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P725 The common homozygous glutathione S-transferase Mu null genotype leads to higher 6-MMP levels in patients treated with azathioprine but not in those treated with 6-mercaptopurine

M. Broekman*1, M. Coenen2, C. van Marrewijk2, G. Wanten1, H. Roelofs1, L. Derijks3, S. Vermeulen2, 4, O. Klungel5, A. Verbeek4, P. Hooymans6, H. Scheffer2, H.-J. Guchelaar7, D. Wong6, D. de Jong1

1Radboud University Medical Centre, Department of Gastroenterology, Nijmegen, Netherlands, 2Radboud University Medical Centre, Department of Human Genetics, Nijmegen, Netherlands, 3Maxima Medical Centre Loc. Veldhoven, Department of Gastroenterology and Hepatology, Veldhoven, Netherlands, 4Radboud University Medical Centre, Radboud Institute for Health Sciences, Nijmegen, Netherlands, 5Utrecht University, Department of Pharmacoepidemiology and Clinical Pharmacotherapy, Utrecht, Netherlands, 6Zuyderland Medical Centre, Department of Clinical Pharmacy, Pharmacology and Toxicology, Sittard, Netherlands, 7Leiden University Medical Centre, Department of Clinical Pharmacy and Toxicology, Leiden, Netherlands

Background

The conversion of azathioprine (AZA) to 6-mercaptopurine (6-MP) by cleavage of the 5-nitro-imidazole group is mainly (90%) regulated by the action of glutathione S-transferases (GST) and for 10% non-enzymatic. Although GST Mu (GSTM1) plays a dominant role, approximately 50% of the population has a homozygous GSTM1 gene deletion (GSTM1 null genotype) leading to an absence of the enzyme. This consequently might limit the conversion of AZA to 6-MP. Our aim was to explore whether the GSTM1 null genotype indeed influences 6-methylmercaptopurine ribonucleotides (6-MMPR) and 6-thioguanine nucleotides (6-TGN) levels in patients treated with AZA, and not in 6-MP-treated individuals.

Methods

We analysed patients included in the TOPIC trial (‘Thiopurine response Optimization by Pharmacogenetic testing in IBD Clinics’). Per-protocol 6-MMPR and 6-TGN levels were assessed at week 8 in the first 377 of 796 randomised patients. For this study, we only included patients without dose adjustments before week 8 (n = 237) and excluded patients with a variant in *2, *3A, *3C of the thiopurine S-methyltransferase (TPMT gene (n = 38), leaving 199 patients for this analysis. Patients with a homozygous GSTM1 deletion were identified by melt curve analysis, performed in duplicate. Herewith, persons with the GSTM1 null genotype (gene deletion) can be distinguished from persons that are heterozygous or homozygous carriers of the gene. The influence of the GSTM1 null genotype on 6-MMPR and 6-TGN levels was analysed using the Mann–Whitney U test. Metabolites were measured in red blood cells (RBC) and values are presented as median (interquartile range) pmol/ 8x10(8).

Results

Of the 199 patients, 126 were treated with AZA, and 73 with 6-MP. The frequency of the GSTM1 null genotype was 56% in AZA-users and 47% in 6-MP-users (p = 0.22). Both for AZA and for 6-MP no differences were seen in thiopurine dosage (mg/kg) and TPMT enzyme activity in patients with and without a GSTM1 null genotype. Patients with a GSTM1 null genotype treated with AZA had lower 6-MMPR levels compared with AZA-users without a GSTM1 null genotype, 2 239 (1 006–4 587) vs 4 371 (1 897–7 369); p < 0.01. In contrast, in patients treated with 6-MP, 6-MMPR levels were not different between patients with and without a GSTM1 null genotype, 6195 (1 551–10 712) vs 6544 (1 717–11 600); p = 0.84. No statistically significant differences were seen in 6-TGN levels in patients with and without a GSTM1 null genotype in both AZA-users 232 (185–315) vs 217 (171–305), as well as patients treated with 6-MP 282 (202–364) vs 267 (207–423).

Conclusion

Homozygous GSTM1 deletion leads to lower 6-MMPR levels in patients treated with AZA, but not in patients treated with 6-MP.