6-Thioguanine

6-Thioguanine (6-TG) is a crucial metabolite derived from the thiopurine drugs azathioprine and mercaptopurine, widely used in the treatment of inflammatory bowel disease (IBD) and acute lymphoblastic leukemia (ALL). 

Understanding and monitoring 6-TG levels alongside 6-methylmercaptopurine (6-MMP) levels is essential for assessing drug exposure, metabolism, and potential toxicity. This process enables personalized dosing strategies and therapeutic drug monitoring, optimizing treatment efficacy while minimizing adverse effects. 

Thiopurine metabolism involves multiple pathways, with 6-TG and 6-MMP being key metabolites. 

Regular measurement of these metabolites in red blood cells helps determine if patients are within their therapeutic window, manage potential side effects, and address issues such as noncompliance or metabolic diversion.

What is 6-Thioguanine (6-TG)?

6-thioguanine (6-TG) is a metabolite of the thiopurine drugs azathioprine and mercaptopurine, which are widely used in the treatment of inflammatory bowel disease (IBD) and acute lymphoblastic leukemia (ALL). 

Measuring 6-TG levels alongside 6-methylmercaptopurine (6-MMP) levels provides valuable information about drug exposure, metabolism, and potential toxicity, enabling therapeutic drug monitoring and personalized dosing strategies.

Thiopurine Drugs Overview [2., 5.]

Thiopurine drugs like azathioprine and mercaptopurine are immunosuppressive agents widely used in the treatment of various autoimmune and inflammatory conditions, as well as certain cancers. Understanding their metabolism is crucial for optimizing therapy and monitoring for potential toxicities.

Mercaptopurine (6-MP)

‍Drug Class

Mercaptopurine is considered a gastrointestinal agent, an antineoplastic agent, a cytotoxic antirheumatic agent, an antimetabolite and a musculoskeletal agent. 

Mechanism of Action

Mercaptopurine is a purine analog that, once inside cells, is activated to form metabolites including thioguanine nucleotides (TGNs), of which 6-TG is one. 

These TGNs incorporate into DNA or RNA, leading to cell-cycle arrest and cell death. 

Additionally, mercaptopurine metabolites inhibit purine synthesis and purine nucleotide interconversions, further disrupting cellular functions and contributing to its therapeutic effects.

Azathioprine

‍Drug Class

Azathioprine is considered a gastrointestinal agent, a cytotoxic antirheumatic, an antimetabolite and a musculoskeletal agent.

Mechanism of Action

Azathioprine, an imidazolyl derivative of and precursor to 6-mercaptopurine, functions as an immunosuppressive antimetabolite. 

While the precise mechanism of its immunosuppressive action is not fully understood due to the complexity of the immune response, azathioprine is known to suppress cell-mediated hypersensitivities and alter antibody production.

Conditions Commonly Treated with Thiopurine Drugs

Conditions Commonly Treated with Mercaptopurine

• Acute Lymphoid Leukemia

• Chronic Myeloid Leukemia

• Crohn's Disease

• Ulcerative Colitis

• Non-Hodgkin's Lymphoma

Conditions Commonly Treated by Azathioprine

• Atopic Dermatitis

• Autoimmune Hepatitis 

• Behcet's Syndrome

• Cardiac Transplant Rejection Prophylaxis

• Crohn's Disease

• Eosinophilic Granulomatosis with Polyangiitis

• Granulomatosis with Polyangiitis

• Lung Transplant Rejection Prophylaxis

• Lupus Nephritis (maintenance following adequate response to induction therapy)

• Microscopic Polyangiitis

• Myasthenia Gravis

• Pemphigus

• Polyarteritis Nodosa

• Renal Transplant Rejection Prophylaxis

• Rheumatoid Arthritis

• Systemic Lupus Erythematosus

• Thyroid Eye Disease, moderate to severe

• Ulcerative Colitis

• Warm Autoimmune Hemolytic Anemia

Thiopurine Metabolism Overview [5.] 

Conversion Process

Azathioprine (AZA) is a prodrug converted to mercaptopurine (6-MP) via biogenic thiols.

6-MP then undergoes three competing metabolic pathways.

Metabolic Pathways of 6-MP

1. Degradation to thiouric acid (TUA), which is excreted.

2. Methylation by thiopurine S-methyltransferase (TPMT) to form 6-methylmercaptopurine (6-MMP).

3. Conversion to thioinosine monophosphate (TIMP) via hypoxanthine phosphoribosyltransferase (HPRT). TIMP is further metabolized to thioguanine nucleotides (TGNs), of which 6-TG is one.

TGNs are the active immunosuppressive metabolites of these drugs, and testing for TGN levels like 6-TG alongside 6-MMP levels can help determine if a patient is within a therapeutic window, is using an alternative (hypermethylation) pathway to break down the drugs, or if the patient is noncompliant with their medication plan. [1., 5.] 

Hyper-methylation, where there is an excessive production of 6-MMP relative to TGNs, can lead to poor therapeutic response and hepatotoxicity.  [5.] 

Strategies to manage side effects include dose splitting or combining low-dose thiopurine with allopurinol, which helps optimize the balance between TGNs and 6-MMP. [5.] 

Clinical Significance of 6-MMP

Regular measurement of TGNs and 6-MMP levels in red blood cells is recommended to determine an individual’s therapeutic window, which is typically reached 3-6 months after initiation of therapy. [1.] 

Monitoring TGN levels can guide dosing to improve clinical outcomes. In cases of subtherapeutic TGNs, dose adjustments are made to reach therapeutic levels. [5.] 

