A Study of the Effects of 6R-BH4 on Blood Pressure in Subjects With Poorly Controlled Systemic Hypertension

Inclusion Criteria: * Willing and able to provide written, signed informed consent after the nature of the study has been explained and prior to any research-related procedures. * At least 18 years of age. * Willing and able to comply with all study-related procedures. * History of documented essential hypertension (BP greater than or equal to 140 mm Hg systolic and/or 90 mm Hg diastolic measured on 2 separate occasions). * Have poorly controlled hypertension despite use of at least two conventional antihypertensive agents with different mechanisms of action taken concurrently and consistently for at least 3 months before randomization. (Note: Antihypertensive agents may be individual or combined into a single medication.) * During the two-week screening period, mean SBP and mean DBP fall within the following ranges: * Mean SBP between 135 and 160 mm Hg * Mean DBP between 85 and 110 mm Hg * Sexually active subjects must be willing to use an acceptable method of contraception while participating in the study. * Females of childbearing potential must have a negative pregnancy test at screening and be willing to have additional pregnancy tests during this study. Individuals in the diabetic cohort must meet this additional criterion: * Documented history of type 2 diabetes that has been treated using the same therapy for at least 3 months. Exclusion Criteria: * Previous treatment with any formulation of BH4. * Known allergy or hypersensitivity to any excipient of 6R-BH4. * K…