An experimental drug called RSO-021 controlled disease in 67% of patients at 12 weeks in a small phase 1 trial for relapsed pleural mesothelioma, according to a study published in Nature Communications. The drug is not approved by the FDA, is not available to U.S. patients outside research studies, and was tested in 15 patients in the United Kingdom.
The paper appeared in July and was recirculated this month in a ScienceDaily research summary, bringing renewed attention to a cancer with few good treatment options. Mesothelioma is most often caused by asbestos exposure.
For U.S. families, the news is best read as a research milestone. About 3,000 Americans are diagnosed with mesothelioma each year, according to the American Cancer Society, and many are older adults whose asbestos exposure happened decades ago at work.
Turning a Tumor's Antioxidant Shield Against It
Cancer cells burn energy quickly, producing unstable molecules called reactive oxygen species that can damage the cells themselves. To survive, mesothelioma cells ramp up a protective enzyme called peroxiredoxin 3, or PRX3, inside their mitochondria, the cell's powerhouses.
RSO-021 is a clinical formulation of thiostrepton, a naturally occurring antibiotic compound that binds to PRX3 and blocks it. Without that protection, the tumor cell's chemical balance is disrupted, and the cell dies.
The strategy reverses an older idea. Earlier trials tried to fight cancer by adding antioxidants, and those largely failed, according to the University of Vermont's announcement. In laboratory work, deleting PRX3 slowed mesothelioma cell growth, and the altered cells lost their ability to form tumors in animal models.
Early Trial Numbers and Their Limits
MedicalDaily Evidence Check: This was a phase 1 trial conducted in 2022 and 2023 under the oversight of the U.K. regulator, the MHRA, and designed mainly to test safety and tolerability. It met those goals at a weekly dose of 90 milligrams, and no patient deaths were attributed to the drug. Disease control reached 67% at 12 weeks, some tumors shrank, and tissue samples showed that the drug reached its target.
The results come with important limits. There was no comparison group, and the sample was small. The University of Vermont reported that patients lived longer than patients given standard treatments, but single-arm data from a small group cannot confirm a survival benefit.
Financial ties also deserve attention. The trial was sponsored by RS Oncology, a private company formed to develop the University of Vermont discoveries. Brian Cunniff, an associate professor at the university's Larner College of Medicine whose lab developed the approach, serves as the company's chief science officer.
The drug is delivered directly into the chest through a catheter that many patients already have, because about 90% of mesothelioma patients develop fluid buildup between the lung and chest wall. That local delivery concentrates the drug near the tumor while limiting exposure elsewhere in the body. Dean Fennell, the trial's principal investigator, presented early phase 1 results at the 2024 American Society of Clinical Oncology Annual Meeting, according to the University of Vermont Cancer Center.
American Patients and Families Weighing Options
Mesothelioma falls hardest on older adults, and men are affected more often, largely because of past workplace asbestos exposure in industries such as shipbuilding, construction, and manufacturing. Veterans who served on ships or in shipyards may also face elevated risk.
Patients and caregivers should not delay proven care while waiting for an experimental option. The National Cancer Institute's mesothelioma information outlines current treatment approaches, including surgery, chemotherapy, immunotherapy, and radiation, and explains how to search for clinical trials.
Families can ask an oncologist whether a mesothelioma specialty center or clinical trial is within reach, whether an existing chest catheter could matter for trial eligibility, and what travel or lodging support trial sponsors provide. Veterans can ask the VA about asbestos-related care and benefits. Cancer centers also employ financial counselors who can explain trial costs, including routine care that Medicare and many private plans cover during qualifying clinical trials. Anyone with a history of asbestos exposure who develops new shortness of breath, chest pain, or unexplained weight loss should see a clinician.
Phase 2 Results Are the Next Test
The phase 2 portion of the trial has been completed, and researchers expect to present results at a global oncology meeting this year. Those data will show whether early disease control holds up in more patients. The team is also developing second-generation PRX3 inhibitors that are more soluble and could potentially be taken as a tablet.
A larger randomized trial and regulatory review would still be required before the drug could reach U.S. patients. The newest data show a new mechanism that appears safe and active in a small group, and the key uncertainty is whether the benefit holds up in larger, controlled testing. MedicalDaily will report on the phase 2 findings once they are released.
Key Questions Answered
What is RSO-021?
RSO-021 is an experimental mesothelioma drug made from thiostrepton, a natural antibiotic compound. It blocks PRX3, an enzyme tumor cells use to survive chemical stress.
What did the trial show?
In a phase 1 trial of patients with relapsed pleural mesothelioma, the drug controlled disease in 67% of patients at 12 weeks and met its safety goals at a weekly 90-milligram dose.
Can U.S. patients get this drug?
Not outside research studies. The drug is not FDA-approved, and the published trial took place in the United Kingdom.
Is this a cure for mesothelioma?
No. The trial was small, had no comparison group, and was designed mainly to test safety. Larger controlled trials are needed.
Who sponsored the research?
The trial was sponsored by RS Oncology, a company formed to develop University of Vermont discoveries. A lead researcher serves as its chief science officer.
What happens next?
Phase 2 results are expected at a global oncology meeting this year, and researchers are developing a second-generation version that could potentially be taken as a tablet.
