We have analyzed pharmacokinetic data for glycerol phenylbutyrate (also GT4P or HPN-100) and sodium phenylbutyrate with respect to possible dosing biomarkers in patients with urea cycle disorders (UCD). Study design: These analyses are based on over 3000 urine and plasma data points from 54 adult and 11 pediatric UCD patients (ages 6-17) who participated in three clinical studies comparing ammonia control and pharmacokinetics during steady state treatment with glycerol phenylbutyrate or sodium phenylbutyrate. All patients received phenylbutyric acid equivalent doses of glycerol phenylbutyrate or sodium phenylbutyrate in a cross over fashion and underwent 24-hour blood samples and urine sampling for phenylbutyric acid, phenylacetic acid and phenylacetylglutamine. Results: Patients received phenylbutyric acid equivalent doses of glycerol phenylbutyrate ranging from 1.5 to 31.8g/day and of sodium phenylbutyrate ranging from 1.3 to 31.7g/day. Plasma metabolite levels varied widely, with average fluctuation indices ranging from 1979% to 5690% for phenylbutyric acid, 843% to 3931% for phenylacetic acid, and 881% to 1434% for phenylacetylglutamine. Mean percent recovery of phenylbutyric acid as urinary phenylacetylglutamine was 66.4 and 69.0 for pediatric patients and 68.7 and 71.4 for adult patients on glycerol phenylbutyrate and sodium phenylbutyrate, respectively. The correlation with dose was strongest for urinary phenylacetylglutamine excretion, either as morning spot urine (r=0.730, p<0.001) or as total 24-hour excretion (r=0.791 p<0.001), followed by plasma phenylacetylglutamine AUC24-hour, plasma phenylacetic acid AUC24-hour and phenylbutyric acid AUC24-hour. Plasma phenylacetic acid levels in adult and pediatric patients did not show a consistent relationship with either urinary phenylacetylglutamine or ammonia control. Conclusion: The findings are collectively consistent with substantial yet variable pre-systemic (1st pass) conversion of phenylbutyric acid to phenylacetic acid and/or phenylacetylglutamine. The variability of blood metabolite levels during the day, their weaker correlation with dose, the need for multiple blood samples to capture trough and peak, and the inconsistency between phenylacetic acid and urinary phenylacetylglutamine as a marker of waste nitrogen scavenging limit the utility of plasma levels for therapeutic monitoring. By contrast, 24-hour urinary phenylacetylglutamine and morning spot urine phenylacetylglutamine correlate strongly with dose and appear to be clinically useful non-invasive biomarkers for compliance and therapeutic monitoring.
Bibliographical noteFunding Information:
This study was sponsored by Hyperion Therapeutics . The authors gratefully acknowledge and thank the efforts of the study coordinators and nursing staff who made these trials possible, including N. Schrager (Mount Sinai School of Medicine), A. Donovan, J. Crawford, Pediatric TRU Staff, K. Defouw, J. Balliet (The Medical College of Wisconsin), M. Keuth, N. O'Donnell (Long Beach Memorial Hospital), M. Hussain, E. Bailey, M. Ambreen (The Hospital for Sick Children, University of Toronto, ON, Canada), C. Bailey, A. Lang (The University of Utah), J. Perry, V. de Leon, A. Niemi, K. Cusmano (Stanford University), T. Carlson, J. Parker (University of Minnesota), S. Burr (Children's Hospital Colorado), K. Simpson (Children's National Medical Center), K. Regis (Nationwide Children's Hospital), A. Behrend, T. Marrone (Oregon Health Sciences University), N. Dorrani (University of California, Los Angeles), C. Heggie (Case Western Reserve University), S. Mortenson (Maine Medical Center), S. Deward (Children's Hospital of Pittsburgh), K. Murray, C. Dedomenico (Tufts Medical Center), C. Gross (University of Florida), L. Brody (Seattle Children's Hospital), M. Mullins, S. Carter, A. Tran, J. Stuff, TCH General Clinical Research Center nursing staff (Baylor), K. Lisam (Hyperion), as well as the Clinical and Translational Science Awards/General Clinical Research Center Grants ( Baylor College of Medicine , M01RR00188 ; Case Western Reserve University , UL1RR024989 ; Clinical and Translational Science Institute at Children's National Medical Center NIH/NCRR , UL1RR31988 ; Medical College of Wisconsin , UL1RR31973 ; Mount Sinai School of Medicine , UL1RR29887 ; Oregon Health & Science University , UL1RR24140 ; Stanford University , UL1RR25744 ; Tufts University , UL1RR25752 ; University of California, Los Angeles , UL1RR33176 ; University of Colorado , UL1RR25780 ; University of Florida , UL1RR29890 ; University of Minnesota , UL1RR33183 ; University of Pittsburgh , UL1RR24153 , UL1TR000005 ; University of Utah , UL1RR25764 ; University of Washington , UL1RR25014 ), the Urea Cycle Disorders Consortium ( NIH Grant U54RR019453 ) and grants from the O'Malley Foundation and Kettering Fund which provided support. SCS. Nagamani is an awardee of the National Urea Cycle Disorders Foundation Research Fellowship .
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- Glycerol phenylbutyrate
- Phenylacetic acid
- Phenylbutyric acid
- Sodium phenylbutyrate
- Urea cycle disorders