PH-10 is an aqueous hydrogel formulation of rose bengal disodium (RB) for topical administration to the skin, and is being studied for the treatment of cutaneous skin disorders, specifically psoriasis and atopic dermatitis. This multicenter phase 2 study of subjects randomized sequentially by center to one of four treatment cohorts will assess efficacy and safety of topical PH-10 applied once daily to areas of mild to moderate plaque psoriasis.

Inclusion Criteria:

• Men or women, age 18 or older.
• Presence of mild to moderate plaque psoriasis on the trunk or extremities (excluding palms, soles, scalp, and facial or intertriginous areas).
• Fitzpatrick skin type I-VI.
• Written informed consent by the subject or legal guardian.

Exclusion Criteria:

• Female subjects of childbearing potential who are pregnant, attempting to conceive, not using effective contraception, or who are nursing an infant.
• Subjects who have received psoralen plus ultraviolet A (PUVA) therapy or systemic antipsoriatic therapy (immunosuppressants, cytostatics, corticosteroids) within 28 days of study initiation (two weeks for methotrexate).
• Subjects who have received ultraviolet B (UVB) light therapy within 14 days of study initiation.
• Subjects who have received topical antipsoriatic therapy (including corticosteroids, tar, anthralin or Vitamin D analogs) to the study plaque areas within 14 days of study initiation.
• Subjects who have received agents posing a clinically significant risk of photosensitivity reaction within 5 half-lives of study initiation.
• Subjects who have received any approved or investigational biologic drug therapy for psoriasis within 90 days or 5 half-lives of study initiation.
• Subjects who have participated in a clinical research study within 28 days of study initiation.
• Subjects with a history of porphyria, systemic lupus erythematosus or xeroderma pigmentosum.
• Subjects with clinical conditions that, in the opinion of the Investigator may pose a health risk to the subject by being involved in the study or detrimentally affect regular follow-up of the subject.