Background and objectives: Glycerol phenylbutyrate (GPB) has demonstrated safety and efficacy in patients with urea cycle disorders (UCDs) by means of its clinical trial program, but there are limited data in clinical practice. In order to analyze the efficacy and safety of GPB in clinical practice, here we present a national Spanish experience after direct switching from another nitrogen scavenger to GPB. Methods: This observational, retrospective, multicenter study was performed in 48 UCD patients (age 11.7 ± 8.2 years) switching to GPB in 13 centers from nine Spanish regions. Clinical, biochemical, and nutritional data were collected at three different times: prior to GPB introduction, at first follow-up assessment, and after one year of GPB treatment. Number of related adverse effects and hyperammonemic crisis 12 months before and after GPB introduction were recorded. Results: GPB was administered at a 247.8 ± 102.1 mg/kg/day dose, compared to 262.6 ± 126.1 mg/kg/day of previous scavenger (46/48 Na-phenylbutyrate). At first follow-up (79 ± 59 days), a statistically significant reduction in ammonia (from 40.2 ± 17.3 to 32.6 ± 13.9 μmol/L, p < 0.001) and glutamine levels (from 791.4 ± 289.8 to 648.6 ± 247.41 μmol/L, p < 0.001) was observed. After one year of GPB treatment (411 ± 92 days), we observed an improved metabolic control (maintenance of ammonia and glutamine reduction, with improved branched chain amino acids profile), and a reduction in hyperammonemic crisis rate (from 0.3 ± 0.7 to less than 0.1 ± 0.3 crisis/patients/year, p = 0.02) and related adverse effects (RAE, from 0.5 to less than 0.1 RAEs/patients/year p < 0.001). Conclusions: This study demonstrates the safety of direct switching from other nitrogen scavengers to GPB in clinical practice, which improves efficacy, metabolic control, and RAE compared to previous treatments.

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