Researchers are seeing encouraging progress with RSO-021, an experimental targeted therapy being studied for mesothelioma. The drug has now finished the second phase of its clinical trial, and researchers plan to share the results later this year.
Early results published in Nature Communications in July 2026 found that 67% of patients followed for 12 weeks had tumors shrink or stop growing.
RSO-021 is being developed by RS Oncology, a biotechnology company focused on treatments for mesothelioma and other aggressive cancers.
We spoke with Dr. Brian Cunniff, the company’s Chief Science Officer and a mesothelioma researcher at the University of Vermont, about the initial results and what could be next.
“We have something that will provide additional years to people’s lives.”
Phase 1 Trial Shows Tumor Shrinkage and Disease Control
RSO-021 is being tested in a clinical trial called MITOPE. The first phase took place in the United Kingdom and was designed to find a safe, effective dose and look for early signs that the drug was working.
All of the patients had pleural mesothelioma, and their cancer had gotten worse after standard treatment.
Here’s what researchers found:
- At 12 weeks, 67% of patients had tumors that shrank or stayed the same size.
- A 90-milligram dose given once a week was well-tolerated by patients.
- Patients went about 4 months on average before their cancer started growing again.
- More than half of the patients in the study were still alive at 80 weeks.
The early results are encouraging, but they don’t tell us yet whether RSO-021 can help patients live longer than current treatments. Larger studies are needed to answer that question.
Looking for more information on mesothelioma treatments? Get our Free Mesothelioma Guide shipped overnight for options that may be available to you.
How Does RSO-021 Treat Mesothelioma?
RSO-021 works differently than immunotherapy and chemotherapy drugs because it targets a protein that mesothelioma cells rely on to survive.
Cancer cells grow and multiply much faster than healthy cells, which puts them under a lot of internal stress. To survive that stress, mesothelioma cells rely heavily on a protein called PRX3.
RSO-021 blocks PRX3, taking away the cell’s ability to protect itself. Without that protection, the stress builds up, and the cancer cell dies.
“A normal cell is like a car driving at 65 mph, but a cancer cell is a car driving at 120 mph. This new drug is like sticking a potato in the tailpipe of the cancer cell.”
RSO-021 is a lab-made version of a naturally occurring antibiotic called thiostrepton. It has the potential to fight cancer in two ways at once: by killing tumor cells directly and helping the immune system recognize and fight them.
RSO-021 May Reduce Tumors and Fluid Around the Lungs
Doctors give RSO-021 directly into the space around the lungs (pleura) through a catheter. This approach, called intrapleural treatment, puts the drug close to pleural mesothelioma tumors.
Researchers are also studying whether RSO-021 can reduce pleural effusion, a buildup of fluid around the lungs that can make breathing harder.
At the 2024 American Society of Clinical Oncology (ASCO) Annual Meeting, researchers highlighted the results of this drug on three patients with pleural mesothelioma.
They found:
- Less fluid buildup: All three patients had less fluid around their lungs after treatment.
- Smaller tumors: RSO-021 shrank tumors in every patient
- Less fluid drainage: Two patients no longer needed fluid drained from around their lungs.
Together, these early findings gave researchers another reason to continue studying RSO-021 in mesothelioma.
“Essentially, we have a drug that is reducing pleural effusion as well as treating the cancer.”
What the RSO-021 Results Mean for Mesothelioma Patients
RSO-021 has shown real promise so far, but there’s still more to learn before it becomes a standard treatment option.
Here’s what patients and families should know:
- Phase 2 is complete: Researchers expect to share the results later in 2026.
- Early results are encouraging: Most patients had tumors that shrank or stopped growing.
- The drug is still experimental: The U.S. Food and Drug Administration (FDA) hasn’t approved RSO-021 to treat mesothelioma.
- More research is needed: Researchers still need to learn who may benefit and how well the drug works.
Dr. Cunniff says it could still be several years before RSO-021 is available outside of clinical trials. Right now, patients can only access this drug through research studies.
If your cancer has gotten worse after standard treatment, a clinical trial may give you access to more options. Our Free Clinical Trial Match can help you find studies that may fit your diagnosis and treatment history.
We’re Here to Help You Find the Right Treatments
RSO-021 is an encouraging new option for mesothelioma patients that will hopefully be available in the near future. In addition, many more treatments are becoming available to help patients live longer and with fewer symptoms.
While trying to sort through mesothelioma treatment options after a diagnosis can be overwhelming, you don’t have to go it alone.
Our Patient Advocates can help you:
- Find mesothelioma specialists near you
- Understand available treatments
- Explore clinical trials that may be a good fit
- Get support with travel and other care needs
- Access financial assistance for medical costs
Use our Free Mesothelioma Doctor Match or call (866) 608-8933 to speak with one of our caring Patient Advocates today.
