DNA incorporation of 6-thioguanine nucleotides during maintenance therapy of childhood acute lymphoblastic leukaemia and non-Hodgkin lymphoma

Abstract

Purpose To explore the DNA incorporation of 6-thioguanine nucleotide levels (DNA-6TGN) during 6-mercaptopurine (6MP) therapy of childhood acute lymphoblastic leukaemia (ALL) and non-Hodgkin lymphoma (NHL) and its relation to erythrocyte levels of their metabolites: 6-thioguanine-nucleotides (E-6TGN), methylated metabolites (E-MeMP), Methotrexate polyglutamates (E-MTX), and to thiopurine methyltransferase activity (TPMT). Methods We studied these metabolites in 229 blood samples from 18 children with ALL (N = 16) or NHL (N = 2) on 6MP/Methotrexate maintenance therapy. Results DNA-6TGN levels were signiWcantly correlated to E-6TGN (r p = 0.66, p = 0.003) with a trend to reach a plateau at high E-6TGN levels. To explore the relative DNA incorporation of 6TGN in relation to cytosol 6TGN levels, a DNA-6TGN index was calculated as DNA-6TGN/ E-6TGN. The DNA-6TGN index was inversely correlated to E-6TGN (r p = ¡0.58, p = 0.012), which implies that with increasing levels of E-6TGN relatively less 6TGN are incorporated into DNA. E-MeMP levels were correlated to the DNA-TGN index (r p = 0.60, p = 0.008), indicating that high levels of MeMP result in enhanced DNA-6TGN incorporation, possibly due to inhibition of purine de novo synthesis, mediated by some of the methylated 6MP metabolites. Conclusions DNA-6TGN may prove to be a more relevant pharmacokinetic parameter for monitoring 6MP treatment intensity than the previously used erythrocyte 6MP metabolites levels. Prospective clinical trials are needed to evaluate the usefulness of DNA-6TGN for individual dose adjustments.

Liquid chromatography–tandem mass spectrometry quantification of 6-thioguanine in DNA using endogenous guanine as internal standard

“…The median (range) TG level of the 18 leukemia patients was 377 (45-1190) fmol TG/g DNA corresponding to 15-400 per 10 6 bases, which is well in line with previous findings [18,20]. Fig.…”

“…As absolute accuracy is not determined because of the lack of a standard, it is reassuring to note the similarity of the clinical values to that of another, dissimilar method, that was based on enzymatic digestion of DNA and fluorescence detection of TG [20]. It suggests that quantification is, at least reasonably, accurate.…”

Mercaptopurine/Methotrexate Maintenance Therapy of Childhood Acute Lymphoblastic Leukemia

“…3). The deoxy form of 6TGN is then incorporated into DNA (DNA-TG) in nucleated cells,72–74 which may activate postreplication mismatch repair systems that lead to DNA strand breaks and apoptosis 24,75…”

“…However, some methylated metabolites, not least methylthioinosine monophosphates (MeMP), are strong inhibitors of purine de novo synthesis 77. As the purine salvage pathway is low in lymphoblasts that primarily depend on purine de novo synthesis,78 the reduced levels of endogenous nucleotides and the resulting enhanced DNA-TG incorporation in the presence of MeMP is likely to play a clinical role 74,79. Still, the impact of these pharmacodynamic interactions on relapse rates and toxicities remains undetermined, partly as a sufficiently sensitive and reliable assay for routine measurements of DNA-TG in nucleated blood has only recently become available 80…”

Identification and genetics of 6-thioguanine secreted by Erwinia species and its interference with the growth of other bacteria

“…6-thioguanine is a guanine analogue with various medical applications. The synthetic compound among other thiopurines has been applied in treatment of acute lymphoblastic leukemia and ulcerative colitis (Hedeland et al 2010, Bar et al 2013. It acts as an antimetabolite, inhibits purine denovo synthesis and can be incorporated into DNA and RNA (McCollister et al 1964, Nelson et al 1975, Somerville et al 2003.…”