Enpatoran, a Toll-like receptor 7/8 inhibitor, in moderate-to-severe systemic lupus erythematosus: findings from Cohort B of a multicentre, international, double-blind, placebo-controlled dose-finding phase 2 trial

BACKGROUND: Toll-like receptors (TLR) 7 and 8 (TLR7/8) are activators of innate and adaptive immunity contributing to lupus pathogenesis. In Cohort B of WILLOW, a phase 2, randomised, placebo-controlled, double-blind, basket, dose-finding study, enpatoran, an oral small molecule inhibitor of TLR7/8, was evaluated in participants with active systemic lupus erythematosus (SLE). METHODS: Participants were eligible if they were aged 18-75 years with moderate-to-severe SLE, with or without cutaneous manifestations, had a disease duration of at least 6 months, and were receiving a stable dose of medication before the screening period. Participants were recruited from 132 centres in 22 countries. In Part 1, participants were randomly allocated in a 1:2 ratio to receive either placebo or 100 mg enpatoran, both twice-daily. Following the enrolment of 60 participants, Part 2 was activated and additional participants were randomly allocated in a 1:1:1:1 ratio to 25 mg, 50 mg, or 100 mg of enpatoran or placebo, all twice-daily, for 24 weeks. Random allocation was stratified by region, biomarker status, and hybrid Safety of Estrogens in Lupus Erythematosus National Assessment-SLE Disease Activity Index score. The primary objective was to evaluate the dose-response relationship of enpatoran, using British Isles Lupus Assessment Group-based Composite Lupus Assessment (BICLA) response rate at week 24, based on multiple comparison procedure-modelling analysis. Study visits were scheduled from week 0 to week 24, followed by a 2-week safety follow-up period for participants who chose not to enter the long-term extension. From weeks 2 to 12, glucocorticoid doses were tapered to a prednisone-equivalent dose of no more than 5 mg/day, as clinically tolerated. Adverse events were monitored continuously throughout the study; safety parameters (including physical examination, vital signs, and routine chemistry and haematology) were assessed at all study visits. The trial was registered at ClinicalTrials.gov (NCT05162586) and a long-term extension study is ongoing. FINDINGS: Between May 4, 2022, and Feb 6, 2024, participants were screened for eligibility for WILLOW cohorts A and B; 715 participants were screened and 354 were randomly allocated and included in the Cohort B safety population (95 to placebo, 71 to 25 mg enpatoran, 74 to 50 mg enpatoran, and 114 to 100 mg enpatoran). One patient allocated to the placebo group was found to be ineligible and was excluded from the full analysis set for the efficacy analyses. 335 (95%) of 353 participants were female, 18 (5%) were male, and median age was 41 years (IQR 33-51). At week 24, the study did not meet its primary objective of identifying a statistically significant dose-response relationship for enpatoran in BICLA response rate (p=0·14). BICLA response rates at week 24 were higher with all doses of enpatoran (25 mg: 41 [58%] of 71; odds ratio [OR] vs placebo 2·2 [95% CI 1·1-4·0], 50 mg: 36 [49%] of 74; OR 1·5 [95% CI 0·8-2·8], and 100 mg: 56 [49%] of 114; OR 1·6 [95% CI 0·9-2·8]) versus placebo (37 [39%] of 94). The most common treatment-emergent adverse event was diarrhoea, in four (6%) of 71, two (3%) of 74, and two (2%) of 114 participants in the 25 mg, 50 mg, and 100 mg enpatoran groups, respectively, and seven (7%) of 95 participants in the placebo group. Serious adverse events were reported in one (1%) of 71, three (4%) of 74, five (4%) of 114, and three (3%) of 95 participants treated with 25 mg, 50 mg, and 100 mg enpatoran and placebo, respectively. INTERPRETATION: In this study of participants with moderate-to-severe SLE, enpatoran improved BICLA response rates versus placebo; however, the primary objective of a statistically significant dose-dependent effect on disease activity based on BICLA response was not met. Enpatoran was well tolerated across all dose groups. FUNDING: Merck Healthcare (Darmstadt, Germany).