Sonelokimab IZAR-1 Phase III Win: Regulatory Path Clear, Market Access Minefield Ahead

MoonLake's Sonelokimab Achieves Primary Endpoint in PsA Phase III Trial

MoonLake Immunotherapeutics announced positive topline results from its Phase III IZAR-1 trial (NCT06641076) for sonelokimab in biologic-naïve adults with active psoriatic arthritis (PsA). The trial met its primary endpoint at week 16, with 42.1% of patients in the 60mg arm achieving an American College of Rheumatology 50 (ACR50) response. Key secondary endpoints, including ACR20 (66.5%), Minimal Disease Activity (41.2%), and PASI90 (61%), were also met, demonstrating broad efficacy across musculoskeletal symptoms, skin disease, and patient quality of life. The data supports sonelokimab's potential as a comprehensive therapy for PsA, consistent with previous IL-17A/F programs.

• The Phase III IZAR-1 trial successfully met its primary endpoint, with 42.1% of biologic-naïve adults with active psoriatic arthritis achieving an ACR50 response at week 16. This significant outcome highlights sonelokimab's efficacy in reducing disease activity in a crucial patient population.
• Beyond the primary endpoint, sonelokimab demonstrated strong performance across multiple key secondary clinical endpoints. These included 66.5% of patients achieving ACR20, 41.2% achieving Minimal Disease Activity (MDA), and 61% achieving PASI90 response, alongside improvements in HAQ-DI and SF-36 PCS scores, indicating comprehensive benefits for patients.
• The positive results from IZAR-1 are consistent with observations from other IL-17A/F programs, reinforcing sonelokimab's mechanism of action and its potential to provide meaningful improvements across musculoskeletal symptoms, skin disease, patient quality of life, and physical function in PsA patients. This broad efficacy profile positions sonelokimab as a promising therapy in the competitive PsA market.

Sonelokimab's Phase III IZAR-1 Success in Biologic-Naïve PsA

Recent clinical trials in psoriatic arthritis (PsA) have evaluated a range of biologic and targeted synthetic DMARDs across biologic-naïve and experienced populations, generating robust safety and efficacy datasets. The studies below represent key evidence informing current and emerging treatment paradigms in PsA.

• BE OPTIMAL and BE COMPLETE (Bimekizumab): These Phase III trials evaluated bimekizumab 160 mg Q4W — a dual IL-17A/F inhibitor — in bDMARD-naïve (BE OPTIMAL) and TNFi-experienced (BE COMPLETE) patients, respectively. Key efficacy endpoints included ACR50 and PASI75 response rates, alongside improvements in HAQ-DI scores. The long-term safety profile (up to 5 years) showed TEAEs at 126.9 per 100 patient-years, with oral candidiasis at 3.8 per 100 PY (predominantly mild/moderate), serious opportunistic infections at 0.1 per 100 PY, and no cases of active tuberculosis. Rates of adjudicated IBD, uveitis, MACE, and suicidal ideation/behaviour remained low, with no new safety signals identified.

• Upadacitinib Phase IIb/III Programme: Across multiple trials enrolling 907 PsA patients (data cutoff August 15, 2024; total exposure 27,164.2 patient-years across indications), upadacitinib — an oral selective JAK inhibitor — demonstrated a long-term safety profile consistent with prior reports. Serious TEAEs ranged from 4.5 to 11.0 per 100 PY; herpes zoster (2.4–6.6 per 100 PY), NMSC, and elevated creatine kinase were numerically higher in PsA versus adalimumab. Rates of serious infection (1.3–4.6 per 100 PY), malignancy, MACE (0–0.5 per 100 PY), and VTE (0–0.9 per 100 PY) remained stable over time, supporting long-term use in chronic disease management.

• REMARCA Study (Methotrexate ± Adalimumab): This treat-to-target study enrolled 44 DMARD-naïve patients with active early peripheral PsA, escalating subcutaneous MTX (10–25 mg/week) with the addition of adalimumab if targets were not met. At 12 months, DAS-defined remission was achieved in 61.4% of patients and MDA in 65.9%. ACR20/50/70 responses were 88%/77%/59%, respectively, and PASI75 was achieved in 88% of patients with BSA ≥3% at baseline. Notably, MTX monotherapy was sufficient in approximately 55% of patients, with significant improvements observed across all PsA clinical domains including arthritis, dactylitis, enthesitis, skin involvement, and quality of life.

