Mucin 16–Directed Therapy in Pediatric Sarcomas: Case Evidence of Ubamatamab Efficacy in Epithelioid Sarcoma and Its Implications for Other Sarcoma Subtypes

Abstract

PURPOSE

Epithelioid sarcoma (ES) and malignant rhabdoid tumor (MRT) are rare soft tissue sarcomas with poor prognoses. Although mucin 16 (MUC16) and its soluble form, cancer antigen 125 (CA125), are established as biomarkers and therapeutic targets in ovarian cancer, emerging data suggest MUC16 may also be expressed in ES and MRT. In this study, we present a patient with ES, who demonstrated a response to ubamatamab, a novel bispecific T-cell engager (MUC16xCD3), and analyze treatment resistance after disease progression. Additionally, we examine MUC16 expression across pediatric and adolescent and young adult (AYA) sarcomas, to evaluate the frequency of this target and explore the broader application of ubamatamab in this population.

MATERIALS AND METHODS

We performed a retrospective clinical case review and immunohistochemical analysis of pediatric and AYA (0-25 years) sarcoma samples from 2015 to 2021, evaluating MUC16 expression using anti-CA125 immunohistochemistry (IHC) on the DAKO Omnis platform.

RESULTS

A 23-year-old female patient with multiply relapsed metastatic ES, harboring MUC16 expression by IHC and elevated serum CA125, received intravenous ubamatamab (250 mg) once per week as part of a single patient study. After 11 weeks of ubamatamab, a RECIST v1.1 partial response was demonstrated, along with serum CA125 normalization, lasting 43 weeks. During the initial step-up dosing, the patient experienced grade 2 cytokine release syndrome. Treatment-emergent adverse events included grade 2 pleural effusion, pericardial effusion, and palmar-plantar erythrodysesthesia, all resolving without intervention. IHC analysis of retrospective samples showed positive MUC16 staining in six of eight (75%) ES and two of four (50%) MRT samples, with no immunoreactivity observed in other pediatric/AYA sarcoma subtypes.

CONCLUSION

MUC16 is frequently detected in ES and MRTs. Ubamatamab is an encouraging anti-MUC16 therapy, demonstrating clinical efficacy. Ongoing trials (ClinicalTrials.gov identifier: NCT06444880) are evaluating ubamatamab in other rare MUC16-positive tumors.

INTRODUCTION

Epithelioid sarcoma (ES) and extracranial malignant rhabdoid tumors (MRTs) are aggressive, rare malignancies, resulting in a poor prognosis despite aggressive treatment regimens.^1,2 ES occurs in approximately 0.1 cases per million people.² By comparison, the incidence of extracranial MRT is notably higher in infancy, with 5-5.7 cases per million people reported, decreasing to 0.1-0.2 per million by 5 years of age.¹ Both entities share a characteristic loss of expression of integrase interactor 1 (INI1), a protein encoded by the SMARCB1 tumor suppressor gene, which plays a critical role in chromatin remodeling.^3,4 Although the biallelic inactivation of SMARCB1 is a hallmark of tumorigenesis in MRT and the majority of ES,^4,5 a small minority of ES retain SMARCB1 (INI1) protein expression and instead exhibit abnormal expression of other SWI/SNF chromatin-remodeling complex members, such as SMARCA4.⁶ Despite the use of multimodal treatment strategies including surgery, radiation, chemotherapy, and targeted therapies,^1,7 the high rate of recurrence and poor outcomes of ES and MRT highlight an urgent need to develop novel targeted therapies.^1,2

CONTEXT

• Key Objective

• Can ubamatamab, a mucin16xCD3 bispecific T-cell engager, serve as a novel therapy for pediatric and young adult patients with sarcomas expressing mucin 16 (MUC16)?

• Knowledge Generated

• MUC16 expression was retrospectively detected in 75% (n = 6/8) of epithelioid sarcoma (ES) and 50% (n = 2/4) of extracranial malignant rhabdoid tumor (MRT) samples, across 91 pediatric and young adult sarcomas at a single institution. One such patient with ES achieved a sustained partial response to ubamatamab. Comparative immunohistochemical and immunofluorescence analysis of this patient's tumor, before ubamatamab and at disease progression, demonstrated loss of MUC16 expression, with increased PD-1 and lymphocyte activation gene-3 (LAG-3) expression on T cells.

