A Study to Evaluate the Efficacy and Safety of Subcutaneous MK-3222, Followed by an Optional Long-Term Safety Extension Study, in Participants With Moderate-to-Severe Chronic Plaque Psoriasis (MK-3222-010)

Inclusion Criteria:

- Clinical diagnosis of moderate-to-severe plaque psoriasis for at least 6 months prior
to study enrollment

- A candidate for phototherapy or systemic therapy

- For the extension study: must have completed Part 3 of the base study

- For the extension study: must have achieved at least a PASI-50 response by the end of
Part 3 of the base study

- For the extension study: must have received active MK-3222 treatment within 12 weeks
prior to the end of Part 3 of the base study

- Premenopausal female participants must agree to abstain from heterosexual activity or
use a medically accepted method of contraception or use appropriate effective
contraception as per local regulations or guidelines

- If enrolled at a Japanese site, participants with psoriatic arthritis using
non-steroidal anti-inflammatory drugs (NSAIDs) must be on a stable dose for at least
4 weeks prior to the first dose of study drug and must not be expected to require an
increase in dose over the course of the study

Exclusion Criteria:

- Has erythrodermic psoriasis, predominantly pustular psoriasis, medication-induced or
medication-exacerbated psoriasis, or new onset guttate psoriasis

- Current or history of severe psoriatic arthritis and is well-controlled on current
treatment

- Women of child-bearing potential that are pregnant, intend to become pregnant within
6 months of completing the trial, or that are breast feeding

- Expected to require topical treatment, phototherapy, or systemic treatment during the
trial

- Presence of any infection

- History of recurrent infection requiring treatment with systemic antibiotics within 2
weeks of screening

- Previous use of MK-3222 or other IL-23/Th-17 pathway inhibitors including P40, p19,
and IL-17 antagonists

- Evidence of active or untreated latent tuberculosis (TB)

- Positive test for human immunodeficiency virus (HIV), hepatitis B surface antigen
(HBs Ag) or hepatitis C virus (HCV)

- At Japanese sites, positive test for HBs antibody and hepatitis B virus (HBV)
deoxyribonucleic acid (DNA)

- At Japanese sites, positive test for the Hepatitis B core (HBc) antibody and HBV DNA

- For the extension study: women of child-bearing potential that are pregnant, intend
to become pregnant within 6 months of completing the trial, or that are breast
feeding

- For the extension study: active or uncontrolled significant organ dysfunction or
clinically significant laboratory abnormalities

- For the extension study: expected to require topical treatment, phototherapy, or
systemic treatment during the extension study

- At Japanese sites, abnormal for Beta D Glucan and/or KL-6 test result(s) at the
screening visit.