A prospective study to determine the minimal biologic drug serum trough concentration required to maintain stable disease activity in patients with moderate to severe chronic plaque psoriasis.

Biologic drugs are commonly used for treatment-resistant moderate to severe
plaque psoriasis. They are licensed with a
*one dose fits all* principle, whereby fixed dose regimens are used. Studies
have shown high inter-patient variability in
serum concentrations of the injected biologic drugs. In patients with high
circulating drug concentrations we expect that the
dose can be reduced or the dose interval can be prolonged. On the other hand,
patients with low circulating drug
concentrations may not respond well to a specific biologic drug. An important
factor to consider is the development of an
immune response resulting in anti-drug antibodies (ADA), which can not only
lead to immune complex formation and
accelerated drug clearance, but also to severe adverse reactions including
anaphylactic shock. Individualization of the
dosing regimen may increase the safety and efficacy of the treatment with
biologic drugs in patients with psoriasis, and may
also be more cost-effective.

To determine the minimal biologic drug serum trough concentration required to
maintain stable disease activity in patients
with moderate to severe chronic plaque psoriasis.

This is an exploratory prospective cohort study that includes patients that are
currently being treated with a biologic drug for
their psoriasis in the department of Dermatology of the Erasmus MC.
To determine whether a patient has stable disease activity, a run-in period of
6 weeks will precede the prolongation of the
dosing interval. In all patients, the dosing interval of the biologic drugs
will be prolonged during every 12 weeks during a 48 week period. For every
patient the total follow-up will be 72 weeks. In case of exacerbation of
disease activity the last effective dose will be resumed, and the dosing
interval will be maintained during the remainder of the study period.

The dosing interval of the biologic will be prolonged with an average of 1 or 2
weeks, during 48 weeks in periods of 12 weeks.

Potential benefits:
- less frequent injections
- reduced risk for infections, because the biologic dose will be reduced

Potential disadvantages:
- hematoma, site of the venepuncture
-collaps because of venepuncture
- in patients in whom dosing interval is prolonged there is a possibility of
exacerbation of psoriasis activity In such cases the
interval of drug administration will be shortened again
- The questionaires will take 3 minutes additional time, also the out-patient
visit will take 10 minutes longer because of the measuring of the PASI score.