides lack.
The cancer rationale was that pulling tumor cells out of the protective marrow niche would expose them to chemotherapy. The Phase 3 failure means this did not translate into better responses in metastatic breast cancer.
Balixafortide has never been approved. Its pivotal Phase 3 trial in breast cancer failed and was terminated, and no new trials have been reported since 2022. Doses listed come from published trials and are not guidance for use. Material sold under this name is research-grade, and supplier molecular weights for its salt forms are inconsistent.
Trials used single infusions (stem cell mobilization) or regimens alongside chemotherapy (breast cancer). No cycling protocol exists.
Development stalled after the 2021 Phase 3 failure. Not an approved or available therapy.
Not approved anywhere. Highest phase reached was Phase 3 (FORTRESS, NCT03786094), which failed and was terminated. Some supplier and aggregator pages still describe it as "in Phase 3", which is out of date. Product sold as "balixafortide" is research-grade material only; reported molecular weights for salt forms (TFA, acetate) vary between suppliers.
Ask anything about Balixafortide — mechanisms, dosing protocols, interactions, or research comparisons.
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