A Phase 3, Multicenter, Open-Label Extension Study of Phenoptin in Subjects With PKU Who Have Elevated Phenylalanine Levels

Inclusion Criteria: * 8 years of age and older * Prior successful participation in Study PKU-003 * Willing and able to provide written informed consent or assent and written informed consent (if required) by a parent or legal guardian * For females of child-bearing potential only: Negative urine pregnancy test within 24 hours prior to enrollment. Women using acceptable birth control measures must agree to continue to use those measures while participating in the study * Willing and able to comply with study procedures * Willing to continue current diet unchanged while participating in the study Exclusion Criteria: * Perceived to be unreliable or unavailable for study participation or, if under the age of 18, have parents or legal guardians who are perceived to be unreliable or unavailable * Withdrew from, or otherwise did not successfully complete, study PKU-003, except for subjects who were removed from the study because their blood Phe exceeded the alert level * Expected to require any investigational agent or vaccine prior to completion of all scheduled study assessments * Pregnant or breastfeeding, or planning pregnancy * Concurrent disease or condition that would interfere with study participation or safety (e.g., seizure disorder, oral steroid-dependent asthma or other condition requiring oral or parenteral corticosteroid administration, or insulin-dependent diabetes) * Requirement for concomitant treatment with any drug known to inhibit folate synthesis (e.g., methotrexate) * Concurrent use of levodopa