Mercaptopurin

Overdose

Symptoms and signs

Gastrointestinal effects, including nausea, vomiting and diarrhoea and anorexia may be early symptoms of overdosage having occurred. The principal toxic effect is on the bone marrow, resulting in myelosuppression. Haematological toxicity is likely to be more profound with chronic overdosage than with a single ingestion of 6-mercaptopurine. Liver dysfunction and gastroenteritis may also occur.

Treatment

As there is no known antidote, blood counts should be closely monitored and general supportive measures, together with appropriate blood transfusion, instituted if necessary. Active measures (such as the use of activated charcoal) may not be effective in the event of 6-mercaptopurine overdose unless the procedure can be undertaken within 60 minutes of ingestion.

Further management should be as clinically indicated or as recommended by the national poisons centre, where available.

Shelf life

60 months

Contraindications

Hypersensitivity to 6-mercaptopurine or to any other component of the preparation.

In view of the seriousness of the indications there are no other absolute contra-indications.

Incompatibilities

None known

List of excipients

Lactose

Maize Starch

Hydrolysed Starch

Stearic Acid

Magnesium Stearate

Purified Water

Mercaptopurin price

Pharmaceutical form

Tablets

Pale yellow, round tablets, biconvex, scored on one side, engraved GX above the score and EX2 below the score and plain on the other side.

Undesirable effects

Summary of the safety profile

For 6-mercaptopurine there is a lack of modern clinical documentation which can serve as support for accurately determining the frequency of undesirable effects. The frequency categories assigned to the adverse drug reactions below are estimates: for most reactions, suitable data for calculating incidence are not available. Undesirable effects may vary in their incidence depending on the dose received and also when given in combination with other therapeutic agents.

The main side effect of treatment with 6-mercaptopurine is bone marrow suppression leading to leucopenia and thrombocytopenia.

Tabulated list of adverse reactions

The following convention has been utilised for the classification of frequency:

Very common >1/10

Common >1/100 and < 1/10

Uncommon >1/1000 and <1/100

Rare >1/10,000 and <1/1000

Very rare <1/10,000

Not known (frequency cannot be estimated from the available data)

# In the paediatric population

Description of selected adverse reactions:

Hepatobiliary disorders

6-mercaptopurine is hepatotoxic in animals and man. The histological findings in man have shown hepatic necrosis and biliary stasis.

The incidence of hepatotoxicity varies considerably and can occur with any dose but more frequently when the recommended dose of 2.5 mg/kg bodyweight daily or 75 mg/m2 body surface area per day is exceeded.

Monitoring of liver function tests may allow early detection of hepatotoxicity. Gamma glutamyl transferase (GGT) levels in plasma may be particularly predictive of withdrawal due to hepatotoxicity. This is usually reversible if 6-mercaptopurine therapy is stopped soon enough but fatal liver damage has occurred.

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

Preclinical safety data

- Carcinogenesis, mutagenesis:

6-Mercaptopurine, in common with other antimetabolites, is potentially mutagenic in man and chromosome damage has been reported in mice, rats and man.

In view of its action on cellular deoxyribonucleic acid (DNA) 6-mercaptopurine is potentially carcinogenic and consideration should be given to the theoretical risk of carcinogenesis with this treatment.

- Teratogenicity:

6-Mercaptopurine causes embryolethality and severe teratogenic effects in mice, rats, hamsters and rabbits at doses that are non-toxic to the mother. In all species, the degree of embryotoxicity and the type of malformations are dependent on the dose and stage of the gestation at the time of administration.

Therapeutic indications

6-mercaptopurine is indicated for the treatment of acute leukaemia in adults, adolescents and children. It may be utilised in:

- Acute lymphoblastic leukaemia (ALL);

- Acute promyelocytic leukaemia (APL)/Acute myeloid leukaemia M3 (AML M3)).

Pharmacotherapeutic group

Pharmacodynamic properties

Pharmacotherapeutic group: antineoplastic agents, antimetabolites, purine analogues, ATC Code: L01BB02

Mechanism of action

6-Mercaptopurine is sulphydryl analogue of the purine bases, adenine and hypoxanthine and acts as a cytotoxic antimetabolite.

6-Mercaptopurine is an inactive pro-drug which acts as a purine antagonist but requires cellular uptake and intracellular anabolism to thioguanine nucleotides (TGNs) for cytotoxicity. The TGNs and other metabolites (e.g. 6-methyl-mecaptopurine ribonucleotides) inhibit de novo purine synthesis and purine nucleotide interconversions. The TGNs are also incorporated into nucleic acids and this contributes to the cytotoxic effects of the medicinal product.

