This Phase 3 study (Study 305) has been designed to determine and compare the efficacy and safety of 188-0551 Spray and Vehicle Spray applied twice daily for up to four weeks in subjects with plaque psoriasis. Subjects will be instructed to apply the test article (188-0551 Spray or Vehicle Spray) to all psoriasis plaques within the designated Treatment Area twice daily for four weeks (Study Day 29), unless the investigator verifies the subject's psoriasis has cleared at Day 15, then test article application will be for 2 weeks (Study Day 15).

Inclusion Criteria:

1. Subject is male or non-pregnant female and is at least 18 years of age at time of informed consent.
2. Subject has provided written informed consent.
3. Subject has a clinical diagnosis of stable plaque psoriasis involving a minimum of 2% and no more than 10% body surface area (BSA) (excluding the face, scalp, groin, axillae, and other intertriginous areas).
4. Subject has moderate to severe plaque psoriasis.
5. Subject is willing and able to apply the test article as directed, comply with study instructions, and commit to all follow-up visits for the duration of the study.
6. Females must be post-menopausal , surgically sterile , or use an effective method of birth control. , Women of childbearing potential (WOCBP) must have a negative urine pregnancy test (UPT) at the Baseline Visit.

Exclusion Criteria:

1. Subject has spontaneously improving or rapidly deteriorating plaque psoriasis.
2. Subject has guttate, pustular, erythrodermic, or other non-plaque forms of psoriasis.
3. Subject has palmar/plantar psoriasis.
4. Subject is pregnant, lactating, or is planning to become pregnant during the study.
5. Subject is currently enrolled in an investigational drug or device study.
6. Subject has been previously enrolled in this study and treated with a test article.

Other protocol defined inclusion or exclusion criteria assessed by the study staff may apply.