Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:2.4.2.8 (hypoxanthine-guanine phosphoribosyltransferase)
 2,527 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Conventional antileukemic chemotherapy in relapsed or refractory acute leukemia or myeloid blast crisis of chronic granulocytic leukemia (CGL) is not curative and remissions, if attained, are usually of short duration. The primary goal of antileukemic therapy in these patients should be the identification of agents that are more selective and better targeted in their action. Tiazofurin is known to inhibit inosine 5'-phosphate dehydrogenase (IMPDH), the rate limiting enzyme of de novo guanine ribonucleotide synthesis. The activity of this enzyme is markedly increased in leukemic cells. To prevent de novo GTP synthesis, it is also necessary to block the guanine-salvaging activity of hypoxanthine-guanine phosphoribosyltransferase (HGPRT). This was achieved by increasing the plasma levels of hypoxanthine through the administration of allopurinol. Twenty-seven patients with end stage leukemia or myeloid blast crisis of CGL were treated with tiazofurin. Assays of IMPDH activity and GTP concentrations in leukemic cells, as well as hypoxanthine levels in the serum, provided a method to monitor the impact of tiazofurin/allopurinol therapy and to adjust drug doses. In these poor prognosis patients seven attained a complete response (CR), 3 had a hematologic improvement and an antileukemic effect was seen in 4. An excellent correlation was observed between biochemical and clinical activity of tiazofurin/allopurinol, with biochemical responses preceding clinical results. However, clinical responses were usually short-lived with IMPDH activity starting to increase soon after discontinuation of therapy, but patients responding again after reinstitution. Tiazofurin therapy was generally well tolerated in patients with less than 15 days of treatment and no other major medical complications. Although an antiproliferative effect was observed in some patients, bone marrows remained cellular in most cases with a marked shift from blasts to granulocytes. Severe neutropenia was absent in the majority of cases and patients could be discharged in good clinical condition immediately after completion of therapy. Tiazofurin/allopurinol therapy provided a rational, biochemically targeted and biochemically monitored approach to the treatment of poor prognosis leukemia and should serve as a paradigm in enzyme pattern-targeted chemotherapy.
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PMID:Biochemically targeted therapy of refractory leukemia and myeloid blast crisis of chronic granulocytic leukemia with Tiazofurin, a selective blocker of inosine 5'-phosphate dehydrogenase activity. 904 9

Dacarbazine (DTIC) is a DNA-methylating drug used in the treatment of malignant melanoma. Among the DNA dducts induced by DTIC are N7-methylguanine (N7-meG) and O6-methylguanine (O6-meG). The latter adduct, in particular, may be important in the mutagenic as well as the cytotoxic activity of DTIC. Repair of O6-meG is carried out by the enzyme O6-alkylguanine-DNA-alkyltransferase (AGT) by a process which results in its autoinactivation. N7-meG is lost from DNA partly spontaneously and partly by enzymatic depurination followed by excision repair of the resulting apurinic site. The purpose of this study was to determine the in vivo kinetics of formation and repair of O6-meG and N7-meG and the changes in AGT in peripheral WBCs with repeated doses of DTIC, and to determine the effects on these processes of concomitant administration of hydroxyurea. In addition, we examined the induction of mutations at the HPRT gene locus. Thirty-four patients with malignant melanoma received 1.0 g/m2 DTIC i.v. every 3 weeks. Hydroxyurea was added to the second and subsequent doses of DTIC in 19 patients. The concentrations of O6-meG, N7-meG, and AGT in peripheral blood lymphocytes were determined up to 24 h after each of the first two doses of DTIC. Mutations at the HPRT gene locus were determined using the T-cell clonal assay. Peak O6-meG levels were detected 1 and 4 h after the first and second dose of DTIC, respectively. AGT concentrations declined to 56.7% (range, 40.3-76.9%) and 55.0% (range, 45.4-58.9%) of pretreatment levels 24 h after the first and second doses of DTIC, respectively, and were still approximately 25%below their initial levels just prior to administration of the second dose of DTIC. An increase in formation of O6-meG was observed at all time points after the second dose of DTIC (P = 0.0001), which was not affected by cotreatment with hydroxyurea (P > 0.5). There was a negative correlation between pretreatment AGT levels and the O6-meG concentration at 24 h after therapy (r = -0.554, P = 0.014). N7-meG levels peaked at 6 h after DTIC therapy and were not significantly influenced by the cycle number. Cotreatment with hydroxyurea tended to be associated with lower levels of N7-meG (P = 0.08). There was no correlation between either O6-meG or N7-meG levels and the grade of neutropenia. On the basis of a limited series of blood samples analyzed, there was no firm evidence that chemotherapy with DTIC resulted in induction of HPRT mutations in lymphocytes. In conclusion, repeated administrations of DTIC resulted in higher concentrations of O6-meG, probably due to reduction in cellular AGT. Hydroxyurea did not significantly influence the kinetics of O6-meG, and N7-meG adduct formation. There was no significant induction of HPRT gene mutations with DTIC. This study suggests that sequencing of DTIC doses should be evaluated using the time course of cellular AGT depletion and DNA adduct formation to achieve higher cytotoxic efficiency.
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PMID:Methyl DNA adducts, DNA repair, and hypoxanthine-guanine phosphoribosyl transferase mutations in peripheral white blood cells from patients with malignant melanoma treated with dacarbazine and hydroxyurea. 981 73