Pharmacodynamic effects

The cytotoxic effect of 6-mercaptopurine can be related to the levels of red blood cell 6-mercaptopurine derived thioguanine nucleotides, but not to the plasma 6-mercaptopurine concentration

Pharmacokinetic properties

Pharmacokinectics

Absorption

The bioavailability of oral 6-mercaptopurine shows considerable inter-individual variability`. When administered at a dosage of 75 mg/m2 to seven paediatric patients, the bioavailability averaged 16% of the administered dose, with a range of 5 to 37%. The variable bioavailability probably results from the metabolism of a significant portion of 6-mercaptopurine during first-pass hepatic metabolism.

After oral administration of 6-mercaptopurine 75 mg/m2 to 14 children with acute lymphoblastic leukaemia, the mean Cmax was 0.89µM, with a range of 0.29 - 1.82µM and Tmax was 2.2 hours with a range of 0.5 - 4 hours.

Distribution

Concentrations of 6-mercaptopurine in cerebrospinal fluid (CSF) are low or negligible after IV or oral administration (CSF: plasma ratios of 0.05 to 0.27). Concentrations in the CSF are higher after intrathecal administration.

Biotransformation

6-mercaptopurine is extensively metabolized by many multi-step pathways to active and inactive metabolites. Because of the complex metabolism, inhibition of one enzyme does not explain all cases of lack of efficacy and/or pronounced myelosuppression. The predominant enzymes responsible for the metabolism of 6-mercaptopurine or its downstream metabolites are: the polymorphic enzyme thiopurine S-methyltransferase (TPMT), xanthine oxidase, inosine monophosphate dehydrogenase (IMPDH) and hypoxanthine guanine phosphribosyltransferase (HPRT). Additional enzymes involved in the formation of active and inactive metabolites are: guanosine monophosphate synthetase (GMPS, which form TGNs) and inosine triphosphate pyrophosphatase (ITPase). There are also multiple inactive metabolites formed via other pathways.

There is evidence that polymorphisms in the genes encoding the different enzyme systems involved with metabolism of 6-mercaptopurine may predict adverse drug reactions to 6-mercaptopurine therapy. For example, individuals with TPMT deficiency develop very high cytotoxic thioguanine nucleotide concentrations.

Elimination

In a study with 22 adult patients the mean 6-mercaptopurine clearance and half-life after IV infusion was 864 mL/min/m2 and 0.9 hours respectively. The mean renal clearance reported in 16 of these patients was 191 mL/min/m2. Only about 20 % of the dose was excreted in the urine as intact medicinal product after IV administration. In a study with 7 children patients the mean 6-mercaptopurine clearance and half-life after IV infusion was 719 (+/-610) ml/min/m2 and 0.9 (+/-0.3) hours respectively.

Special patient populations

- Older population

- Renal impairment

Studies with a pro-drug of 6-mercaptopurine have shown no difference in 6-mercaptourine pharmacokinetics in uremic patients compared to renal transplant patients.).

6-mercaptopurine and/or its metabolites are eliminated by haemodialysis, with approximately 45 % of radioactive metabolites eliminated during dialysis of 8 hours.

- Hepatic impairment

A study with a pro-drug of 6-mercaptopurine was performed in three groups of renal transplant patients: those without liver disease, those with hepatic impairment (but no cirrhosis) and those with hepatic impairment and cirrhosis.).

Date of revision of the text

07/2017

Name of the medicinal product

Mercaptopurine 50 mg tablets

Marketing authorisation holder

Aspen Pharma Trading Limited

3016 Lake Drive,

Citywest Business Campus,

Dublin 24,

Ireland

Special precautions for storage

Store below 25°C. Keep the bottle tightly closed.

Nature and contents of container

Amber glass bottles with child resistant high density polyethylene closures with induction heat seal liners.

Pack size: 25 tablets

Marketing authorisation number(s)

PL 39699/ 0047

Qualitative and quantitative composition

6-mercaptopurine.

Each tablet contains 50 mg of the active substance 6-mercaptopurine.

Excipients with known effect:

Lactose

Special warnings and precautions for use

6-mercaptopurine is an active cytotoxic agent for use only under the direction of physician experienced in the administration of such agents.

Immunisation using a live organism vaccine has the potential to cause infection in immunocompromised hosts. Therefore, immunisations with live organism vaccines are not recommended in patients with ALL or AML. In all cases, patients in remission should not receive live organism vaccines until the patient is deemed to be able to respond to the vaccine. The interval between discontinuation of chemotherapy and restoration of the patient's ability to respond to the vaccine depends on the intensity and type of immunosuppression-causing medications used, the underlying disease, and other factors.

