2.1 Recommended Dose

Sodium phenylacetate and sodium benzoate injection must be diluted with sterile 10% Dextrose Injection (D10W) before administration. The dilution and dosage of sodium phenylacetate and sodium benzoate injection are determined by weight for neonates, infants and young children, and by body surface area for larger patients, including older children, adolescents, and adults (Table 1). Discard unused portion.

2.2 Administration

Sodium phenylacetate and sodium benzoate injection is a concentrated solution and must be diluted before intravenous administration via a central venous catheter. Administration through a peripheral intravenous catheter may cause burns. Sodium phenylacetate and sodium benzoate injection may not be administered by any other route.

Sodium phenylacetate and sodium benzoate injection should be administered as a loading dose infusion over 90 to 120 minutes, followed by the same dose repeated as a maintenance infusion administered over 24 hours. Because of prolonged plasma levels achieved by phenylacetate in pharmacokinetic studies, repeat loading doses of sodium phenylacetate and sodium benzoate injection should not be administered. Maintenance infusions may be continued until elevated plasma ammonia levels have been normalized or the patient can tolerate oral nutrition and medications. An antiemetic may be administered during sodium phenylacetate and sodium benzoate injection infusion to aid control of infusion-associated nausea and vomiting. Administration of analogous oral drugs, such as sodium phenylbutyrate, should be terminated prior to sodium phenylacetate and sodium benzoate injection infusion.

Sodium phenylacetate and sodium benzoate injection infusion should be started as soon as the diagnosis of hyperammonemia is made. Treatment of hyperammonemia also requires caloric supplementation and restriction of dietary protein. Non-protein calories should be supplied principally as glucose (8–10 mg/kg/min) with an intravenous fat emulsion added. Attempts should be made to maintain a caloric intake of greater than 80 kcal/kg/day. During and after infusion of sodium phenylacetate and sodium benzoate injection, ongoing monitoring of the following clinical laboratory values is crucial: plasma ammonia, glutamine, quantitative plasma amino acids, blood glucose, electrolytes, venous or arterial blood gases, AST and ALT. On-going monitoring of the following clinical responses is also crucial to assess patient response to treatment: neurological status, Glasgow Coma Scale, tachypnea, CT or MRI scan or fundoscopic evidence of cerebral edema, and/or of gray matter and white matter damage. Hemodialysis should be considered in patients with severe hyperammonemia or who are not responsive to sodium phenylacetate and sodium benzoate injection administration [see Warnings and Precautions (5)]. In the non-neonatal study patient population treated with sodium phenylacetate and sodium benzoate injection, dialysis was required in 13% of hyperammonemic episodes. Standard hemodialysis was the most frequently used dialysis method. High levels of ammonia can be reduced quickly when sodium phenylacetate and sodium benzoate injection is used with hemodialysis, as the ammonia-scavenging of sodium phenylacetate and sodium benzoate injection suppresses the production of ammonia from catabolism of endogenous protein and hemodialysis eliminates the ammonia and ammonia conjugates.

Sodium phenylacetate and sodium benzoate injection solutions are physically and chemically stable for up to 24 hours at room temperature and room lighting conditions. No compatibility information is presently available for sodium phenylacetate and sodium benzoate injection infusion solutions except for Arginine HCl Injection, 10%, which may be mixed in the same container as sodium phenylacetate and sodium benzoate injection. Other infusion solutions and drug products should not be administered together with sodium phenylacetate and sodium benzoate injection infusion solution. Sodium phenylacetate and sodium benzoate injection solutions may be prepared in glass and PVC containers.

5.1 Decreased Potassium Levels

Because urine potassium loss is enhanced by the excretion of the non‑reabsorbable anions, phenylacetylglutamine and hippurate, plasma potassium levels should be carefully monitored and appropriate treatment given when necessary.

5.2 Conditions Associated with Fluid Overload

Sodium phenylacetate and sodium benzoate injection contains 30.5 mg of sodium per mL of undiluted product. Thus, sodium phenylacetate and sodium benzoate injection should be used with great care, if at all, in patients with congestive heart failure or severe renal insufficiency, and in clinical states in which there is sodium retention with edema. Discontinue administration of sodium phenylacetate and sodium benzoate injection, evaluate the patient, and institute appropriate therapeutic countermeasures if an adverse event occurs.

