PURINETHOL

Clinical safety rating: caution

Comprehensive clinical and safety monograph for PURINETHOL (PURINETHOL).

How it works

Mercaptopurine is a purine antimetabolite that inhibits purine nucleotide synthesis and metabolism. It is converted intracellularly to 6-thioguanine nucleotides (6-TGNs), which incorporate into DNA and RNA, inhibiting their synthesis and function. It also inhibits de novo purine synthesis via feedback inhibition.

What the body does with it

Metabolism	Primarily metabolized by xanthine oxidase (XO) to 6-thiouric acid (inactive), and via thiopurine methyltransferase (TPMT) to 6-methylmercaptopurine (inactive). Activity of TPMT and NUDT15 affects toxicity. Allopurinol inhibits XO, leading to increased mercaptopurine levels.
Excretion	Primarily hepatic metabolism; renal excretion of metabolites accounts for approximately 50% of elimination. Biliary excretion contributes to a minor extent (<10%).
Half-life	The terminal elimination half-life of mercaptopurine is approximately 1.5 hours. However, the active metabolite 6-thioguanine nucleotides have a half-life of 5-7 days, correlating with pharmacological effects.
Protein binding	Approximately 19% bound to plasma proteins, primarily albumin.
Volume of Distribution	Volume of distribution is 0.9 L/kg, indicating distribution into total body water.
Bioavailability	Oral bioavailability is highly variable, ranging from 5% to 37% (mean approximately 16%), due to extensive first-pass metabolism.
Onset of Action	Oral: Therapeutic effects in leukemia may be observed within 2-3 weeks of continuous dosing.
Duration of Action	Oral: Duration of action is prolonged due to accumulation of active metabolites; therapeutic effects persist for several weeks after discontinuation.

Classification & Brands

Dosing & administration

1.5-2.5 mg/kg orally once daily. Initial dose typically 50-75 mg/m²/day.

Dosage form	TABLET
Renal impairment	GFR 50-80 mL/min: reduce dose by 25-50%. GFR 10-50 mL/min: reduce dose by 50-75%. GFR <10 mL/min: administer 50% of normal dose every 48 hours or consider alternative.
Liver impairment	Child-Pugh Class A: no adjustment. Child-Pugh Class B: reduce dose by 25-50%. Child-Pugh Class C: avoid use or reduce dose by 75%.
Pediatric use	Induction: 50-75 mg/m² orally once daily. Maintenance: 50-75 mg/m² orally once daily. Adjust based on tolerance and disease response.
Geriatric use	Start at lower end of dosing range (1.5 mg/kg/day). Monitor renal function and hematologic parameters closely. Reduce dose if significant renal impairment present.

Use during pregnancy

1st trimester	Consult provider
2nd trimester	Consult provider
3rd trimester	Consult provider

Clinical note

Comprehensive clinical and safety monograph for PURINETHOL (PURINETHOL).

Breastfeeding	Present in breast milk in low concentrations. M/P ratio not established. Potential for infant myelosuppression and immunosuppression. Contraindicated in breastfeeding or use with caution; monitor infant for neutropenia and thrombocytopenia.
Teratogenic Risk	First trimester: Increased risk of congenital malformations including craniofacial defects, limb anomalies, and cardiovascular defects. Second and third trimesters: Risk of intrauterine growth restriction, preterm delivery, and fetal myelosuppression. Overall, considered teratogenic in humans; avoid use unless benefit outweighs risk.

Warnings & precautions

■ FDA Black Box Warning

Severe myelosuppression, especially with TPMT or NUDT15 deficiency. The drug is myelotoxic; fatal myelosuppression can occur. Monitor blood counts frequently.

