This is an open label, single center, controlled study with each subject's two treatment plaque areas assigned by the investigator 1:1 to (a) PH-10 with ambient light exposure and (b) PH-10 with 544 nm LED light illumination at 10 J/cm2. A third plaque area will receive no treatment and serve as a control.

Subjects with at least three distinct, stable study plaque areas will receive the experimental therapy to two treatment plaque areas twice a week (2-5 days apart) for the lesser of 12 weeks or until remission is observed in the treatment plaque areas. If remission is observed in a treatment plaque area then treatment of that area will be discontinued and the area assessed weekly. A third plaque area (control plaque area) will receive no drug or light treatment and serve as an internal control.

Primary efficacy will be assessed 12 weeks after initial PH-10 treatment. Subjects will be followed for a total of 16 weeks to allow assessment of Durability of Response of treated lesions and comprehensive follow-up of adverse events.

Inclusion Criteria:

• Stable, moderate to severe plaque psoriasis in at least three distinct plaque areas, each separated by at least a 2.5-cm band of normal skin. Study plaque areas, each covering a contiguous area up to 15 x 35 cm in size, should have a minimum plaque size of 2 cm2. All study plaque areas must be on the trunk or extremities (excluding palms, soles, scalp, and facial or intertriginous sites).
• Fitzpatrick skin type I-VI.
• Ability to understand and sign the informed consent document.

Exclusion Criteria:

• Female subjects of childbearing potential who are pregnant, attempting to conceive, or nursing an infant.
• Subjects who have received PUVA or UVB light therapy or systemic antipsoriatic therapy within 4 weeks of study treatment (two weeks for methotrexate).
• Subjects who have received topical antipsoriatic therapy (including corticosteroids, tar, anthralin, or Vitamin D analogs) to the study plaque areas within 2 weeks of study treatment.
• Subjects who have received any photosensitizing or phototoxic drug within 4 weeks of study treatment.
• Subjects who have received any approved biologic drug therapy for psoriasis within 3 months or 5 half-lives of study treatment.
• Subjects who have participated in a clinical research study within 4 weeks of study treatment.
• Subjects with a history of porphyria, systemic lupus erythematosus or xeroderma pigmentosum.
• Subjects with clinical conditions that, in the opinion of the Principal Investigator, may pose a health risk to the subject by being involved in the study or detrimentally affect regular follow-up of the subject.