Extended Thiopurine Metabolite Assessment During 6‐Thioguanine Therapy for Immunomodulation in Crohn's Disease

Abstract

The proposed metabolic advantage of 6-thioguanine (6-TG) is the direct conversion into the pharmacologically active 6-thioguaninenucleotides (6-TGN). The authors assessed metabolic characteristics of 6-TG treatment in patients with Crohn's disease (N = 7) on therapy with 20 mg 6-TG. 6-thioguanine-monophosphate (6-TGMP), 6-thioguanine-diphosphate (6-TGDP), and 6-thioguanine-triphosphate (6-TGTP) were measured by high-performance liquid chromatography analysis in erythrocytes. Thiopurine S-methyltransferase activity and total 6-TGN levels were determined by standard methods. High interindividual variance in metabolite measurements was observed. Main metabolites were 6-TGTP (median = 531 pmol/8 x 10(8) red blood cells) and 6-TGDP (median = 199 pmol/8 x 10(8) red blood cells). Traces of 6-TGMP (median = 39 pmol/8 x 10(8) red blood cells) and 6-TG (2 patients) could be detected. 6-TGN levels correlated with 6-TGTP levels (r = 0.929, P = .003) and with the sum of separate nucleotides (r = 0.929, P = .003). No correlations were established between TPMT activity (median = 13 pmol/h/10(7)) and 6-TG metabolites. The 1-step metabolism of 6-TG still leads to high interindividual variance in metabolite concentrations. Total 6-TGN level monitoring may suffice for clinical practice.

Acquired resistance to 6-thioguanine in melanoma cells involves the repair enzyme O6- methylguanine-DNA methyltransferase (MGMT).

“…38,39 In addition to its therapeutic effect in non-malignant diseases, systemic exposure to 6-TG in these cases may increase MGMT levels in different tissues providing protection from environmental genotoxic insults. Conversely, elevated MGMT levels induced in normal or premalignant cells may promote intrinsic drug resistance when malignantly transformed.…”

Novel Strategies in the Thiopurine Treatment of Inflammatory Bowel Disease

“…[13] Routine measurement of 6-TGN includes codetermination of thioguanine mono-, di-, and triphosphates and corresponds to 69-77% of the sum obtained from separate determination of these metabolites ( Figure 5). [14,15] In patients under thiopurine treatment, thioguanine triphosphate (6-TGTP) constitutes the largest fraction of 6-TGN irrespective of which of the GST, gluthatione S-transferase; GSH, gluthatione; HPRT, hypoxanthine (guanine) phosphoribosyltransferase; PRPP, phospho-ribosyl-pyrophosphate; TIMP, thioinosine monophosphate; TPMT, thiopurine methyltransferase; SAM, S-adenosyl methionine; IMPDH, inosine 5 -monophosphate dehydrogenase; NAD, nicotinamide adenine dinucleotide; GMPS, guanosine 5 -monophosphate synthetase; IMP, inosine monophosphate. )…”

“…Adapted from. [14,15] thiopurine drugs the patient is treated. [14][15][16] Besides incorporation of 6-TGN in nucleic acid, the thiopurine responsiveness is mediated through the formation of 6-TGTP acting via Rac1-blockade to induce apoptosis in activated lymphocytes (Figure 4).…”

How Should Thiopurine Treatment be Monitored?— Methodological Aspects

“…Instead TGN is mainly made up by thioguanosine mono-(TGMP), di-(TGDP), and triphosphates (TGTP) [10,[13][14][15] . It is likely that these substances differ in their clinical effects, and therefore it is possible that individual patients might be more or less skewed towards the more potent substances, affecting the clinical outcome.…”

“…It is likely that these substances differ in their clinical effects, and therefore it is possible that individual patients might be more or less skewed towards the more potent substances, affecting the clinical outcome. Some evidence from studies comparing TGMP, TGDP, and TGTP concentrations suggest that this is indeed the case [10,13,15] . More specifically, this indicates that the concentration of TGN might not accurately reflect the concentrations of TGTP and deoxyTGTP, which are the substances believed to be incorporated into RNA and DNA, respectively.…”