• Achievement of PASI 50 [ Time Frame: at week 24 ] [ Designated as safety issue: No ]
• Achievement of a static PGA score of clear (0) or almost clear (1) [ Time Frame: at week 12 and week 24 ] [ Designated as safety issue: No ]

The aim of this study is to evaluate the safety and effectiveness of an investigational drug called doxercalciferol in patients with moderate to severe chronic plaque psoriasis, in comparison with a placebo ("sugar pill"). All study related care is provided including doctor visits, physical exams, laboratory tests and study medication. Total length of participation is 28 weeks.

This is a multicenter, randomized, double-blind, placebo-controlled, dose-ranging, parallel group study to evaluate the efficacy and safety of doxercalciferol given orally, once daily for 24 weeks to patients with moderate to severe chronic plaque psoriasis. Subjects will be randomized and stratified by site and Psoriasis Area Severity Index (PASI) score to one of three active treatment groups or to the placebo group.

• Drug: doxercalciferol
• Other: N/A - placebo

Inclusion Criteria:

• Subject must have moderate to severe chronic plaque psoriasis with a body surface area (BSA) involvement ≥10%.
• Subject must have static PGA of moderate or severe.
• Subject must have a minimum PASI score of ≥10.

Exclusion Criteria:

• Use of drugs known to influence serum calcium (such as lithium, digoxin, thiazide diuretics, teriparatide, bisphosphonates, and calcitonin) and multivitamin supplements containing >400 IU vitamin D or calcium and/or calcium-containing antacids exceeding a total of 1 gram/day.
• Use of other psoriasis medications or light therapy during the study except for shampoos containing coal tar or salicylic acid and low potency topical corticosteroids (Classes VI and VII) on the groin, scalp, palms, soles and face.
• History of kidney stones, chronic kidney disease, symptomatic coronary or cerebral vascular disease, HIV, active viral hepatitis, or any other clinically significant, unstable medical condition that would interfere with the completion of the study.
• Any evidence of active malignancy except for basal cell carcinoma of the skin. A history of malignancy is not an exclusion.
• Active ethanol or drug abuse.
• Pregnant or breast-feeding women.