Background Infliximab (IFX) is a chimeric anti-TNFα monoclonal antibody used in the treatment of psoriasis. Due to the large interindividual variability in IFX, measurement of serum concentrations and correlation with disease activity and different covariables could be useful for psoriasis management.

Purpose The primary endpoint was to assess the relation between IFX trough levels (C_min) and treatment efficacy. A secondary endpoint was to identify variables that could affect C_min.

Material and methods Prospective study of patients with psoriasis treated with IFX between October 2013 and August 2015 at a tertiary level hospital. C_min (mg/L) and antibodies against IFX (ATI) were determined at steady state by enzyme linked immunosorbent assay (ELISA) (Promonitor). Data recorded: sex, weight, BSA, PASI, dose regimen and cigarette smoking. Dose adjusted C_min values were statistically compared after log transformation. Statistical analysis was performed using SPSS v.19.

Results 16 patients (25% women) were included. Median weight (kg) was 83.4 (Q1-Q3 65.5–93.8), median age (years) was 45 (Q1-Q3 38–54) years and median BSA (m²) was 1.96 (1.66–1.96). 40 serum samples were available for analysis. Median IFX dose was 5 mg/kg/8 weeks (range 4 mg/kg/8 weeks to 5 mg/kg/6 weeks). All patients receiving dose intensified IFX had a BSA >1.7 m². Median C_min (mg/L) and dose adjusted C_min (C_min/D) (mg/L/mg/kg/month) were 1.59 (Q1-Q3 0.86–2.63) and 0.66 (Q1-Q3 0.37–1.1), respectively. 3 samples were positive for ATI. All patients who developed ATI had undetectable C_min. Patients achieving PASI75 had a 23% higher C_min/D compared with those not achieving PASI75. In patients with BSA >1.7 m², median C_min and C_min/D were 45% and 15% higher, respectively. Only 63% of patients with BSA >1.7 m²achieved PASI75 (compared with 100% of patients with BSA ≤1.7, p = 0.026); patients with BSA >1.7 m²and achieving PASI75 had a 36% higher C_min/D compared with those not achieving PASI75. Median C_min was 13.7% lower in cigarette smoking patients.

Conclusion Higher C_min and C_min/D values were associated with better treatment response in all patients. Patients with SC ≥1.7 showed a tendency to lower treatment response. Lower C_min was found in smoking patients. More studies with a higher number of patients are needed to define the target levels and assess the influence of covariables.

No conflict of interest.