5.3 Extravasation

Administration must be through a central venous catheter. Administration through a peripheral line may cause burns. Bolus infusion flow rates are relatively high, especially for infants [see Dosage and Administration (2)]. Extravasation of sodium phenylacetate and sodium benzoate injection into the perivenous tissues may lead to skin necrosis. If extravasation is suspected, discontinue the infusion and resume at a different infusion site, if necessary. The infusion site must be monitored closely for possible infiltration during drug administration. Do not administer undiluted product.

5.4 Neurotoxicity of Phenylacetate

Because of prolonged plasma levels achieved by phenylacetate in pharmacokinetic studies, repeat loading doses of sodium phenylacetate and sodium benzoate injection should not be administered. Additionally, neurotoxicity was reported in cancer patients receiving intravenous phenylacetate, 250–300 mg/kg/day for 14 days, repeated at 4-week intervals. Manifestations were predominantly somnolence, fatigue, and lightheadedness, with less frequent headaches, dysgeusia, hypoacusis, disorientation, impaired memory, and exacerbation of a pre‑existing neuropathy. The acute onset of symptoms upon initiation of treatment and reversibility of symptoms when the phenylacetate was discontinued suggest a drug effect [see Animal Toxicology and/or Pharmacology (13.2)].

5.5 Hyperventilation and Metabolic Acidosis

Due to structural similarities between phenylacetate and benzoate to salicylate, sodium phenylacetate and sodium benzoate injection may cause side effects typically associated with salicylate overdose, such as hyperventilation and metabolic acidosis. Monitoring of blood chemistry profiles, blood pH and pCO2 should be performed.

6.1 Clinical Trials Experience

Because clinical trials are conducted under widely varying conditions, adverse reaction rates observed in the clinical trials of a drug cannot be directly compared to rates in the clinical trials of another drug and may not reflect the rates observed in clinical practice.

The safety data were obtained from 316 patients who received sodium phenylacetate and sodium benzoate injection as emergency (rescue) or prospective treatment for hyperammonemia as part of an uncontrolled, open-label study. The study population included patients between the ages of 0 to 53 years with a mean (SD) of 6.2 (8.54) years; 51% were male and 49% were female who had the following diagnoses: OTC (46%), ASS (22%), CPS (12%), ASL (2%), ARG (< 1%), THN (< 1%), and other (18%).

Adverse reactions were reported with similar frequency in patients with OTC, ASS, CPS, and diagnoses categorized as “other.” Nervous system disorders were more frequent in patients with OTC and CPS, compared with patients with ASS and patients with “other” diagnoses. Convulsions and mental impairment were reported in patients with OTC and CPS. These observations are consistent with literature reports that patients with enzyme deficiencies occurring earlier in the urea cycle (i.e., OTC and CPS) tend to be more severely affected.

Adverse reactions profiles differed by age group. Patients ≤ 30 days of age had more blood and lymphatic system disorders and vascular disorders (specifically hypotension), while patients > 30 days of age had more gastrointestinal disorders (specifically nausea, vomiting and diarrhea).

Less common adverse reactions (< 3% of patients) that are characterized as severe are listed below by body system.

Blood and Lymphatic System Disorders: coagulopathy, pancytopenia, thrombocytopenia

Cardiac Disorders: atrial rupture, bradycardia, cardiac or cardiopulmonary arrest/failure, cardiogenic shock, cardiomyopathy, pericardial effusion

Eye Disorders: blindness

Gastrointestinal Disorders: abdominal distension, gastrointestinal hemorrhage

General Disorders and Administration-Site Conditions: asthenia, brain death, chest pain, multiorgan failure, edema

Hepatobiliary Disorders: cholestasis, hepatic artery stenosis, hepatic failure/hepatotoxicity, jaundice

Infections and Infestations: sepsis/septic shock

Injury, Poisoning and Procedural Complications: brain herniation, subdural hematoma, overdose

Investigations: blood carbon dioxide changes, blood glucose changes, blood pH increased, cardiac output decreased, pCO2 changes, respiratory rate increased

Metabolism and Nutrition Disorders: alkalosis, dehydration, fluid overload/retention, hypoglycemia, hyperkalemia, hypernatremia, alkalosis, tetany

Neoplasms Benign, Malignant and Unspecified: hemangioma acquired

Nervous System Disorders: areflexia, ataxia, brain infarction, brain hemorrhage, cerebral atrophy, clonus, depressed level of consciousness, encephalopathy, nerve paralysis, intracranial pressure increased, subdural hematoma, tremor

Psychiatric Disorders: acute psychosis, aggression, confusional state, hallucinations

