Sodium Phenylacetate and Sodium Benzoate (Page 4 of 5)

12.2 Pharmacodynamics

In patients with hyperammonemia due to deficiencies in enzymes of the urea cycle, sodium phenylacetate and sodium benzoate injection has been shown to decrease elevated plasma ammonia levels. These effects are considered to be the result of reduction in nitrogen overload through glutamine and glycine scavenging by sodium phenylacetate and sodium benzoate injection in combination with appropriate dietary and other supportive measures.

12.3 Pharmacokinetics

The pharmacokinetics of intravenously administered sodium phenylacetate and sodium benzoate injection was characterized in healthy adult volunteers. Both benzoate and phenylacetate exhibited nonlinear kinetics. Following 90-minute intravenous infusion mean AUC last for benzoate was 20.3, 114.9, 564.6, 562.8, and 1599.1 mcg/mL following doses of 1, 2, 3.75, 4, and 5.5 g/m 2 , respectively. The total clearance decreased from 5.19 to 3.62 L/h/m 2 at the 3.75 and 5.5 g/m 2 doses, respectively.

Similarly, phenylacetate exhibited nonlinear kinetics following the priming dose regimens. AUC last was 175.6, 713.8, 2040.6, 2181.6, and 3829.2 mcg⋅h/mL following doses of 1, 2, 3.75, 4, and 5.5 g/m 2 , respectively. The total clearance decreased from 1.82 to 0.89 mcg⋅h/mL with increasing dose (3.75 and 4 g/m 2 , respectively).

During the sequence of 90-minute priming infusion followed by a 24-hour maintenance infusion, phenylacetate was detected in the plasma at the end of infusion (T max of 2 hr at 3.75 g/m 2) whereas, benzoate concentrations declined rapidly (T max of 1.5 hr at 3.75 g/m 2) and were undetectable at 14 and 26 hours following the 3.75 and 4 g/m 2 dose, respectively.

A difference in the metabolic rates for phenylacetate and benzoate was noted. The formation of hippurate from benzoate occurred more rapidly than that of phenylacetylglutamine from phenylacetate, and the rate of elimination for hippurate appeared to be more rapid than that for phenylacetylglutamine.

Pharmacokinetic observations have also been reported from twelve episodes of hyperammonemic encephalopathy in seven children diagnosed (age 3 to 26 months) with urea cycle disorders who had been administered sodium phenylacetate and sodium benzoate injection intravenously. These data showed peak plasma levels of phenylacetate and benzoate at approximately the same times as were observed in healthy adults. As in healthy adults, the plasma levels of phenylacetate were higher than benzoate and were present for a longer time.

The pharmacokinetics of intravenous phenylacetate have been reported following administration to adult patients with advanced solid tumors. The decline in serum phenylacetate concentrations following a loading infusion of 150 mg/kg was consistent with saturable enzyme kinetics. Ninety-nine percent of administered phenylacetate was excreted as phenylacetylglutamine.


13.1 Carcinogenesis, Mutagenesis, Impairment of Fertility

Long-term studies in animals have not been performed to evaluate the carcinogenic potential of sodium phenylacetate and sodium benzoate injection. Studies to evaluate the possible impairment of fertility or mutagenic potential of sodium phenylacetate and sodium benzoate injection have not been performed. Results indicate that sodium benzoate is not mutagenic or carcinogenic, and does not impair fertility.

13.2 Animal Toxicology and/or Pharmacology

In animal studies, subcutaneous administration to rat pups of 190 — 474 mg/kg of phenylacetate caused decreased proliferation and increased loss of neurons, and reduced central nervous system (CNS) myelin. Cerebral synapse maturation was retarded, and the number of functioning nerve terminals in the cerebrum was reduced, which resulted in impaired brain growth. Pregnant rats were given phenylacetate at 3.5 μmol/g/day subcutaneously from gestation day 7 through normal delivery. Prenatal exposure of rat pups to phenylacetate produced lesions in layer 5 cortical pyramidal cells; dendritic spines were longer and thinner than normal and reduced in number.


