Thiostrepton

Summary

Thiostrepton is a thiopeptide antibiotic approved for veterinary topical use against Gram‑positive bacteria. Research has explored its activity against malaria parasites, bacterial ribosomes, and, more recently, as a cancer‑targeting agent that inhibits the transcription factor FoxM1 and the mitochondrial antioxidant enzyme PRX3. In a phase‑1 study, intrapleural thiostrepton achieved disease control in two‑thirds of participants at 12 weeks.

Mechanism of Action

Thiostrepton binds to the L11 protein–rRNA complex of the bacterial 70S ribosome, preventing the association of GTP‑binding elongation factors and halting protein synthesis. In eukaryotic cells it forms covalent adducts with the mitochondrial peroxiredoxin‑3 (PRX3) enzyme, disrupting redox homeostasis, and cross‑links the oncogenic transcription factor FoxM1, leading to its functional inhibition and proteasome‑mediated degradation. These interfacial interactions collectively trigger oxidative stress‑induced apoptosis in cancer cells.

What the Research Shows

The peptide is a potent inhibitor of bacterial protein translation and has been used as a veterinary antibiotic. In vitro studies show it can disrupt protein synthesis in the apicoplast of Plasmodium falciparum, suggesting antimalarial potential, although no animal or clinical malaria data are reported. Pre‑clinical cancer work demonstrates that thiostrepton suppresses FoxM1 activity, inhibits PRX3, and induces apoptosis across a range of tumor cell lines and mouse xenograft models. A phase‑1 trial (NCT05278975) evaluated a locally administered intrapleural formulation (RSO‑021) in patients with relapsed mesothelioma, reporting tolerability and disease control in 67% of participants at 12 weeks, with ongoing pharmacokinetic and efficacy assessments.

Reported Benefits

Evidence from bacterial studies confirms strong antibacterial activity, especially against Gram‑positive organisms. Laboratory and animal cancer models indicate that thiostrepton can down‑regulate FoxM1, impair mitochondrial redox balance via PRX3 inhibition, and reduce tumor growth. Early human data from a phase‑1 intrapleural trial suggest a favorable safety profile and disease‑control rates of two‑thirds at 12 weeks, providing preliminary support for therapeutic benefit in malignant pleural disease.

Limitations of the Evidence

Clinical experience is limited to a small phase‑1 cohort; efficacy and long‑term safety remain unproven. Systemic formulation is challenging due to poor solubility, and most pre‑clinical work relies on local or nanoparticle delivery. Resistance mechanisms, such as up‑regulation of SLC7A11, have been identified, and the antimalarial activity is only demonstrated in vitro without in vivo validation. Consequently, broader therapeutic conclusions are premature.

Safety Considerations

The phase‑1 intrapleural study reported that the thiostrepton formulation was well tolerated, with no major adverse events noted in the short‑term follow‑up. However, comprehensive safety data for systemic exposure are lacking, and the compound’s known poor pharmacokinetic properties raise concerns about off‑target toxicity. Formulation difficulties and potential immunogenicity of micelle‑based carriers also warrant caution pending further investigation.

How It Is Administered

Thiostrepton is officially approved for topical veterinary use. Experimental human applications have employed a locally delivered intrapleural injection of the RSO‑021 formulation at 90 mg weekly. Pre‑clinical studies have also used micelle‑nanoparticle encapsulation for intravenous delivery in animal tumor models. Formulation remains a key barrier to broader clinical use.

Routes of Administration

Goals & Uses

• Anti‑cancer researchOncologyModerate
• Anti-protozoal activity (malaria)Antiparasitic / ResearchLow
• Antibacterial activityInfectious DiseaseHigh
• Treatment of bacterial skin and ear infections (veterinary)Antimicrobial / VeterinaryHigh
• Anticancer activity via FOXM1 inhibitionOncology / ResearchLow

Contraindications

• Application to perforated tympanic membraneAnatomicalModerate
• Known hypersensitivity to thiostrepton or thiopeptide antibioticsAllergyHigh

Adverse Effects

• Local skin irritationDermatologicalUncommon
• Hypersensitivity reactionImmunologicRare

Drug Interactions

• Other aminoglycoside or ototoxic agents (otic use)Moderate

Population Constraints

• Humans (systemic use)Species/RouteAbsolute
• Pregnant animalsReproductiveRelative

Regulatory Status

• European UnionUnapprovedNot authorized for clinical use
• United StatesUnapprovedInvestigational only, no FDA approval
• United KingdomUnapprovedNo marketing authorization

Not approved for human therapy; limited to in‑vitro and animal research. No marketed formulation.

Evidence & Sources

• Antimalarial peptides: the long and the short of it
  Journal ArticleModerateBell A2011-01-01T00:00:00.000000Z
• Preclinical characterization and phase 1 clinical testing of targeting mitochondrial peroxiredoxin 3 in cancer
  Journal ArticleModerateGibson V, et al.2026-01-01T00:00:00.000000Z
• Interfacial inhibitors of protein-nucleic acid interactions
  Journal ArticleModeratePommier Y, Marchand C2005-01-01T00:00:00.000000Z
• The bacterial thiopeptide thiostrepton. An update of its mode of action, pharmacological properties and applications
  Journal ArticleModerateBailly C2022-01-01T00:00:00.000000Z
• Suppression of the Oncogenic Transcription Factor FOXM1 by Proteasome Inhibitors
  Journal ArticleModerateGartel AL2014-01-01T00:00:00.000000Z
• A new target for proteasome inhibitors: FoxM1
  Journal ArticleModerateGartel AL2010-01-01T00:00:00.000000Z

Frequently Asked Questions