WT1 Immunotherapy for Mesothelioma: The #1-Ranked Cancer Target With 0 Approved Treatments in 2026

What Are the Key Facts About WT1 Immunotherapy in Mesothelioma?

• WT1 is the top-ranked cancer antigen. An NCI project ranked it #1 of 75 candidates for immunotherapy targeting [1].
• It is overexpressed in most epithelioid mesotheliomas and is already one of the four standard markers pathologists use to diagnose the disease.
• No WT1 therapy is approved for mesothelioma — not the galinpepimut-S vaccine, nor any WT1 T-cell therapy.
• Galinpepimut-S is the most advanced WT1 program — a tetravalent peptide vaccine developed at Memorial Sloan Kettering and licensed by SELLAS Life Sciences [11].
• The 2017 adjuvant Phase 2 was encouraging — a survival signal in patients who had completed surgery and chemotherapy [2].
• The 2024 Phase 1 combination was disappointing — 7.4-month median overall survival and zero objective responses in pretreated patients [3].
• The mesothelioma program is effectively paused. SELLAS's lead asset is now the Phase 3 REGAL trial in acute myeloid leukemia [8].
• WT1 cannot be targeted by conventional CAR-T because it is intracellular; mesothelioma CAR-T trials target mesothelin instead [12].
• "WT1-positive" carries a modest adverse prognosis (pooled hazard ratio ~1.48 for overall survival) but does not change current treatment [10][4].
• Approved first-line immunotherapy is nivolumab + ipilimumab [6] or pembrolizumab + chemotherapy [7].

Why Is WT1 Such an Attractive Target in Mesothelioma?

The comprehensive review of active immunotherapy targeting the Wilms' tumor protein 1 (WT1) [1] placed WT1 at the very top, scoring it on therapeutic function, immunogenicity, oncogenic role, specificity, expression level, and the number of antigen-positive patients [1]. Three properties explain the ranking.

First, selectivity: WT1 is highly overexpressed in tumors but restricted to a few normal tissues at low levels, which limits the risk of an immune attack damaging healthy organs. Second, functional dependence: mesothelioma and other cancer cells rely on WT1 to keep proliferating, so they rarely shed the antigen to escape immune recognition — a failure mode that defeats many other targeted therapies. Third, immunogenicity: across 21 early-phase WT1 vaccine trials in various cancers, measurable clinical responses (including stable disease) occurred in roughly 46% of evaluable solid-tumor patients and 64% of patients with blood cancers [1].

"On paper, WT1 is close to an ideal target — the tumor needs it, it's visible to the immune system, and it's already in nearly every mesothelioma pathology report. That's exactly why the gap between the biology and the clinical results is so frustrating. A great target is necessary, but it has never been sufficient on its own."

— David Foster, Executive Director of Client Services, Danziger & De Llano

WT1 is also part of the standard diagnostic immunohistochemistry panel for mesothelioma, alongside calretinin, cytokeratin 5/6, and podoplanin (D2-40). It is worth understanding what distinguishes these markers, because patients often see all of them on a pathology report and assume they are interchangeable. They are not.

How Does WT1 Differ From Calretinin and Mesothelin?

These three proteins play different roles — diagnosis, prognosis, and therapeutic targeting are not the same job.

Protein	Primary role	Can it be targeted by immunotherapy?
WT1	Diagnosis + prognosis; potential vaccine/TCR target	Yes — but only through HLA peptide presentation (vaccines, TCR T-cells), not CAR-T
Calretinin	Diagnostic workhorse marker (~90% sensitivity in epithelioid disease)	No — it is an intracellular calcium-binding protein, not a viable target
Mesothelin	Diagnosis + the leading cell-surface therapeutic target	Yes — it is the antigen for current mesothelioma CAR-T and antibody programs

The practical distinction is that calretinin is diagnostic only, mesothelin is the dominant surface target for CAR-T cell therapy, and WT1 sits in a unique niche as an intracellular antigen suited to vaccines and T-cell-receptor-engineered therapy. Because WT1 lives inside the cell, conventional CAR-T — which recognizes surface proteins directly — cannot see it [12].

