RSO-021 and PRDX3 Inhibition in Pleural Mesothelioma

RSO-021 and PRDX3 Inhibition in Pleural Mesothelioma: 1 Completed Phase 1/2 Trial, 67% Disease Control at 12 Weeks (2026)

RSO-021 Quick Facts
An experimental drug infused directly into the chest for pleural mesothelioma
What It Is	RSO-021, a drug formulation of the natural compound thiostrepton
Target	Mitochondrial peroxiredoxin 3 (PRX3), an antioxidant enzyme tumors rely on
Route	Weekly infusion into the pleural space, not through a vein
Trial	NCT05278975 (MITOPE), phase 1/2, registry status Completed
Reported Result	Disease control in 67% of patients at 12 weeks at the 90 mg dose
Status	Investigational — not approved for mesothelioma

Executive Summary

RSO-021 is an experimental mesothelioma drug that is infused directly into the chest cavity rather than given through a vein. It is a formulation of thiostrepton, a naturally occurring compound that permanently disables peroxiredoxin 3 (PRX3) — an antioxidant enzyme inside mitochondria that cancer cells depend on to survive the oxidative stress their own metabolism creates. Switching that enzyme off is intended to let damaging hydrogen peroxide accumulate to a level the tumor cell cannot tolerate, while normal cells, which generate less of it, are better able to cope. The approach was tested in a phase 1/2 study, NCT05278975, known as MITOPE, in patients with malignant pleural effusion from advanced solid tumors including mesothelioma. In the phase 1 portion, weekly intrapleural RSO-021 at 90 mg was well tolerated in patients with relapsed pleural mesothelioma and malignant pleural effusion, and was associated with disease control in 67% of patients at 12 weeks. The trial's registry record is now listed as Completed. RSO-021 is investigational and is not an approved treatment for mesothelioma.

Why This Matters

Mesothelioma that has returned after first-line treatment has few effective options, and that is precisely the setting this drug was tested in. RSO-021 is also notable for two reasons beyond its results. First, it attacks a target — mitochondrial redox balance — that is different from chemotherapy and different from the immune checkpoints most current mesothelioma research is aimed at. Second, it is delivered locally into the pleural space, the same cavity that has to be drained when fluid builds up around the lung, which means the treatment reaches the tumor with minimal exposure to the rest of the body. For families researching what is genuinely in development, the distinction between an experimental agent with published human trial data and one still confined to the laboratory is the distinction that matters most.

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At a Glance

RSO-021 and PRDX3 inhibition in pleural mesothelioma as of 2026:

• The target — peroxiredoxin 3 (PRX3) is a mitochondrial antioxidant enzyme that manages reactive oxygen species and helps tumor cells survive; cancers upregulate antioxidant networks like this one to counteract oxidative stress.
• The drug — thiostrepton covalently inhibits PRX3, disrupting the tumor cell's redox balance and selectively inducing tumor cell death; RSO-021 is the formulated drug product used in patients.
• Why it is selective — thiostrepton inactivates PRX3 by covalently crosslinking its two catalytic cysteine residues, a specific chemical reaction rather than general toxicity.
• Mesothelioma origins — malignant mesothelioma was the model disease in which combinatorial PRX3 targeting was first shown to be an effective therapeutic approach, in work published in 2015.
• Preclinical evidence — genetic deletion of PRX3 impairs mitochondrial bioenergetics and suppresses mesothelioma growth, and thiostrepton induces apoptosis in patient-derived mesothelioma explants.
• The trial — NCT05278975 (MITOPE) was a phase 1/2 dose-escalation and expansion study in malignant pleural effusion due to advanced or metastatic solid tumors including mesothelioma, sponsored by RS Oncology LLC across ten United Kingdom sites, with 50 patients actually enrolled.
• Human result — weekly intrapleural RSO-021 at 90 mg was well tolerated, met its primary endpoints of safety, tolerability and dose finding, and was associated with disease control in 67% of patients at 12 weeks and with tumor reductions.
• Known resistance mechanism — genomic screening identified solute carrier family 7 member 11 (SLC7A11) as a mediator of thiostrepton resistance, suggesting combination strategies may be needed.
• What is next — a new generation of mechanism-based covalent PRX3 inhibitors has been reported, developed alongside the clinical work in mesothelioma.
• Regulatory status — RSO-021 is investigational. It is not approved by the U.S. Food and Drug Administration for mesothelioma and is not available outside a clinical study.