• Relevance

• This proof-of-concept report illustrates ubamatamab's potential as a novel and effective therapy for patients with MUC16 expressing ES and MRT. Increased PD-1 and LAG-3 expression after disease progression, suggesting T-cell exhaustion, highlights LAG-3 as a prospective target for future ubamatamab combination strategies to overcome resistance.

Mucin 16 (MUC16) is a transmembrane glycoprotein, where proteolytic cleavage results in the release of its extracellular portion, known as cancer antigen 125 (CA125).⁸ MUC16 has been established as an important biomarker in several adult epithelial malignancies, including ovarian carcinoma.⁸ Similarly, immunohistochemistry (IHC) studies have shown strong expression of MUC16 in ES compared with other soft tissue tumors,⁹ and elevated serum CA125 levels have correlated with ES progression.¹⁰

Ubamatamab, a bispecific T-cell engager, targeting MUC16 and CD3, has demonstrated its ability to induce T-cell activation against MUC16-expressing tumor cells in vitro.¹¹ In Phase I/II trials, ubamatamab exhibited notable antitumor activity in patients with recurrent ovarian carcinoma.^11,12 The efficacy of ubamatamab, either as monotherapy or in combination with cemiplimab (an anti–PD-1 immune checkpoint inhibitor), is currently under investigation (ClinicalTrials.gov identifier: NCT03564340) in adult patients with recurrent ovarian carcinoma, primary peritoneal, fallopian tube, and endometrial carcinoma.¹³

Here, we report a clinical response to ubamatamab in a young adult patient with heavily pretreated ES, highlighting its potential as a novel therapeutic approach for a non-ovarian indication. Additionally, this study presents exploratory data on MUC16 immunohistochemical staining across various pediatric and young adult tumor types.

MATERIALS AND METHODS

Case Report

Our patient received ubamatamab, a MUC16xCD3 bispecific T-cell engager, in the context of a single-patient study (C17-POPM-007-SK), a prospective clinical trial designed for this individual patient, approved by Health Canada and the Hospital for Sick Children's Research Ethics Board, with written consent for reporting of results. Disease status was assessed radiographically, using computed tomography and/or magnetic resonance imaging every 3 months, with response assessed according to RECIST v1.1. Serum CA125 was determined every 6 weeks, with adverse events (AEs) collected continuously and graded according to Common Terminology Criteria for Adverse Events v5.0. Comparative IHC and immunofluorescence analyses of the patient's tumor were performed on tissue obtained before treatment and from percutaneous biopsy at disease progression.¹⁴

Sarcoma Histology Series

An IHC analysis of pediatric and young adult (0-25 years) sarcoma samples from 2015 to 2021 was conducted using tissue specimens stored at Hospital for Sick Children. Ethical approval was obtained from the Hospital for Sick Children Research Ethics Board for this retrospective review. H&E stained tumor specimens were reviewed and MUC16 expression was evaluated by IHC using an anti-CA125 antibody (clone M11; DAKO GA701) on 4-μm formalin-fixed, paraffin-embedded tissue sections with the EnVision FLEX detection kit (DAKO GV800) on the DAKO Omnis platform. CA125 IHC slides were blindly scored by one pathologist (R.C.) using QuPath (version v0.5.1) in a semiautomated manner. Tumor regions were manually selected, and QuPath's automated cell detection and staining quantification were applied. The H-score was calculated as follows: (1× % of weakly staining cells +2× % of moderately staining cells +3× % of strongly staining cells), ranging from 0 (negative staining) to 300 (intense, widespread positive staining), and provided a semiquantitative measure of MUC16 expression. Results were manually reviewed to ensure accuracy. Positive and negative controls appropriate for CA125 IHC were included in each run, with fallopian tube epithelium serving as the positive control and tonsils as the negative control, ensuring staining validity and consistency across all samples. Samples with H scores of 0 were considered negative for MUC16 expression, while those with H scores above 0 were reported using their actual values and classified as MUC16-expressing, as no validated threshold for MUC16 positivity currently exists for pediatric and adolescent and young adult (AYA) sarcomas. Descriptive statistics summarized patient characteristics and staining outcomes, with data analysis identifying histologic subtypes showing notable MUC16 expression.¹⁴