Safe handling of 6-mercaptopurine Tablets

Monitoring:

Since 6-mercaptopurine is strongly myelosuppressive full blood counts must be taken daily during remission induction. Patients must be carefully monitored during therapy.

Bone marrow suppression

Treatment with 6-mercaptopurine causes bone marrow suppression leading to leukopenia and thrombocytopenia and, less frequently, to anaemia. Full blood counts must be taken frequently during remission induction. During maintenance therapy, complete blood counts, including platelets, should be regularly monitored and more frequently if high dosage is used or if severe renal and/or hepatic disorder is present.

Increased haematological monitoring of the patient is advised when switching between different pharmaceutical formulations of mercaptopurine.

The leukocyte and platelet counts continue to fall after treatment is stopped, so at the first sign of an abnormally large fall in the counts, treatment should be interrupted immediately.

Bone marrow suppression is reversible if 6-mercaptopurine is withdrawn early enough.

During remission induction in acute myelogenous leukaemia, the patient may frequently have to survive a period of relative bone marrow aplasia and it is important that adequate supportive facilities are available.

Hepatotoxicity

6-mercaptopurine is hepatotoxic and liver function tests should be monitored weekly during treatment. Gamma glutamyl transferase (GGT) levels in plasma may be particularly predictive of withdrawal due to hepatotoxicity. More frequent monitoring may be advisable in those with pre-existing liver disease or receiving other potentially hepatotoxic therapy. The patient should be instructed to discontinue 6-mercaptopurine immediately if jaundice becomes apparent.

Tumour lysis syndrome

During remission induction when rapid cell lysis is occurring, uric acid levels in blood and urine should be monitored as hyperuricaemia and/or hyperuricosuria may develop, with the risk of uric acid nephropathy.

TPMT Deficiency

There are individuals with an inherited deficiency of the enzyme thiopurine methyltransferase (TPMT) who may be unusually sensitive to the myelosuppressive effect of 6-mercaptopurine and prone to developing rapid bone marrow depression following the initiation of treatment with 6-mercaptopurine. This problem could be exacerbated by co-administration with medicinal products that inhibit TPMT, such as olsalazine, mesalazine or sulfazalazine. Also a possible association between decreased TPMT activity and secondary leukaemias and myelodysplasia has been reported in individuals receiving 6-mercaptopurine in combination with other cytotoxics. Approximately 0.3 % (1:300) of patients have little or no detectable enzyme activity. Approximately 10 % of patients have low or intermediate TPMT activity and 90 % of individuals have normal TPMT activity. There may also be a group of approximately 2 % who have very high TPMT activity. Some laboratories offer testing for TPMT deficiency, although these tests have not been shown to identify all patients at risk of severe toxicity. Therefore close monitoring of blood counts is still necessary.

Patients with NUDT15 variant

Patients with inherited mutated NUDT15 gene are at increased risk for severe 6-mercaptopurine toxicity, such as early leukopenia and alopecia, from conventional doses of thiopurine therapy. They generally require dose reduction, particularly those being NUDT15 variant homozygotes (see 4.2). The frequency of NUDT15 c.415C>T has an ethnic variability of approximately 10 % in East Asians, 4 % in Hispanics, 0.2 % in Europeans and 0 % in Africans. In any case, close monitoring of blood counts is necessary.

Cross Resistance

Cross resistance usually exists between 6-mercaptopurine and 6-thioguanine.

Hypersensitivity

Patients suspected to have previously presented with a hypersensitivity reaction to 6-mercaptopurine should not be recommended to use its pro-drug azathioprine, unless the patient has been confirmed as hypersensitive to 6-mercaptopurine with allergological tests, and tested negative for azathioprine. As azathioprine is a pro-drug of 6-mercaptopurine, patients with a previous history of hypersensitivity to azathioprine must be assessed for hypersensitivity to 6-mercapopurine prior to initiating treatment.

Renal and/or hepatic impairment

Caution is advised during the administration of 6-mercaptopurine in patients with renal impairment and/or hepatic impairment.2 Pharmacokinetic properties: Special populations).

Mutagenicity and carcinogenicity

Patients receiving immunosuppressive therapy, including mercaptopurine are at an increased risk of developing lymphoproliferative disorders and other malignancies, notably skin cancers (melanoma and non-melanoma), sarcomas (Kaposi's and non-Kaposi's) and uterine cervical cancer in situ. The increased risk appears to be related to the degree and duration of immunosuppression. It has been reported that discontinuation of immunosuppression may provide partial regression of the lymphoproliferative disorder.

