Sapropterin

How it works

It is a synthetic analogue of the natural cofactor tetrahydrobiopterin, which acts as a cofactor for phenylalanine hydroxylase; in responsive patients it enhances residual enzyme activity, increasing conversion of phenylalanine to tyrosine.

Prescribing in practice

• It must be used together with continued dietary phenylalanine restriction and is only effective in patients shown to be BH4-responsive on testing, not as a replacement for diet.
• Phenylalanine control should be reviewed after a response trial, with treatment continued only where a meaningful reduction in blood phenylalanine is demonstrated.
• Prescribe within a specialist metabolic service according to the SPC and a children's formulary, with diet adjusted in light of treatment effect.

Monitoring

Monitor blood phenylalanine (and tyrosine) levels regularly to confirm responsiveness and guide dietary and dose adjustments under metabolic team supervision.

Counselling the patient

• Explain that the medicine helps lower phenylalanine but must be taken alongside the prescribed low-phenylalanine diet.
• Regular blood tests are needed to check phenylalanine control and to confirm the medicine is helping.
• Do not relax the diet without specialist advice, as good phenylalanine control protects brain development.

Evidence & guidelines

Sapropterin is established for BH4-responsive phenylketonuria as an adjunct to dietary management, with trials demonstrating reductions in blood phenylalanine and improved dietary tolerance in responders.