Furthermore, fully automated fluorescent multiplex IHC was performed on the Ventana Discovery ULTRA platform (Ventana Medical Systems, Oro Valley, AZ) as previously described.¹⁵ All antibody concentration, assay conditions, and fluorophore combinations were optimized and applied in a specific sequence (Appendix Table A1). Subsequently, stained tissue sections were counterstained and coverslipped with Invitrogen ProLong Gold Antifade Mountant with NucBlue. Whole-slide image acquisition was performed on the Zeiss Axioscan 7, equipped with Colibri light source and appropriate filter sets, and obtained fluorescent images were visualized with Zeiss Zen Imaging software. Quantitative image analysis using HALO Indica Labs Hyperplex module could not be performed because dense erythrocyte accumulation in the stroma and tumor tissue (Figs 1C and 1G; white asterisk) caused strong autofluorescence in the fluorescent DCC channel. Therefore, only visual interpretation of immune cell infiltrate and expression of immune checkpoint inhibitor molecules PD-1 and LAG3 was performed on the primary tumor and progressive pulmonary metastatic lesion.

FIG 1.

(A) Summative timeline of the patient course from diagnosis to demise, demonstrating the multi-modality approach to treatment over a 12-year period (Figure created in BioRender.com. Connolly, D, 2025, https://BioRender.com/dfuirte). (B) Comparative trend of relative tumor size (as per RECIST v1.1) and serum CA125 levels over the course of ubamatamab treatment, demonstrating initial tumor response with reduction in tumor size, which subsequently progressed and was biopsied. CA125, cancer antigen 125.

RESULTS

Case Report

A 23-year-old woman with multiply relapsed and extensively treated (Fig 1A) metastatic ES was treated with ubamatamab as part of a single-patient study.

At 12 years of age, she presented with a 2-week history of a 2-cm mass adjacent to her right wrist, which was biopsied, showing ES with INI1 loss. Staging confirmed localized disease involving the right radius. She received preoperative radiotherapy (50.4 Gy in 28 fractions), followed by an R0 resection and staged forearm reconstruction, yielding a 19-month remission. A localized epitrochlear nodal recurrence in her right arm marked her first relapse, which responded to further radiotherapy (54 Gy in 30#) to her right axilla and forearm. This achieved 16 months of disease control before a second relapse, involving her primary site and elbow, that was treated for 6 months with the EZH2-inhibitor tazemetostat (800 mg orally twice a day) as part of a clinical trial and complicated by noncompliance. Her disease, involving her right forearm, axilla, and new pulmonary metastases, subsequently progressed and she was treated with anti–PD-1 pembrolizumab (2 mg/kg/dose intravenously once every 3 weeks) on trial, until a new soft tissue component adjacent to this right lower lobe lesion suggested possible progression 13 months later.

A definitive picture of progressive locoregional and pulmonary disease declared itself over 3.5 months, prompting a transition to combined anti–PD-1 and anti–cytotoxic T-lymphocyte–associated protein 4 immune checkpoint blockade. This regime included intravenous nivolumab (3 mg/kg/dose) and ipilimumab (1 mg/kg/dose), administered once every 3 weeks for the first four cycles. In view of significant ongoing infection and ulceration (that preceded initiation of immune checkpoint inhibitor therapy), she underwent a forequarter amputation after cycle 2 of nivolumab and ipilimumab. She subsequently received intravenous nivolumab (480 mg) monotherapy once every month from cycle 5 onward until cycle 10, at which point pneumonitis was identified. The patient elected to resume tazemetostat therapy (800 mg orally twice a day) and continued it for 6 months until clinical disease progression. She then received a single dose of iopofosine I-131 (2,592 megabecquerel) as part of a phase I trial; however, further progressive disease involving her amputation site and right breast necessitated external-beam radiotherapy (20 Gy in 5#) to her right chest wall.

