Mol Genet Metab. 2013 Dec;110(4):446-53. doi: 10.1016/j.ymgme.2013.09.017. Epub 2013 Oct 8.

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.

Author information

1: Hyperion Therapeutics, 601 Gateway Blvd., Suite 200, South San Francisco, CA 94080, USA. Electronic address: Masoud.mokhtarani@hyperiontx.com.

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