Tetrahydrobiopterin, (BH4), is a radical new approach to treatment of PKU, it works on the principle of providing replacement parts for the enzyme to allow it to react naturally. This means that a PKU sufferer could in theory consume a lot more protein exchanges each day, however tests are still ongoing. We know that maintaining the restricted diet is beneficial if not essential to preventing brain damage, but disagreements as how long this diet should last.
While still facing technical difficulties to replace the defective gene/enzyme that causes PKU, one new approach to treat at least some PKU patients seems to be a close reality. A relatively high percentage of those with mild PKU may benefit from a substitution with tetrahydrobiopterin (BH4) added to the amino acid supplement already taken each day which reduces plasma phenylalanine levels.
"BH4 can activate the specific mutated enzyme PAH, by increasing the need for the BH4 substrate, by three- dimension structure stabilization, or by its chaperone-like activity. It has been shown that a number of DNA mutations correlate with BH4 responsiveness" - Nenad Blau
The main problem with BH4 is the relatively expensive costs of the drug and so far the lack of regulations by societies governing it's use.
For more information and BH4 responsive strains check the BIOPKU database