Sodium Phenylbutyrate

Sodium Phenylbutyrate is indicated as adjunctive therapy in the chronic management of urea cycle disorders, involving deficiencies of carbamylphosphate synthetase, ornithine transcarbamylase or argininosuccinate synthetase.

It is indicated in all patients with neonatal-onset presentation (complete enzyme deficiencies, presenting within the first 28 days of life). It is also indicated in patients with late-onset disease (partial enzyme deficiencies, presenting after the first month of life) who have a history of hyperammonaemic encephalopathy.

Sodium Phenylbutyrate treatment should be supervised by a physician experienced in the treatment of urea cycle disorders.

The use of Sodium Phenylbutyrate tablets is indicated for adults and children who are able to swallow tablets. Sodium Phenylbutyrate is also available as granules for infants, children who are unable to swallow tablets and for patients with dysphagia.

The daily dose should be individually adjusted according to the patient's protein tolerance and the daily dietary protein intake needed to promote growth and development.

The usual total daily dose of sodium phenylbutyrate in clinical experience is:

- 450 - 600 mg/kg/day in children weighing less than 20 kg

- 9.9 - 13.0 g/m²/day in children weighing more than 20 kg, adolescents and adults.

The safety and efficacy of doses in excess of 20 g/day (40 tablets) have not been established.

Therapeutic monitoring: Plasma levels of ammonia, arginine, essential amino acids (especially branched chain amino acids), carnitine and serum proteins should be maintained within normal limits. Plasma glutamine should be maintained at levels less than 1,000 µmol/l.

Nutritional management: Sodium Phenylbutyrate must be combined with dietary protein restriction and, in some cases, essential amino acid and carnitine supplementation.

Citrulline or arginine supplementation is required for patients diagnosed with neonatal-onset form of carbamyl phosphate synthetase or ornithine transcarbamylase deficiency at a dose of 0.17 g/kg/day or 3.8 g/m²/day.

Arginine supplementation is required for patients diagnosed with deficiency of argininosuccinate synthetase at a dose of 0.4 - 0.7 g/kg/day or 8.8 - 15.4 g/m²/day.

If caloric supplementation is indicated, a protein-free product is recommended.

The total daily dose of Sodium Phenylbutyrate should be divided into equal amounts and given with each meal (e.g. three times per day). The tablets should be taken with a large volume of water.

Sodium Phenylbutyrate tablets should not be used in patients with dysphagia due to the potential risk of oesophageal ulceration if tablets are not promptly delivered to the stomach.

Each Sodium Phenylbutyrate tablet contains 62 mg (2.7 mmol) of sodium, corresponding to 2.5 g (108 mmol) of sodium per 20 g of sodium phenylbutyrate, which is the maximum daily dose. Sodium Phenylbutyrate should therefore be used with caution in patients with congestive heart failure or severe renal insufficiency, and in clinical conditions where there is sodium retention with oedema.

Since the metabolism and excretion of sodium phenylbutyrate involves the liver and kidneys, Sodium Phenylbutyrate should be used with caution in patients with hepatic or renal insufficiency.

Serum potassium should be monitored during therapy since renal excretion of phenylacetylglutamine may induce a urinary loss of potassium.

Even on therapy, acute hyperammonaemic encephalopathy may occur in a number of patients.

Sodium Phenylbutyrate is not recommended for the management of acute hyperammonaemia, which is a medical emergency.

In children unable to swallow tablets, it is recommended to use Sodium Phenylbutyrate granules instead.

No studies on the effects on the ability to drive and use machines have been performed.

In clinical trials with Sodium Phenylbutyrate, 56 % of the patients experienced at least one adverse event and 78 % of these adverse events were considered as not related to Sodium Phenylbutyrate.

Adverse reactions mainly involved the reproductive and gastrointestinal system.

The adverse reactions are listed below, by system organ class and by frequency. Frequency is defined as very common (>1/10), common (>1/100 to <1/10), uncommon (>1/1,000 to <1/100), rare (>1/10,000 to <1/1,000), very rare (<1/10,000), not known (cannot be estimated from the available data). Within each frequency grouping, undesirable effects are presented in order of decreasing seriousness.