What Is Galinpepimut-S (GPS), and What Did the Trials Show?

Galinpepimut-S is a tetravalent, non–HLA-restricted, heteroclitic WT1 peptide vaccine originally designed at Memorial Sloan Kettering and licensed by SELLAS Life Sciences [11]. In plain terms, it contains four WT1-derived peptides — including synthetic "heteroclitic" versions engineered to bind the immune system's HLA molecules more tightly and break the body's tolerance to its own WT1 — and is given with immune-stimulating adjuvants to provoke both helper and killer T-cell responses [3].

The 2017 adjuvant Phase 2 — the encouraging signal

The mesothelioma development path began with a randomized Phase II trial of adjuvant galinpepimut-S WT-1 analogue peptide vaccine after multimodality therapy in patients who had completed multimodality therapy — surgery plus chemotherapy, with or without radiation [2]. Patients were randomized to GPS plus adjuvants versus adjuvants alone. The control arm was stopped early for futility, and the most encouraging summary figures reported a median overall survival of about 22.8 months in the GPS arm versus 18.3 months in the control group, with the strongest benefit in patients who had a complete surgical resection and those who mounted a measurable WT1 immune response. SELLAS announced plans in 2016 to launch a pivotal Phase 2b/3 mesothelioma trial — a trial that was never initiated.

The 2024 Phase 1 combination — the disappointing reality

To try to boost the response, MSK ran a small Phase 1 study combining a WT1 vaccine (galinpepimut-S) with checkpoint inhibition (nivolumab) in diffuse pleural mesothelioma in WT1-positive, previously-treated diffuse pleural mesothelioma (NCT04040231), published in early 2025 [3]. Ten patients enrolled. The results were sobering:

Outcome	Result
Median progression-free survival	3.9 months
Median overall survival (from enrollment)	7.4 months
Best tumor response	0 partial responses; 3 patients with prolonged stable disease
WT1-specific T-cell response	3 of 10 patients (30%)

The combination was tolerable, with no unexpected toxicity attributed to the vaccine — but it showed limited clinical activity in this small, heavily pretreated, second-line group. The authors themselves noted that the benefit was "limited possibly owing to the small sample size" [3]. The 30% immune-response rate also fell short of expectations; in the company's acute myeloid leukemia program, an interim analysis reported an 80% WT1-specific T-cell response rate [8].

"Families ask me whether there's a mesothelioma vaccine they can get. I tell them the truth: there's a promising idea with a real 2017 signal, a disappointing 2024 result, and no open trial today. Hope is not a treatment plan. The honest move is to put a patient on the therapy that has Phase 3 evidence right now and bank tissue for the science that might mature later."

— David Foster, Executive Director of Client Services, Danziger & De Llano

Where Does WT1 Treatment Development Stand Today?

SELLAS has redirected its lead development entirely to acute myeloid leukemia, where the Phase 3 REGAL trial of galinpepimut-S is the company's flagship program [8]. An independent monitoring committee recommended continuing the trial after a January 2025 interim analysis, and final results have been anticipated around the end of 2025. The company's pipeline still lists mesothelioma, but with no active or planned mesothelioma trial beyond the published Phase 1 [11].

Other WT1 approaches exist but, again, almost entirely outside mesothelioma. WT1-specific T-cell-receptor (TCR) gene therapy has been tested in acute myeloid leukemia [5], and WT1-pulsed dendritic-cell vaccines have been studied in leukemia, glioblastoma, and ovarian cancer — but no WT1 TCR-T or dendritic-cell trial is currently recruiting mesothelioma patients. The realistic path forward for WT1 in mesothelioma now depends on the REGAL leukemia readout succeeding and unlocking the investment to revisit the disease, ideally testing the vaccine alongside today's standard of care rather than in the failed second-line salvage setting.

How Does WT1 Therapy Compare to Approved Mesothelioma Immunotherapy?

This is the comparison that matters for patients, because it is the difference between a hypothetical and a prescription. Three regimens form the approved immunotherapy and device backbone for unresectable pleural mesothelioma, and none of them target WT1 or require WT1 testing.

