Sonelokimab Phase III Win Puts the IL-17A/F Nanobody Strategy in Focus

MoonLake Immunotherapeutics has placed the IL-17 pathway back in the spotlight with positive topline Phase III results for sonelokimab, a humanized single-domain antibody designed to inhibit both IL-17A and IL-17F. In the global IZAR-1 study, which enrolled biologic-naive adults with active psoriatic arthritis, the trial successfully met its primary endpoint and delivered improvements across multiple clinical measures ^[1].

This is not just another IL-17 success story. It is a critical clinical readout testing whether a compact, multi-domain Nanobody format can offer a differentiated approach in one of immunology’s most competitive therapeutic areas.

IL-17 Is a Proven Target, but Differentiation Is Getting Harder

IL-17 has become one of the central therapeutic pathways in autoimmune and inflammatory disease. IL-17A, a major effector cytokine associated with Th17 biology, contributes to inflammatory processes involved in diseases including psoriasis, psoriatic arthritis and ankylosing spondylitis.

Figure 1. IL-17 signalling in inflammation and regeneration ^[2].

The commercial and clinical success of IL-17 blockade has consequently attracted extensive drug development activity. The source material notes that multiple IL-17-targeting monoclonal antibodies are already approved globally, while therapies such as secukinumab have established the value of directly inhibiting IL-17A. Bimekizumab subsequently extended the strategy by inhibiting both IL-17A and IL-17F, demonstrating the clinical potential of broader blockade within the IL-17 family.

This success also creates a development challenge. Once a pathway has been strongly validated, new therapeutics must demonstrate more than target engagement. Developers increasingly need meaningful differentiation in efficacy, dosing, tissue exposure, safety, patient stratification or molecular format. For IL-17, simply combining it with another inflammatory target has not consistently produced that differentiation.

Why Combining IL-17 With Other Targets Is Not Straightforward

The biology of IL-17 proves that rational combination strategies do not guarantee superior clinical outcomes.

One example is dual inhibition of TNF-α and IL-17A. AbbVie’s ABT-122, a bispecific antibody targeting TNF-α and IL-17A, failed to demonstrate a clear overall efficacy advantage over adalimumab in psoriatic arthritis Phase II trials^[3].

The relationship between IL-23 and IL-17 is even more complex. IL-23 is generally regarded as an important upstream regulator of IL-17-producing immune responses, which might suggest that simultaneous inhibition of both nodes would provide stronger pathway suppression. Clinical observations, however, indicate that the IL-23/IL-17 axis behaves differently across tissues and diseases.

In axial inflammatory disease, IL-17 inhibition can be effective even where IL-23-targeted therapies have failed, suggesting that some local IL-17 production may occur through mechanisms that are not strictly dependent on IL-23 ^[4]. In the gut, the situation is different again. The source material highlights evidence that IL-17 can contribute to intestinal barrier integrity, creating the possibility that blocking IL-17 in gastrointestinal disease could have context-dependent or even detrimental biological effects ^[5]. These examples reinforce an important principle for drug developers: cytokine networks are not simple linear cascades. The therapeutic value of inhibiting an inflammatory mediator depends on anatomical location, immune-cell context, disease mechanism and the degree of pathway suppression.

Figure 2. Context-dependent roles of IL-17 across tissues, illustrating distinct IL-17/IL-23 regulation and the contrasting effects of IL-17 inhibition on joint inflammation and intestinal barrier homeostasis ^[6].

IL-17 biology therefore presents a paradox. It is highly druggable and clinically validated, but aggressive expansion of pathway blockade may also narrow the therapeutic window. That makes molecular format an increasingly interesting source of innovation.

Sonelokimab Tests a Different Approach to IL-17A/F Blockade

Sonelokimab is described as a trivalent humanized camelid-derived Nanobody that targets IL-17A, IL-17F and human serum albumin.

Compared with a conventional full-length IgG antibody of approximately 150 kDa, single-domain antibodies are substantially smaller. Their compact architecture can potentially improve access to sterically restricted epitopes and influence tissue distribution. Sonelokimab combines IL-17A/F inhibition with an albumin-binding domain intended to extend exposure and support localization within inflamed tissues.

The IZAR-1 Phase III study evaluated subcutaneous sonelokimab in biologic-naive adults with active psoriatic arthritis. At week 16, the source material reports that 42.1% of participants achieved ACR50, while 66.5% achieved ACR20 and 41.2% reached minimal disease activity. Among patients with concomitant psoriasis, 61% reportedly achieved PASI90 ^[1]. The study did not include an active comparator, and more complete analyses remain important for interpreting how the therapy ultimately compares with established treatment options.

That distinction matters. Positive placebo-controlled Phase III results validate clinical activity, but they do not by themselves prove that a Nanobody is superior to conventional IL-17 antibodies. Questions surrounding comparative efficacy, durability, safety, pharmacokinetics and performance in treatment-experienced populations remain important.

Nevertheless, IZAR-1 provides another clinical test of the idea that single-domain antibody engineering can create new development opportunities even against highly established targets.

From Target Discovery to Fully Human Single-Domain Antibodies

The clinical progress of sonelokimab highlights a pressing preclinical challenge: traditional VHH discovery typically relies on camelid-derived sequences, necessitating complex and time-consuming humanization during development.

To bypass this bottleneck, developers are turning to fully human in vivo generation. Cyagen’s HUGO-Nano™ Fully Human Single-Domain Antibody Discovery Platform is engineered precisely for this strategy. By silencing endogenous mouse light-chain expression, this genetically engineered model directly produces fully human heavy-chain-only antibodies incorporating human heavy-chain variable-region sequences.

