An experimental drug called RSO-021 kept pleural mesothelioma from growing in two out of three evaluable patients after 12 weeks of treatment in an early clinical trial. The drug works by shutting down an enzyme that mesothelioma cells rely on to survive their own internal stress. It is not approved for general use and is available only through clinical research.
The results, published in July 2026 by researchers at the University of Vermont and the University of Leicester, come from patients whose mesothelioma had already returned after standard treatment. That is the group with the fewest options today, which is why an early signal in these patients draws attention even from a small study.
What is RSO-021 and how does it work?
Every cell produces unstable, oxygen-based molecules as a byproduct of turning fuel into energy. In normal amounts they are harmless. In excess they damage the cell from the inside. Tumor cells run their metabolism at a much higher rate than healthy tissue, so they generate far more of these molecules and live under constant internal strain.
To survive that strain, mesothelioma cells lean heavily on protective enzymes that neutralize the damaging molecules before they build up. One of the most important is peroxiredoxin 3, usually shortened to PRX3. It works inside the mitochondria, the part of the cell that produces most of its energy.
RSO-021 is a clinical formulation of thiostrepton, a naturally occurring antibiotic that binds to PRX3 and disables it. With that enzyme out of action, hydrogen peroxide accumulates inside the tumor cell’s mitochondria until the damage becomes more than the cell can withstand, and it dies.
This reverses an older line of thinking. For years, researchers tested whether giving cancer patients extra antioxidants could slow tumors by lowering those damaging molecules. Most of those trials failed, and some evidence suggested added antioxidants may even help tumors grow. The Vermont team asked the opposite question: what happens if a tumor loses one of the defenses it depends on most?
Why would this approach affect cancer cells more than healthy cells?
Because mesothelioma cells already operate close to their limit. They produce more of the damaging molecules than normal cells do, and PRX3 turns over faster in tumor tissue. Removing that protection pushes cancer cells past the breaking point while healthy cells, which carry a much lighter load, have more room to absorb the change.
Laboratory work supported the idea. When researchers removed the PRX3 gene entirely from mesothelioma cells, the cells grew far more slowly, their energy production dropped, and they failed to form tumors when implanted in mice. Separate research has shown that mice born without PRX3 develop and function largely normally. That finding matters because mitochondria are essential to nearly every cell in the body, and some scientists had doubted they could be targeted safely.
How is the drug given?
RSO-021 is delivered directly into the chest rather than through a vein. Most people with pleural mesothelioma develop a pleural effusion, a buildup of fluid between the lung and the chest wall, and many already have a small drainage tube called an indwelling pleural catheter to manage it. In the trial, the drug was given once a week through that same catheter after the fluid was drained.
Delivering the drug this way concentrates it where the tumor is. Blood levels measured after each dose were extremely low, which helps explain why side effects stayed limited.
What did the clinical trial show?
The phase 1 trial was run in the United Kingdom between 2022 and 2023 under the oversight of the Medicines and Healthcare products Regulatory Agency, the British counterpart to the FDA. Fifteen patients received the drug. Twelve had pleural mesothelioma, all of the epithelioid type, and all had seen their disease progress after earlier treatment. The other three had lung or colorectal cancer that had spread to the chest lining.
The main purpose of a phase 1 trial is to find a safe dose. At 90 milligrams a week, no patient had a dose-limiting reaction, and that became the recommended dose. Higher doses caused serious breathing problems and inflammation in some patients. At the recommended dose, the most common side effects were fatigue and fever, and nearly all side effects were mild to moderate. No deaths were attributed to the drug.
Researchers also looked for early signs that the drug was working. At 12 weeks, the disease was controlled in six of nine patients whose results could be measured, or 67 percent. One patient’s tumor shrank substantially, and that response held until week 30. Tissue samples confirmed the drug was binding to PRX3 in human tumors as it had in the lab.
The median time before the cancer began growing again was about 18 weeks, roughly four months, which is in line with other treatments used for relapsed disease. The more encouraging figure was overall survival. When the data were reviewed about 80 weeks into the study, 7 of the 12 mesothelioma patients were still alive, which means the point at which half the group had died had not yet been reached.
How should patients and families read these results?
With cautious interest. This was a small study built primarily to test safety, and it had no comparison group. That means the survival figures cannot be directly measured against standard treatment, even though they look favorable next to what is typically seen in relapsed pleural mesothelioma. Results from a dozen patients can shift considerably as more people are treated.
A larger phase 2 portion of the same trial has now been completed. It tested RSO-021 on its own and in combination with the chemotherapy drug paclitaxel, and it included some patients who had not yet started standard treatment. The University of Vermont has said those results are expected at a major oncology meeting later this year. Those findings will say much more than the phase 1 data can.
Research is also moving in other directions. Scientists are developing second-generation PRX3 inhibitors that dissolve more easily and could eventually be taken as a pill, which would open the approach to cancers that do not involve the chest lining. A separate effort at the University of Vermont is studying thiostrepton in cancers of the abdominal lining, including peritoneal mesothelioma.
Can patients take part in RSO-021 research?
The trial is registered on ClinicalTrials.gov under the identifier NCT05278975, and it has been conducted at centers in the United Kingdom. Participation in the completed stages required a pleural effusion with an indwelling catheter in place. Anyone interested in emerging treatments, including this one, should raise it with their oncology team, who can check whether a related study is enrolling and whether they would qualify.
Why this research matters for people exposed to asbestos
Mesothelioma often appears 20 to 50 years after asbestos exposure, and many of the people diagnosed today worked in shipyards, refineries, power plants, and manufacturing decades ago. Treatment has improved in recent years, but options remain limited once the disease returns after first-line therapy. A drug that works by a completely different mechanism gives researchers, and eventually patients, another path to pursue.
RSO-021 is not a cure, and it is not yet a treatment anyone can request outside a trial. It is a credible early result built on more than a decade of laboratory work, and it deserves close attention as the phase 2 data arrive. Many former workers may be at an increased risk for mesothelioma and asbestos-related lung cancer.