Renal and Urinary Disorders: anuria, renal failure, urinary retention

Respiratory, Thoracic and Mediastinal Disorders: acute respiratory distress syndrome, dyspnea, hypercapnia, hyperventilation, Kussmaul respiration, pneumonia aspiration, pneumothorax, pulmonary hemorrhage, pulmonary edema, respiratory acidosis or alkalosis, respiratory arrest/failure

Skin and Subcutaneous Tissue Disorders: alopecia, blister, pruritus generalized, rash, urticaria

Vascular Disorders: flushing, hemorrhage, hypertension, phlebothrombosis/thrombosis

8.1 Pregnancy

8.2 Lactation

8.4 Pediatric Use

Sodium phenylacetate and sodium benzoate injection has been used as a treatment for acute hyperammonemia in pediatric patients including patients in the early neonatal period [see Dosage and Administration (2)].

8.5 Geriatric Use

Clinical studies of sodium phenylacetate and sodium benzoate injection did not include any patients aged 65 and over to determine whether they respond differently from younger patients. Urea cycle disorders are presently diseases of the pediatric and younger adult populations. No pharmacokinetic studies of sodium phenylacetate and sodium benzoate injection have been performed in geriatric patients. In general, dose selection for an elderly patient should be cautious, usually starting at the low end of the dosing range, reflecting the greater frequency of decreased hepatic, renal, or cardiac function, and concomitant disease or other drug therapy in this patient population.

8.6 Gender

Pharmacokinetic parameters of sodium phenylacetate and sodium benzoate injection were compared in healthy males and females. Bioavailability of both benzoate and phenylacetate was slightly higher in females than in males. However, conclusions cannot be drawn due to the limited number of subjects in this study.

8.7 Hepatic Insufficiency

Limited information is available on the metabolism and excretion of sodium phenylacetate and sodium benzoate in patients with impaired hepatic function. However, metabolic conjugation of sodium phenylacetate and sodium benzoate is known to take place in the liver and kidney. Therefore, caution should be used in administering sodium phenylacetate and sodium benzoate injection to patients with hepatic insufficiency.

8.8 Renal Impairment

The drug metabolites of sodium phenylacetate and sodium benzoate injection (phenylacetylglutamine and hippurate) and subsequently ammonia are primarily excreted by the kidney. Therefore, use caution and closely monitor patients with impaired renal function who receive sodium phenylacetate and sodium benzoate injection.

12.1 Mechanism of Action

Urea cycle disorders can result from decreased activity of any of the following enzymes: N-acetylglutamate synthetase (NAGS), carbamyl phosphate synthetase (CPS), argininosuccinate synthetase (ASS), ornithine transcarbamylase (OTC), argininosuccinate lyase (ASL), or arginase (ARG).

Sodium phenylacetate and sodium benzoate are metabolically active compounds that can serve as alternatives to urea for the excretion of waste nitrogen. Figure 2 is a schematic illustrating how the components of sodium phenylacetate and sodium benzoate injection, phenylacetate and benzoate, provide an alternative pathway for nitrogen disposal in patients without a fully functioning urea cycle. Phenylacetate conjugates with glutamine in the liver and kidneys to form phenylacetylglutamine, via acetylation. Phenylacetylglutamine is excreted by the kidneys via glomerular filtration and tubular secretion. The nitrogen content of phenylacetylglutamine per mole is identical to that of urea (both contain two moles of nitrogen). Two moles of nitrogen are removed per mole of phenylacetate when it is conjugated with glutamine. Similarly, preceded by acylation, benzoate conjugates with glycine to form hippuric acid, which is rapidly excreted by the kidneys by glomerular filtration and tubular secretion. One mole of hippuric acid contains one mole of waste nitrogen. Thus, one mole of nitrogen is removed per mole of benzoate when it is conjugated with glycine.

Figure 2.

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 AUClast 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/m2, respectively. The total clearance decreased from 5.19 to 3.62 L/h/m2 at the 3.75 and 5.5 g/m2 doses, respectively.

Similarly, phenylacetate exhibited nonlinear kinetics following the priming dose regimens. AUClast 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/m2, respectively. The total clearance decreased from 1.82 to 0.89 mcg⋅h/mL with increasing dose (3.75 and 4 g/m2, 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 (Tmax of 2 hr at 3.75 g/m2) whereas, benzoate concentrations declined rapidly (Tmax of 1.5 hr at 3.75 g/m2) and were undetectable at 14 and 26 hours following the 3.75 and 4 g/m2 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.