Patients will continue to receive tildrakizumab according to the standard dosing schedule: subcutaneous injections at weeks 0 and 4, then every 12 weeks (100mg)

Blood samples will be collected to determine the serum trough levels and anti-drug antibodies of tildrakizumab.

To assess the predictive value of early serum trough levels of tildrakizumab (µg/ml), the clinical response (PASI) at week 12 and/or week 24 will be correlated with serum trough concentrations of tildrakizumab measurements taken from week 0,1,2,3 and/or 4.

To assess the predictive value of early anti-drug antibodies of tildrakizumab (µg/ml), the clinical response (PASI) at week 12 and/or week 24 will be correlated with serum trough concentrations of tildrakizumab measurements taken from week 0,1,2,3 and/or 4.

Defining a therapeutic window for tildrakizumab based on serum trough concentrations corresponding with adequate clinical response (ROC analysis and concentration-effect curve).

The DLQI (Dermatology Life Quality Index) (range 0-30) is a dermatology specific quality of life (QoL) instrument designed to assess the impact of the disease on participant's QoL. It is a ten item questionnaire that, in addition to evaluated overall QoL, can be used to assess six different aspects that may affect QoL: 1) symptoms and feelings, 2) daily activities, 3) leasure, 4) work or school performances, 5) personal relationships, and 6) treatment.

The scoring of each question is as follows:

Very much - scored 3; A lot - scored 2; A little - scored 1; Not at all - scored 0; Not relevant - scored 0; Question 7, 'prevented work or studying' - scored 3. The DLQI is calculated by summing the score of each question resulting in a maximum of 30 and a minimum of 0. The higher the score, the more quality of life is impaired.