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Key Facts

Essential RSO-021 Information
Drug	RSO-021, a formulated drug product of thiostrepton (TS)
Molecular Target	Mitochondrial peroxiredoxin 3 (PRX3), also written PRDX3
Mechanism	Covalent inhibition of PRX3, disrupting reactive oxygen species (ROS) balance and inducing tumor cell death
Route of Administration	Weekly local intrapleural infusion into the pleural space
Dose Reported in Patients	90 mg weekly
Trial Identifier	NCT05278975, referred to as MITOPE
Trial Design	Translational phase 1/2 dose-escalation and expansion study
Patient Population	Malignant pleural effusion (MPE) due to advanced or metastatic solid tumors including mesothelioma
Enrollment	50 patients (actual)
Sponsor and Sites	RS Oncology LLC; ten sites in the United Kingdom
Registry Status	Completed; primary completion November 2025, record last updated July 2026
Primary Endpoints	Safety, tolerability and dose finding — reported as met
Reported Efficacy Signal	Disease control in 67% of patients at 12 weeks, with associated tumor reductions
Regulatory Status	Investigational — not FDA-approved for mesothelioma

What Is PRDX3 and Why Would Blocking It Kill Mesothelioma Cells?

Cancer cells run their metabolism hard, and one consequence is that they generate more reactive oxygen species (ROS) — chemically aggressive molecules, hydrogen peroxide chief among them — than normal cells do. Too much of it is lethal to any cell. Tumors survive by counteracting that oxidative stress through upregulation of antioxidant networks. Peroxiredoxin 3 (PRX3), encoded by the PRDX3 gene, is one of those defenses: a mitochondrial antioxidant enzyme that regulates reactive oxygen species homeostasis and promotes tumor cell survival.

That dependence is what makes PRX3 a target. If the enzyme is disabled, the tumor cell loses the mechanism it uses to keep its own hydrogen peroxide under control, and the resulting redox imbalance pushes it past the threshold it can survive. The therapeutic logic is a deliberate inversion of the usual one: rather than protecting cells from oxidative damage, the goal is to let it run.

The evidence that this works in mesothelioma specifically is not new. Malignant mesothelioma cells were the model system in which combinatorial targeting of PRX3 was first characterized as an effective therapeutic approach, in work published in 2015 showing that thiostrepton irreversibly disables PRX3 through covalent crosslinking of the enzyme's peroxidatic and resolving cysteine residues. More recent work has demonstrated that genetic deletion of PRX3 impairs mitochondrial bioenergetics and suppresses mesothelioma growth — separating the target's importance from any particular drug — and that pharmacological inhibition with thiostrepton induces apoptosis in patient-derived mesothelioma explants, tumor tissue taken from actual patients rather than a cell line.

How Does Thiostrepton Work as a Drug?

Thiostrepton (TS) is a natural product. What distinguishes it as a drug candidate is that it is a covalent inhibitor: it does not merely bind PRX3 and let go, it forms a permanent chemical bond. Specifically, it inhibits the peroxidase activity of PRX3 by forming a covalent crosslink between the enzyme's two active-site cysteine residues. Detailed biochemical work has established why the reaction is specific to this enzyme, identifying the cellular and biochemical features of human mitochondrial PRX3 that form the molecular basis for its particular reaction with thiostrepton.

