IL-17 vs IL-23: Insights from the BE BOLD PsA Trial

UCB has announced positive topline results from the BE BOLD trial, the first head-to-head study comparing bimekizumab with risankizumab in adults with active psoriatic arthritis (PsA).¹ Bimekizumab, the only approved therapy to selectively inhibit both interleukin 17A (IL-17A) and interleukin 17F (IL-17F), demonstrated statistically significant superiority in the ACR50 primary endpoint at week 16. The treatment was generally well tolerated, with no new safety signals observed to week 16.²

To provide context for clinicians and patients, Peter Nash, MBBS, FRACP, FQAAS, from the School of Medicine at Griffith University, Brisbane, Australia, answers key questions about the implications of the trial.

Dermatology Times: How clinically meaningful is the ACR50 advantage seen with bimekizumab at week 16?

Nash: I think of ACR20, which was used in previous trials as a primary outcome measure, as the minimal clinically important improvement needed to separate a drug from placebo. By contrast, ACR50 requires a 50% improvement in joint tenderness and swelling, along with a 50% improvement in other outcomes such as pain, physical function, and both patient and physician global assessments. It therefore represents a clear, noticeable, moderate to major improvement.

With so many therapies now available, modern clinicians are demanding at least this level of response for their patients. In some countries, including mine, if at least a 50% improvement is not achieved by 16 weeks, the patient cannot continue to access reimbursed therapy.

DT: Did improvements in joint symptoms correlate with skin clearance in the BE BOLD population?

Nash: So far, we only have a limited press release, and important questions like this will need to await formal presentation of the data, hopefully at the next major meeting, followed by publication in a peer-reviewed paper. Anecdotally, from our trial experience, patients who did well clinically from a joint perspective also tended to do well from a psoriasis perspective, although, of course, we were blinded to the treatment arm.

DT: What does this head-to-head trial suggest about the relative roles of the IL-17 and IL-23 pathways in psoriatic arthritis?

Nash: We know, from a pathophysiological perspective, that both pathways are important and that both are effective therapeutic targets in psoriatic disease. IL-23 is the upstream regulator that drives the process—initiating and sustaining it, if you like—whereas IL-17 is more directly responsible for the clinical effects, executing the inflammation and damage.

DT: Do the results support earlier use of dual IL-17A/IL-17F inhibition in patients with both skin and joint disease?

Nash: Whilst there is suggestive evidence from other trials to support this, the BE-BOLD trial did not stratify by disease duration. However, provided there are sufficient numbers of patients with early disease (e.g. <2 years) and established disease (e.g. >2 years), a post hoc analysis should help shed light on this question.

DT: Where might bimekizumab fit in treatment sequencing as clinicians choose between IL-17, IL-23, and TNF inhibitors?

Nash: Thus far, head-to-head trials in PsA have compared IL-17 inhibition with TNF inhibition, and this is the first trial to compare IL-17A and IL-17F inhibition directly with IL-23 inhibition. It will provide important evidence for treatment algorithms based on direct efficacy and safety data, rather than on network meta-analyses.

Conclusion

The BE BOLD trial adds important evidence supporting bimekizumab’s potential to deliver clinically meaningful improvements in PsA, particularly in achieving ACR50 responses, which represent moderate to major improvements in joint and overall disease activity. While full data on domain-specific outcomes and correlations with skin clearance are pending, the trial provides a foundation for clinicians to make informed decisions regarding treatment sequencing among IL-17, IL-23, and TNF inhibitors.

References

1. BIMZELX[®]▼(bimekizumab) superior to SKYRIZI[®] (risankizumab) in BE BOLD: first head-to-head study in active psoriatic arthritis (PsA) to demonstrate superiority in ACR50. News release. UCB. Published March 11, 2026. Accessed March 31, 2026. https://www.ucb.com/newsroom/press-releases/article/bimzelxrvbimekizumab-superior-to-skyrizir-risankizumab-in-be-bold-first-head-to-head-study-in-active-psoriatic-arthritis-psa-to-demonstrate-superiority-in-acr50
2. Thaçi D, Asahina A, Boehncke WH, et al. Bimekizumab efficacy and safety in patients with psoriatic arthritis with substantial skin and nail psoriasis to 1 year. Dermatol Ther (Heidelb). 2026;16(2):953-976. doi:10.1007/s13555-025-01599-5