Azathioprine

Three Pathways for Azathioprine Metabolism ( Table 15.3, Fig. 15.2 )

Azathioprine is rapidly converted to 6-MP upon absorption; conceptually, azathioprine is a ‘prodrug’. This conversion occurs mainly in erythrocytes. Q15.1 The fate of 6-MP is determined by one of the following three competing pathways:

• 1.

  Anabolized to its active form, a purine analog 6-TG, by the enzyme hypoxanthine-guanine phosphoribosyltransferase (HGPRT);

• 2.

  Degraded by TPMT to inactive metabolites; or

• 3.

  Degraded by xanthine oxidase (XO) to inactive metabolites.

The anabolic pathway leads to the active metabolites, the purine analogs thioguanine monophosphate and other 6-TG metabolites (including diphosphate and triphosphate metabolites). Both degradative pathways lead to inactive, nontoxic metabolites. Reduced activity of either of the degradative pathways will shift more of the 6-MP into the HGPRT active (or anabolic) pathway, leading to excessive clinical immunosuppression, with an increased risk of myelosuppression.

Thiopurine Methyltransferase Polymorphism

Q15.1 TPMT activity is reduced or absent in certain patients with a genetic polymorphism. TPMT enzyme function testing and enzyme allele sequencing are readily available. The enzyme function assay is available for clinical use and measures the activity of TPMT in red blood cells (RBC) and correlates well with systemic TPMT activity. Through this functional assay system, three groups of patients have been identified: those with high activity, intermediate activity, or low activity. Patients with low TPMT activity have markedly increased accumulation of 6-TG metabolites, which increases the risk of catastrophic myelosuppression; these patients should not be treated with azathioprine. Patients with high levels of this enzyme are commonly therapeutically underdosed, unless the clinician appropriately adjusts the traditional dose upward. Research into ethnic variation at the TPMT locus revealed that certain mutations appear to be unique to a given ethnic group. This diversity among ethnicities makes it difficult to create a genetic test to identify all possible mutations in TPMT leading to a low-activity phenotype. The functional enzyme assay is a relatively simple solution to identify patients with varying levels of TPMT activity phenotype. In addition, it appears that TPMT is inducible and that the enzyme activity is not a constant but has an activity range. Induction of TPMT may contribute to nonresponse. Despite the imperfections of these tests, the literature supports genotypic and, more importantly, phenotypic testing (enzyme activity) of TPMT activity to minimize the risk of myelosuppression and optimally dose azathioprine.

Q15.2 Should a laboratory provide a genotypic result for TPMT without quantitative enzymatic levels, the genotype can be easily interpreted by the number of wild type alleles. Because the TPMT alleles exhibit codominance, the number of wild type alleles are directly proportional to enzymatic activity. Effectively, homozygous wild type alleles equate to high TPMT activity, heterozygous wild type to medium activity, and homozygous mutant alleles to low activity. Therefore, dosing is easily translatable (see Table 15.1 ). Of note, a small proportion of heterozygous wild type individuals may tolerate homozygous dosing.

Xanthine Oxidase Catabolic Pathway

Q15.3 Decreased activity of XO occurs as the result of a drug interaction with allopurinol. Allopurinol inhibits the XO pathway and coadministration of allopurinol or febuxostat with azathioprine results in the production of more active metabolite 6-TG analogs. This in turn leads to excessive immunosuppression and increased risk for myelosuppression. In patients receiving allopurinol or the similar drug febuxostat, who also require azathioprine therapy, the dose of azathioprine should be reduced by 75%.

Hypoxanthine-Guanine Phosphoribosyltransferase Anabolic Pathway

Pharmacological inhibition or genetically reduced activity of either catabolic pathway respectively (XO or TPMT) will result in excessive HGPRT conversion of azathioprine (via 6-MP), with clinically enhanced immunosuppression and risk of myelosuppression.