Bimekizumab Beats Risankizumab Head-to-Head in Psoriatic Arthritis: BE BOLD Trial

Psoriatic arthritis (PsA) sits at the intersection of skin and joint disease, driven primarily by IL-17 and IL-23 pathway dysregulation. The biologic landscape for PsA has expanded rapidly: TNF inhibitors, IL-17A inhibitors (secukinumab, ixekizumab), IL-23 p19 inhibitors (guselkumab, risankizumab, ustekinumab), and now bimekizumab — the only agent that blocks both IL-17A and IL-17F simultaneously. The BE BOLD trial is the first phase 3b head-to-head trial powered to demonstrate superiority on a joint endpoint between these mechanisms.[1]

ACR50 — the American College of Rheumatology response criterion requiring 50% improvement across joint counts, patient-reported outcomes, and inflammatory markers — was the primary endpoint at 16 weeks. Bimekizumab achieved this in 49.1% of patients, compared with 38.4% for risankizumab. The absolute difference of approximately 11 percentage points was statistically significant. Skin clearance outcomes (PASI 90, IGA 0/1) were comparable between arms, reflecting that both IL-17A inhibition and IL-23 p19 inhibition are similarly effective for psoriasis. The joint superiority is where the mechanisms diverge: dual IL-17A/F blockade appears to provide more complete suppression of synovial inflammation than IL-23 inhibition alone.[1]

The mechanistic rationale is straightforward. IL-17F, often overlooked relative to its better-studied sibling IL-17A, contributes independently to joint inflammation through separate receptor-binding and downstream signaling. Blocking only IL-17A (as secukinumab and ixekizumab do) leaves residual IL-17F activity. Blocking IL-23 p19 reduces both IL-17A and IL-17F at the source, but with somewhat slower and less complete kinetics. Bimekizumab blocks both downstream effectors simultaneously and directly, and the BE BOLD data suggest this mechanism advantage translates to better joint outcomes.[1, 2]

Clinical Context

Psoriatic arthritis treatment has been stratified since the 2010s by mechanism: TNF inhibitors (etanercept, adalimumab) became standard first-line biologics, but up to 30–40% of patients do not achieve ACR50 or have inadequate skin clearance. The IL-17A inhibitors secukinumab and ixekizumab demonstrated superiority to placebo and, in some datasets, to TNF inhibitors for both joint and skin outcomes — driven by the central role of IL-17 in PsA synovitis, enthesitis, and plaque formation.[2] The IL-23 p19 inhibitors (guselkumab, risankizumab) offer a distinct approach: blocking upstream IL-23 that drives Th17 differentiation suppresses both IL-17A and IL-17F production chronically, with quarterly dosing convenience. Indirect treatment comparisons from network meta-analyses have consistently placed IL-17 inhibition slightly ahead of IL-23 inhibition on joint endpoints — but head-to-head data in PsA were lacking.[3]

Bimekizumab is unique among approved biologics in that it blocks both IL-17A and IL-17F simultaneously. IL-17F, while less potent than IL-17A on a molar basis, is produced in larger quantities and contributes to joint inflammation through shared receptor signaling. The BE OPTIMAL trial (bimekizumab vs placebo in biologic-naïve PsA) and BE COMPLETE (bimekizumab vs placebo in TNF-inadequate responders) established bimekizumab's efficacy across PsA domains. BE BOLD is the first head-to-head comparison of this dual-IL-17 blockade against an approved IL-23 inhibitor, directly testing whether blocking both IL-17 isoforms confers measurable joint benefit over blocking only the IL-23 signal upstream.[2]

Why It Matters Clinically

The broader significance of BE BOLD is that it legitimizes head-to-head design in the crowded biologic market for inflammatory arthritis. For too long, PsA treatment decisions were guided primarily by comparison to placebo, with manufacturers avoiding head-to-head trials that could disadvantage their products. A positive head-to-head result for a joint endpoint in PsA is exactly the kind of evidence that allows rational sequencing decisions. The 2025 EULAR updated RA guidelines — which now incorporate comparative effectiveness data — signal that this is the direction the field is heading.[3]

Limitations

The 16-week timepoint is relatively short; IL-23 inhibitors often show continued improvement beyond 24–52 weeks, and the gap may narrow with longer follow-up. Subgroup analyses by baseline disease severity, prior biologic use, and enthesitis/dactylitis presence would be clinically valuable. Full peer-reviewed publication is awaited.