Systemic Immune–Inflammation Index (SII) as a Predictive Biomarker of Therapeutic Response in Psoriasis: A Retrospective Comparative Analysis of Anti-TNF, Anti-IL-17, and Anti-IL-23 Agents

Background/Objectives: The Systemic Immune–Inflammation Index (SII), derived from routine blood counts, has emerged as a potential marker of systemic inflammation in psoriasis. However, its longitudinal behavior across different systemic and biologic therapies remains poorly characterized. This study aimed to evaluate changes in SII over time, assess its relationship with Psoriasis Area and Severity Index (PASI) scores, and compare SII trajectories among different treatment classes. Methods: A retrospective single-center study included 210 adults with psoriasis treated for 12 months with cyclosporine, anti-TNF-α, anti-IL-17, or anti-IL-23 agents. SII and PASI were recorded at baseline, 16, 36, and 52 weeks. Correlations between SII and PASI were assessed using Spearman’s analysis. Longitudinal changes were evaluated using the Friedman test, and treatment-group differences were assessed using Kruskal–Wallis analysis. An adjusted multivariable linear regression model including age, sex, body mass index, psoriatic arthritis, baseline PASI, and treatment group was performed to identify factors associated with Δ%SII. Results: SII correlated with PASI at baseline (ρ = 0.406, p < 0.001) and at 52 weeks (ρ = 0.186, p = 0.007), whereas no significant associations were observed at intermediate timepoints. Longitudinal analyses demonstrated significant differences in SII trajectories among treatment groups (p < 0.001). SII increased over time in the cyclosporine and anti-TNF-α groups, while anti-IL-17 and anti-IL-23 therapies were associated with marked and sustained reductions. In the adjusted model, anti-IL-17 (β = −90.7, 95% CI −119.6 to −61.8, p < 0.001) and anti-IL-23 therapies (β = −97.9, 95% CI −126.2 to −69.6, p < 0.001) remained independently associated with greater reductions in SII compared with cyclosporine, whereas anti-TNF therapy showed no significant difference. Conclusions: SII is a dynamic marker of systemic inflammatory changes in psoriasis and exhibits distinct longitudinal patterns according to treatment class. The pronounced reductions observed with IL-17 and IL-23 inhibitors support the potential value of SII as an adjunctive measure of systemic inflammation. However, prospective studies are required to clarify its clinical utility and determine its role in routine patient management.