Conventional chemotherapy was anticipated to provide, at best, a few months of disease control, with an unfavorable toxicity profile. However, upon learning that MUC16 was highly expressed in other ES patients,^9,10 and could be targeted therapeutically,¹¹ further IHC of her tumor sample from her forequarter amputation was pursued. This IHC demonstrated MUC16 expression in 100% of these tumor cells, alongside an elevated serum CA125 of 793 U/mL (normal range, 0-35), providing the rationale for pursuing treatment with ubamatamab.

The selected dose of ubamatamab (250 mg once per week) was previously demonstrated to be tolerable in adult patients with ovarian cancer.¹¹ This dose was reached through a gradual intrapatient dose escalation in Cycle 1, with 1 mg on Day 1; 10 mg on Day 8 and 9; 50 mg on Day 15 with 200 mg on Day 16; and full 250 mg dose on Days 22, 29, and 36, completing a 6-week cycle. Subsequently, ubamatamab 250 mg was administered once per week, for 162 weeks, without dose modifications, until Cycle 28 Day 1, when the final dose was given.

At baseline, two measurable pulmonary lesions were selected as target lesions, to facilitate radiologic assessment of disease response, every two cycles, using RECIST v1.1. The best observed treatment response was a partial response (40% reduction) noted after 11 weeks of ubamatamab, alongside a reduction in serum CA125 (Fig 1B). Progressive disease, initially involving a single left lower lobe lesion, was identified on week 55, while the remaining target lesions and her serum CA125 remained stable.

This left lower lobe lesion underwent percutaneous biopsy, with IHC showing a significant reduction of MUC16 expression, from 100% in the primary tumor to <5% in the lung metastasis, with corresponding changes in the tumor's immune infiltrate (Fig 2). Further analysis of this infiltrate within the biopsied pulmonary metastasis illustrated high levels of PD-1 and lymphocyte activation gene-3 (LAG-3) expression on surrounding T cells (Fig 3), which were not present before starting ubamatamab. Additionally, beyond MUC16/CA125 significant expression loss, there was increased stromal fibrosis, but no tumor necrosis identified.

FIG 2.

A comparison of changes in (A-D) tumor microenvironment, (E, F) MUC16 expression, and (G, H) immune cell infiltrate in the patient's primary tumor before ubamatamab treatment (left column) and her pulmonary metastases during disease progression on ubamatamab (right column). (A) Primary tumor with two major lesions of invasive epithelioid sarcoma (A, green dashed line) surrounded by stromal and scar tissue. Large tissue areas are necrotic and contain a high number of erythrocytes (C, G, asterisk). High MUC16 expression was observed on 82% of primary tumor cells before ubamatamab treatment (E, brown color). Specific stromal tissue areas with moderate to strong immune cell infiltrate, including T cell subsets, B cells, and myeloid cells (C, G, arrows), are located close to tumor boundaries (C, green dashed line). (B) Progressive metastatics tissue with four major lesions of invasive epithelioid sarcoma cells (B, green dashed line) surrounded by lung, stromal, and scar tissue (D). (F) Pulmonary metastatic lesions are almost exclusively MUC16-negative and only 5% of tumor cells show low to weak MUC16 expression. (H) Most analyzed tumor areas exhibited an immune-excluded phenotype, with low immune cell infiltrate. Present tumor infiltrating lymphocytes include regulatory T cells (H, white arrows) and CD3/CD8+ T cells. Scale bar: (A, B) = 1 mm, (C-H) = 50 μm. Immune cell phenotypes from FL Mx IHC assay A: T cell subsets = CD3: CD3+, CD8: CD3+CD8+FOXP3-, Treg: CD3+CD8-FOXP3+; B cell = CD20+; myeloid cell = CD68+.

FIG 3.

