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Does the 48-hour BH4 loading test miss responsive PKU patients?

Annemiek M J van Wegberg¹, Roeland A F Evers¹, Esther van Dam¹

¹Department of Metabolic Diseases, Beatrix Children's Hospital, University of Groningen, University Medical Centre Groningen, the Netherlands.

Molecular Genetics and Metabolism

|January 12, 2020

Summary

A 7-day tetrahydrobiopterin (BH4) loading test may be necessary to identify all phenylketonuria (PKU) patients responsive to BH4 treatment. A 48-hour test may miss some responders, and a 20% phenylalanine reduction threshold is not reliable.

Keywords:

BH4 Loading test PKU Phenylketonuria Responsiveness Tetrahydrobiopterin

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Area of Science:

Metabolic disorders
Genetics
Pharmacology

Background:

Phenylketonuria (PKU) is an inherited metabolic disorder affecting phenylalanine (Phe) metabolism.
Tetrahydrobiopterin (BH4) is a treatment option for some PKU patients.
The efficacy of the standard 48-hour BH4 loading test in identifying all responsive patients is under investigation.

Purpose of the Study:

To determine if the 48-hour BH4 loading test misses BH4-responsive PKU patients.
To evaluate the utility of a 20% Phe reduction cut-off versus the standard 30%.
To assess the benefit of extending the BH4 loading test to 7 days.

Main Methods:

22 PKU patients underwent a 7-day oral BH4 loading test (20 mg/kg/day).
Blood Phe levels were monitored at 13 time points.
Potential responders were identified based on Phe reduction criteria and confirmed with a long-term treatment trial.

Main Results:

Of 22 patients completing the 7-day test, 12 were potential BH4 responders.
5 potential responders were false positives, and 6 were true responders.
A 20% Phe reduction threshold within 48 hours was not a reliable predictor of responsiveness.

Conclusions:

A 7-day BH4 loading test may be required to detect all BH4-responsive PKU patients.
The 48-hour test might miss responders, especially if a clear trend of Phe reduction is observed.
A 20% Phe reduction threshold is not recommended for predicting true BH4 responsiveness.