# Characterization and treatment monitoring of ureagenesis disorders using stable isotopes

## TL;DR

A new method using stable isotopes helps monitor urea cycle disorders and track treatment effectiveness in patients.

## Contribution

A UHPLC-HRMS method for assessing ureagenesis using [15N]ammonium chloride as a tracer is introduced for UCD monitoring.

## Key findings

- The method successfully detected ureagenesis variations in UCD patients based on their metabolic defects.
- Therapy follow-up showed restoration of [15N]urea production and reduced [15N]glutamine levels.
- The method is safe, efficient, and requires minimal tracer and sample amounts with short preparation time.

## Abstract

Urea cycle disorders (UCDs) are a group of rare conditions, possibly life-threatening and without definitive cure besides liver transplantation. Traditional biochemical analyses/biomarkers cannot reliably determine changes in the UC-function from baseline to post-intervention. We describe a UHPLC-HRMS method to assess ureagenesis in plasma and dried blood spots for [15N]urea and [15N]amino acids, using [15N]ammonium chloride as tracer. [15N]enrichment of urea and amino acids was studied in controls (n = 22) and patients (n = 59), the latter showing characteristic ureagenesis variations according to their underlying metabolic defect. Follow-up of therapies was successful, as we observed restoration of [15N]urea production and lowering of [15N]glutamine. There were no adverse events, and only minimal amounts of tracer and samples required with a short sample preparation time and analysis. Thus, the method proved to be safe and efficient to monitor UCD patients of variable severity pre- and post-therapy, being suitable as physiological endpoint for development of therapies.