Genotyping—most common mutant alleles (TPMT*2, TPMT*3A, TPMT*3B, and TPMT*3C) are routinely available through reference laboratories.
Enzymatic analysis—less commonly available, but is the preferred method as it will also detect rare mutations not identified by genetic screening. Recent blood transfusions (within 90 days) can produce incorrect results.
Thiopurine Drugs and Metabolic Pathway
AZA, 6-MP, and 6-TG are thiopurine-based prodrugs that have no intrinsic biologic activity, and require extensive metabolism for activity (Fig. 5-1). After oral administration of AZA or 6-MP, between 27% and 83% is available as biologically active metabolites. AZA is often used clinically, as it is more stable and soluble than 6-MP. AZA doses are higher because the molecular weight of 6-MP is 55% of that of AZA.
Metabolic Pathways of Thiopurine Drugs.
Reproduced with permission from Agency for Healthcare Research and Quality. Assessment of Thiopurine Methyltransferase Activity in Patients Prescribed Azathioprine or Other Thiopurine-Based Drugs. Evidence Report No 196, Published December 2010.
In the gut, approximately 90% of AZA is converted to 6-MP, a thiopurine analogue of the purine base hypoxanthine, by cleavage of the imidazolyl moiety which is thought to be catalyzed through the action of glutathione transferase. 6-MP is then enzymatically converted to its active metabolite, deoxy-6-thioguanosine 5’ triphosphate (6-tGN), through successive enzymatic conversions by hypoxanthine-guanine phosphoribosyl transferase (HGPRT) and inosine monophosphate dehydrogenase (IMPDH). Inactivation of 6-MP (and hence AZA) occurs primarily through S-methylation by thiopurine S-methyltransferase (TPMT), and to a minor degree by catabolism, to thiouric acid by xanthine oxidase (XO). 6-TG is converted to its active metabolite (6-tGN) in a single step involving HGPRT, while inactivation occurs through two pathways. ...