Performed fluorescent IHC analysis of ubamatamab-treated pulmonary metastasis to investigate the relationship between single/locally present MUC16-positive tumor cells and the PD-1/LAG3 immune checkpoint status of tumor cell engaging T cells. To address this question, two serial tissue sections were used to analyze (A) MUC16 expression on tumor cells and (B-G) PD-1/LAG3 expression on T cells. (A) Small tissue areas within post-treatment pulmonary metastasis contain individual MUC16-positive tumor cells (A, white arrows). Most of the remaining tumor cells are MUC16-negative. (B, F) Same tissue area of the adjacent tissue section illustrates CD3 and CD8 T cell infiltrates with very close tumor cell proximity, suggesting potential MUC16xCD3-driven T-cell/tumor cell engagement (B, white dashed circles). (C-E, G) Tumor-infiltrating T cells show notable expression or coexpression of immune checkpoint inhibitor receptors PD-1 and LAG3. Especially T cells with close tumor cell proximity (white dashed circles) are predominantly PD-1–positive and/or LAG3-positive, indicating T cell exhaustion as a likely mechanism of evolving tumor resistance to ubamatamab (in addition to MUC16 loss). Scale bar: (A-E) = 50 μm; (F, G) = 20 μm. Immune cell phenotypes from FL Mx IHC assay B: T cell subsets = CD3: CD3+, CD8: CD3+CD8+; T cell/immune checkpoint subsets = CD3+PD-1+, CD3+LAG3+, CD3+PD-1+LAG3+, CD3+CD8+PD-1+, CD3+CD8+LAG3+, CD3+CD8+PD-1+LAG3+. MUC16, mucin 16.

Given the patient's excellent treatment tolerance and the lack of alternative curative options, ubamatamab was continued. Ultimately, disease in the right axilla/chest wall and both lungs continued to progress, and patient died from progressive disease during Cycle 28, after receiving 162 weeks of treatment with ubamatamab.

Ubamatamab was well tolerated. The patient maintained a Karnofsky performance status of 70%-90% until her final 2 months of life. During the initial step-up dosing, the patient experienced grade 2 cytokine release syndrome (CRS) as evidenced by fever and hypoxia. Treatment-related AEs were exclusively limited to a grade 2 pleural effusion and grade 2 pericardial effusion after cycle 2 that self-resolved, in addition to grade 1 palmar-plantar erythrodysesthesia syndrome. No suspected unexpected serious adverse reactions occurred.

Sarcoma Histology Series

Ninety-one sarcoma samples from pediatric and young adult patients across 14 different subtypes were analyzed with MUC16 staining patterns and associated H scores as described in Tables 1 and 2. ES accounted for eight of these 91 sarcoma samples, with corresponding patients diagnosed between 4 and 22 years of age (median age: 8 years). Six of these eight (75%) ES samples had MUC16 immunoreactivity. Notably, four of these samples had widespread intense MUC16 staining (H scores of 300), while the remaining two exhibited moderate heterogeneous staining (H scores 85 and 110). MRT accounted for four of the 91 sarcoma samples, with corresponding patients diagnosed between 6 days and 2.5 years of age (median age: 1 year and 4 months). Two of these four MRT samples (50%), both of which were extrarenal in origin, had H scores ranging from 170 to 300, reflecting moderate to strong MUC16 staining. Specimens from all other examined pediatric sarcoma subtypes, as well as atypical teratoid rhabdoid tumor (ATRT)—the central nervous system equivalent to MRT—had H scores of 0, showing no evidence of MUC16 staining.

TABLE 1.

