Low-Dose Interleukin-2 as Adjunct to Riluzole in ALS: Insights from the Phase 2b MIROCALS Trial

Highlights

• Low-dose IL-2 (2 MIU) combined with riluzole is safe and significantly enhances regulatory T-cell (Treg) counts in ALS patients.
• Unadjusted primary analysis showed non-significant mortality reduction; adjusted Cox regression accounting for prognostic covariates revealed significant survival benefit.
• CSF-phosphorylated neurofilament heavy-chain (CSF-pNFH), a biomarker reflecting neurodegeneration, stratifies response, with low levels identifying patients with nearly 50% risk reduction in death on IL-2LD therapy.
• Findings highlight the critical importance of controlling for ALS disease heterogeneity and incorporating biomarkers in clinical trial design and interpretation.

Background

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disorder marked by selective loss of upper and lower motor neurons, leading to muscle weakness, respiratory failure, and death typically within 3-5 years. Current treatments are limited, with riluzole offering modest survival benefit. Neuroinflammation and immune dysregulation contribute to ALS pathophysiology, implicating regulatory T cells (Tregs) as potential therapeutic targets to modulate neuroinflammatory processes.

Interleukin-2 (IL-2) at low doses selectively expands Tregs, restoring immune homeostasis. Prior phase 2a trials (e.g., IMODALS) demonstrated safety, Treg enhancement, and favorable biomarker modulation, supporting further investigation into IL-2LD as an add-on therapy to riluzole in ALS.

Key Content

Chronological Development of Low-Dose IL-2 in ALS

Initial early-phase trials assessed IL-2’s immunomodulatory effects in ALS. The IMODALS phase 2a trial (2015) randomized riluzole-treated ALS patients to placebo, 1 MIU, or 2 MIU IL-2, confirming dose-dependent Treg increases and reductions in plasma CCL2 without severe adverse events. However, clinical efficacy signals remained preliminary.

Building on this, the MIROCALS phase 2b trial (Bensimon et al., 2025) was designed to rigorously evaluate survival impact over 21 months in riluzole-naive ALS patients initiating riluzole, then randomized to IL-2LD or placebo adjunct.

Study Design and Population

The MIROCALS trial enrolled 220 patients diagnosed per revised El Escorial criteria (possible to definite ALS), aged 18-76, with symptom duration ≤24 months and slow vital capacity ≥70%. After a 12-18 week riluzole run-in, participants were randomized (1:1) to subcutaneous 2 MIU IL-2 daily for 5 consecutive days every 28 days or placebo over 18 months.

Primary and Secondary Outcomes

The primary endpoint was survival at 640 days (21 months). Secondary outcomes included ALSFRS-R decline, safety, and biomarkers: regulatory T-cell counts, CSF-phosphorylated neurofilament heavy-chain (CSF-pNFH; marker of neuronal injury), and plasma and CSF-chemokine ligand 2 (CCL2; inflammation marker).

Findings and Interpretation

– Of 220 randomized, 90 (41%) died during follow-up; all included in intention-to-treat (ITT) analysis.
– Unadjusted log-rank analysis showed a non-significant 19% risk reduction with IL-2LD (HR 0.81; 95% CI 0.54-1.22; p=0.33).
– Adjusted Cox regression accounting for pre-defined prognostic covariates revealed a significant 68% risk reduction (HR 0.32; 95% CI 0.14-0.73; p=0.007).
– Significant interaction between treatment effect and CSF-pNFH levels (p=0.001) was found.
– In patients with low CSF-pNFH (750-3700 pg/mL, ~70% of cohort), IL-2LD reduced risk of death by 48% (HR 0.52; 95% CI 0.30-0.89; p=0.016); no benefit in the high CSF-pNFH subgroup.
– IL-2LD was well tolerated; injection-site reactions and flu-like symptoms were mild to moderate.
– Treatment increased Tregs and decreased plasma CCL2 consistently throughout treatment.

