Highlights
– In a small cohort of patients with metastatic melanoma (n=14 evaluable), low‑dose subcutaneous IL‑2 after lymphodepletion and TIL infusion produced clinical outcomes comparable to high‑dose intravenous IL‑2 when followed by pembrolizumab.
– Overall response rates were low in both arms; one durable ongoing partial response (>76 months) occurred in the low‑dose arm.
– IL‑2 dose did not significantly alter circulating T‑cell frequency, phenotype, or proliferation; anti‑PD‑1 (pembrolizumab) administered starting 21 days after TIL infusion did not augment peripheral T‑cell proliferation.
– Low‑dose IL‑2 was associated with slightly lower rates of grade 3 febrile neutropenia and shorter hospitalization, suggesting potential safety and resource advantages.
Background
Adoptive transfer of tumor‑infiltrating lymphocytes (TILs) is an established immunotherapy for metastatic melanoma with durable responses in a subset of patients. Historically, TIL therapy has been administered with lymphodepletion and high‑dose interleukin‑2 (IL‑2) to support infused T‑cell survival and expansion, but high‑dose IL‑2 causes substantial toxicity and requires intensive inpatient monitoring. The optimal post‑TIL cytokine regimen and the best way to integrate checkpoint blockade (e.g., anti‑PD‑1 antibodies) remain unresolved. The study by Hasanov et al. examines whether lower‑intensity IL‑2 can preserve immunologic and clinical benefit while reducing toxicity when combined with pembrolizumab started three weeks after TIL infusion.
Study design
This investigation enrolled patients with metastatic melanoma who underwent lymphodepletion with cyclophosphamide and fludarabine, followed by infusion of unengineered TILs. Two IL‑2 dosing strategies were compared in parallel arms:
- Arm‑1 (High‑dose IL‑2, HD): IL‑2 720,000 IU/kg IV every 8 hours, up to 15 doses.
- Arm‑2 (Low‑dose IL‑2, LD): IL‑2 2 million IU subcutaneously daily for 14 days.
All patients received pembrolizumab 200 mg IV starting 21 days after TIL infusion and every 3 weeks thereafter for up to 2 years. The primary endpoint was overall response rate (ORR) by RECIST 1.1. Longitudinal peripheral blood sampling was performed for flow cytometry and cytokine analysis to assess immune correlates.
Key findings
Clinical outcomes
The study reports small, balanced cohorts (n=7 per arm) with the following best responses:
- Arm‑1 (HD IL‑2): 1 partial response (PR) lasting 10 months, 2 patients with stable disease (SD), 3 with progressive disease (PD), and 1 not evaluable (NE).
- Arm‑2 (LD IL‑2): 1 ongoing PR (>76 months and counting), 1 SD, and 5 PD.
Because of the small sample size, formal statistical comparison of ORR was not reported as definitive. The single durable PR in the low‑dose arm is notable but must be interpreted cautiously given the numbers.
Safety and resource use
Toxicity profiles were broadly comparable between arms. However, patients receiving low‑dose IL‑2 had a lower incidence of grade 3 febrile neutropenia (57% vs. 71% in the high‑dose arm) and shorter median hospitalization (16 days vs. 18 days). These differences suggest potential advantages in patient safety, tolerability, and health‑care utilization with subcutaneous low‑dose IL‑2, although both regimens remain associated with significant peri‑procedural risk due to lymphodepletion and TIL infusion.
Immune correlates
Peripheral blood flow cytometry and cytokine analyses found no consistent differences between IL‑2 dosing arms in the frequency, phenotype, or proliferation of circulating T‑cell subsets. The investigators also report that initiation of pembrolizumab at day 21 post‑TIL did not produce measurable increases in peripheral T‑cell proliferation. No clear correlation between the overall TIL phenotype and clinical response was observed across the cohort; however, patients who achieved partial responses received larger absolute numbers of TILs with relatively high CD8+/CD4+ ratios.
Interpretation and expert commentary
This study addresses a clinically important question: can lower‑intensity IL‑2 support TIL engraftment and anti‑tumor activity while reducing toxicity and resource demands? The principal finding — no major differences in peripheral immune readouts or clinical responses between high‑ and low‑dose IL‑2 when combined with TILs and delayed pembrolizumab — supports the hypothesis that subcutaneous low‑dose IL‑2 may be an acceptable alternative to intensive inpatient high‑dose IL‑2 in this setting.
