TPMT and NUDT15 Variants Predict Discontinuation of Azathioprine for Myelotoxicity in Patients with Inflammatory Disease: Real‐World Clinical Results

Abstract

Azathioprine is used frequently to treat several inflammatory conditions. However, treatment is limited by adverse events-in particular, myelotoxicity. Thiopurine-S-methyltransferase (TPMT) and nudix hydrolase-15 (NUDT15) are enzymes involved in azathioprine metabolism; variants in the genes encoding these enzymes increase the risk for azathioprine myelotoxicity. The Clinical Pharmacogenetics Implementation Consortium (CPIC) has recommended dose adjustments based on the results of TPMT and NUDT15 genotyping. However, little is known about the importance of this genetic information in routine clinical care. We hypothesized that in patients with inflammatory diseases, TPMT and NUDT15 genotype data predict the risk of discontinuing azathioprine due to myelotoxicity. This was a retrospective cohort study in 1,403 new adult azathioprine users for the management of inflammatory conditions for whom we had genetic information and clinical data. Among patients who discontinued azathioprine, we adjudicated the reason(s). Genotyping was performed using the Illumina Infinium Expanded Multi-Ethnic Genotyping Array plus custom content. We used CPIC guidelines to determine TPMT and NUDT15 metabolizer status; patients were grouped as either: (i) poor/intermediate, or (ii) normal/indeterminate metabolizers. We classified 110 patients as poor/intermediate, and 1,293 patients as normal/indeterminate metabolizers. Poor/intermediate status was associated with a higher risk for azathioprine discontinuation due to myelotoxicity compared to normal/indeterminate metabolizers (hazard ratio (HR) = 2.90, 95% confidence interval (CI): 1.58-5.31, P = 0.001). This association remained significant after adjustment for race, age at initiation, sex, primary indication, and initial daily dose of azathioprine (adjusted HR (aHR) = 2.67, 95% CI: 1.44-4.94, P = 0.002). In conclusion, TPMT and NUDT15 metabolizer status predicts discontinuation due to myelotoxicity for patients taking azathioprine for inflammatory conditions.

Predicted expression of genes involved in the thiopurine metabolic pathway and azathioprine discontinuation due to myelotoxicity

“…The metabolism of azathioprine is complex and involves multiple enzymes and transporters. 3 Whereas TPMT and NUDT15 are included in current clinical guidelines, 4 , 28 most patients who discontinue azathioprine due to myelotoxicity are normal TPMT and NUDT15 metabolizers, 6 underscoring the need to examine other genes encoding enzymes involved in the thiopurine pathway. This novel proof‐of‐concept study indicates that the combined genetically predicted expression of NME1 and AOX1 among TPMT and NUDT15 normal metabolizers is associated with higher risk for discontinuation of azathioprine due to myelotoxicity.…”

“…We prepared genotyping data for imputation using McCarthy tools 18 and imputed additional variants using Michigan Imputation Server 19 with HRC version r1.1 reference panel and phasing with Eagle. 20 Following standard quality control, as described previously, 6 we estimated genetically predicted gene expression of the candidate proteins in the thiopurine metabolic pathway using PrediXcan (GTEx version 8) with MASHR version 8 expression quantitative trait locus (eQTL) weights to perform the imputation. 11 , 21 , 22 , 23 Because most drug metabolism occurs in the liver, we prespecified use of liver tissue‐specific estimations.…”

“…4 Whereas certain TPMT and NUDT15 polymorphisms are associated with several‐fold increased risk for myelotoxicity among azathioprine users, 2 , 5 they explain fewer than 25% of bone marrow toxicity cases in routine clinical practice. 6 …”

“…Currently, the Pharmacogenomics Knowledgebase (PharmGKB) lists 27 genes involved in thiopurine metabolism and transport, 3 but only thiopurine S‐methyltransferase ( TPMT ) and nudix hydrolase 15 ( NUDT15 ) are included in clinical guidelines for thiopurine use 4 . Whereas certain TPMT and NUDT15 polymorphisms are associated with several‐fold increased risk for myelotoxicity among azathioprine users, 2,5 they explain fewer than 25% of bone marrow toxicity cases in routine clinical practice 6 …”

“…4 Whereas certain TPMT and NUDT15 polymorphisms are associated with severalfold increased risk for myelotoxicity among azathioprine users, 2,5 they explain fewer than 25% of bone marrow toxicity cases in routine clinical practice. 6 The Clinical Pharmacogenetics Implementation Consortium (CPIC) provides evidence-based recommendations regarding pharmacogenetic tests for patient care. Their guidelines for azathioprine include dose reductions or alternative medication options based on TPMT and NUDT15 metabolizer status.…”