Navigating the IL-17A/F Landscape in Psoriatic Arthritis

Sonelokimab's dual IL-17A/F inhibition places it in a mechanistic class shared by at least one other agent currently under clinical investigation. Bimekizumab, a monoclonal IgG1 antibody that selectively neutralises both IL-17A and IL-17F, has been evaluated across multiple phase 3 trials spanning psoriasis and axial spondyloarthritis (axSpA) — indications that overlap with sonelokimab's development programme. The intervention models across these trials vary in randomisation ratios, comparator arms, and dose-transition designs.

Beyond PsA: Sonelokimab's Expanding Pipeline in HS

Sonelokimab's clinical development extends beyond psoriatic arthritis into dermatological indications with shared IL-17 pathway pathophysiology. The most substantiated evidence base covers plaque psoriasis, with emerging data positioning sonelokimab as a candidate therapy in hidradenitis suppurativa (HS).

• Plaque Psoriasis — Phase 2b RCT: A multicentre, randomised, placebo-controlled phase 2b trial was conducted across 41 sites in Bulgaria, Canada, Czech Republic, Germany, Hungary, Poland, and the USA, using a parallel assignment design with six treatment arms: placebo, sonelokimab 30 mg, 60 mg, 120 mg (normal load), 120 mg (augmented load), and secukinumab 300 mg as an active comparator.

• Plaque Psoriasis — Randomisation and Trial Structure: Participants were allocated 1:1:1:1:1:1 via a centralised interactive response technology (IRT) system. The trial comprised four sequential periods: a 4-week screening phase, a 12-week placebo-controlled induction period, a 12-week dose maintenance or escalation period, and a 24-week response assessment or dose-holding period.

• Hidradenitis Suppurativa — Emerging Candidacy: Sonelokimab has been identified as a novel therapeutic candidate for HS, with key findings demonstrating performance on disease-specific endpoints including the Hidradenitis Suppurativa Clinical Response (HiSCR). Detailed intervention model specifications for HS trials are not yet fully characterised in the available literature.

Sonelokimab's Phase III Success: A New Frontier in PsA Treatment

The recent announcement of positive topline Phase III results for sonelokimab in biologic-naïve adults with active psoriatic arthritis (PsA) marks a significant moment for patients and clinicians grappling with this complex, multidomain disease. Sonelokimab, a novel trivalent nanobody, targets both interleukin (IL)-17A and IL-17F, a dual inhibition strategy that research suggests may offer enhanced efficacy over single-target therapies. This mechanism is particularly relevant given the established role of IL-17 cytokines in PsA pathogenesis.

The Phase III IZAR-1 trial demonstrated compelling efficacy, with 42.1% of patients achieving an ACR50 response at week 16, alongside robust improvements in key secondary endpoints such as ACR20 (66.5%), Minimal Disease Activity (41.2%), and PASI90 (61%). These results underscore sonelokimab's potential to provide comprehensive relief across the diverse manifestations of PsA, including joint inflammation, skin lesions, and overall patient quality of life. This broad impact is a critical differentiator in a disease where patients often present with varied symptoms.

However, as with any emerging therapy, certain considerations remain. While generally well-tolerated, previous studies have noted a higher incidence of Candida infections with sonelokimab compared to some other biologics, a known class effect of IL-17 inhibition that will require careful monitoring in real-world settings. Furthermore, while the short-term data are impressive, the long-term efficacy and safety profile beyond 16 weeks will be crucial for establishing its enduring value and market position. The competitive landscape for PsA is robust, with several established biologics and other dual IL-17A/F inhibitors like bimekizumab already on the market or in advanced development. Without direct head-to-head comparisons against all competitors, particularly other dual inhibitors, differentiating sonelokimab will rely heavily on its comprehensive efficacy, nanobody-specific advantages like improved tissue penetration, and its safety profile as more data emerge. These results position sonelokimab as a strong contender to become a leading therapeutic option, potentially reshaping treatment paradigms for biologic-naïve PsA patients.