Biologics, such as brodalumab, are currently the most effective treatment option for patients with moderate-to-severe psoriasis. But they are costly for health care systems and prescribed according to a 'one dose fits all' dosing regimen, leading to potential over- and undertreatment. Within this study we aim to investigate the predictive value of early serum trough levels of brodalumab and determine the therapeutic window of brodalumab in psoriasis patients.

Patients will be included after siging informed consent. After inclusion, patients will continue on standard dosing schedule of brodalumab (i.e. one loading dose first (210mg), once a week for 2 weeks (210mg), then a regular dose regimen (210mg) every 2 weeks). During each study visit, blood will be taken in order to quantify Ctroughs and/or anti-drug antibodies towards brodalumab. In addition, the Psoriasis Severity and Area Index (PASI) and the Investigator's Global Assessment (IGA) score will be evaluated by a physician. Patients complete the Dermatology Life Quality Index (DLQI) and European quality of life EQ-5D instrument at each visit.

Inclusion Criteria:

1. Participants must have a clinical or histological diagnosis of chronic plaque-type psoriasis
2. Participants must sign an ICF indicating that he or she understands the purpose of, and procedures required for, the study and is willing to participate in the study

Exclusion Criteria:

1. Participants who have currently a predominant nonplaque form of psoriasis
2. Participants who are pregnant, nursing or planning a pregnancy
3. Participants who are unable or unwilling to undergo multiple venapunctures
4. Participants who are treated according to a different dosing schedule than standard dosing of brodalumab