Sodium phenylacetate and sodium benzoate are metabolically active compounds that can serve as alternatives to urea for the excretion of waste nitrogen. 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). 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. It has been shown that phenylacetylglutamine and hippurate can serve as alternative vehicles to effectively reduce waste nitrogen levels in patients with deficiencies of urea cycle enzymes and, thus, attenuate the risk of ammonia and glutamine-induced neurotoxicity.
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). The most frequently observed initial presenting symptoms in neonates include lethargy, seizures, poor feeding, neurologic changes, edema, and respiratory distress. Patients with milder forms of enzyme deficiencies may not present until late childhood, adolescence, or adulthood. Hyperammonemic crisis with lethargy, delirium, and coma, in these patients, are often precipitated by viral illness, high protein diet, stress, or trauma.
Plasma and urine amino acid analyses are used to diagnose ASS and ASL and to provide a preliminary diagnosis of CPS, OTC, or ARG. Blood citrulline levels are very low or absent in OTC and CPS, very high in ASS, and normal to moderately high in ASL and ARG. ASL may be distinguished by the presence of high levels of the unusual amino acid argininosuccinic acid (ASA) in the urine. It should be noted, however, that ASA tends to co-elute initially with other amino acids (such as leucine and isoleucine) in chromatographs, and may be missed on initial examination. ARG is characterized by high urine levels of arginine. A definitive diagnosis of CPS and OTC require a liver biopsy, and red blood cell enzyme analysis is needed to confirm a diagnosis of ARG. Patients suspected of having a urea cycle disorder, based on family history, should have documented hyperammonemia prior to administration of AMMONUL®.
Mechanism of Action
Figure 2 is a schematic illustrating how the components of AMMONUL®, phenylacetate and benzoate, provide an alternative pathway for nitrogen disposal in patients without a fully functioning urea cycle. Two moles of nitrogen are removed per mole of phenylacetate when it is conjugated with glutamine, and one mole of nitrogen is removed per mole of benzoate when it is conjugated with glycine.
CPS = carbamyl phosphate synthetase; OTC = ornithine transcarbamylase;
ASS = argininosuccinate synthetase; ASL = argininosuccinate lyase;
ARG = arginase; NAGS = N-acetylglutamate synthetase
The pharmacokinetics of intravenously administered AMMONUL® were 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 AMMONUL® intravenously. These data showed peak plasma levels of phenylacetate and benzoate at approximately the same times as were observed in adults. As in 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 [2,3].
Pharmacokinetic parameters of AMMONUL® 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.
Limited information is available on the metabolism and excretion of sodium phenylacetate and sodium benzoate in patients with impaired hepatic function. However, as the liver is one of the two organs (the other is the kidney) in which the metabolic conjugation of sodium phenylacetate and sodium benzoate is known to take place, care should be used in administering AMMONUL® to patients with hepatic insufficiency.
For effective AMMONUL® drug therapy, renal clearance of the drug metabolites and subsequently ammonia is required. Therefore, patients with impaired renal function should be closely monitored.
Intravenous use of AMMONUL® is complementary with the use of dialysis [4,5]. In the non-neonatal study patient population treated with AMMONUL®, dialysis (standard hemodialysis, peritoneal dialysis, arteriovenous hemofiltration, or other 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 AMMONUL® is used with dialysis, as the ammonia-scavenging of AMMONUL® suppresses the production of ammonia from catabolism of endogenous protein  and dialysis eliminates the ammonia and ammonia conjugates.
Formal drug interaction studies have not been performed with AMMONUL®.
In patients with hyperammonemia due to deficiencies in enzymes of the urea cycle, AMMONUL® has been shown to decrease elevated plasma ammonia levels and improve encephalopathy and survival outcome compared to historical controls. These effects are considered to be the result of reduction in nitrogen overload through glutamine and glycine scavenging by AMMONUL® in combination with appropriate dietary and other supportive measures.
The efficacy of AMMONUL® in improving patient survival of acute hyperammonemic episodes was demonstrated in an analysis of 316 patients (1045 episodes of hospitalization) treated between 1981 and 2003.
The demographic characteristics and diagnoses of the patient population are shown in Table 1.
Table 1 Baseline Characteristics and Diagnoses of Study Population
|PatientsFor the summary at the patient level, data obtained at first episode used.|
|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|
|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||OTC||146 (46%)|
|ARG||2 (< 1%)|
|THN||2 (< 1%)|
|OtherDiagnosis 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). ||56 (18%)|
On admission to the hospital, patients with hyperammonemia or a potential urea cycle disorder (UCD) were treated with a bolus dose of 0.25 g/kg (or 5.5 g/m2) sodium phenylacetate + 0.25 g/kg (or 5.5 g/m2) 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 AMMONUL® treatment compared with historical values (estimated 14% 1-year survival rate with dietary therapy alone)  and with dialysis (estimated 43% survival of acute hyperammonemia) .
Ninety-four percent (981 of 1045) of hyperammonemic episodes treated with AMMONUL® resulted in patients being discharged from the hospital. 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 AMMONUL®, 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 significantly within four hours of initiation of AMMONUL® therapy and were maintained. Dialysis is recommended for those patients who fail to have a significant reduction in plasma ammonia levels within 4 to 8 hours after receiving AMMONUL®. A shift from high (≤ 4 times ULN) to very high (> 4 times ULN) levels was observed in only 4% of the episodes.
Improvements in neurological status endpoints were observed in most episodes and patients. 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).