A treatment regimen containing multiple immunosuppressants (including thiopurines) should therefore be used with caution as this could lead to lymphoproliferative disorders, some with reported fatalities. A combination of multiple immunosuppressants, given concomitantly increases the risk of Epstein-Barr virus (EBV)-associated lymphoproliferative disorders.

Increases in chromosomal aberrations were observed in the peripheral lymphocytes of leukaemic patients, in a hypernephroma patient who received an unstated dose of 6-mercaptopurine and in patients with chronic renal disease treated at doses of 0.4 to 1.0 mg/kg/day.

Two cases have been documented of the occurrence of acute non-lymphatic leukaemia in patients who received 6-mercaptopurine, in combination with other medicinal products, for non-neoplastic disorders. A single case has been reported where a patient was treated for pyoderma gangrenosum with 6-mercaptopurine and later developed acute non-lymphatic leukaemia, but it is not clear whether this was part of the natural history of the disease or if the 6-mercaptopurine played a causative role.

A patient with Hodgkin's disease treated with 6-mercaptopurine and multiple additional cytotoxic agents developed acute myelogenous leukaemia.

Twelve and a half years after 6-mercaptopurine treatment for myasthenia gravis, a female patient developed chronic myeloid leukaemia.

Reports of hepatosplenic T-cell lymphoma in the Inflammatory Bowel Disease (IBD) population (unlicensed indication) have been received when 6-mercaptopurine is used in combination with anti-TNF agents.

Macrophage activation syndrome

Macrophage activation syndrome (MAS) is a known, life-threatening disorder that may develop in patients with autoimmune conditions, in particular with inflammatory bowel disease (IBD) (unlicensed indication), and there could potentially be an increased susceptibility for developing the condition with the use of mercaptopurine. If MAS occurs, or is suspected, evaluation and treatment should be started as early as possible, and treatment with mercaptopurine should be discontinued. Physicians should be attentive to symptoms of infection such as EBV and cytomegalovirus (CMV), as these are known triggers for MAS.

Paediatric population

Cases of symptomatic hypoglycaemia have been reported in children with ALL receiving 6-mercaptopurine. The majority of reported cases were in children under the age of six or with a low body mass index.

Infections

Patients treated with 6-mercaptopurine alone or in combination with other immunosuppressive agents, including corticosteroids, have shown increased susceptibility to viral, fungal and bacterial infections, including severe or atypical infection, and viral reactivation. The infectious disease and complications may be more severe in these patients than in non-treated patients.

Prior exposure to or infection with varicella zoster virus should be taken into consideration prior to starting treatment. Local guidelines may be considered, including prophylactic therapy if necessary. Serologic testing prior to starting treatment should be considered with respect to hepatitis B. Local guidelines may be considered, including prophylactic therapy for cases which have been confirmed positive by serologic testing. Cases of neutropenic sepsis have been reported in patients receiving 6-mercaptopurine for ALL.

Lesch-Nyhan syndrome

Limited evidence suggests that neither 6-mercaptopurine nor its pro-drug azathioprine are effective in patients with the rare inherited condition complete hypoxanthine-guanine-phosphoribosyltransferase deficiency (Lesch-Nyhan syndrome). The use of 6-mercaptopurine or azathioprine is not recommended in these patients.

UV exposure

Patients treated with 6-mercaptopurine are more sensitive to the sun. Exposure to sunlight and UV light should be limited, and patients should be recommended to wear protective clothing and to use a sunscreen with a high protection factor.

Lactose

Patients with rare hereditary problems of galactose intolerance, complete lactase deficiency or glucose-galactose malabsorption should not take this medicine.

Xanthine oxidase inhibitors

Anticoagulants

Inhibition of the anticoagulant effect of warfarin and acenocoumarol has been reported when co-administered with 6-mercaptopurine; therefore higher doses of the anticoagulant may be needed.

Effects on ability to drive and use machines

There are no data on the effect of 6-mercaptopurine on driving performance or the ability to operate machinery. A detrimental effect on these activities cannot be predicted from the pharmacology of the medicinal product.

Dosage (Posology) and method of administration

Posology

6-mercaptopurine treatment should be supervised by a physician or other healthcare professional experienced in the management of patients with ALL and APL (AML M3).

6-mercaptopurine may be taken with food or on an empty stomach, but patients should standardise the method of administration. The dose should not be taken with milk or dairy products. 6-mercaptopurine should be taken at least 1 hour before or 2 hours after milk or dairy products.