MUC16 Expression, Identified on IHC, Across a Pediatric and Young Adult Sarcoma Cohort (N = 91)

Tumor Type	No. of Patients	No. of Patients With MUC16 Expression on IHC
ES (with diffuse loss of INI-1 expression)	8	6
Renal and soft-tissue rhabdoid tumor (with diffuse loss of INI-1 expression)	4	2
Atypical rhabdoid teratoid tumor (non-sarcoma, INI-1 loss entity)	8	0
Alveolar rhabdomyosarcoma	7	0
Embryonal rhabdomyosarcoma	17	0
Synovial sarcoma	9	0
Ewing sarcoma	19	0
Angiosarcoma	2	0
BCOR-altered sarcoma	5	0
Malignant peripheral nerve sheet tumor	3	0
Low-grade fibromyxoid sarcoma	5	0
Desmoplastic small round cell tumor	2	0
Alveolar soft part sarcoma	1	0
CIC-rearranged sarcoma	1	0

NOTE. Samples with an H score of 0 were considered negative for MUC16 staining. Patients with H scores above 0 were reported using their actual values and classified as MUC16-expressing, as no threshold defining MUC16 positivity has been established in this population.

Abbreviations: CA125, cancer antigen 125; CIC, capicua transcriptional repressor; ES, epithelioid sarcoma; IHC, immunohistochemistry; INI1, integrase interactor 1; MRT, malignant rhabdoid tumor; MUC16, mucin 16.

TABLE 2.

Analysis of Sarcomas Expressing MUC16, Specifically Epithelioid Sarcoma and Extracranial Malignant Rhabdoid Tumors, by Measuring Histochemical (H) Scores in Matched Primary Tumor and Metastatic Samples, Where Available

Tumor Sample, Type, and Age at Diagnosis	Primary Tumor H-Score	Metastatic Site H-Score
ES proximal type, 16 years	110	—
ES proximal type, 4 years	0	—
ES distal type, 12 years	300	—
ES distal type, 8 years	300	—
ES proximal type, 14 years	0	0
ES distal type, 12 yearsa	300	300
ES distal type, 8 years	300	—
ES type NS, 22 years	85 (sample NS)b
MRT kidney, 2.5 years	0	0
MRT extrarenal, 6 days	300	—
MRT extrarenal, 8 months	170	300
MRT kidney, 2 years	0	—

NOTE. These H scores semiquantitatively assess the degree of MUC16 expression, ranging from 0, indicating negative staining, to 300, indicating intense, widespread positive MUC16 staining.

Abbreviations: ES, epithelioid sarcoma; H-score, histochemical score; MRT, malignant rhabdoid tumor; MUC16, mucin 16; NS, sample origin not specified.

^aPatient described in case report.

^b NS: Type of epithelioid sarcoma and sample origin not specified.

DISCUSSION

This patient represents the first proof-of-concept use of ubamatamab in MUC16-positive metastatic ES. Their clinical course reflects the typical presentation and pattern of progression observed in patients with distal ES,² with our patient receiving seven lines of therapy before ubamatamab.

Ubamatamab achieved a sustained partial response and a significant clinical response, providing prolongation of life for this patient with advanced disease without significantly affecting quality of life. In ES, doxorubicin yields an objective response rate (ORR) of 15%-27%, with a median overall survival of 10-16 months and gemcitabine offers an ORR of 27%, but with a median overall survival of 2.5 months.¹⁶ Tazemetostat, an oral EZH2 inhibitor, has similar response rates, with an ORR of 15%. However, a subset of locally advanced/metastatic patients with ES have demonstrated durable responses to tazemetostat, with a reported 12-month progression-free survival of 21%.² Variable results have been achieved with immune checkpoint inhibitors suggesting that success may relate to individual patient factors,⁷ underscoring the importance of patient selection on the basis of their tumor profile.

Our understanding of the efficacy of ubamatamab is based upon a cohort of 42 adult patients (age range, 36-85 years) with recurrent ovarian carcinoma.¹¹ A range of doses from 20 mg to 800 mg of ubamatamab was administered to this cohort once per week, with some patients receiving a dose higher than that received by our patient (250 mg).¹¹ This adult cohort of patients with recurrent ovarian carcinoma demonstrated an ORR of 14.3% (95% CI, 5.4% to 28.5%), a disease control rate of 57.1%, and a median duration of response of 12.2 months.¹¹ Additionally, these adult trial data and our patient show that ubamatamab is generally well tolerated. Our patient experienced grade 2 CRS initially, which was also common in this adult cohort with recurrent ovarian carcinoma, where 73% experienced either grade 1 or 2 CRS.¹¹