Specificity matters for a drug whose entire mechanism is causing oxidative damage. A compound that raised reactive oxygen species indiscriminately would be difficult to give safely. A compound that reacts with one enzyme in one compartment of the cell, in tumors that depend on that enzyme more than normal tissue does, is a more targeted proposition — and the trial's stated tolerability at the tested dose is consistent with that.

RSO-021 is the formulated drug product of thiostrepton that was administered to patients.

How Is RSO-021 Delivered Into the Pleural Space?

RSO-021 is given as a weekly local intrapleural treatment — infused directly into the pleural cavity, the thin space between the lung and the chest wall where pleural mesothelioma grows and where malignant pleural effusion collects. It is not given intravenously.

The route follows from the patient population. The trial enrolled patients with malignant pleural effusion (MPE) — the fluid buildup around the lung that is common in advanced pleural mesothelioma and that frequently requires drainage for symptom relief. Those patients already have, or need, access to the pleural space for clinical reasons. That access doubles as a delivery route, allowing the drug to be placed where the tumor is.

This places RSO-021 in the same broad category as other locoregional strategies under study in mesothelioma, including intrapleural immunotherapy approaches such as oncolytic virotherapy and intrapleural photodynamic therapy. The shared premise is that the pleural cavity is an accessible, high-value delivery target. What separates RSO-021 is the mechanism: it is a small-molecule agent aimed at the tumor cell's mitochondrial redox machinery rather than an agent aimed at the immune system.

What Did the MITOPE Trial Find?

NCT05278975, referred to as MITOPE, was registered as a translational phase 1/2 dose-escalation and expansion study to determine the safety, tolerability and recommended phase 2 dose of RSO-021 in patients with malignant pleural effusion due to advanced or metastatic solid tumors including mesothelioma. It was sponsored by RS Oncology LLC and conducted across ten sites in the United Kingdom, with an actual enrollment of 50 patients. The registry lists the study as Completed, with a primary completion date of November 2025 and a record last updated in July 2026.

The phase 1 results were published in Nature Communications in July 2026. In patients with relapsed pleural mesothelioma and malignant pleural effusion, weekly local intrapleural treatment with RSO-021 at 90 mg was well tolerated, leading to disease control in 67% of patients at 12 weeks and associated with tumor reductions. The primary endpoints of safety, tolerability and dose finding were met, and secondary endpoints — pharmacokinetics (PK), objective response rate (ORR), disease control rate (DCR), and progression-free survival (PFS) — were explored.

Two cautions belong alongside those numbers. A phase 1 study is designed principally to establish safety and dose, not to prove that a drug extends life; efficacy measures at this stage are exploratory signals, not conclusions. And a disease control rate describes the proportion of patients whose disease did not progress over a defined window — here 12 weeks — which is a different and more modest claim than tumor shrinkage or survival benefit.

Is There a Known Way Mesothelioma Resists This Drug?

Yes, and it was identified in the same body of work. Genomic screening identified solute carrier family 7 member 11 (SLC7A11) as a mediator of thiostrepton resistance, and the investigators suggest that combined targeting may further enhance the pro-oxidant activity of RSO-021.

This is a meaningful finding rather than a footnote. SLC7A11 is a transporter that supports a cell's antioxidant capacity, so a tumor that increases it has an alternative route to managing the oxidative stress RSO-021 is trying to impose. Identifying that escape route during the first human study, rather than after a later trial fails, points toward rational combination strategies as the next step.

Separately, medicinal chemistry work published in 2025 reported the identification of mechanism-based peroxiredoxin 3 inhibitors exploiting the same covalent warhead chemistry, developed explicitly alongside the mesothelioma clinical program using direct pleural infusion. The PRX3 target, in other words, is being pursued beyond thiostrepton itself.

What Does This Mean for Patients and Families?

