Patients with recurrent or refractory gynecologic malignancies who progress after immunotherapy face limited treatment options and a poor prognosis. Denileukin diftitox (Lymphir®; Citius Oncology)* is a recombinant fusion protein composed of interleukin-2 and diphtheria toxin that targets IL-2R-expressing cells and transiently depletes regulatory T-cells, potentially enhancing the efficacy of immune checkpoint inhibitors (ICIs).
Denileukin diftitox selectively binds to IL-2 receptors on the cell surface, allowing entry of diphtheria toxin fragments, which subsequently inhibit protein synthesis in target cells. This mechanism enables denileukin diftitox to target both malignant T-cells and immunosuppressive regulatory T-cells (Tregs). Transient depletion of Tregs may enhance the patient’s immune response against tumor cells.
Preclinical studies have shown that denileukin diftitox effectively depletes murine Tregs in vivo and human Tregs ex vivo. Furthermore, combining denileukin diftitox with anti-mouse PD-1 therapy resulted in improved tumor responses and significantly prolonged survival in a syngeneic mouse solid-tumor model compared with either agent alone.
Study design
An open-label, dose-escalation Phase 1/2, investigator-initiated study (NCT05200559) evaluated the safety and preliminary efficacy of denileukin diftitox in combination with pembrolizumab (Keytruda®; Merck & Co/MSD) in this patient population.
The study, conducted at UPMC Magee-Womens Hospital and funded by Citius Oncology, enrolled 25 patients with recurrent or metastatic solid tumors, predominantly gynecologic cancers. Eligible patients had received a median of five prior systemic therapies; over half had prior anti-PD-1/PD-L1 exposure. Denileukin diftitox was administered intravenously on Days 1–3 of each 21-day cycle at escalating doses (3, 6, 9, 12 mcg/kg), combined with pembrolizumab (200 mg IV) on Day 1. Patients completing eight cycles of combination therapy continued on pembrolizumab monotherapy until progression. Efficacy was assessed using RECIST v1.1 criteria.
Results:
Among 21 efficacy-evaluable patients:
- Overall Response Rate (ORR): 24% (5 partial responses); no complete responses.
- Median Duration of Response (mDOR): Not reached; current range 4.2–35 months (median: 21.1 months); 80% of responders maintained benefit at data cutoff.
- Clinical Benefit Rate (CR + PR + SD ≥6 months): 48% (10/21); median progression-free survival (mPFS) among these patients was 20.5 months (95% CI: 6.5–NA).
- Overall mPFS: 5.8 months (95% CI: 2.2–NA).
- Endometrial Cancer Cohort: 33% ORR in patients previously treated with ICIs, including one ongoing response >3 years.
- Safety: Of 24 patients evaluable for DLTs, one patient with a reversible grade 3 capillary leak syndrome was observed; the maximum tolerated dose was not reached. No new safety signals or grade ≥3 immune-related adverse events were observed.
Combination
The combination of denileukin diftitox and pembrolizumab demonstrated promising clinical activity and a manageable safety profile in heavily pre-treated patients with recurrent or refractory gynecologic malignancies, including those previously exposed to ICIs. Durable responses and prolonged disease control were observed, supporting continued clinical investigation. Translational analyses are underway to further elucidate immunologic effects and identify predictive biomarkers.
A Phase 2 expansion is planned.
Denileukin diftitox plus pembrolizumab shows potential as a novel, chemo-free immunotherapeutic strategy for recurrent or refractory gynecologic cancers, warranting further study in larger, less heavily pretreated populations.
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Note:* Denileukin diftitox is a recombinant fusion protein composed of human interleukin-2 (IL-2) linked to diphtheria toxin fragments A and B, utilized as an antineoplastic agent for the treatment of cutaneous T-cell lymphomas expressing IL-2 receptors. Administration at higher doses may lead to mild-to-moderate increases in serum liver enzymes and bilirubin; however, clinically significant acute liver injury is rare.
Clinical trials
T-regulatory Cell Depletion With E7777 Combined With Pembrolizumab in Recurrent or Metastatic Solid Tumors – ClinicalTrials.gov ID NCT05200559
Highlights of Prescribing Information
Denileukin diftitox (Lymphir®; Citius Oncology)[Prescribing Information]
Pembrolizumab (Keytruda®; Merck & Co/MSD)[Prescribing Information]
References:
[1] Siegel RL, Kratzer TB, Wagle NS, Sung H, Jemal A. Cancer statistics, 2026. CA Cancer J Clin. 2026;e70043. doi:10.3322/caac.70043
[2] National Cancer Institute. SEER Cancer Stat Facts: Uterine and Ovarian Cancer. Online. Last Accessed on June 1, 2026.
Featured image courtesy © 2016 – 2026 ASCO. Used with permission
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