On the limitation of 6‐tioguaninenucleotide monitoring during tioguanine treatment

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Abstract
Summary: Background : Tioguanine (thioguanine) has been proposed as a rescue thiopurine for azathioprine or mercaptopurine intolerant inflammatory bowel disease patients. The use of tioguanine leads to high 6‐tioguaninenucleotide (6‐thioguaninenucleotide) levels in red blood cells but, contra‐intuitively, these have yet not been associated with an increased risk of myelotoxicity.Aim : To assess the role of 6‐tioguaninenucleotide concentrations in developing myelotoxicity during tioguanine treatment.Methods : Database analysis of 25 patients treated with tioguanine. Clinical findings and laboratory parameters were related to 6‐tioguaninenucleotide levels.Results : One patient developed a myelodepression (21 mg TG/day for 3 months and 6‐tioguaninenucleotide 714 pmol/8 × 108 red blood cells). 6‐Tioguaninenucleotide levels varied greatly between individuals (mean 6‐tioguaninenucleotide level 621 pmol/8 × 108 red blood cells, s.d. 340 pmol/8 × 108 red blood cells and range 34–1653 pmol/8 × 108 red blood cells). The TG dosages (mean 20.6 mg/day and median 20 mg/day) did not correlate with 6‐tioguaninenucleotide levels (r = 0.31, N.S.). High 6‐tioguaninenucleotide levels (>450 pmol/8 × 108 red blood cells) did not effect haemoglobin concentrations (mean 8 mmol/L), peripheral leucocyte (mean 7.5 × 109/L) or platelet counts (mean 298 × 109/L). No correlations were established between laboratory parameters, type of disease and 6‐tioguaninenucleotide level.Conclusion : High 6‐tioguaninenucleotide levels in erythrocytes (>450 pmol/8 × 108) during TG treatment compared to azathioprine or mercaptopurine treatment are not indicative for (developing) myelotoxicity.

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