Azathioprine is a widely used immunosuppressant medication. It is prescribed for various conditions where the immune system needs to be regulated, such as in organ transplantation to prevent rejection. A specific enzyme, Thiopurine S-methyltransferase (TPMT), plays a role in how the body processes azathioprine. The interaction between azathioprine and TPMT is a key factor in ensuring the safety and effectiveness of the treatment.
How Azathioprine Works in the Body
Azathioprine functions as a “prodrug,” meaning it is inactive until converted in the body. It is transformed into 6-mercaptopurine (6-MP) through a non-enzymatic pathway. This 6-MP then undergoes further metabolic steps inside cells.
A series of reactions convert 6-MP into active thioguanine nucleotides (6-TGNs), which are the forms responsible for its therapeutic effects. These 6-TGNs work by interfering with the synthesis of nucleic acids, by incorporating into DNA to suppress the immune system. The body also inactivates 6-MP and its metabolites to prevent harmful accumulation.
TPMT is an enzyme that inactivates 6-MP by methylating it into 6-methylmercaptopurine (6-MMP). This reduces 6-MP available for conversion into active 6-TGNs. Xanthine oxidase (XO) also inactivates 6-MP by oxidizing it to 6-thiouric acid.
Understanding TPMT and Genetic Differences
Thiopurine S-methyltransferase (TPMT) is an enzyme that breaks down thiopurine drugs like azathioprine. Its activity varies among individuals due to genetic variations, also known as polymorphisms, in the TPMT gene. These genetic differences influence how quickly and efficiently the enzyme can metabolize thiopurines.
Over 40 variant alleles exist for the TPMT gene, but four common variants—TPMT2, TPMT3A, TPMT3B, and TPMT3C—account for over 90% of cases with reduced or absent enzyme activity. The TPMT1 allele is associated with normal enzyme activity. Individuals inherit two copies of the TPMT gene, one from each parent, and the combination of these alleles determines their TPMT activity level.
Based on their genetic makeup and subsequent enzyme activity, individuals are categorized into different metabolizer phenotypes. Normal metabolizers typically have two functional TPMT1 alleles and exhibit high TPMT activity, accounting for approximately 86-97% of patients. Intermediate metabolizers possess one functional and one non-functional allele, resulting in reduced TPMT activity, found in about 3-14% of patients. Poor metabolizers have two non-functional alleles, leading to very low or absent TPMT activity, occurring in approximately 0.25-0.6% of individuals.
The Importance of TPMT Activity for Azathioprine Safety
TPMT enzyme activity directly impacts the safety of azathioprine treatment. When TPMT activity is reduced, the enzyme cannot efficiently break down the active metabolites of azathioprine, such as 6-mercaptopurine. This leads to an accumulation of these active compounds, particularly 6-thioguanine nucleotides (6-TGNs), in the body’s cells.
High concentrations of 6-TGNs can become toxic, causing severe adverse reactions. The most significant side effect is myelosuppression, which is the suppression of bone marrow activity. Myelosuppression can result in dangerously low counts of various blood cells, including leukopenia (low white blood cells), neutropenia, thrombocytopenia, and anemia.
Individuals with absent TPMT activity, or “poor metabolizers,” are at an extremely high risk of life-threatening myelosuppression if given standard doses of azathioprine. Even intermediate metabolizers, who have reduced enzyme activity, face an increased risk of myelosuppression compared to normal metabolizers. While myelosuppression is the primary concern, other potential side effects like hepatotoxicity (liver damage) and pancreatitis can also occur, though these are not directly linked to TPMT activity.
Using TPMT Testing for Personalized Treatment
To mitigate the risks associated with varying TPMT activity, healthcare providers often recommend TPMT testing before initiating azathioprine therapy. This testing can involve genetic analysis to identify specific TPMT gene variants or a direct measurement of TPMT enzyme activity in red blood cells. Identifying a patient’s TPMT metabolizer phenotype—normal, intermediate, or poor—allows for a personalized approach to azathioprine dosing.
For individuals identified as poor metabolizers, alternative therapies are typically considered due to the high risk of severe toxicity. If azathioprine must be used, an extremely low dose, sometimes as low as 10% of the usual starting dose, may be administered with very close monitoring. Intermediate metabolizers usually require a reduced starting dose, often between 30-70% of the normal dose, and careful monitoring of blood cell counts.
Normal metabolizers can typically begin with standard azathioprine doses, usually ranging from 0.5 to 2.5 mg/kg/day, and adjust based on their response and tolerance. The goal of this personalized dosing strategy is to achieve therapeutic levels of the active drug metabolites while minimizing the risk of severe adverse reactions like myelosuppression. This proactive approach aims to maximize the benefits of azathioprine while protecting the patient from potentially harmful side effects.
Other Genes Affecting Azathioprine Response
While TPMT is a factor in how individuals respond to azathioprine, it is not the only genetic influence. Other genes also play a role in the complex metabolism of this medication and can impact its efficacy and potential for toxicity. One such gene is NUDT15 (Nudix Hydrolase 15), which encodes an enzyme that converts active thioguanine nucleotide metabolites into inactive forms.
Variations in the NUDT15 gene can lead to reduced enzyme activity, similar to TPMT, resulting in higher levels of active drug metabolites and an increased risk of myelosuppression. NUDT15 polymorphisms are particularly relevant in certain ethnic populations, such as East Asian populations. Research continues to uncover additional genetic factors that contribute to individual differences in drug response, further refining the understanding of azathioprine pharmacogenomics.