Carglumic acid enhances rapid ammonia detoxification in classical organic acidurias with a favourable risk-benefit profile: a retrospective observational study

Organic acidurias (OAs) are a group of rare inherited diseases characterised by the excretion of non-amino organic acids in the urine. They result from the impaired metabolism of organic acids, leading to abnormal and toxic levels of organic acids in the blood, urine and tissues, causing serious health problems [1, 2]. Three classical OAs have been identified, namely isovaleric aciduria (IVA), propionic aciduria (PA) and methylmalonic aciduria (MMA). IVA is caused by a deficiency of isovaleryl-CoA dehydrogenase, resulting in defective breakdown of leucine. PA occurs due to a deficiency of propionyl-CoA carboxylase and MMA is caused by a defect in the conversion of methylmalonyl-CoA to succinyl-CoA, both of which affect the metabolism of amino acids isoleucine, valine, methionine and threonine [1].

OAs usually develop in the neonatal period, characterised by toxic encephalopathy within the first few days of life, with symptoms including vomiting, poor feeding, seizures and abnormal tone, and lethargy progressing to coma [3, 4]. This metabolic decompensation is a medical emergency that can be fatal or result in brain damage if patients do not receive immediate medical interventions [2]. Mechanisms for acute encephalopathy include direct neurotoxicity of metabolites and hyperammonaemia [1, 5]. Normally the level of ammonia is controlled by conversion to urea in the liver and subsequent excretion in urine. In the classical OAs, propionyl CoA, methylmalonyl CoA and isovaleryl CoA accumulate and inhibit N-acetylglutamate (NAG) synthase (NAGS). NAGS catalyses the formation of NAG, an activator required by carbamoyl-phosphate synthetase I (CPS-I). CPS-I is a key enzyme of ureagenesis that catalyses the first and rate-limiting step of the urea cycle [6‐9]. Propionyl CoA and methylmalonyl CoA also inhibit the pathway by depleting hepatic acetyl CoA, which is required for NAG synthesis. Figure 1 provides an overview of the mechanism of hyperammonaemia in OAs.

Reducing ammonia levels as quickly as possible is one of the medical priorities in acute decompensation episodes of OAs as a longer duration and higher levels of hyperammonaemia are associated with worse neurological outcomes [10]. First-line management includes the promotion of anabolism through the administration of high calorie energy sources, such as intravenous glucose and a protein-restricted diet [7]. Ammonia scavengers such as sodium benzoate and sodium phenylbutyrate (excreted as hippurate and phenylacetylglutamine after conjugation with glycine or glutamine, respectively [11, 12]) have a higher renal clearance than ammonia itself, and are therefore used for accelerating ammonia elimination in the urine [7]. Haemodialysis or peritoneal dialysis may be started if these interventions do not generate significant reductions in blood ammonia levels within a few hours.

Carglumic acid is a synthetic structural analogue of NAG, and as such is a specific intervention for hyperammonaemia in these patients. This was approved as Carbaglu® by the European Medicines Agency (EMA) for the treatment of hyperammonaemia due to primary NAGS deficiency, and hyperammonaemia due to IVA, MMA and PA [13]. Carglumic acid accelerates ammonia detoxification by mimicking the effects of NAG on CPS-I, thereby driving the urea cycle forward independent of other mechanisms for detoxifying the organic acids (Fig. 1). In the first published clinical study, Ah Mew et al. showed that single dose/short-term treatment with carglumic acid was associated with reduced ammonia and glutamine levels and an increase in ureagenesis [8].

Carglumic acid has been used in humans since 1981 in the acute and long-term treatment of NAGS deficiency. Although a number of publications describe the response to, and safety of carglumic acid in the treatment of hyperammonaemia due to OA decompensation episodes or NAGS primary deficiency [7, 8, 14, 15], carglumic acid’s orphan drug status highlights the need for further research into its effectiveness in treating OA-induced hyperammonaemia in various age groups and in patients with different aetiologies of hyperammonaemia.

This retrospective, multicentre, open-label, uncontrolled, phase IIIb study was designed to evaluate the efficacy and safety of carglumic acid in OA patients with hyperammonaemia during metabolic decompensation episodes. The primary objective of this study was to study the change in plasma ammonia levels as the main biological response to carglumic acid treatment during episodes of hyperammonaemia in OA. Secondary objectives include describing the demographic profile of the patient population, evaluating the clinical and biological responses to treatment and assessing the safety of carglumic acid.

