This is a 2 part, Phase 2, open-label dose-finding study in approximately 35 subjects with PKU. Seven dose cohorts are planned, each consisting of 5 subjects. In Part 1, the planned starting dose levels are those tested in PAL 001 (0.001, 0.003, 0.01, 0.03, 0.1, 0.3, and 1.0 mg/kg), provided no dose limiting toxicity was observed in PAL 001. In Parts 1 and 2, study drug will be administered by clinic staff.