The purpose of the study is to see if PS433540 lowers blood pressure better than placebo and to see how safe PS433540 is compared to placebo.

Inclusion Criteria:

• Males or females 30 - 80 years
• Mean seated Systolic Blood Pressure (SBP) ≥ 150 - ≤ 179 mmHg and mean seated DBP < 110 mmHg at two consecutive qualifying visits. The mean difference in SBP between the two consecutive qualifying visits must be ≤ 10 mmHg.
• Mean daytime (8AM - 4PM) ambulatory SBP must be between ≥ 140 - ≤ 179 mmHg and mean daytime (8AM - 4PM) Diastolic Blood Pressure (DBP) ≤ 110 mmHg
• Subjects must have a usual daytime schedule. Night shift workers are excluded from participation.
• Women of child-bearing potential (WOCBP) and male subjects must use two reliable forms of contraception if sexually active. Alternatively, female subjects must be postmenopausal (for at least 1 year)

Exclusion Criteria:

• Subjects with serious disorders which may limit the ability to evaluate the efficacy or safety of PS433540, including cardiovascular, renal (including the absence of one kidney), pulmonary, hepatic, gastrointestinal (including clinically significant malabsorption), endocrine/metabolic, hematologic/oncologic (including an active malignancy other than basal cell carcinoma), neurologic and psychiatric diseases.
• Subjects with a history of myocardial infarction, angina, coronary angioplasty, bypass surgery or New York Heart Association (NYHA) class II-IV heart failure within the last 6 months.
• Subjects with a history of cerebrovascular accident or transient ischemic attack within the last 1 year.
• Subjects with diabetes mellitus (type I and type II).