Glycerol phenylbutyrate

How it works

It is hydrolysed to phenylbutyrate and then phenylacetate, which conjugates with glutamine to form phenylacetylglutamine; this is excreted renally, providing an alternative route for waste nitrogen elimination that bypasses the deficient urea cycle.

Prescribing in practice

• It does not treat acute hyperammonaemic crises, which require emergency management; it is a long-term measure to maintain control alongside dietary protein restriction.
• It is used as part of a specialist regimen including a low-protein diet and, where appropriate, amino acid supplementation under metabolic medicine supervision.
• Reduced exposure can occur in hepatic impairment, and gastrointestinal symptoms are commonly reported.

Monitoring

Monitor plasma ammonia together with the metabolic and nutritional parameters guided by the specialist metabolic team to confirm adequate nitrogen control.

Counselling the patient

• This medicine helps control your condition long term and must be taken alongside the prescribed diet; it is not a treatment for sudden episodes.
• Do not stop or change the dose without advice from your metabolic team.

Evidence & guidelines

Nitrogen-scavenging therapy is an established component of chronic urea cycle disorder management used under specialist supervision.