Mercapton

Mercapton Injection is indicated in adults and children for the treatment of acute poisoning by certain heavy metals, arsenic, mercury, gold, bismuth, antimony and possibly thallium. Although Mercapton has not been successful in the treatment of lead poisoning when used alone, there is evidence that used in conjunction with sodium calcium edetate, it can be used successfully in the treatment of lead poisoning, particularly in children.

Posology

Adults

400 - 800 mg, in divided doses, on the first day.

200 - 400 mg, in divided doses, on the second and third days.

100 - 200 mg, in divided doses, on the subsequent days.

Within the above dose range, individual dosage should be calculated on a bodyweight basis and will depend upon the severity of symptoms and the causative agent. As a general guide, single doses should not exceed 3 mg per kg bodyweight. However, in severe acute poisoning, single doses up to 5 mg per kg bodyweight may be required initially.

Paediatric population

Mercapton Injection is well tolerated by children and the dosage should be calculated on the basis of bodyweight, using the same unit dose per kg of bodyweight as for an adult under similar clinical circumstances.

Elderly

There are no specific data on the use of Mercapton in the elderly but since it is eliminated via the kidney, it should be used with caution in this age group.

Method of administration

For intramuscular injection.

Poisoning by iron, cadmium, or selenium.

Impaired hepatic function unless due to arsenic poisoning.

Mercapton Injection should be used with care in patients with hypertension or impaired renal function. It should be discontinued or continued with extreme caution if acute renal insufficiency develops during therapy. Mercapton Injection may not be effective in cases of concomitant renal failure, e.g. in arsine poisoning and some cases of arsenic poisoning. Any abnormal reaction (e.g. pyrexia) occurring after the initial injection of Mercapton should be assessed before continuing treatment. The use of Mercapton Injection does not eliminate the need for the general treatment of poisoning due to the particular heavy metal.

A reaction apparently peculiar to children is fever which may persist during therapy. It occurs in approximately 30% of children.

A transient reduction of the percentage of polymorphonuclear leukocytes may also be observed.

Mercapton has been reported to induce hemolysis (which may be severe) in individuals with glucose-6-phosphate dehydrogenase (G6PD) deficiency. Therefore, high-risk individuals should be screened for G6PD deficiency, and susceptible patients should be monitored for hemolysis during therapy with Mercapton.

Dimecaprol Injection contains Arachis oil (peanut oil)

Mercapton Injection should not be given to patients known to be allergic to peanut. As there is a possible relationship between allergy to peanut and allergy to Soya, patients with Soya allergy should also avoid Mercapton Injection.

Mercapton Injection contains benzyl benzoate

May increase the risk of jaundice in newborn babies.

No adverse effects known.

Side effects are relatively frequent, but at the therapeutic dosage employed, are seldom severe enough to warrant cessation of treatment and are almost invariably reversible. There is some evidence to indicate that 30-60 mg of ephedrine sulphate by mouth, given half an hour before each injection of Mercapton, will reduce these reactions. Also, a minimum interval of four hours between doses appears to reduce side effects.

Mercapton may cause the following side effects, particularly at the higher dosage levels:

Blood and lymphatic system disorders

Haemolysis, transient reductions in leukocyte count have also been reported.

Psychiatric disorders

Anxiety, restlessness

Nervous system disorders

Headache, tingling of the hands and other extremities, tremor. High doses have produced hypertensive encephalopathy with convulsions and coma.

Eye disorders

Burning sensation of the eyes, lacrimation, conjunctivitis, blepharospasm

Cardiac disorders

Elevation of blood pressure accompanied by tachycardia

Respiratory, thoracic and mediastinal disorders

Rhinorrhoea, a feeling of constriction in the chest and throat

Gastrointestinal disorders

Nausea and possibly vomiting, salivation, abdominal pain, burning sensation of the lips, mouth and throat

Hepatobiliary disorders

Hepatotoxicity/liver injury

Skin and subcutaneous tissue disorders

Sweating of the forehead and hands

Musculoskeletal and connective tissue disorders

Muscle pain and spasm, pain in jaw

Renal and urinary disorders

Renal impairment

Reproductive system and breast disorders

Burning sensation in the penis

General disorders and administration site conditions

Local pain may occur at the site of injection and gluteal abscess has occasionally been encountered. Pyrexia.

Investigations

Activated partial thromboplastin time prolonged, blood zinc decreased

Paediatric population

A side effect apparently peculiar to children is a fever which develops after the second or third injection, and persists until treatment with Mercapton is terminated.

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 the Yellow Card Scheme (website: www.mhra.gov.uk/yellowcard).

Symptoms

Symptoms of over dosage include malaise, nausea, vomiting, lacrimation and salivation, burning sensation of lips, mouth, throat and eyes with headache. A sense of constriction of the throat and chest. Increased blood pressure maximal after 15-20 minutes. Transient effects lasting about four hours.

Management

Treatment consists of the subcutaneous administration of diphenhydramine 50 mg or ephedrine 30 mg or ephedrine in a dosage of 30-60 mg orally if time permits.

Pharmacotherapeutic group: Antidote

ATC code: V03AB

Mercapton is a chelating agent used in the treatment of acute poisoning by heavy metals.

Mechanism of action

The sulphydryl groups of Mercapton compete with endogenous sulphydryl groups on proteins such as enzymes to combine with these metals; chelation by Mercapton therefore prevents or reverses any inhibition of the sulphydryl enzymes by the metal and the Mercapton-metal complex formed is readily excreted by the kidney.

Absorption

After intramuscular injection, maximum plasma concentrations of Mercapton may be attained within one hour.

Distribution

Mercapton is widely distributed to all body tissues, with the highest concentrations found in the kidneys and liver.

Biotransformation

Mercapton is rapidly metabolised and the metabolites and Mercapton-metal chelates are excreted in the urine and bile.

Elimination

Elimination is essentially complete within four hours of a single dose.

None stated.

Not applicable.

Special precautions for disposal: react with weak aqueous solution (up to 15% of calcium hypochlorite). Leave for 24 hours. Neutralise and discharge to drain with copious quantities of water.