This study was conducted in accordance with Good Clinical Practice, the Principles of the Declaration of Helsinki and the requirements of local ethics committees (Spain: Comité Etico de Investigacion Clinica de Aragon, Avda. San Juan Bosco, Zaragoza; Comité Etico de Investigacion Clinica Hospital Clinico Universitarion de Valencia, Avda. Vicente Blasco Ibanez, Valencia. France: Comité de Protection des Personnes Ile De France II, Centre Universitaire des Saints-Pères, rue des Saints-Pères, Paris. Italy : Comitato Etico dell Azienda, Ospedaliera Policlinico Consorziale di Bari; Comitato Etico Locale per la sperimentazione dei farmaci, dell’azienda ospedaliero-Universitaria Anna Meyer de Firenze. UK: Leicestershire, Northamptonshire and Ruland Ethics Committee 2, Nottingham. Germany: Ethik-kommission des Fachbereichs Medizin der Johann Wolfgang Goethe-Universitat, Frankfurt. Netherlands: Stichting CGR Hanzeweg, Goud.)

Informed consent was obtained for each patient prior to commencement of the planned data collection, review and analysis. Cases where circumstances made it necessary to waive informed consent (eg if the patient was dead or lost to follow-up) were dealt with on a case by case basis and in accordance with local regulations.

Data were collected from 21 centres in Italy, France, Germany, the Netherlands, Spain, Turkey and the United Kingdom from patients treated between January 1995 and October 2009.

This study demonstrated that carglumic acid, with or without additional therapies such as ammonia scavengers, induced a rapid and effective reduction of plasma ammonia during decompensation episodes in OAs. It represents the largest body of evidence to date on the efficacy and safety of carglumic acid in the management of hyperammonaemia in patients with OAs. Mean reductions to similar ammonia levels at study endpoint were seen in patients with IVA, MMA and PA. Furthermore, carglumic acid treatment of hyperammonaemia episodes reduced plasma NH₃ concentrations to similar levels in neonates and non-neonates, despite neonatal episodes having higher ammonia concentrations at treatment initiation. These data support the use of carglumic acid in the treatment of hyperammonaemia in neonates and non-neonates with any of the three classical OA types. Reductions in plasma NH₃ concentration following carglumic acid treatment were accompanied by improvements in clinical symptoms and neurological status.

Mean plasma NH₃ concentration at endpoint was similar in patients treated with scavengers compared to those not treated with scavenger medication (55.6 vs 60.8 μmol/L). However, mean baseline NH₃ concentration was higher in the group of patients who received scavengers (466.1 vs 261.0 μmol/L). When the higher baseline NH₃ level is considered, there was a higher rate of change in plasma NH₃ levels in patients treated with scavengers (-410.5 vs -200.2 μmol/L). Reduction in plasma NH₃ concentration was more rapid in episodes of hyperammonaemia treated with carglumic acid and scavengers compared with those not treated with scavengers. These analyses were repeated excluding one outlier, who experienced a very rapid and large decrease in ammonia levels, and yielded similar results. However, the hyperammonaemia episodes treated with scavengers were more severe at presentation and scavenger use was more frequent in neonates, thus the observed differences between these groups are unsurprising. As expected, the use of haemodialysis prior to or concomitantly with carglumic acid was also associated with a more rapid response. Although use of scavengers may be beneficial by increasing reduction of plasma NH₃ levels, there are a number of potential disadvantages to be considered. Sodium phenylacetate may induce glutamine depletion as it conjugates with glutamine to yield phenylacetylglutamine, which is subsequently excreted in the urine [16]. Additionally, sodium phenylacetate and sodium benzoate may potentiate ammonia toxicity by blocking the urea cycle through sequestration of CoA [17, 18] and caution when used in organic acidemias was suggested by recent guidelines [19]. Moreover, treatment with these scavenging drugs at high doses may increase serum sodium and decrease serum potassium levels [18]. Consequently, measuring plasma levels of sodium benzoate is recommended in the neonatal period, particularly in jaundiced infants; however, this analysis is not available in most centres [18].

The response to carglumic acid with or without ammonia scavengers in neonates was faster than in non-neonates; however, it is difficult to separate the effects of scavenger treatment from the effects due to age. The rapidity of response in non-neonates was similar with or without scavengers; however, more data are needed as only two non-neonates were treated with scavengers in this study. The time to response following carglumic acid administration was similar in patients with MMA and PA. The small number of patients with IVA and the high plasma NH₃ concentration at treatment initiation in these patients make it difficult to draw conclusions on the time to response compared with the other OAs.

