Structural basis for differential p19 targeting by IL-23 biologics

IL-23 is central to the pathogenesis of psoriasis, and is structurally comprised of p19 and p40 subunits. "Targeted" IL-23 inhibitors risankizumab, tildrakizumab, and guselkumab differ mechanistically from ustekinumab because they bind p19, whereas ustekinumab binds p40; however, a knowledge gap exists regarding the structural composition of their epitopes and how these molecular properties relate to their clinical efficacy.

To characterize and differentiate the structural epitopes of the IL-23 inhibitors risankizumab, guselkumab, tildrakinumab, and ustekinumab, and correlate their molecular characteristics with clinical response in plaque psoriasis therapy.

We utilized epitope data derived from hydrogen-deuterium exchange studies for risankizumab, tildrakizumab, and guselkumab, and crystallographic data for ustekinumab to map drug epitope locations, hydrophobicity, and surface charge onto the IL-23 molecular surface (Protein Data Bank ID Code 3D87) using UCSF Chimera. PDBePISA was used to calculate solvent accessible surface area (SASA). Epitope composition was determined by classifying residues as acidic, basic, polar, or hydrophobic and calculating their contribution to epitope SASA. Linear regression and analysis of variance was performed.

All the p19-specific inhibitor epitopes differ in location and size, with risankizumab and guselkumab having large epitope surface areas (SA), and tildrakizumab and ustekinumab having smaller SA. The tildrakizumab epitope was mostly hydrophobic (56%), while guselkumab, risankizumab, and ustekinumab epitopes displayed >50% non-hydrophobic residues. Risankizumab and ustekinumab exhibited acidic surface charges, while tildrakizumab and guselkumab were net neutral. Each inhibitor binds an epitope with a unique size and composition, and with mostly distinct locations except for a 10-residue overlap region that lies outside of the IL-23 receptor epitope. We observed a strong correlation between epitope SA and PASI-90 rates (R² = 0.9969, p = 0.0016), as well as between epitope SA and K_D (R² = 0.9772, p = 0.0115). In contrast, we found that total epitope hydrophobicity, polarity, and charge content do not correlate with clinical efficacy.

Structural analysis of IL-23 inhibitor epitopes reveals strong association between epitope SA and early drug efficacy in plaque psoriasis therapy, exemplifying how molecular data can explain clinical observations, inform future innovation, and help clinicians in specific drug selection for patients.