Influence of 5-aminosalicylic acid on 6-thioguanosine phosphate metabolite levels: a prospective study in patients under steady thiopurine therapy

Abstract

Background and purpose: 5-aminosalicylate (5-ASA) raises levels of 6-thioguanine nucleotides (6-TGN), the active metabolites of thiopurines such as azathioprine (AZA). Changes in levels of each individual TGN - 6-thioguanosine mono-, di- and triphosphate (6-TGMP, 6-TGDP, 6-TGTP) - and of 6-methylmercaptopurine ribonucleotides (6-MMPR) after 5-ASA are not known.

Experimental approach: Effects of increasing 5-ASA doses on AZA metabolites were investigated prospectively in 22 patients with inflammatory bowel disease in 4-week study periods. Patients started with 2 g 5-ASA daily, and then were increased to 4 g daily and followed by a washout period. Thiopurine doses remained unchanged throughout the entire study. Levels of 6-TGMP, 6-TGDP, 6-TGTP and 6-MMPR as well as of 5-ASA and N-acetyl-5-aminosalicylic acid (N-Ac-5-ASA) were determined each study period.

Key results: Median baseline levels in 17 patients of 6-TGDP, 6-TGTP and 6-MMPR were 52, 319 and 1676 pmol per 8 x 10(8) red blood cells respectively. After co-administration of 2 g 5-ASA daily, median 6-TGDP and 6-TGTP levels increased but median 6-MMPR levels were unchanged. Increasing 5-ASA to 4 g daily did not affect median 6-TGDP and 6-TGTP levels, but median 6-MMPR levels decreased. After discontinuation of 5-ASA, both 6-TGDP and 6-TGTP levels decreased and median 6-MMPR levels increased. The 6-TGTP/(6-TGDP+6-TGTP)-ratio did not change during the study, but 6-MMPR/6-TGN ratios decreased.

Conclusions and implications: Individual 6-TGN metabolites increased after addition of 5-ASA, but 6-MMPR-levels and the 6-MMPR/6-TGN ratios decreased. Further studies are needed to decide whether this pharmacokinetic interaction would result in improvement of efficacy and/or increased risk of toxicity of AZA.

Figure 1

Thiopurine metabolism (simplified). Azathioprine (AZA) is non-enzymatically degraded to 6-mercaptopurine (6-MP). Xanthine oxidase (XO) inactivates 6-MP by the formation of 6-thiouric-acid (6-TUA). Thiopurine S-methyltransferase (TPMT) methylates 6-MP into 6-methylmercaptopurine (6-MMP). Via hypoxanthine phosphoribosyl transferase (HPRT), 6-MP is converted to 6-thioinosine-monophosphate (6-TIMP). Via two other enzymatic steps, inosine monophosphate dehydrogenase (IMPDH) and guanosine monophosphate synthetase (GMPS), a pool of 6-thioguanine nucleotides (6-TGN) is ultimately generated, consisting of 6-thioguanine monophosphate (6-TGMP), 6-thioguanine diphosphate (6-TGDP) and 6-thioguanine triphosphate (6-TGTP). 6-TIMP may also be methylated by TPMT leading to 6-methylmercaptopurine ribonucleotides (6-MMPR) (consisting of 6-methyl-thioinosine monophosphate, 6-methyl-thioinosine diphosphate and 6-methyl-thioinosine triphosphate). In a closed cycle, 6-TIMP may be phosphorylated to 6-thioinosine-diphosphate (6-TIDP), subsequently to 6-thioinosine triphosphate (6-TITP) and ultimately back to 6-TIMP, via the inosine triphosphate pyrophosphatase (ITPase).

Figure 2

Trial design. The effect of increasing 5-ASA doses on AZA metabolite levels was investigated in three study periods of 4 weeks. Patients started with 2 g 5-ASA daily, which was subsequently increased to 4 g daily and followed by a washout period. Thiopurine doses remained unchanged throughout the entire study. Levels of 6-TGMP, 6-TGDP, 6-TGTP and 6-MMPR as well as those of 5-ASA and N-acetyl-5-aminosalicylic acid (N-Ac-5-ASA) were determined in each study period. 5-ASA, 5-aminosalicylate; 6-MMPR, 6-methylmercaptopurine ribonucleotides; 6-TGDP, 6-thioguanosine diphosphate; 6-TGMP, 6-thioguanosine monophosphate; 6-TGN, 6-thioguanine nucleotides; 6-TGTP, 6-thioguanosine triphosphate; AZA, azathioprine.

Figure 3

Influence of 5-ASA on individual thiopurine metabolite levels. The influence of different doses of 5-ASA on the median thiopurine metabolite levels of 6-TGMP, 6-TGDP, 6-TGTP and 6-MMPR in patients under steady-state thiopurine therapy. T1, baseline, no. 5-ASA; T2, 2 g 5-ASA daily; T3, 4 g 5-ASA daily; T4, washout, no. 5-ASA; note that thiopurine doses were not altered over these study periods. 5-ASA, 5-aminosalicylate; 6-MMPR, 6-methylmercaptopurine ribonucleotides; 6-TGDP, 6-thioguanosine diphosphate; 6-TGMP, 6-thioguanosine monophosphate; 6-TGN, 6-thioguanine nucleotides; 6-TGTP, 6-thioguanosine triphosphate.

Figure 4

Influence of 5-ASA on different thiopurine metabolite ratios. Contribution of the individual levels of 6-TGDP and 6-TGTP to total 6-TGN-levels and the changes in 6-MMPR/6-TGN-ratio under co-administration of different 5-ASA doses in patients under steady-state thiopurine therapy. T1, baseline, no. 5-ASA; T2, 2 g 5-ASA daily; T3, 4 g 5-ASA daily; T4, washout, no. 5-ASA; note that thiopurine doses were not altered over these study periods. 5-ASA, 5-aminosalicylate; 6-MMPR, 6-methylmercaptopurine ribonucleotides; 6-TGDP, 6-thioguanosine diphosphate; 6-TGMP, 6-thioguanosine monophosphate; 6-TGN, 6-thioguanine nucleotides; 6-TGTP, 6-thioguanosine triphosphate.