Genotyping of TPMT Polymorphisms: Safe and Personalized Thiopurine Therapy

Thiopurine S-methyltransferase (TPMT) is a crucial enzyme responsible for the inactivation of thiopurine drugs through S-methylation. These drugs — including azathioprine, mercaptopurine, and thioguanine — are widely used in clinical practice, especially in hematology, oncology, gastroenterology, and transplantology.

Thiopurines are prodrugs that require metabolic activation to exert their cytotoxic or immunosuppressive effects. The balance between therapeutic efficacy and toxicity is largely influenced by TPMT enzyme activity, which is determined by genetic polymorphisms in the TPMT gene.

Clinical Significance of TPMT Genotyping

Genetic variations in the TPMT gene lead to reduced or absent enzyme activity, which in turn increases the intracellular levels of active thioguanine nucleotides (TGNs). In patients with reduced TPMT activity, standard doses of thiopurines can lead to severe, potentially life-threatening myelotoxicity, particularly leukopenia, thrombocytopenia, and anemia.

Conversely, individuals with high TPMT activity may rapidly inactivate thiopurines, resulting in reduced therapeutic efficacy. Therefore, TPMT genotyping is a critical tool in optimizing thiopurine dosing and minimizing adverse events.

The gb PHARM TPMT is a CE IVD-certified kit designed for allelic discrimination by real-time PCR, enabling the detection of the most clinically relevant TPMT alleles:

• TPMT*2 (c.238G>C)
• TPMT*3A (c.460G>A and c.719A>G)
• TPMT*3B (c.460G>A)
• TPMT*3C (c.719A>G)

The kit determines these alleles in either homozygous or heterozygous form. It allows for accurate identification of intermediate or poor metabolizers, who are at higher risk of toxicity if treated with conventional thiopurine doses.

It is important to note that the kit does not distinguish **TPMT1/3A (two mutations in cis) from the rare **TPMT3B/3C (mutations in trans) genotype, although the latter is exceedingly rare in Caucasian populations.

Recommendations for Clinical Practice

Based on TPMT genotype, patients can be stratified as follows:

Genotyping prior to therapy is endorsed by multiple clinical guidelines, including CPIC and EMA recommendations, particularly in the treatment of acute lymphoblastic leukemia (ALL), inflammatory bowel disease (IBD), and rheumatologic diseases.

Conclusion

The gb PHARM TPMT kit provides a reliable, fast, and standardized method to determine key TPMT variants, enabling personalized thiopurine therapy and minimizing the risk of severe adverse effects. It is an essential tool in the implementation of precision medicine in clinical routine.

Radovan Haluza