A Phase 2 Study of Cellular and Immunologic Changes in the Skin of Subjects Receiving PH-10

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). * At least one target plaque having a minimum diameter of 5 cm (2 inches) with uniform mild to moderate plaque psoriasis. * 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 PH-10. * Subjects who have received PUVA or systemic antipsoriatic therapy (immunosuppressants, cytostatics, corticosteroids) within 28 days of study initiation (14 days for methotrexate). * Subjects who have received 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 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 because of involvement in the study or detrimentally affect regular follow-up of the subject.