RSO-021 is a research drug. It is not approved by the U.S. Food and Drug Administration for mesothelioma, it is not something an oncologist can prescribe, and the study that generated the results above is listed as complete rather than open. Anyone encountering this drug in a news article or a search result should understand it as evidence that a new mechanism is being pursued in mesothelioma, not as a treatment currently available.

For patients considering experimental options generally, the useful questions for a care team are whether any trial is open and appropriate for the specific disease situation, what the realistic goal of a phase 1 or phase 2 study is, and how participation would interact with standard treatment already received or planned. The broader landscape of studies is covered on the mesothelioma clinical trials page, and related locoregional approaches on the intrapleural immunotherapy page.

Paying for Mesothelioma Care

Trial participation does not remove the cost of mesothelioma care, and families generally document treatment and travel expenses as they go. Compensation for those costs is often available through asbestos trust funds, U.S. Department of Veterans Affairs benefits, and legal claims against the companies responsible for the asbestos exposure that caused the disease. Which of those channels applies depends on the specific exposure history — where and when it occurred, and whether military service was involved. Firms that concentrate in asbestos litigation, such as Danziger & De Llano, assess exposure histories to determine which channels are available in a given case.

Because settlement and trust recoveries frequently fund the treatment and travel that experimental care requires, the scale of those recoveries is relevant context. Reported mesothelioma settlement and verdict outcomes vary widely with exposure evidence and jurisdiction; the settlement process is documented separately on this wiki.

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Frequently Asked Questions

What is RSO-021 for mesothelioma?

RSO-021 is an experimental drug formulation of thiostrepton that is infused weekly directly into the pleural space rather than given through a vein. It works by covalently inhibiting peroxiredoxin 3, a mitochondrial antioxidant enzyme mesothelioma cells rely on, which disrupts the tumor cell's redox balance and induces cell death. It is investigational and not approved for mesothelioma.

What were the results of the MITOPE trial?

In the phase 1 portion, weekly intrapleural RSO-021 at 90 mg was well tolerated in patients with relapsed pleural mesothelioma and malignant pleural effusion, leading to disease control in 67% of patients at 12 weeks and associated with tumor reductions. Primary endpoints of safety, tolerability and dose finding were met. The registry lists NCT05278975 as Completed.

Can I get RSO-021 as a mesothelioma treatment today?

No. RSO-021 is investigational and not approved by the U.S. Food and Drug Administration for mesothelioma, so it cannot be prescribed as standard care. The phase 1/2 study that tested it, NCT05278975, is listed in the registry as Completed rather than recruiting. Patients interested in experimental options should ask their oncologist what studies are currently open to them.

Are there other experimental drugs delivered into the pleural space?

Yes. Locoregional delivery into the pleural cavity is an active research direction in pleural mesothelioma. Other approaches under study include oncolytic virotherapy and intrapleural photodynamic therapy combined with checkpoint immunotherapy, covered on the intrapleural immunotherapy page. All are investigational, and none is an approved standard treatment for mesothelioma.

Why does mesothelioma resist thiostrepton in some cases?

Genomic screening identified solute carrier family 7 member 11, or SLC7A11, as a mediator of thiostrepton resistance. SLC7A11 supports a cell's antioxidant capacity, giving the tumor an alternative way to manage the oxidative stress the drug is meant to impose. The investigators suggest combined targeting may further enhance the drug's pro-oxidant activity.

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External Links

• ClinicalTrials.gov: NCT05278975 — the registry record for the MITOPE phase 1/2 study of RSO-021.
• PubMed: PMID 42448681 — the Nature Communications report of the preclinical characterization and phase 1 clinical testing.
• Danziger & De Llano: national mesothelioma and asbestos law firm; telephone (855) 699-5441.

See Also

• Intrapleural Immunotherapy for Mesothelioma
• Mesothelioma Clinical Trials
• Gene Therapy for Mesothelioma
• Pleural Mesothelioma
• Mesothelioma Treatment
• Mesothelioma Settlements