Quantitative Fluorescence Molecular Imaging of Ustekinumab-800CW to Elucidate the Drug Distribution Throughout Inflamed Tissue in Crohn's Disease and Psoriasis.

Inclusion Criteria: * Established diagnosis of CD or PsO * Non-responsive to conventional therapy * Active disease: * CD cohort: clinically active disease of the bowel defined either clinically as at least moderate activity using dedicated scoring indices (for definitions of disease activity, see below) or biochemically active disease as defined by a faecal calprotectin \> 200 ug/g; * PsO: clinically active disease of at least PASI ≥ 10 or clinically active disease as assessed by a dermatologist; * Age ≥ 18 years; * Written informed consent. Exclusion Criteria: * Medical or psychiatric conditions that compromise the patient's ability to give informed consent according to treating medical physician; * Prior ustekinumab treatment within the last 15 weeks (except for patients in the treatment arm and control group); * Ustekinumab contraindicated as therapy; * Pregnancy or breast feeding. A negative pregnancy test must be available for women of childbearing potential (i.e. premenopausal women with intact reproductive organs and women less than two years after menopause);