Dynamics of leucocyte DNA thioguanine nucleotide levels during maintenance therapy of childhood acute lymphoblastic leukemia

Purpose Methotrexate (MTX)/6-Mercaptopurine (6MP)-based maintenance therapy is crucial to cure childhood acute lymphoblastic leukemia (ALL). Cytotoxicity is mediated by incorporation of thioguanine nucleotides (TGN) into DNA (DNA-TG) with higher levels in leucocytes being associated with reduced relapse risk. To further understand the dynamics of DNA-TG formation, we measured DNA-TG levels in leucocyte subsets during maintenance therapy and in the months following its discontinuation. Methods DNA-TG levels were measured in leucocytes (DNA-TG Total ), polymorph nucleated granulocytes (neutrophils, eosinophils, basophils [DNA-TG PMN ]) and mononucleated cells (lymphocytes, monocytes [DNA-TG MNC ]) in 1013 samples from 52 patients on ALL maintenance therapy (951 samples during therapy and 62 samples after therapy discontinuation, respectively). Results Median DNA-TG Total , DNA-TG PMN and DNA-TG MNC during maintenance therapy were 539, 563 and 384 fmol/µg DNA, respectively. DNA-TG PMN displayed more pronounced fluctuation than DNA-TG MNC (range 0–3084 [interquartile range IQR 271–881] versus 30–1411 [IQR 270–509] fmol/µg DNA). DNA-TG Total was more strongly correlated with DNA-TG PMN ( r S = 0.95, p < 0.0001) than DNA-TG MNC ( r S = 0.73, p < 0.0001). DNA-TG PMN correlated less with DNA-TG MNC ( r S = 0.64, p < 0.0001) and to a much lesser extent with absolute neutrophil count ( r S = 0.35, p < 0.0001). Following discontinuation of therapy, DNA-TG PMN was rapidly eliminated, and not measurable beyond day 22 after discontinuation, whereas DNA-TG MNC was slowly eliminated, and five patients demonstrated a measurable DNA-TG MNC more than 365 days after therapy discontinuation. Conclusion Fluctuations in DNA-TG Total are predominantly caused by corresponding fluctuations in DNA-TG PMN , thus DNA-TG Total measures recent TGN incorporation in these short-lived cells. Measurement of DNA-TG Total at 2–4 weeks intervals provides a reliable profile of DNA-TG levels.