Comparison with Prior Evidence

MIROCALS findings extend the IMODALS trial results, confirming the immunological efficacy of IL-2LD and introducing survival benefit evidence after adjustment for disease heterogeneity. These convergent results emphasize the immunomodulatory mechanism of IL-2LD and encourage biomarker-directed patient selection.

Mechanistic Insights

Low-dose IL-2 selectively promotes expansion and function of Tregs, a critical immune subset that attenuates neuroinflammation. Increased Tregs may reduce microglial activation and inflammatory cytokine production, potentially protecting motor neurons. Decreased plasma CCL2 indicates reduced chemokine-mediated inflammatory cell recruitment.

CSF-pNFH reflects axonal injury severity; patients with lower baseline neurodegeneration biomarkers may retain greater neural reserve, hence showing improved survival outcomes with immunomodulation. This biomarker-driven stratification underscores ALS heterogeneity in pathogenic mechanisms and treatment responses.

Expert Commentary

The MIROCALS trial is a pivotal study demonstrating the feasibility and potential benefit of modulating the immune system in ALS via IL-2LD. The discordance between unadjusted and adjusted analyses highlights the challenge of ALS clinical heterogeneity and the essential role of prognostic biomarker-guided analyses to uncover true treatment effects.

Current ALS therapeutic landscape is limited; riluzole and edaravone provide modest benefit. IL-2LD, acting via Treg enhancement, offers a novel immune-based modality targeting neuroinflammation, a critical pathogenic pathway.

Key strengths of MIROCALS include its large sample size, well-characterized cohort, long duration, and robust biomarker integration. Limitations include the absence of functional outcome differences reported at primary analysis and the need for replication in larger, more diverse populations. Survival benefit confined to low CSF-pNFH subgroups suggests a window of therapeutic opportunity before extensive neurodegeneration.

Future research should focus on:
– Confirming efficacy in phase 3 trials with stratification by CSF-pNFH or other neurodegeneration biomarkers.
– Exploring combination therapies integrating immunomodulation and neuroprotection.
– Investigating IL-2LD effects on functional decline and quality of life.

Guidelines currently do not recommend IL-2LD outside clinical trials. However, these data form a strong rationale for biomarker-guided, precision medicine trials in ALS.

Conclusion

The MIROCALS study advances ALS therapeutics by demonstrating, for the first time, that low-dose IL-2 add-on to riluzole is safe, immunologically active, and may reduce mortality in a biomarker-defined ALS subgroup. This trial underscores the necessity of accounting for disease heterogeneity in ALS research and highlights CSF-pNFH as a critical stratification marker.

Continued investigation is warranted to establish IL-2LD as a viable immunotherapy with potential to modify disease trajectory, ultimately improving outcomes in this devastating disorder with limited treatment options.

References

• Bensimon G, Leigh PN, Tree T, et al.; MIROCALS Study Group. Efficacy and safety of low-dose IL-2 as an add-on therapy to riluzole (MIROCALS): a phase 2b, double-blind, randomised, placebo-controlled trial. Lancet. 2025;405(10492):1837-1850. doi:10.1016/S0140-6736(25)00262-4. PMID: 40354799.
• Beers DR, Henkel JS, Zhao W, et al. Regulatory T cells in amyotrophic lateral sclerosis: potential neuroprotective mechanism. Front Immunol. 2011;2:50. doi:10.3389/fimmu.2011.00050
• Levine M, Asselin C, Boutin H, et al. Low-dose IL-2 in ALS (IMODALS): a phase 2a double-blind placebo-controlled study. EBioMedicine. 2020;59:102844. doi:10.1016/j.ebiom.2020.102844. PMID: 32651161
• Zetterberg H, Skillbäck T, Mattsson N, et al. Association of cerebrospinal fluid neurofilament light concentration with amyotrophic lateral sclerosis progression. JAMA Neurol. 2016;73(1):60-67. doi:10.1001/jamaneurol.2015.2984