In this JCO PO Article insights episode, Dr. Jiasen He summarized the JCO PO article "Mucin 16–Directed Therapy in Pediatric Sarcomas: Case Evidence of Ubamatamab Efficacy in Epithelioid Sarcoma and Its Implications for Other Sarcoma Subtypes" by Connolly et al.

TRANSCRIPT

Jiasen He: Hello, and welcome to JCO Precision Oncology Article Insights. I'm your host, Jiasen He, and today we'll be discussing the JCO Precision Oncology article, "Mucin 16-Directed Therapy in Pediatric Sarcomas: Case Evidence of Ubamatamab Efficacy in Epithelioid Sarcoma and Its Implication for Other Sarcoma Subtypes" by Connolly et al.

Epithelioid sarcoma and malignant rhabdoid tumor are rare pediatric soft tissue sarcomas, characterized by INI1 loss, high recurrence rates, and poor outcome despite multimodal treatments. Emerging evidence has shown that Mucin 16 is expressed in both tumor types. Mucin 16 is a transmembrane glycoprotein whose extracellular domain can be cleaved and released as CA-125. Both Mucin 16 and CA-125 are well-established biomarkers in several adult epithelioid malignancies, particularly ovarian cancer. Ubamatamab is a specific T-cell engager targeting CD3 and Mucin 16. It has demonstrated antitumor activity in patients with recurrent ovarian cancer, and clinical trials are ongoing to evaluate its efficacy as monotherapy or in combination regimens.

In this manuscript, Connolly et al. present the first reported case of a heavily pretreated patient with epithelioid sarcoma who responded to ubamatamab, followed by an investigation into mechanisms of resistance after disease progression. Furthermore, the authors retrospectively assessed Mucin 16 expression in a cohort of pediatric and young adult sarcomas, finding high expression in both epithelioid sarcoma and malignant rhabdoid tumor.

In this case report, the authors describe a 23-year-old woman with relapsed metastatic epithelioid sarcoma. Initially diagnosed at age 12, she had received multiple lines of treatments, including surgery, radiotherapy, targeted therapy, and immunotherapy. Following disease progression after all these treatments, her tumor was tested for Mucin 16 expression and it demonstrated 100% positivity with markedly elevated CA-125 levels, providing a rationale for treatment with the Mucin 16-CD3 bispecific T-cell engager, ubamatamab.

Ubamatamab was administered in an escalating dose schedule up to 250 mg once weekly during cycle one and continued for a total of 162 weeks. The best response was observed at week 11, with a 40% reduction and a marked decline in CA-125 levels. Disease progression was first detected in a single left lower lobe lung nodule, which on biopsy showed a reduction in Mucin 16 expression from 100% to less than 5%. Post-treatment analysis revealed changes in the tumor microenvironment, including increased expression of T-cell exhaustion markers such as PD-1 and LAG-3.

Ubamatamab was generally well tolerated. Cytokine release syndrome occurred during the escalating phase of cycle one, presenting with fever and hypoxia. Other notable adverse events included pleural and pericardial effusion, both of which resolved spontaneously. Given its favorable safety profile and limited alternative treatment options, ubamatamab was continued beyond the initial progression. The patient ultimately received 28 cycles of treatment before she passed away due to disease progression.

In the second part of the paper, the authors examined Mucin 16 expression in a cohort of pediatric and young adult sarcomas. Among 91 samples, Mucin 16 was expressed in six out of eight epithelioid sarcomas and two out of four malignant rhabdoid tumors. H-score analysis showed that all Mucin 16-positive tumors showed moderate to high expression levels.

In conclusion, this manuscript presents the first reported use of a Mucin 16-CD3 bispecific T-cell engager for epithelioid sarcoma, along with an investigation into resistance mechanisms following progression. The treatment achieved a substantial partial response with a favorable safety profile. The findings suggest that resistance may be associated with loss of Mucin 16 expression and T-cell exhaustion. Follow-up studies are needed to confirm these mechanisms. Notably, the study also identifies INI1-deficient sarcoma as a group with high Mucin 16 expression, warranting additional validation and mechanism exploration. These findings offer valuable insight for future therapeutic strategies and support the use of Mucin 16/CA-125 as both a treatment target and a biomarker for patient selection and disease monitoring.

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JCO PO Article Insights: MUC16 Directed BiTE Therapy in Epithelioid Sarcoma

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