TGNs such as 6-TG should be measured 4 weeks after starting thiopurines or following any dose changes. Routine checks help ensure effective therapy and manage any potential side effects early. [5.] 

Therapeutic ranges for 6-TG are between 235-450 pmol/8×10^8 RBCs. High 6-MMP levels (>5700 pmol/8×10^8 RBCs) are associated with hepatotoxicity. [5.] 

For unresponsive patients, consider the following: [1.] 

• Noncompliance: low 6-MMP and low 6-TG levels indicate patients not taking their medications.

• Metabolite Diversion: low 6-TG and normal or increased 6-MMP suggest diversion of metabolites away from 6-TG production.

• Refractory to Treatment: adequate 6-TG levels but lack of clinical response indicates treatment resistance.

• Hepatotoxicity Risk: increased 6-MMP or a high 6-MMP:6-TG ratio with adequate 6-TG levels may lead to increased hepatotoxicity.

Laboratory Testing for 6-TG

Test Information, Sample Collection and Preparation 

6-TG levels can be measured in various biological samples including whole blood and red blood cells (RBCs). Whole blood and RBC samples are commonly used, as 6-TG is primarily present in erythrocytes due to its high affinity for red blood cell proteins.

Sample collection requires a venipuncture procedure. Speak to the ordering provider prior to sample collection, as a trough specimen (sample collection within one hour of next recommended dose) may be required. [3.] 

Interpretation of 6-TG Test Results

Optimal Levels of 6-TG Levels

Test results must be interpreted within the context of an individual’s clinical context and alongside levels of 6-MMP.  

Optimal RBC levels are given by one laboratory as: [3.] 

‍6-TG 235-400 pmol/8x10(8) RBC

6-MMP <5700 pmol/8x10(8) RBC

Another resource states that the recommended therapeutic range for 6-TG is 235-450 pmol/8×10^8 RBCs. [5.] 

Clinical Significance of Increased 6-TG Levels

Increased 6-TG levels are commonly associated with hepatotoxicity, although it may also indicate metabolic diversion. [1., 5.] 

Clinical Significance of Decreased 6-TG Levels

Decreased 6-TG levels are associated with noncompliance or altered thiopurine metabolism. [1., 5.] 

FAQ: Understanding 6-TG

6-TG (6-thioguanine) is an active metabolite of thiopurine drugs, commonly used in the treatment of certain types of leukemia and autoimmune diseases. Monitoring 6-TG levels is crucial for optimizing therapy and minimizing adverse effects. 

This FAQ section addresses common questions about 6-TG, its significance, and related health concerns.

What is 6-TG?

6-TG (6-thioguanine) is an active metabolite produced during the metabolism of thiopurine drugs, such as 6-mercaptopurine (6-MP) and azathioprine. 

These drugs are used to treat acute lymphoblastic leukemia (ALL), inflammatory bowel disease (IBD), and other autoimmune conditions. 6-TG levels are measured to monitor and adjust treatment efficacy and safety.

Why is it Important to Monitor 6-TG Levels?

Monitoring 6-TG levels is important because it helps ensure the effectiveness of thiopurine therapy while minimizing the risk of adverse effects, such as myelosuppression (bone marrow suppression). 

By measuring 6-TG levels, healthcare providers can adjust dosages to maintain therapeutic effectiveness and reduce the risk of toxicity.

What are Considered Normal 6-TG Levels?

Normal 6-TG levels typically range from 235 to 400 or 450 pmol/8x10^8 red blood cells (RBCs). [3., 5.] 

However, the optimal range may vary depending on individual patient factors and specific treatment protocols. Healthcare providers use these levels to ensure effective and safe therapy.

What Does an Elevated 6-TG Level Indicate?

An elevated 6-TG level indicates a higher concentration of the active metabolite, which can increase the risk of myelosuppression. 

Symptoms of myelosuppression include fatigue, increased susceptibility to infections, and easy bruising or bleeding. Monitoring and managing 6-TG levels is crucial to prevent these adverse effects.

How Can Elevated 6-TG Levels Be Managed?

Elevated 6-TG levels can be managed by adjusting the dosage of the thiopurine drug or switching to alternative therapies if necessary. 

Healthcare providers may also recommend regular monitoring of blood counts and other parameters to ensure patient safety and effective treatment.

What are the Potential Side Effects of High 6-TG Levels?

High 6-TG levels can lead to side effects such as myelosuppression, which may manifest as anemia, leukopenia (low white blood cell count), thrombocytopenia (low platelet count), and increased risk of infections. [5.] 

Regular monitoring and appropriate dose adjustments can help mitigate these risks.

Are There Any Ways to Prevent Toxicity from High 6-TG Levels?

Preventing toxicity from high 6-TG levels involves regular monitoring of 6-TG levels and blood counts, adhering to prescribed dosages, and promptly reporting any symptoms of myelosuppression to healthcare providers. [5.]

Personalized dosing strategies based on metabolite monitoring can help minimize the risk of toxicity.

When Should I Consult a Healthcare Provider About 6-TG Levels?

Consult a healthcare provider if you are undergoing treatment with thiopurine drugs and experience symptoms such as unusual fatigue, frequent infections, easy bruising, or bleeding. 

Regular follow-ups and monitoring of 6-TG levels and blood counts are essential for managing therapy effectively and safely.

How Can I Stay Informed About 6-TG and Related Health Issues?

Stay informed by following scientific publications, attending relevant medical conferences, and monitoring updates from health organizations and research institutions. Journals focused on oncology, gastroenterology, and pharmacology often publish the latest findings on 6-TG and its clinical implications.

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