A Study to Assess the Effect of Ustekinumab (Stelara®) and Etanercept (Enbrel®) in Participants With Moderate to Severe Psoriasis (MK-0000-206)

This is a two-part study. The purpose of the pilot study (Part 1) is to optimize the acquisition, handling and shipping procedure for skin biopsies obtained from participants with plaque psoriasis. No treatment will be administered. Part 2 will include 2 cohorts. In Cohort 1, the effects of 16 weeks of treatment with either ustekinumab or etanercept on biomarkers in lesional skin in participants with moderate to severe psoriasis will be evaluated. In Cohort 2, biomarkers of lesional skin from participants with moderate to severe psoriasis who are not treated with biologic therapy will be evaluated over 16 weeks. The primary hypothesis is that treatment with ustekinumab reduces messenger RNA (mRNA) expression of genes in the interleukin 12 (IL-12) pathway that are modulated by interferon gamma (IFN-γ).