High MUC16 expression (100%) was present in our patient's tumor before receiving ubamatamab. This is a common finding in both proximal and distal ES, as shown by both our institutional analysis of sarcoma samples and published case series, such as that of Kato et al, where MUC16 IHC staining was evident in 10 of 11 (90.9%) ES tumors.⁹ At progression, repeat biopsy of the progressing lung lesion showed marked loss of MUC16 expression suggesting that selective pressure on this target led to antigen escape through loss of expression.¹⁷ However, in the absence of a pre-ubamatamab biopsy of the lung metastases for comparison, this change could also represent inherent biological differences between the primary tumor and lung metastases.

The loss of MUC16 at the tumor level can also affect our ability to interpret serum CA125 levels. Elevated serum CA125 levels, ranging from hundreds to thousands of U/mL, have been reported in approximately 70% of patients with ES, and generally tend to correlate with MUC16 expression.¹⁰ As such CA125 levels usually normalize after effective treatment, as demonstrated by our patient initially, but can rise rapidly during disease progression, making it a valuable biomarker for tumor response. However, as demonstrated by the latter period of our patient's clinical course, there are instances where serum CA125 levels are incongruent with tumor burden. Variability in MUC16 expression across different tumor cells,¹⁷ the shedding of the extracellular domain of MUC16, and its delayed clearance may also contribute to these dynamics.⁸

In addition to immune escape through loss of MUC16, the increased expression of PD-1 and LAG-3 on tumor-infiltrating T cells suggests other mechanisms of therapy resistance such as T-cell exhaustion. However, it was not feasible to reliably quantify these markers, because of red blood cell autofluorescence in the tumor tissue, which overlapped with fluorophores used for PD-1 and LAG-3 detection. Nevertheless, the presence of such markers reinforces the rationale for exploring novel combinations of ubamatamab with other therapeutics. An adult phase I/II study (ClinicalTrials.gov identifier: NCT03564340) is examining the efficacy of an anti–PD-1 antibody, cemiplimab, in combination with ubamatamab in patients with recurrent ovarian carcinoma and/or advanced MUC16+ endometrial carcinoma.¹³ Furthermore, another phase II trial (ClinicalTrials.gov identifier: NCT06444880) will assess the efficacy of ubamatamab alone and in combination with cemiplimab in adult patients with locally advanced or metastatic MUC16-expressing SMARCB1-deficient malignancies, including ES and renal medullary carcinoma.¹⁸ If no trial-limiting toxicity is identified by the interim analysis, subsequent enrollment may open to adolescents.¹⁸

Our institutional series identified MRT as the second most common sarcoma subtype exhibiting high MUC16 immunoreactivity, albeit on the basis of small patient numbers. Similarly, Hua et al¹⁹ demonstrated significant upregulation of MUC16 across two ATRT cell lines and a kidney MRT cell line, when compared with nonmalignant controls. MUC16 has also been implicated in several tumorigenic processes in MRT including invasion and metastasis. MUC16 interacts with mesothelin, another highly expressed biomarker in MRT, which supports the development of peritoneal and pleural metastasis.¹⁹ MUC16 also activates AKT/ERK pathways, promoting cell proliferation and survival.¹⁹ Although our case series suggests a relationship between INI-1 loss and MUC16 expression, this connection is not yet established in ES and MRT. However, SMARB1 loss in renal medullary carcinoma has been shown to cause epigenetic dysregulation, causing upregulation of MUC16 expression.²⁰

This case report and single institutional review of MUC 16 expression across a series of pediatric and AYA sarcoma tumor samples support the inclusion of MUC16 IHC in the routine diagnostic workup of all advanced or metastatic INI1-deficient sarcomas, particularly those that have exhausted conventional therapeutic options, while providing a rationale for targeting MUC16, with limited generalizability. Interpatient variability of MUC16 expression within ES and MRT cohorts could also affect the efficacy of ubamatamab and the durability of its responses.

