IL-17A/F bispecific vs IL-17A inhibitors: bimekizumab

Why blocking both IL-17A and IL-17F changes the biology

IL-17A and IL-17F share approximately 50% amino acid sequence homology and both signal through IL-17 receptor A (IL-17RA), yet they are not interchangeable. IL-17A binds IL-17RA with roughly ten-fold higher affinity and is the more potent driver of neutrophil recruitment and keratinocyte activation. IL-17F, while less potent, independently induces pro-inflammatory cytokine production — including IL-6, IL-8, and TNF-α — from synoviocytes, keratinocytes, and dermal fibroblasts, albeit at higher concentrations than IL-17A.

Critically, IL-17A and IL-17F do not only act as homodimers. They form IL-17A/F heterodimers in vivo, creating a third signaling entity that selective IL-17A inhibitors leave untouched. This structural reality underpins the compensatory upregulation hypothesis: when IL-17A is blocked selectively, residual IL-17F-driven inflammation — via homodimers and heterodimers — may sustain disease activity. Evidence from psoriatic arthritis synovial tissue supports this directly: dual neutralization with bimekizumab reduced IL-6 and IL-8 production more completely than secukinumab (IL-17A-only), demonstrating that IL-17F contributes meaningfully to tissue inflammation when IL-17A is blocked.

Bimekizumab, developed by UCB and the leading approved dual inhibitor, is a humanized IgG1 antibody achieving neutralization of all three molecular forms — IL-17A homodimers, IL-17F homodimers, and IL-17A/F heterodimers — with IC50 values in the low picomolar range. Foundational patent families for cross-reactive IL-17A/F antibodies were filed between 2007 and 2010 by Zymogenetics, Genentech, and UCB (including WO2010025400A2 and US20100055103A1), establishing the technical feasibility of a single binding domain neutralizing all three species.

Izokibep takes a structurally distinct approach. Rather than a monoclonal antibody, it is an Affibody®-based engineered protein scaffold of approximately 7 kDa — roughly one-tenth the size of a conventional monoclonal antibody at ~150 kDa. This small molecular size is central to its proposed clinical differentiation: enhanced tissue penetration, particularly into poorly vascularized or fibrotic compartments, and convenient subcutaneous dosing. Izokibep is a selective IL-17A inhibitor, not a dual blocker, but its pharmacokinetic profile distinguishes it from existing mAb-based IL-17A inhibitors such as secukinumab and ixekizumab.

Psoriasis: where head-to-head data settles the debate

In moderate-to-severe plaque psoriasis, dual IL-17A/F blockade demonstrates clear, statistically significant superiority over selective IL-17A inhibition — a conclusion now supported by a prospective head-to-head randomized controlled trial rather than indirect comparisons. The BE RADIANT Phase IIIb trial compared bimekizumab 320 mg Q4W directly against secukinumab 300 mg Q4W, with complete skin clearance (PASI 100) as the primary endpoint.

“Patients initially randomized to secukinumab who switched to bimekizumab at Week 48 achieved comparable PASI 100 rates by Year 3 — demonstrating rescue efficacy from dual blockade even after prior selective IL-17A exposure.”

Izokibep’s Phase 2 psoriasis data adds a further data point at the upper range of IL-17 inhibitor efficacy. At the 160 mg Q2W dose, izokibep achieved PASI 100 in 71% of patients at Week 12 and PASI 90 in 86% — with PASI 75 responses evident in the majority of patients by Week 2. For context, selective IL-17A inhibitors such as ixekizumab and secukinumab typically achieve PASI 100 rates of 40–50% at Weeks 12–16 in pivotal trials. Izokibep’s Phase 2 results, while from a smaller study and not yet confirmed in Phase 3, place it at the top of the observed efficacy range for IL-17 pathway inhibition in psoriasis.

Axial spondyloarthritis: a more modest differential

In axial spondyloarthritis (axSpA) — encompassing both ankylosing spondylitis and non-radiographic axial SpA — the incremental benefit of dual IL-17A/F blockade over selective IL-17A inhibition appears substantially more modest than in psoriasis. Bimekizumab achieved ASAS40 response rates of 47–52% in Phase 2b/3 trials, a range that overlaps with historical secukinumab data (42–44%) and ixekizumab data (48–52%). Partial remission rates with bimekizumab were 20–25%, compared to 15–20% for secukinumab and 18–22% for ixekizumab historically.

The attenuated differential in axSpA likely has a mechanistic explanation. Preclinical evidence suggests that IL-17F contributes meaningfully to synovial inflammation in peripheral joints but plays a lesser role in axial enthesitis — the dominant pathological process in ankylosing spondylitis. Patent literature describes bimekizumab’s ability to reduce IL-6 and matrix metalloproteinase production from spinal ligament cells, but the magnitude of IL-17F’s independent contribution to axial inflammation remains uncertain. Ceiling effects in clinical response measures (ASAS criteria) may also mask genuine differences, if they exist.

According to data reviewed by the European Medicines Agency, bimekizumab demonstrated significant reduction in spinal MRI inflammation (SPARCC score) in axSpA, comparable in direction to what has been observed with secukinumab and ixekizumab. Bimekizumab also showed ASAS partial remission rates of 20–25%, versus 15–20% for secukinumab and 18–22% for ixekizumab historically — a numerically positive but not yet validated differential in the absence of a head-to-head trial.