Side Effect Profile

Serious Effects

Absolute Contraindications

["Hypersensitivity to mercaptopurine","Prior resistance to mercaptopurine (ineffective)","Severe myelosuppression (unless benefits outweigh risks)","Concomitant use with allopurinol (unless dose-adjusted due to toxicity risk)","Pregnancy (absolute contraindication in some contexts)"]

Clinical Precautions

Precautions

["Myelosuppression: monitor CBCs regularly; reduce dose if severe.","TPMT/NUDT15 deficiency: increased risk of severe myelosuppression; consider testing before therapy.","Hepatotoxicity: monitor liver function tests; can cause hepatic veno-occlusive disease.","Immunosuppression: increased risk of infections.","Carcinogenicity: risk of secondary malignancies, especially with prolonged use.","Pregnancy: Category D; may cause fetal harm."]

Clinical Tips & Counseling

Drug interactions

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PURINETHOL

Clinical safety rating: caution

Comprehensive clinical and safety monograph for PURINETHOL (PURINETHOL).

How it works

What the body does with it

Metabolism	Primarily metabolized by xanthine oxidase (XO) to 6-thiouric acid (inactive), and via thiopurine methyltransferase (TPMT) to 6-methylmercaptopurine (inactive). Activity of TPMT and NUDT15 affects toxicity. Allopurinol inhibits XO, leading to increased mercaptopurine levels.
Excretion	Primarily hepatic metabolism; renal excretion of metabolites accounts for approximately 50% of elimination. Biliary excretion contributes to a minor extent (<10%).
Half-life	The terminal elimination half-life of mercaptopurine is approximately 1.5 hours. However, the active metabolite 6-thioguanine nucleotides have a half-life of 5-7 days, correlating with pharmacological effects.
Protein binding	Approximately 19% bound to plasma proteins, primarily albumin.
Volume of Distribution	Volume of distribution is 0.9 L/kg, indicating distribution into total body water.
Bioavailability	Oral bioavailability is highly variable, ranging from 5% to 37% (mean approximately 16%), due to extensive first-pass metabolism.
Onset of Action	Oral: Therapeutic effects in leukemia may be observed within 2-3 weeks of continuous dosing.
Duration of Action	Oral: Duration of action is prolonged due to accumulation of active metabolites; therapeutic effects persist for several weeks after discontinuation.

Classification & Brands

Dosing & administration

1.5-2.5 mg/kg orally once daily. Initial dose typically 50-75 mg/m²/day.

Dosage form	TABLET
Renal impairment	GFR 50-80 mL/min: reduce dose by 25-50%. GFR 10-50 mL/min: reduce dose by 50-75%. GFR <10 mL/min: administer 50% of normal dose every 48 hours or consider alternative.
Liver impairment	Child-Pugh Class A: no adjustment. Child-Pugh Class B: reduce dose by 25-50%. Child-Pugh Class C: avoid use or reduce dose by 75%.
Pediatric use	Induction: 50-75 mg/m² orally once daily. Maintenance: 50-75 mg/m² orally once daily. Adjust based on tolerance and disease response.
Geriatric use	Start at lower end of dosing range (1.5 mg/kg/day). Monitor renal function and hematologic parameters closely. Reduce dose if significant renal impairment present.

Use during pregnancy

1st trimester	Consult provider
2nd trimester	Consult provider
3rd trimester	Consult provider

Clinical note

Comprehensive clinical and safety monograph for PURINETHOL (PURINETHOL).

Breastfeeding	Present in breast milk in low concentrations. M/P ratio not established. Potential for infant myelosuppression and immunosuppression. Contraindicated in breastfeeding or use with caution; monitor infant for neutropenia and thrombocytopenia.
Teratogenic Risk	First trimester: Increased risk of congenital malformations including craniofacial defects, limb anomalies, and cardiovascular defects. Second and third trimesters: Risk of intrauterine growth restriction, preterm delivery, and fetal myelosuppression. Overall, considered teratogenic in humans; avoid use unless benefit outweighs risk.

Warnings & precautions

■ FDA Black Box Warning

Severe myelosuppression, especially with TPMT or NUDT15 deficiency. The drug is myelotoxic; fatal myelosuppression can occur. Monitor blood counts frequently.