Subcutaneous Mosunetuzumab Shows Strong Efficacy in First-Line High-Tumor Burden Follicular Lymphoma Care

In the phase 2 MorningSun study, subcutaneous mosunetuzumab was given to patients with untreated high-tumor burden follicular lymphoma (FL). Mosunetuzumab’s mechanism of action involves connecting B-cells from cancer with T-cells for elimination by the immune system. Its efficacy has been studied in early stages of disease and at hospitals and clinics, and has received regulatory approval for use in patients with disease which has recurred despite several treatments. 

Results from the study were discussed at the 2025 American Society for Clinical Oncology (ASCO) Annual Meeting, which took place from May 30 to June 3, 2025 in Chicago, Illinois 

“The Fixed-duration SC [subcutaneous] Mosun [mosunetuzumab] treatment showed good early benefits and was well received among high-risk FL patients,” wrote lead author Ian Flinn, MD, of Tennessee Oncology in Nashville, and colleagues. 

A total of 102 patients with high numbers of tumors were enrolled in the study. The median age of the patients was 65 years, 91.2% had Ann Arbor stage III/IV disease, and 78.4% had a FLIPI score of 2 or greater.  

To minimize cytokine release syndrome (CRS), the first treatment cycle with mosunetuzumab involved increasing the dose after which a fixed dose was given every 21 days for 17 cycles, having a duration of approximately one year. Patients who responded to treatment could continue to take maintenance doses every 8 weeks for one more year. 

As of the analysis, 55 patients had completed the study treatment, 16 were still taking the drug, and 31 discontinued early due to worsening disease. From the respondents, 42 patients entered maintenance treatment and this was continued in 38. 

The median duration of follow-up in the cohort was 13.9 months, 89 patients had response, and the median time to response was 2.7 months. Regarding efficacy, the 12-month progression-free survival (PFS) rate was 82.8%, objective response rate (ORR) was 87.3%, and 60.8% of patients had complete metabolic response. 

Regarding side effects, injection-site reactions, fatigue, CRS, headache, and nausea, affected 57.8%, 42.2%, 34.3%, 31.4%, and 30.4% of patients, respectively. All the CRS in this study were grade 1 or 2 severity, but 10.8% of patients experienced serious adverse events cases.