This study will evaluate the safety and efficacy of NGGT002 gene therapy with two dose cohorts in adult subjects with diagnosis of classic PKU, a condition characterized by severe PAH deficiency with no residual enzyme activity. NGGT002 will be administered through intravenous infusion. In Part 1 of the study, subjects will receive NGGT002 at the low dose. Dosing of the first 3 subjects will be staggered. Following evaluation of data from the first 3 subjects, a decision can be made to either escalate to the high dose level or expand the low dose cohort with additional 3 subjects. Upon completion of Part 1 study, based on the evaluation of and safety and efficacy, the study may be stopped or proceed to Part 2. In Part 2, the same process will be conducted with 3 -6 subjects dosed at the high dose.