Figure 3. VDJ rearrangement expression analysis of heavy-chain antibody sequences in splenic B cells from HUGO-Nano™ mice. The results demonstrate that HUGO-Nano™ mice exhibit a highly diverse heavy-chain antibody repertoire.

Because the resulting candidates use a single-domain architecture, they can serve as modular building blocks for several therapeutic formats, including bispecific and multispecific antibodies, antibody-drug conjugates and other engineered biologics. Their compact structure may also facilitate access to epitopes that are difficult for conventional IgG molecules to reach.

For programs exploring IL-17 or other crowded immune targets, this type of platform offers a way to investigate whether format innovation, rather than simply additional target combinations, can create meaningful therapeutic differentiation.

👉 Explore our HUGO-Nano™ Fully Human Single-Domain Antibody Discovery Platform

Translational Models: The Final Piece of the Puzzle

Antibody discovery is only one part of the development process. Advancing sophisticated IL-17 programs requires precise in vivo translational pharmacology models. Cyagen addresses this with fully humanized IL-17 models for this purpose, including IL-17A and IL-17F single-humanized models as well as dual-target humanized configurations.

Proof of Concept: In our huIL-17A mouse model, imiquimod induces psoriasis-like skin inflammation accompanied with upregulated human IL-17A. Therapeutic intervention with the anti-IL-17A antibody ixekizumab improved skin manifestations in a dose-dependent manner, reduced PASI scores and improved histopathological abnormalities associated with IMQ-induced inflammation.

Figure 4. PASI scores of 8-week-old female huIL-17A mice and wild-type mice (B6N) after IMQ treatment and Ixekizumab treatment.

Such models can help researchers connect molecular design with in vivo pharmacology when evaluating antibodies targeting human cytokine pathways.

👉 View Full Validation of the huIL17A Humanized Model

What the Sonelokimab Result Means for the IL-17 Field

The IL-17 therapeutic landscape is unlikely to be transformed simply by adding more molecules against the same cytokine. Instead, future differentiation may increasingly come from how IL-17 is targeted: broader IL-17A/F coverage, alternative antibody architectures, optimized tissue exposure, multispecific engineering and better alignment between mechanism and disease biology.

Sonelokimab’s Phase III progress makes single-domain antibodies particularly relevant to that discussion. It does not yet establish Nanobodies as the preferred format for IL-17 therapy, but it provides an important clinical example of how antibody engineering may extend innovation within a mature target class.

For researchers developing next-generation single-domain antibodies or evaluating IL-17-targeted therapeutics, Cyagen provides HUGO-Nano™ fully human single-domain antibody discovery, humanized IL-17 models and in vivo pharmacology support to help move programs from antibody generation toward functional validation.

Explore HUGO-Nano™ and Cyagen’s humanized models, or contact our scientific team to discuss an IL-17 or single-domain antibody development program.

👉 Consult with Our Experts to Design Your Next High-Impact Project

Reference

[1] MoonLake Immunotherapeutics. Positive topline results from the Phase 3 IZAR-1 trial of sonelokimab in psoriatic arthritis [Internet]. Zug (Switzerland): MoonLake Immunotherapeutics; 2026 Aug 10 [cited 2026 Aug 13]. Available from: https://ir.moonlaketx.com/news-releases/news-release-details/moonlake-announces-positive-topline-results-phase-3-izar-1-trial

[2] Adamopoulos IE, Kuchroo V. IL-17A and IL-17F in tissue homeostasis, inflammation and regeneration. Nat Rev Rheumatol. 2023 Sep;19(9):535-536. doi: 10.1038/s41584-023-01004-5. PMID: 37488297; PMCID: PMC10709714.

[3] Genovese MC, Weinblatt ME, Aelion JA, Mansikka HT, Peloso PM, Chen K, Li Y, Othman AA, Khatri A, Khan NS, Padley RJ. ABT-122, a Bispecific Dual Variable Domain Immunoglobulin Targeting Tumor Necrosis Factor and Interleukin-17A, in Patients With Rheumatoid Arthritis With an Inadequate Response to Methotrexate: A Randomized, Double-Blind Study. Arthritis Rheumatol. 2018 Nov;70(11):1710-1720. doi: 10.1002/art.40580. Epub 2018 Oct 10. PMID: 29855172; PMCID: PMC6704363.

[4] Li H, Tsokos GC. IL-23/IL-17 Axis in Inflammatory Rheumatic Diseases. Clin Rev Allergy Immunol. 2021 Feb;60(1):31-45. doi: 10.1007/s12016-020-08823-4. Epub 2020 Nov 13. PMID: 33185790; PMCID: PMC8018566.

[5] Țiburcă L, Bembea M, Zaha DC, Jurca AD, Vesa CM, Rațiu IA, Jurca CM. The Treatment with Interleukin 17 Inhibitors and Immune-Mediated Inflammatory Diseases. Curr Issues Mol Biol. 2022 Apr 26;44(5):1851-1866. doi: 10.3390/cimb44050127. PMID: 35678656; PMCID: PMC9164043.

[6] Fauny M, Moulin D, D'Amico F, Netter P, Petitpain N, Arnone D, Jouzeau JY, Loeuille D, Peyrin-Biroulet L. Paradoxical gastrointestinal effects of interleukin-17 blockers. Ann Rheum Dis. 2020 Sep;79(9):1132-1138. doi: 10.1136/annrheumdis-2020-217927. Epub 2020 Jul 21. PMID: 32719044.