Strengths
- Prospectively collected clinical outcomes and longitudinal immune monitoring allow for a direct comparison of IL‑2 strategies following a uniform lymphodepletion and TIL infusion protocol.
- The long, ongoing durable response (>76 months) in the LD arm provides proof of principle that durable remissions are achievable without high‑dose IL‑2 in selected patients.
Limitations
- Very small sample size (n=14 treated patients) limits statistical power and the ability to draw definitive efficacy conclusions.
- The study design and reporting do not provide clarity on randomization or stratification; potential selection bias cannot be excluded.
- Peripheral blood assays may not fully capture intratumoral T‑cell dynamics, trafficking, and functional states of infused TILs.
- Delayed start of pembrolizumab (day 21) may have reduced the potential for synergistic expansion of infused TILs; alternative timing or concurrent approaches deserve study.
Biological plausibility and mechanistic considerations
IL‑2 supports T‑cell survival and proliferation but also expands regulatory T cells at certain doses and can induce activation‑induced cell death. High‑dose IV IL‑2 has been used historically to maximize expansion of transferred T cells but at the cost of systemic toxicity. Subcutaneous low‑dose IL‑2 may provide sufficient trophic support for transferred T cells without excessive toxicity and may preferentially support effector memory and exhausted populations when combined with checkpoint blockade. The absence of a measurable rise in peripheral proliferation after pembrolizumab suggests either that pembrolizumab’s main effects were intratumoral or that timing and systemic indicators used were not optimal to detect PD‑1 blockade effects on transferred cells.
Clinical implications and future directions
For clinicians and trialists, these data provide a rationale for further study of lower‑intensity IL‑2 regimens after TIL therapy, especially where resource constraints or patient comorbidity make high‑dose IL‑2 unattractive. Larger, randomized trials are required to establish non‑inferiority of low‑dose IL‑2 for efficacy and to quantify safety and quality‑of‑life benefits. Key questions for future research include:
- Is there an optimal timing for checkpoint blockade relative to TIL infusion to maximize synergy?
- Can biomarkers (e.g., TIL neoantigen specificity, T‑cell receptor clonality, intratumoral persistence) predict which patients can do well with low‑dose IL‑2?
- Should alternative cytokines (IL‑7, IL‑15, or engineered IL‑2 variants that favor CD8+ effectors over Tregs) be tested to improve potency with lower toxicity?
- How generalizable are these findings across centers with variable TIL manufacturing practices and patient populations?
Conclusion
This small prospective study indicates that subcutaneous low‑dose IL‑2 administered after lymphodepletion and TIL infusion, followed by delayed pembrolizumab, yields similar peripheral immune readouts and broadly comparable clinical outcomes to traditional high‑dose IV IL‑2, with modest reductions in some toxicities and shorter hospital stays. The results support further evaluation of lower‑intensity IL‑2 regimens in larger, controlled trials, with attention to sequencing with checkpoint blockade and to robust intratumoral immune monitoring and predictive biomarkers.
Funding and clinicaltrials.gov
Funding sources and clinicaltrial registration identifiers should be consulted directly from the published article citation below. The primary report for these data is:
References
Hasanov M, Kiany S, Forget MA, Bassett R, Davies MA, Diab A, Gershenwald JE, Glitza IC, Lee JE, Lucci A, McQuade JL, Patel SP, Ross MI, Tawbi HA, Wargo JA, Wong MK, Bernatchez C, Hwu P, Haymaker C, Amaria RN. Lymphodepletion, tumor-infiltrating lymphocytes, and high versus low dose IL-2 followed by pembrolizumab in patients with metastatic melanoma. Oncoimmunology. 2025 Dec;14(1):2546402. doi: 10.1080/2162402X.2025.2546402. Epub 2025 Aug 15. PMID: 40815607; PMCID: PMC12360205.
For additional context on TIL therapy and IL‑2 historically, readers should consult current guideline documents and comprehensive reviews in the field.
Thumbnail image prompt
Close‑up scene of a modern oncology infusion suite: a middle‑aged patient seated calmly receiving an IV infusion, clinicians in the background reviewing imaging on a tablet, and semi‑transparent scientific overlays of T‑cells and an illustrated melanoma lesion. Style: realistic medical photography with soft clinical lighting, muted blues and whites, high detail, editorial center composition.