Special populations

Adults and paediatric population

For adults and children the usual dose is 2.5 mg/kg bodyweight per day, or 50 to 75 mg/m2 body surface area per day, but the dose and duration of administration depend on the nature and dosage of other cytotoxic agents given in conjunction with 6-mercaptopurine.

The dosage should be carefully adjusted to suit the individual patient.

6-mercaptopurine has been used in various combination therapy schedules for acute leukaemia and the literature and current treatment guidelines should be consulted for details.

Studies carried out in children with acute lymphoblastic leukaemia suggested that administration of 6-mercaptopurine in the evening lowered the risk of relapse compared with morning administration.

Older population

It is advisable to monitor renal and hepatic function in these patients, and if there is impairment, consideration should be given to reducing the 6-mercaptopurine dosage.

Renal impairment

Hepatic impairment

Medicinal product interactions

When the xanthine oxidase inhibitors, such as allopurinol, oxipurinol or thiopurinol and 6-mercaptopurine are administered concomitantly it is essential that only 25 % of the usual dose of 6-mercaptopurine is given since these agents decrease the rate of catabolism of 6-mercaptopurine.).

TPMT-deficient patients

Patients with NUDT15 variant

Patients with inherited mutated NUDT15 gene are at increased risk for severe 6-mercaptopurine toxicity (see 4.4). These patients generally require dose reduction; particularly those being NUDT15 variant homozygotes (see 4.4). Genotypic testing of NUDT15 variants may be considered before initiating 6-mercaptopurine therapy. In any case, close monitoring of blood counts is necessary.

Special precautions for disposal and other handling

Safe handling:

It is recommended that 6-mercaptopurine tablets should be handled following the prevailing local recommendations and/or regulations for the handling and disposal of cytotoxic agents.

Disposal

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

Date of first authorisation/renewal of the authorisation

Date of first authorisation: 01/05/2012

Interaction with other medicinal products and other forms of interaction

Anticoagulants

Inhibition of the anticoagulant effect of warfarin and acenocoumarol has been reported when co-administered with 6-mercaptopurine; therefore higher doses of the anticoagulant may be needed.

The administration of 6-mercaptopurine with food may decrease systemic exposure slightly. 6-mercaptopurine may be taken with food or on an empty stomach, but patients should standardise the method of administration to avoid large variability in exposure. The dose should not be taken with milk or dairy products since they contain xanthine oxidase, an enzyme which metabolises 6-mercaptopurine and might therefore lead to reduced plasma concentrations of mercaptopurine.

Effect of concomitant medicinal products on 6-mercaptopurine

Ribavirin

Ribavirin inhibits the enzyme, inosine monophosphate dehydrogenase (IMPDH), leading to a lower production of the active 6-thioguanine nucleotides.2 Pharmacokinetic properties: metabolism).

Myelosuppressive agents

Allopurinol/oxipurinol/thiopurinol and other xanthine oxidase inhibitors

Xanthine oxidase activity is inhibited by allopurinol, oxipurinol and thiopurinol, which results in reduced conversion of biologically active 6-thioinosinic acid to biologically inactive 6-thiouric acid.: Medicinal product interactions).

Other xanthine oxidase inhibitors, such as febuxostat, may decrease the metabolism of 6-mercaptopurine. Concomitant administration is not recommended as data are insufficient to determine an adequate dose reduction.

Aminosalicylates

There is in vitro and in vivo evidence that aminosalicylate derivatives (e.g. olsalazine, mesalazine or sulfazalazine) inhibit the TPMT enzyme.).

Methotrexate

Methotrexate (20 mg/m2 orally) increased 6-mercaptopurine AUC by approximately 31% and methotrexate (2 or 5 g/m2 intravenously) increased 6-mercaptopurine AUC by 69 and 93%, respectively. Therefore, when 6-mercaptopurine is administered concomitantly with high dose methotrexate, the dose should be adjusted to maintain a suitable white blood cell count.

Infliximab

Interactions have been observed between azathioprine, a pro-drug of 6-mercaptopurine, and infliximab. Patients receiving ongoing azathioprine experienced transient increases in 6-TGN (6-thioguanine nucleotide, an active metabolite of azathioprine) levels and decreases in the mean leukocyte count in the initial weeks following infliximab infusion, which returned to previous levels after 3 months.

Effect of 6-mercaptopurine on other medicinal products

Anticoagulants

Inhibition of the anticoagulant effect of warfarin and acenocoumarol has been reported when co-administered with 6-mercaptopurine; therefore higher doses of the anticoagulant may be needed. It is recommended that coagulation tests are closely monitored when anticoagulants are concurrently administered with 6-mercaptopurine.