Merck Presents Results on Efficacy and Safety of Enpatoran in Systemic Lupus Erythematosus (SLE) at EULAR 2025

Cohort B of the Phase 2 study indicates all doses of enpatoran were associated with higher BICLA response rates compared with placebo, though the primary endpoint of BICLA dose-response relationship at Week 24 was not metData show encouraging efficacy in a large subgroup of SLE patients with active cutaneous manifestations at baseline, including improved response rates in key disease activity measurementsThere is a substantial unmet treatment need for the estimated 70-80% of SLE patients who experience active cutaneous manifestations, which can profoundly impact quality of life1

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Merck, a leading science and technology company, today announced the presentation of detailed results from Cohort B of the global Phase 2 WILLOW study (NCT05162586) evaluating enpatoran, an investigational, oral, novel TLR7/8 inhibitor in systemic lupus erythematosus (SLE). Although it did not meet the primary endpoint of dose-response relationship, when compared to placebo, enpatoran demonstrated improvements in measures of both systemic and cutaneous disease activity in prespecified SLE subpopulations despite standard of care (SoC), including those with active cutaneous manifestations at baseline [(CLASI-A) ≥8], and was overall well tolerated. These findings will be presented in a late-breaking oral presentation at the 2025 European Congress of Rheumatology (EULAR) in Barcelona (Abstract # LB0004).

“Analyses of Cohort B contribute to our understanding of enpatoran’s potential to address the critical unmet needs for patients living with lupus, including those experiencing significant skin manifestations. These manifestations are often part of the systemic activity or flare, which can be painful and have a considerable impact on quality of life,” said principal investigator Prof. Eric Morand, from Monash University and Monash Health. “The improvements observed in key disease measures represent a meaningful advancement in our ongoing investigation of the TLR7/8 inhibition approach for patients insufficiently managed by current therapies.”

WILLOW is a global, multicenter, randomized, placebo-controlled Phase 2 study evaluating three doses of oral enpatoran taken twice daily (25 mg, 50 mg and 100 mg) versus placebo plus SoC over 24 weeks. The study features a unique design across two lupus cohorts, including both patients with active SLE and cutaneous lupus erythematosus (CLE). Cohort B of the study was designed to evaluate the dose-response relationship of enpatoran in reducing disease activity, based on the British Isles Lupus Assessment Group (BILAG)-based Composite Lupus Assessment (BICLA) response rate at Week 24 and enrolled SLE patients who had moderate or severe active disease despite SoC.

Cohort B showed positive results in secondary and exploratory endpoints and within prespecified patient subpopulations. In patients with active skin disease (CLASI-A ≥8), BICLA response rates were up to 58.6% while placebo response rates were 31.7%, and up to 60.5% of patients receiving enpatoran showed a CLASI-70 response, compared with 26.8% for placebo, at Week 24. In addition, the subgroups of patients with high corticosteroid (prednisone-equivalent ≥10 mg/day) use and those with high interferon gene signature (IFN-GS) at baseline also showed higher and relevant BICLA response rates for enpatoran compared to placebo.

As presented earlier this year at LUPUS 2025, Cohort A analyses from the WILLOW study showed clinically meaningful improvement in disease activity in patients with CLE and mild SLE with active lupus rash at Weeks 16 and 24. Overall, for skin-related signs and symptoms, comparable improvements were observed in Cohort B relative to Cohort A, reinforcing the potential efficacy of enpatoran in patients with cutaneous manifestations of lupus erythematosus with or without systemic disease.

“The efficacy and tolerability results from Cohort B, including among those with active skin involvement—a manifestation that affects most lupus patients—are consistent with our observations from Cohort A. The lupus rash is not only a visible symptom but is also closely linked to the underlying systemic activity of lupus,” said Jan Klatt, Head of Development Unit Neurology & Immunology for the Healthcare business of Merck. “We are set to initiate regulatory discussions with key health authorities to determine the most effective pathway for bringing enpatoran to patients.”

