Experimental Drug Shows Early Promise for Pleural Mesothelioma
Written by Amy Zappone on November 12, 2026
In an early clinical trial, an experimental drug kept cancer stable or shrank tumors in 67% of evaluable patients with pleural mesothelioma after 12 weeks. Researchers published the findings in a July 2026 issue of Nature Communications. In October 2026, the sponsor reported that the phase II trial had completed with peer-reviewed results yet to be published.
The phase I clinical trial included 15 study participants. Among the participants, 12 had pleural mesothelioma, and 3 had other types of cancer. All 15 patients received RSO-021 at doses of 90, 120, or 180 milligrams. At 12 weeks, researchers could assess disease control in 9 patients. Of these patients, 5 had stable disease, and 1 had a partial response, meaning that the patient's tumors had decreased in size.
The findings warranted further study in a phase II trial, according to researchers.
How RSO-021 Targets Mesothelioma Cells
RSO-021 is a formulation of the antibiotic thiostrepton. It blocks an enzyme called PRX3, which helps protect mesothelioma cells from damage.
Mesothelioma cells produce high levels of potentially harmful molecules as they use energy. PRX3 helps keep these molecules under control. Blocking PRX3 allows them to build up to levels that can kill the cancer cells.
This approach grew from laboratory research at the University of Vermont. RS Oncology developed the clinical formulation process of creating RSO-021 and sponsored the phase I trial in the United Kingdom.
Patients received RSO-021 weekly through a pleural catheter already in the chest. The drug was delivered directly to the pleural space between the lung and the chest wall. This process placed a concentrated dose at the tumor site but limited exposure in other parts of the body.
What the Early Results Mean for Patients
The phase I results showed promise, but more research is needed. Researchers used this initial study to address safety and patient drug tolerance, and it helped establish correct dosing for the phase II trial. It did not directly compare RSO-021 with other treatments.
The most commonly reported adverse events were fatigue and fever. At higher doses, dose-limiting reactions included a severe acute inflammatory response and severe shortness of breath. Adverse events are health problems reported during treatment and are not necessarily caused by the drug.
What the Company Reported From Phases I and II
In October 2026, RS Oncology announced results from the phase II portion of the trial. The company shared them in a conference poster at the European Society for Medical Oncology Congress 2026. These company-reported findings have not been published in a peer-reviewed journal. The analysis also includes phase I patients.
According to the company, 41 patients with fluid buildup around the lungs caused by cancer, including pleural mesothelioma, who participated in the phase I and phase II trials received weekly doses of RSO-021. Patients received doses of either 45 or 90 milligrams. Among the 32 patients who received the medication before standard treatment, 70% saw their tumors shrink or remain stable.
Researchers measured how long patients survived without their cancer getting worse:
- Progression-free survival for 90-milligram dose: A median of 4.1 months
- Progression-free survival for 45-milligram dose: A median of 3.1 months
The company reported no treatment-related deaths. Serious adverse events occurred in 61% of patients on the higher dose and 44% on the lower dose. RS Oncology selected the 90-milligram dose for further development and said it is working toward a phase III study.
What Can Patients Do Now?
Patients can ask their mesothelioma doctor how the study relates to their own care and treatment options. Their care team can also help the patient find other clinical trials that are accepting participants and discuss eligibility, potential benefits, and risks.
RSO-021 offers an encouraging direction for mesothelioma treatments, but researchers need more evidence to understand its benefits and risks. Patients can work with their specialists to review available treatments and clinical trials accepting new patients.
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Amy Zappone brings nearly two decades of experience in education and research-based communication. At Mesothelioma.com, she translates complex topics into understandable resources that help patients and families make informed decisions.