This study represents a promising example of disease response to ubamatamab, and the first demonstration of therapeutic targeting of MUC16 in sarcoma. Repeat biopsy at progression on ubamatamab has offered several key insights into the dynamics of MUC16 and CA125 in this context, underscoring the overall importance of serial tumor sampling to understand the underlying mechanisms of immune escape that can inform our future therapeutic strategies.

Future strategies in this area should prioritize validating the safety and efficacy of ubamatamab in larger cohorts of patients, including children and adolescents with MUC16-positive ES and potentially MRT. Concurrent correlative biology studies are necessary to establish meaningful biomarkers for monitoring treatment response and early resistance for patients receiving MUC16-directed therapy, in addition to developing further understanding about mechanisms of tumor resistance that can be leveraged to identify effective therapeutic combinations.

APPENDIX

TABLE A1.

Summary of Fluorescent IHC Assays

Antibody		Clone	Company and Catalog No.	Detection Method	Part No.
Fluorescent multiplex IHC assay—A
Rabbit anti-CD20	L26	Abcam, ab9475	DISCOVERY Rhodamine 6G	7988168001
Rabbit anti-CD3		SP162	Abcam, ab135372	DISCOVERY DCC	7988192001
Rabbit anti-CD8		SP239	Abcam, ab178089	DISCOVERY Rhodamine 610	7988176001
Rabbit anti-CD68		SP251	Abcam, ab192847	DISCOVERY Cy5	7551215001
Rabbit anti-FOXP3		SP97	Abcam, ab99963	DISCOVERY FAM	7988150001
Mouse anti-pan keratin		AE1/AE3/PCK26	Ventana, 5266840001	Cy7 Goat-Anti Mouse	Bioquest, 16856
Fluorescent multiplex IHC assay—B
Mouse anti-PD1		NAT105	Abcam, ab52587	DISCOVERY Rhodamine 6G	7988168001
Mouse anti-LAG3		17B4	Abcam, ab40466	DISCOVERY DCC	7988192001
Rabbit anti-CD28		EPR22076	Abcam, ab243228	DISCOVERY Rhodamine 610	7988176001
Rabbit anti-CD8		SP239	Abcam, ab178089	DISCOVERY Cy5	7551215001
Rabbit anti-CD3		SP162	Abcam, ab135372	DISCOVERY FAM	7988150001
Mouse anti-pan Keratin		AE1/AE3/PCK26	Ventana, 5266840001	Cy7 Goat-Anti Mouse	Bioquest, 16856
Single fluorescent IHC assay—C
Mouse anti-MUC16		X325	Abnova, MAB0388	DISCOVERY Cy5	7551215001

Abbreviations: IHC, immunohistochemistry; MUC16, mucin 16.

DATA SHARING STATEMENT

A data sharing statement provided by the authors is available with this article at DOI https://doi.org/10.1200/PO-25-00404.

AUTHOR CONTRIBUTIONS

Conception and design: Gabriel Revon-Rivière, Rose Chami, Thomas S. Uldrick, David A. Knorr, Priscila Goncalves, Michael Dobosz, Daniel A. Morgenstern

Financial support: Daniel A. Morgenstern

Administrative support: Daniel A. Morgenstern

Provision of study materials or patients: Gabriel Revon-Rivière, Daniel A. Morgenstern

Collection and assembly of data: Denise M. Connolly, Gabriel Revon-Rivière, Rose Chami, Denise Mills, Thomas S. Uldrick, David A. Knorr, Priscila Goncalves, Michael Dobosz, Sumreen Jalal, Daniel A. Morgenstern

Data analysis and interpretation: Denise M. Connolly, Gabriel Revon-Rivière, Rose Chami, Ailish C. Coblentz, Thomas S. Uldrick, David A. Knorr, Priscila Goncalves, Michael Dobosz, Sarah Cohen-Gogo, Daniel A. Morgenstern

Manuscript writing: All authors

Final approval of manuscript: All authors

Accountable for all aspects of the work: All authors

AUTHORS' DISCLOSURES OF POTENTIAL CONFLICTS OF INTEREST

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