Regimen	FDA approval	Median overall survival
Nivolumab + ipilimumab (Opdivo + Yervoy)	October 2020, first-line	18.1 months vs. 14.1 months with chemotherapy (CheckMate 743) [6]
Pembrolizumab + pemetrexed/platinum	September 2024, first-line	17.3 months vs. 16.1 months; response rate 52% vs. 29% [7]
Tumor Treating Fields (Optune Lua) + chemotherapy	May 2019, device	18.2 months vs. ~12 months historical (STELLAR)

The GPS-plus-nivolumab second-line survival of 7.4 months cannot be fairly compared to first-line CheckMate 743's 18.1 months — these are entirely different patient populations [3][6]. The honest benchmark is second-line salvage after chemotherapy failure, where historical survival runs roughly 5 to 9 months, placing GPS in the lower-middle of that range without a clear improvement. The 2017 adjuvant data remain the strongest scientific rationale for ever returning to WT1 in this disease. For a fuller view of the approved options, see our overview of the immunotherapy standard of care and the WikiMesothelioma immunotherapy reference.

What Should Mesothelioma Patients and Caregivers Actually Do?

The most important thing to understand is that WT1 immunotherapy is not a treatment you can pursue today — but the WT1 status in your pathology report still matters, and there are concrete, useful steps to take now.

• Confirm your pathology at a high-volume mesothelioma center. WT1 should already appear on the standard four-marker panel with calretinin, CK5/6, and D2-40.
• Start evidence-based first-line therapy. In 2026 that means nivolumab + ipilimumab or pembrolizumab + chemotherapy, with Tumor Treating Fields available for unresectable disease [6][7].
• Ask about tissue banking and HLA typing. Preserving tissue keeps the door open to TCR-based therapies and future WT1 trials, which depend on both antigen expression and your HLA type.
• Consider open immunotherapy trials — for example, mesothelin-targeted CAR-T programs (such as the NCI's NCT04577326) — and a different cancer vaccine, the UV1 telomerase vaccine, which targets telomerase rather than WT1 [12].
• Watch the REGAL Phase 3 readout. A positive result in leukemia could catalyze renewed mesothelioma development of GPS [8].

"WT1-positive on a pathology report is not bad news you can act against today — but it's worth saving tissue over. The patients who will benefit when this science matures are the ones whose oncologists banked a usable sample and typed their HLA early. That's a fifteen-minute conversation that can matter years later."

— David Foster, Executive Director of Client Services, Danziger & De Llano

Frequently Asked Questions

Is the galinpepimut-S WT1 vaccine available for mesothelioma patients in 2026?

No. Galinpepimut-S is not FDA-approved and is not available outside a clinical trial. The most recent mesothelioma study (NCT04040231) is complete and published, and no WT1-targeted mesothelioma trial is currently recruiting [3][9].

Did the WT1 vaccine work in mesothelioma?

The results are mixed. A 2017 randomized Phase 2 trial after surgery and chemotherapy showed an encouraging survival signal [2], but a 2024 Phase 1 trial of GPS plus nivolumab in pretreated patients reported only 7.4-month median overall survival and no objective responses [3]. The program has not advanced to Phase 3 in mesothelioma.

What immunotherapy is approved for mesothelioma right now?

Nivolumab plus ipilimumab (approved October 2020) and pembrolizumab plus chemotherapy (approved September 2024) are FDA-approved first-line regimens; Tumor Treating Fields has been available with chemotherapy since 2019 [6][7]. None require WT1 testing.

What does "WT1-positive" mean for my prognosis?

WT1-positive tumors carry a modest adverse prognosis on average (pooled hazard ratio about 1.48 for overall survival), but the effect is small and should not change standard treatment decisions [10][4].

References

This article draws on peer-reviewed publications, FDA approval records, and ClinicalTrials.gov trial registrations. Trial outcome figures are reported as published; the 2017 Phase 2 survival summary reflects the early-termination context noted in the original report. Citations are listed in the references section above.