Blood and lymphatic system disorders

Common: Anaemia, thrombocytopenia, leukopenia, leukocytosis, thrombocytosis

Uncommon: Aplastic anaemia, ecchymosis

Metabolism and nutrition disorders

Common: Metabolic acidosis, alkalosis, decreased appetite

Psychiatric disorders

Common: Depression, irritability

Nervous system disorders

Common: Syncope, headache

Cardiac disorders

Common: Oedema

Uncommon: Arrhythmia

Gastrointestinal disorders

Common: Abdominal pain, vomiting, nausea, constipation, dysgeusia

Uncommon: Pancreatitis, peptic ulcer, rectal haemorrhage, gastritis

Skin and subcutaneous tissue disorders

Common: Rash, abnormal skin odour

Renal and urinary disorders

Common: Renal tubular acidosis

Reproductive system and breast disorders

Very common: Amenorrhoea, irregular menstruation

Investigations

Common: Decreased blood potassium, albumin, total protein and phosphate. Increased blood alkaline phosphatase, transaminases, bilirubin, uric acid, chloride, phosphate and sodium. Increased weight.

A probable case of toxic reaction to Sodium Phenylbutyrate (450 mg/kg/d) was reported in an 18-year old anorectic female patient who developed a metabolic encephalopathy associated with lactic acidosis, severe hypokalaemia, pancytopaenia, peripheral neuropathy, and pancreatitis. She recovered following dose reduction except for recurrent pancreatitis episodes that eventually prompted treatment discontinuation.

Reporting of suspected adverse reactions

Reporting suspected adverse reactions after authorisation of the medicinal product is important. It allows continued monitoring of the benefit/risk balance of the medicinal product. Healthcare professionals are asked to report any suspected adverse reactions via:

United Kingdom

Yellow Card Scheme

Website: www.mhra.gov.uk/yellowcard

Ireland

HPRA Pharmacovigilance

Earlsfort Terrace

IRL - Dublin 2

Tel: +353 1 6764971

Fax: +353 1 6762517

Website: www.hpra.ie

e-mail: [email protected]

One case of overdose occurred in a 5-month old infant with an accidental single dose of 10 g (1370 mg/kg). The patient developed diarrhoea, irritability and metabolic acidosis with hypokalaemia. The patient recovered within 48 hours after symptomatic treatment.

These symptoms are consistent with the accumulation of phenylacetate, which showed dose-limiting neurotoxicity when administered intravenously at doses up to 400 mg/kg/day. Manifestations of neurotoxicity were predominantly somnolence, fatigue and light-headedness. Less frequent manifestations were confusion, headache, dysgeusia, hypacusis, disorientation, impaired memory and exacerbation of a pre-existing neuropathy.

In the event of an overdose, discontinue the treatment and institute supportive measures. Haemodialysis or peritoneal dialysis may be beneficial.

Pharmacotherapeutic group: various alimentary tract and metabolism products, ATC code: A16A X03.

Sodium phenylbutyrate is a pro-drug and is rapidly metabolised to phenylacetate. Phenylacetate is a metabolically active compound that conjugates with glutamine via acetylation to form phenylacetylglutamine which is then excreted by the kidneys. On a molar basis, phenylacetylglutamine is comparable to urea (each containing 2 moles of nitrogen) and therefore provides an alternate vehicle for waste nitrogen excretion. Based on studies of phenylacetylglutamine excretion in patients with urea cycle disorders it is possible to estimate that, for each gram of sodium phenylbutyrate administered, between 0.12 and 0.15 g of phenylacetylglutamine nitrogen are produced. As a consequence, sodium phenylbutyrate reduces elevated plasma ammonia and glutamine levels in patients with urea cycle disorders. It is important that the diagnosis is made early and treatment is initiated immediately to improve the survival and the clinical outcome.

Previously, neonatal-onset presentation of urea cycle disorders was almost universally fatal within the first year of life, even when treated with peritoneal dialysis and essential amino acids or their nitrogen-free analogues. With haemodialysis, use of alternative waste nitrogen excretion pathways (sodium phenylbutyrate, sodium benzoate and sodium phenylacetate), dietary protein restriction, and, in some cases, essential amino acid supplementation, the survival rate in new-borns diagnosed after birth (but within the first month of life) increased to almost 80 % with most deaths occurring during an episode of acute hyperammonaemic encephalopathy. Patients with neonatal-onset disease had a high incidence of mental retardation.