The relatively small number of IVA cases in this study (n = 4) is consistent with the frequency observed in clinical practice. Clinical experience suggests that patients with IVA have less severe hyperammonaemia [5, 20] and are easier to manage compared to patients with other OAs.The higher baseline ammonaemia in the IVA episodes in this study suggests that only the most severe cases were treated with carglumic acid. In addition, this small sample population had an artificially high mean and median plasma NH₃ concentration due to an outlier (1633 μmol/L). The larger number of neonates (n = 28) compared with non-neonates (n = 13) included in the study reflects the more severe hyperammonaemia seen in neonates at presentation [3]. Unsurprisingly, higher doses of carglumic acid were administered to neonates, as in general the more severe hyperammonaemia episodes were treated with high doses. Similarly episodes in neonates vs non-neonates were more frequently concomitantly treated with scavengers, as they were more severe cases with higher baseline plasma NH₃ levels.

Dietary management is often used as first-line treatment in the management of decompensation episodes [19, 21]; however, the impact is limited as an extremely restricted diet cannot be maintained for more than 48 hours [19]. Carglumic acid treatment might allow a faster reintroduction of protein into the diet and thus limit muscular catabolism contributing to hyperammonaemia as seen in the context of NAGS deficiency [22].

The clinical presentation of patients with metabolic decompensation at baseline was consistent with other reports in the literature [19], with marked neurological and gastrointestinal symptoms as well as anaemia, ketosis and metabolic acidosis. Eight patients were comatose prior to treatment with carglumic acid. A total of 23 patients presented with lethargy. At study endpoint, gastrointestinal and neurological symptoms, ketoacidosis and hyperventilation were significantly reduced compared to baseline.

Analysis of data from the safety population showed carglumic acid to have a good safety profile. The number of deaths (n = 7) is not unexpected, as OA has a high mortality rate. Mortality rates of approximately 50 % have been reported, however these are related to long-term outcomes and cannot be directly compared with this study [1, 23]. The time between start of an episode and initiation of carglumic acid treatment was similar for the three diagnostic categories. The mean time between start of an episode and initiation of carglumic acid treatment was longer in non-neonates compared to neonates, however the median time was similar. The mean duration of treatment for hyperammonaemia was similar in both neonates and non-neonates. Treatment with carglumic acid was largely initiated at the EMA label recommended dose (100–250 mg/kg). While dose was decreased over time, this decrease was not pronounced. A high first dose of carglumic acid, albeit within the recommended dose range of 100–250 mg/kg, is consistent with the fact that treatment often commences with a loading dose for rapid normalisation of plasma ammonia levels [24] and subsequently a daily dose based on weight and hyperammonaemia severity is administered [13]. There was significant variation between regimens (eg whether the loading dose was high or low) making an analysis by dose quite difficult.

This retrospective study has a number of limitations, including lack of control patients treated only with scavengers, patient data variability, the inability to adjust for confounders (including concomitant drugs and diet) and the small number of patients in analysis subgroups.

We are currently looking into a new follow-up retrospective study designed to compare carglumic acid to ammonia scavengers and/or hemofiltration, provided that an equivalent number of hyperammonemic episodes would be retrieved in the same period of time with inclusion criteria being treatment by ammonia scavengers or hemofiltration. Such a study would still present limitations, as it would be unlikely to match the same numbers of episodes with the same treatment modalities (i.e. 1 or 2 scavengers, with or without hemofiltration).

Also, a prospective study with ammonia scavengers only would raise ethical questions since carglumic acid is now an approved drug for treating hyperammonemia in organic acidemias.

Nevertheless, the current study with its observational design provides valuable information regarding real-life clinical practice and the response of patients who have been treated according to local protocols and subject to the clinical judgement of physicians on a case-by-case basis.

Data from this study are consistent with other published data supporting the use of carglumic acid in PA [8]. Furthermore, Daniotti et al. reported that carglumic acid improves acute therapy and reduces the need for the duration of peritoneal dialysis and haemodialysis [7], and recommended that carglumic acid be initiated as soon as possible, even before confirmation of the exact diagnosis. Indeed, although recent guidelines focus on the use of carglumic acid in NAGS deficiency, the use of carglumic acid in hyperammonaemic patients with suspected NAGS deficiency is also recommended [19].

The results of this study indicate that carglumic acid at the recommended initial dose of 100–250 mg/kg/day, with or without ammonia scavengers, is an effective and safe treatment to restore normal plasma ammonia concentrations (<60 μmol/L) in episodes of hyperammonaemia occurring during metabolic decompensation in patients with IVA, MMA, and PA.