Extended thiopurine metabolite assessment during 6-thioguanine therapy for immunomodulation in Crohn's disease.

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.

Reversal of methylmercaptopurine ribonucleoside cytotoxicity by purine ribonucleosides and adenine.

6-Methylmercaptopurine ribonucleoside-5'-phosphate (MeSPuRMP), the sole metabolite of 6-methylmercaptopurine ribonucleoside (MeSPuRib), is a strong inhibitor of purine de novo synthesis, inducing depletion of intracellular purine nucleotides and subsequent cell death in several tumor cell lines. In this study prevention of MeSPuRib cytotoxicity by compounds of the purine salvage pathway was studied in Molt F4 human malignant T-lymphoblasts. Adenosine, adenine and inosine were able to prevent depletion of the adenine nucleotide pool when used in combination with 0.5 microM MeSPuRib, but had virtually no effect on depletion of guanine nucleotides. Nevertheless, these three purine compounds were able to reduce the cytotoxic effects induced by MeSPuRib. Addition of guanosine to cells treated with 0.5 microM MeSPuRib normalized the guanine nucleotide pool, but adenine nucleotides remained depleted. Under these conditions, inhibition of cell growth was significantly decreased. With the combination of guanosine and 10 microM MeSPuRib, cytotoxicity was increased compared to 10 microM MeSPuRib alone, associated with a depletion of adenine nucleotides to 9% of untreated cells. Since cell growth and cell viability of Molt F4 cells are less inhibited by MeSPuRib under conditions where adenine nucleotide depletion is prevented by purine compounds (and where the other nucleotides are depleted) we conclude that depletion of adenine nucleotides is an important factor in MeSPuRib cytotoxicity.

Inhibition of IMP dehydrogenase by mycophenolic acid in Molt F4 human malignant lymphoblasts.

The effects of inhibition of inosine monophosphate dehydrogenase (IMPDH), the rate-limiting enzyme in guanine nucleotide de novo synthesis, on cell growth, cell viability, endogenous nucleotide concentrations and concentrations of extracellular nucleosides and bases were studied in Molt F4 human malignant lymphoblasts. Mycophenolic acid (MPA) was used as a specific inhibitor of the enzyme activity. IMPDH activity was maximally inhibited with 0.5 microM MPA. After a 2 h exposure of the cells to 0.5 microM MPA, guanine nucleotides were depleted to approximately 50% of control values, whereas 5-phosphoribosyl-1-pyrophosphate levels increased to approximately 200%. Under these conditions, cytotoxicity became obvious after 24 h. Depletion of guanine nucleotides and cytotoxicity were prevented by addition of guanosine to MPA treatment. Daily supplements of guanosine were required to prevent MPA cytotoxicity during the entire incubation period of 72 h. We conclude that depletion of guanine nucleotides, induced by treatment with MPA, induces a severe and rapid cytotoxicity in Molt F4 cells.