Hidradenitis suppurativa: the tissue penetration hypothesis

Hidradenitis suppurativa (HS) presents a distinct biological challenge: dense, hypoxic, fibrotic lesion microenvironments and draining tunnels that may physically limit antibody penetration. This makes HS the indication where izokibep’s small molecular size (~7 kDa) carries the most mechanistically compelling hypothesis, and where the comparison between selective and dual IL-17 inhibition is still evolving.

Secukinumab became the first IL-17 inhibitor approved for HS, receiving FDA and EMA approval in 2023–2024 based on the SUNSHINE and SUNRISE Phase 3 trials. In those trials, secukinumab 300 mg Q2W achieved HiSCR (Hidradenitis Suppurativa Clinical Response) in 43.4% of patients at Week 16, versus 39.5% for Q4W dosing and 31.5% for placebo. Real-world evidence for secukinumab in HS shows HiSCR response rates of 50–64% in biologic-naïve patients, with higher failure rates in Hurley Stage III disease.

Bimekizumab received EMA approval for HS in 2024. Phase 2 data showed HiSCR at Week 12 of approximately 55–60%, with potentially superior efficacy in draining tunnel resolution — a key unmet need where TNF inhibitors and selective IL-17A mAbs show limited benefit. A real-world meta-analysis comparing secukinumab and bimekizumab in HS found an adverse event rate of 5.45% for bimekizumab versus 8.22% for secukinumab, with most events being mild mucocutaneous infections.

Izokibep’s Phase 3 trials in HS are underway at 160 mg Q2W dosing. The patent basis includes WO2025054498A1, which claims methods for treating HS with sustained IL-17A neutralization via Affibody® technology. The scientific hypothesis — that a 7 kDa protein can achieve higher local drug concentrations in poorly vascularized, fibrotic HS lesions and draining tunnels than a 150 kDa monoclonal antibody — is mechanistically plausible but requires direct measurement of drug concentrations in HS tissue to confirm. As PubMed-indexed clinical data for izokibep in HS remains limited to Phase 2 and ongoing Phase 3 work, this hypothesis remains to be validated.

Safety trade-offs: candidiasis and the redundancy cost

The most clinically significant safety differential between dual and selective IL-17 inhibition is oral candidiasis. Both IL-17A and IL-17F contribute to mucosal antifungal immunity by promoting neutrophil recruitment and antimicrobial peptide production at mucosal surfaces. Dual blockade eliminates this functional redundancy, increasing susceptibility to Candida albicans colonization in a dose-dependent and mechanistically predictable way.

Despite the elevated candidiasis incidence with bimekizumab (10.0 per 100 patient-years versus 3–5 for secukinumab and 4–6 for ixekizumab), the clinical impact is generally manageable. Infections are mostly mild-to-moderate, responsive to topical antifungals, and rarely lead to treatment discontinuation — with discontinuation rates below 2% across all IL-17 inhibitors. Serious fungal infections remain rare. Over three years of follow-up in the BE RADIANT extension, bimekizumab showed no increased rate of serious infections versus secukinumab.

Other safety signals are broadly comparable across the class. Paradoxical inflammatory bowel disease (IBD) or IBD flares are reported with all IL-17 inhibitors, with rates of approximately 0.5–1% across agents — no meaningful differential between dual and selective blockade. Bimekizumab shows low anti-drug antibody formation (below 5%), comparable to secukinumab. For practitioners making formulary decisions, the candidiasis trade-off is the primary distinguishing safety consideration, particularly in patients with a history of recurrent oral candidiasis or immunocompromise. Standards from the World Health Organization on antimicrobial stewardship are relevant context for managing this risk in practice.

Patent landscape and the next innovation wave

The IL-17 inhibitor patent landscape reflects three distinct innovation eras, each building on the last. Foundational cross-reactive IL-17A/F antibody patents filed between 2007 and 2010 — from Zymogenetics, Genentech, and UCB — established the technical feasibility of a single binding domain neutralizing IL-17A homodimers, IL-17F homodimers, and IL-17A/F heterodimers simultaneously. These early filings (including WO2010025400A2 and US20100055103A1) set the IP foundation for the entire dual-blockade field.

UCB’s bimekizumab-specific patent estate covers humanized antibody sequences, stable liquid formulations with controlled oxygen headspace, and dosing regimens including Q4W and Q8W maintenance schedules. The formulation and dosing patents extend commercial exclusivity beyond the sequence patents and represent a key competitive moat. According to the European Patent Office, such formulation and dosing patents have increasingly been the subject of post-grant opposition proceedings in the biologics space.

Izokibep’s Affibody®-based patent family (2020–2025) operates in a structurally distinct space from traditional antibody patents, providing genuine freedom-to-operate. Key claims cover engineered protein domains with picomolar IL-17A binding affinity, subcutaneous formulations enabling Q1W or Q2W dosing, and methods of treating psoriasis, psoriatic arthritis, and HS with doses of 160 mg or above. The HS-specific patent WO2025054498A1 represents a recent filing aimed at establishing method-of-treatment exclusivity in a high-unmet-need indication before Phase 3 data matures.

The emerging frontier is oral small-molecule IL-17 inhibitors. Recent patents including CN119708121A and WO2025131062A1 describe oral small-molecule IL-17A inhibitors, though none have advanced to late-stage clinical development. If successful, oral IL-17 inhibition would represent a fundamental shift in the treatment paradigm — removing the subcutaneous administration requirement that currently defines the class. The PatSnap Insights blog has tracked the acceleration of small-molecule IL-17 patent filings as a leading indicator of this next innovation wave. For teams conducting freedom-to-operate analysis or competitive intelligence in this space, PatSnap’s patent database covers all seven of the key patent families referenced in this article.