Enpatoran was well-tolerated and exhibited a manageable safety profile consistent with previous studies, with no new safety signals identified. Rates of treatment-emergent adverse events (TEAEs) were comparable between all enpatoran arms and placebo, ranging from 60.6% to 64.2%, and the most frequently reported TEAEs were infections and infestations. These results further support the anticipated favorable safety profile of enpatoran.

About Enpatoran

Enpatoran is a selective Toll-like receptor (TLR)7/8 inhibitor under investigation for the treatment of systemic lupus erythematosus (SLE) and cutaneous lupus erythematosus (CLE). By inhibiting TLR7/8 activation, enpatoran may help reduce pro-inflammatory cytokines and autoantibody production, potentially addressing underlying mechanisms of chronic inflammation and disease progression in lupus. With its novel proposed mechanism of action and oral administration, enpatoran has the potential to be a first-in-class treatment for patients across lupus conditions. Enpatoran is currently under clinical investigation and is not approved for any use anywhere in the world.

About the Phase 2 WILLOW Clinical Study

WILLOW (NCT05162586) is a randomized, double-blind, placebo-controlled Phase 2 proof of concept and dose-finding study designed to evaluate the efficacy and safety of enpatoran in patients with systemic lupus erythematosus (SLE) and cutaneous lupus erythematosus (CLE). The study incorporates a basket design, including two cohorts – Cohort A including patients with CLE or SLE with active lupus rash and Cohort B including patients with active SLE. The WILLOW study aims to advance the understanding of enpatoran’s therapeutic potential and to help address significant unmet needs in lupus treatment.

About Lupus Erythematosus

Lupus erythematosus is a chronic autoimmune disease that can affect various parts of the body, including the skin, joints, kidneys, and other organs. It occurs when the immune system mistakenly attacks healthy tissues, leading to inflammation, pain, and potential organ damage. There are multiple types of lupus, with systemic lupus erythematosus (SLE) and cutaneous lupus erythematosus (CLE) being two primary forms. Symptoms can range from mild to life-threatening, often including fatigue, joint pain, rashes, and organ involvement. Lupus disproportionately impacts women and people of color, and despite available treatments, many patients experience unmet needs due to limited efficacy or side effects.

Merck in Neurology and Immunology

Merck has a long-standing legacy in neurology and immunology, with significant R&D and commercial experience in multiple sclerosis (MS). The company’s current MS portfolio includes two products for the treatment of relapsing MS – Rebif^® (interferon beta-1a) and MAVENCLAD^® (cladribine tablets). Merck aims to improve the lives of patients by addressing areas of unmet medical needs. In addition to Merck’s commitment to MS, the company also has a pipeline focusing on discovering new therapies that have potential in other neuroinflammatory and immune-mediated diseases, including systemic lupus erythematosus (SLE), cutaneous lupus erythematosus (CLE) and generalized myasthenia gravis (gMG).

About Merck

Merck, a leading science and technology company, operates across life science, healthcare and electronics. More than 62,000 employees work to make a positive difference to millions of people’s lives every day by creating more joyful and sustainable ways to live. From providing products and services that accelerate drug development and manufacturing as well as discovering unique ways to treat the most challenging diseases to enabling the intelligence of devices – the company is everywhere. In 2024, Merck generated sales of € 21.2 billion in 65 countries.

The company holds the global rights to the name and trademark “Merck” internationally. The only exceptions are the United States and Canada, where the business sectors of Merck KGaA, Darmstadt, Germany, operate as MilliporeSigma in life science, EMD Serono in healthcare and EMD Electronics in electronics. Since its founding in 1668, scientific exploration and responsible entrepreneurship have been key to the company’s technological and scientific advances. To this day, the founding family remains the majority owner of the publicly listed company.

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¹ Cojocaru M, Cojocaru IM, Silosi I, Vrabie CD. Manifestations of systemic lupus erythematosus. Maedica (Bucur). 2011 Oct;6(4):330-6. PMID: 22879850; PMCID: PMC3391953.

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