The efficacy of sodium phenylacetate and sodium benzoate injection in improving patient survival of acute hyperammonemic episodes was demonstrated in an analysis of 316 patients (1,045 episodes of hospitalization) treated between 1981 and 2003.

The demographic characteristics and diagnoses of the patient population are shown in Table 3.

Table 3. Baseline Characteristics and Diagnoses of Study Population
OTC = ornithine transcarbamylase deficiency
ASS = argininosuccinate synthetase deficiency
CPS = carbamyl phosphate synthetase deficiency
ASL = argininosuccinate lyase deficiency
ARG = arginase deficiency
THN = transient hyperammonemia of the newborn
For the summary at the patient level, data obtained at first episode used.
Diagnosis unknown or pending (33 episodes), acidemia (14 episodes), HHH syndrome (6 episodes), carnitine translocase deficiency (4 episodes), liver disease (3 episodes), HMG CoA lyase deficiency (1 episode), non-ketotic hyperglycinemia (1 episode), suspected fatty acid oxidation deficiency (1 episode), and valproic-acid-induced hyperammonemia (1 episode).

Patients *

N = 316



158 (51%)


150 (49%)

Age (years)



Mean (SD)

6.2 (8.54)



Age groups

0–30 days

104 (34%)

31 days–2 years

55 (18%)

> 2–12 years

90 (29%)

> 12–16 years

30 (10%)

> 16 years

31 (10%)

Enzyme deficiency


146 (46%)


71 (22%)


38 (12%)


7 (2%)


2 (< 1%)


2 (< 1%)


56 (18%)

On admission to the hospital, patients with hyperammonemia and a suspected or confirmed urea cycle disorder (UCD) diagnosis were treated with a bolus dose of 0.25 g/kg (or 5.5 g/m 2) sodium phenylacetate + 0.25 g/kg (or 5.5 g/m 2) sodium benzoate over a period of 90 minutes to 6 hours, depending on the specific UCD. Infusions also contained arginine; the dose of arginine depended on the specific UCD. After completion of the bolus dose, maintenance infusions of the same dose over 24 hours were continued until the patient was no longer hyperammonemic or oral therapy could be tolerated. The mean (SD) duration of treatment was 4.6 (6.45) days per episode, and ranged from 1 to 72 days.

Survival was substantially improved after sodium phenylacetate and sodium benzoate injection treatment compared with historical values (estimated 14% 1-year survival rate with dietary therapy alone) and with dialysis (estimated 43% survival of acute hyperammonemia).

Eighty percent of patients (252 of 316) survived their last episode. Of the 64 patients who died, 53 (83%) died during their first hyperammonemic episode. Of the 104 neonates (< 30d) treated with sodium phenylacetate and sodium benzoate injection, 34 (33%) died during the first hyperammonemic episode.

Ammonia levels decreased from very high levels (> 4 times the upper limit of normal [ULN]) to lower levels in 91% of episodes after treatment. In patients responding to therapy, mean ammonia concentrations decreased from 200.9 μmol/L at hour zero to 101.6 μmol/L within four hours of initiation of sodium phenylacetate and sodium benzoate injection therapy and were maintained. Hemodialysis is recommended for those patients whose plasma ammonia levels fail to fall below 150 μmol/L or by more than 40% within 4 to 8 hours after receiving sodium phenylacetate and sodium benzoate injection. A shift from high (≤ 4 times ULN) to very high (> 4 times ULN) levels was observed in only 4% of the episodes.

Overall, investigators rated neurological status as improved, much improved, or the same in 93% of episodes, and overall status in response to treatment as improved, much improved, or the same in 97% of episodes. Recovery from coma was observed in 97% of episodes where coma was present at admission (111 of 114 episodes).

All resources are included in as near-original form as possible, meaning that the information from the original provider has been rendered here with only typographical or stylistic modifications and not with any substantive alterations of content, meaning or intent.

This site is provided for educational and informational purposes only, in accordance with our Terms of Use, and is not intended as a substitute for the advice of a medical doctor, nurse, nurse practitioner or other qualified health professional.

Privacy Policy | Copyright © 2023. All Rights Reserved.