In patients diagnosed during gestation and treated prior to any episode of hyperammonaemic encephalopathy, survival was 100 %, but even in these patients, many subsequently demonstrated cognitive impairment or other neurologic deficits.

In late-onset deficiency patients, including females heterozygous for ornithine transcarbamylase deficiency, who recovered from hyperammonaemic encephalopathy and were then treated chronically with dietary protein restriction and sodium phenylbutyrate, the survival rate was 98 %. The majority of the patients who were tested had an IQ in the average to low average/borderline mentally retarded range. Their cognitive performance remained relatively stable during phenylbutyrate therapy.

Reversal of pre-existing neurologic impairment is not likely to occur with treatment, and neurologic deterioration may continue in some patients.

Sodium Phenylbutyrate may be required life-long unless orthotopic liver transplantation is elected.

Phenylbutyrate is known to be oxidised to phenylacetate which is enzymatically conjugated with glutamine to form phenylacetylglutamine in the liver and kidney. Phenylacetate is also hydrolysed by esterases in liver and blood.

Plasma and urine concentrations of phenylbutyrate and its metabolites have been obtained from fasting normal adults who received a single dose of 5 g of sodium phenylbutyrate and from patients with urea cycle disorders, haemoglobinopathies and cirrhosis receiving single and repeated oral doses up to 20 g/day (uncontrolled studies). The disposition of phenylbutyrate and its metabolites has also been studied in cancer patients following intravenous infusion of sodium phenylbutyrate (up to 2 g/m²) or phenylacetate.

Absorption

Phenylbutyrate is rapidly absorbed under fasting conditions. After a single oral dose of 5 g of sodium phenylbutyrate, in the form of tablets, measurable plasma levels of phenylbutyrate are detected 15 minutes after dosing. The mean time to peak concentration is 1.35 hour and the mean peak concentration 218 µg/ml. The elimination half-life was estimated to be 0.8 hours.

The effect of food on absorption is unknown.

Distribution

The volume of distribution of phenylbutyrate is 0.2 l/kg.

Biotransformation

After a single dose of 5 g of sodium phenylbutyrate, in the form of tablets, measurable plasma levels of phenylacetate and phenylacetylglutamine are detected 30 and 60 minutes respectively after dosing. The mean time to peak concentration is 3.74 and 3.43 hours, respectively, and the mean peak concentration is 48.5 and 68.5 µg/ml, respectively. The elimination half-life was estimated to be 1.2 and 2.4 hours, respectively.

Studies with high intravenous doses of phenylacetate showed non linear pharmacokinetics characterised by saturable metabolism to phenylacetylglutamine. Repeated dosing with phenylacetate showed evidence of an induction of clearance.

In the majority of patients with urea cycle disorders or haemoglobinopathies receiving various doses of phenylbutyrate (300 - 650 mg/kg/day up to 20 g/day) no plasma level of phenylacetate could be detected after overnight fasting. In patients with impaired hepatic function the conversion of phenylacetate to phenylacetylglutamine may be relatively slower. Three cirrhotic patients (out of 6) who received repeated oral administration of sodium phenylbutyrate (20 g/day in three doses) showed sustained plasma levels of phenylacetate on the third day that were five times higher than those achieved after the first dose.

In normal volunteers gender differences were found in the pharmacokinetic parameters of phenylbutyrate and phenylacetate (AUC and C_max about 30 - 50 % greater in females), but not phenylacetylglutamine. This may be due to the lipophilicity of sodium phenylbutyrate and consequent differences in volume of distribution.

Elimination

Approximately 80 - 100 % of the medicinal product is excreted by the kidneys within 24 hours as the conjugated product, phenylacetylglutamine.

Sodium phenylbutyrate was negative in 2 mutagenicity tests, i.e. the Ames test and the micronucleus test. Results indicate that sodium phenylbutyrate did not induce any mutagenic effects in the Ames test with or without metabolic activation.

Micronucleus test results indicate that sodium phenylbutyrate was considered not to have produced any clastogenic effect in rats treated at toxic or non-toxic dose levels (examined 24 and 48 hours after a single oral administration of 878 to 2800 mg/kg). Carcinogenicity and fertility studies have not been conducted with sodium phenylbutyrate.

Not applicable.

Any unused medicinal product or waste material should be disposed of in accordance with local requirements.