With the introduction of the newest biologicals like secukinumab, achieving excellent response (PASI < 2) has become a realistic goal. However, real life clinical practice data reveal that response to biologics varies from optimal to non-response and loss of response upon treatment. With a therapeutic window, the most optimal outcome can be achieved for each patient.
We aimed to determine the optimal therapeutic window of secukinumab in psoriasis in order to achieve the best clinical outcome for each patient. 
In this cross-sectional pilot study, sera from psoriasis patients on secukinumab were collected at trough during maintenance phase. Secukinumab trough levels (Ctrough) were determined using an in-house developed ELISA and disease severity was assessed through the Psoriasis Area and Severity Index (PASI). 
A negative correlation was observed between Ctrough and absolute PASI at trough (ρ = - 0.442; p = 0.004) and a positive correlation was found between Ctrough and relative PASI (ρ = 0.325; p = 0.041). Ctrough were significantly higher in optimal (PASI ≤ 2) compared to suboptimal responders (PASI > 2) (p = 0.004). Smoking, treatment duration, former biological treatment and BMI correlated inversely with Ctrough and/or PASI. Based on a receiver operating characteristic curve analysis, a minimal effective Ctrough of 33.2 μg/ml during maintenance (sensitivity: 78.3%; specificity: 76.5%) was determined for achieving PASI ≤ 2 (p = 0.005). 
Patients with insufficient clinical response and Ctrough below 33.2 µg/ml are potentially undertreated and could benefit from dose intensification.

Defining a minimal effective serum trough concentration of secukinumab in psoriasis: a step towards personalized therapy