Elevated phenylacetic acid levels do not correlate with adverse events in patients with urea cycle disorders or hepatic encephalopathy and can be predicted based on the plasma PAA to PAGN ratio

Abstract

Background: Phenylacetic acid (PAA) is the active moiety in sodium phenylbutyrate (NaPBA) and glycerol phenylbutyrate (GPB, HPN-100). Both are approved for treatment of urea cycle disorders (UCDs) - rare genetic disorders characterized by hyperammonemia. PAA is conjugated with glutamine in the liver to form phenylacetyleglutamine (PAGN), which is excreted in urine. PAA plasma levels ≥ 500 μg/dL have been reported to be associated with reversible neurological adverse events (AEs) in cancer patients receiving PAA intravenously. Therefore, we have investigated the relationship between PAA levels and neurological AEs in patients treated with these PAA pro-drugs as well as approaches to identifying patients most likely to experience high PAA levels.

Methods: The relationship between nervous system AEs, PAA levels and the ratio of plasma PAA to PAGN were examined in 4683 blood samples taken serially from: [1] healthy adults [2], UCD patients of ≥ 2 months of age, and [3] patients with cirrhosis and hepatic encephalopathy (HE). The plasma ratio of PAA to PAGN was analyzed with respect to its utility in identifying patients at risk of high PAA values.

Results: Only 0.2% (11) of 4683 samples exceeded 500 μg/ml. There was no relationship between neurological AEs and PAA levels in UCD or HE patients, but transient AEs including headache and nausea that correlated with PAA levels were observed in healthy adults. Irrespective of population, a curvilinear relationship was observed between PAA levels and the plasma PAA:PAGN ratio, and a ratio>2.5 (both in μg/mL) in a random blood draw identified patients at risk for PAA levels>500 μg/ml.

Conclusions: The presence of a relationship between PAA levels and reversible AEs in healthy adults but not in UCD or HE patients may reflect intrinsic differences among the populations and/or metabolic adaptation with continued dosing. The plasma PAA:PAGN ratio is a functional measure of the rate of PAA metabolism and represents a useful dosing biomarker.

Trial registration: ClinicalTrials.gov NCT00551200 NCT00947297 NCT00947544 NCT00992459 NCT00999167 NCT01347073.

Keywords: BUPHENYL; GEE; GPB; Glycerol phenylbutyrate; HE; HPN-100; NaPBA; Neurological adverse events; PAA; PAA:PAGN ratio; PAGN; PBA; RAVICTI; SE; SO; Sodium phenylbutyrate; UCD; generalized estimating equations; glycerol phenylbutyrate (generic name for glyceryl tri (4-phenylbutyrate), also referred to as HPN-100 or RAVICTI(®)); hepatic encephalopathy; phenylacetic acid; phenylacetylglutamine; phenylbutyric acid; ratio of the concentrations in μg/mL of PAA to PAGN in plasma; safety extension; sodium phenylbutyrate (BUPHENYL(®)); switchover; urea cycle disorder.

Figure 1. Lack of Relationship Between PAA Levels and Neurological AEs in UCD and HE Patients

The top and middle panels depict box-and-whisker plots for the mean maximal (Cmax) concentration of PAA during dosing of UCD patients with sodium phenylbutyrate and glycerol phenylbutyrate, respectively. There was no statistical difference in maximal PAA levels between UCD patients who did or did not report neurological AEs. The bottom panel depicts mean PAA concentrations (mean [SD] = 61.4 [75.3] vs. 36.4 [55.6]; p = 0.77) among patients with cirrhosis and hepatic encephalopathy randomized to treatment with glycerol phenylbutyrate who reported neurological adverse events. The range of PAA concentrations as reflected by the box (25^th to 75^th percentile) are similar for patients who did or did not report an AE. The dots depict individual values. (See Table 2 for statistical summary)

Figure 1. Lack of Relationship Between PAA Levels and Neurological AEs in UCD and HE Patients

The top and middle panels depict box-and-whisker plots for the mean maximal (Cmax) concentration of PAA during dosing of UCD patients with sodium phenylbutyrate and glycerol phenylbutyrate, respectively. There was no statistical difference in maximal PAA levels between UCD patients who did or did not report neurological AEs. The bottom panel depicts mean PAA concentrations (mean [SD] = 61.4 [75.3] vs. 36.4 [55.6]; p = 0.77) among patients with cirrhosis and hepatic encephalopathy randomized to treatment with glycerol phenylbutyrate who reported neurological adverse events. The range of PAA concentrations as reflected by the box (25^th to 75^th percentile) are similar for patients who did or did not report an AE. The dots depict individual values. (See Table 2 for statistical summary)

Figure 2. PAA Levels in Healthy Adults Reporting a Nervous System Adverse Event (AE) Grouped by Dose

The maximum PAA value (Cmax) is displayed in relation to dose of glycerol phenylbutyrate for patients who did or did not report a neurological adverse event (AE), regardless of relationship to study drug or timing relative to blood draw for PAA. The box and whisker plots depict mean (horizontal line), 25-75 percentiles (box) and 10 and 90% confidence intervals. Note that a wide range of PAA levels was observed at each dose and among patients with or without AEs. PAA levels were significantly higher among patients with AEs as compared to those without at the 6mL TID dose, but not at the 4 mL TID dose. All but 1 subject in the 9 and 12 mL dose groups reported a neurological AE. (See Table 2 for statistical summary)

Figure 3. Plasma PAA Intra-subject Variability

Healthy subjects and patients with UCD or HE underwent serial blood sampling over 12 to 24 hours. The figure depicts the coefficient of variation (CV%) as an indicator of intra-subject variability. Regardless of the dose or population, there is high degree of variability among all subjects.

Figure 4. Plasma PAA vs. Plasma PAA:PAGN Ratio

PAA levels in μg/mL (Y axis) are plotted in relation to the ratio of PAA to PAGN concentration (both expressed as μgμg/mL) in plasma (X axis) in that same sample. This plot includes >3500 samples from all populations, including healthy adults (Healthy), patients with cirrhosis and hepatic encephalopathy (Hepatic), and patients with urea cycle disorders (UCD). All populations exhibit a similar relationship, with the upward inflection point occurring at ratios exceeding approximately 2.5 and PAA concentrations in the range of 100-200 μg/ml.