Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound   Target Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound

---

Query: HUMANGGP:024500 (thymidylate synthase)
2,970 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Leucovorin (LV) increased the growth inhibition produced by iododeoxyuridine (IdUrd), a halogenated analogue of thymidine, in a murine tumor cell line (L1210) and three human tumor cell lines (HL-60, HT-29, and MCF-7). This increased growth inhibition was associated with increased incorporation of IdUrd into DNA. Consistent with previous reports, IdUrd (as iododeoxyuridine monophosphate) inhibited thymidylate synthase (TS) and was dehalogenated intracellularly by TS to generate thymidine nucleotides. In all four cell lines, LV decreased the dehalogenation of IdUrd, producing a 3-fold increase in the labeled iododeoxyuridine triphosphate/dTTP ratios in cytoplasm and labeled IdUrd/thymidine in DNA derived from tritiated IdUrd. In intact L1210 cells, apparent TS activity was inhibited 50% by 3 microM IdUrd alone and 75% by the combination of 3 microM IdUrd and 20 microM LV. Apparent TS activity was unchanged with 20 microM LV alone. In all cell lines except HL-60, the ratios of labeled iododeoxyuridine triphosphate/dTTP derived from tritiated IdUrd were 3-fold lower than the labeled IdUrd/thymidine ratios in DNA. This observation suggests that replicative DNA polymerases preferentially incorporate iododeoxyuridine triphosphate into DNA compared to the endogenous substrate dTTP. This preferential incorporation was independent of the effect of LV. These novel findings suggested that a potential mechanism for the effects of LV on IdUrd was increased inhibition of TS analogous to the interaction between fluoropyrimidines and LV. Enzyme inhibition studies using L1210 cell extracts showed that iododeoxyuridine monophosphate was a weak inhibitor of TS (Ki greater than 10 microM) when compared to 5-fluorodeoxyuridine monophosphate (Ki less than 10 nM). Despite the major differences in potency of these two halogenated pyrimidines, LV appears to modulate the activity of IdUrd as well as 5-fluorodeoxyuridine. LV may provide a clinically useful approach to improve the radiosensitizing and/or cytotoxic properties of IdUrd.
...
PMID:Effects of leucovorin on idoxuridine cytotoxicity and DNA incorporation. 220 28

We previously reported that fluorouracil (5FU) accumulation and metabolism in human livers and tumors can be studied by in vivo nuclear magnetic resonance spectroscopy (NMRS). We have extended these observations by evaluating the pharmacokinetics of 5FU in the tumors of 11 patients with carcinoma of the breast, colon, endometrium, cervix, and kidney, using 19F-NMRS in a 1.5 Magnetom (Siemens Medical Systems, Cerrito, CA) magnetic resonance imaging unit (MRI). These NMRS measurements detected a long-lived tumor pool of 5FU in six of 11 tumors in our patients including carcinomas in the pelvis, breast, lung, and liver. The half-life (T1/2) of this tumor pool of "trapped" 5FU was 0.33 to 1.3 hours (20 to 78 minutes), much longer than the T1/2 of 5FU in blood (5 to 15 minutes). Neither the anabolites of 5FU (fluorinated nucleosides, nucleotides, 5FU-RNA, or 5FU-thymidylate synthase) nor the catabolites (eg, fluorobetaalanine [FBAL]) were detectable by 19F NMRS. Patient response to chemotherapy appeared to correlate with the extent of trapping of free 5FU in the human tumors: in the seven patients receiving 5FU, or 5FU or FUdR plus leucovorin, four of four patients whose tumors trapped 5FU responded to fluorinated pyrimidine chemotherapy, whereas three patients in whom there was a failure to detect tumor trapping were resistant to 5FU. We conclude that NMRS is clinically feasible, and enables investigators to study 5FU pharmacokinetics and metabolism in tumors in vivo. 19F-NMRS of 5FU allows for in vivo evaluation of 5FU metabolic modulation and might be able to guide therapeutic decisions.
...
PMID:Human tumor fluorouracil trapping: clinical correlations of in vivo 19F nuclear magnetic resonance spectroscopy pharmacokinetics. 223 Aug 74

Previously, we identified an altered structural form of thymidylate synthase (TS) in a human colonic tumor cell line. This form, which is encoded by a variant structural gene, renders cells relatively resistant to 5-fluoro-2'-deoxyuridine as a result of the reduced affinity of the enzyme for the active metabolite 5-fluoro-2'-deoxyuridylic acid. We have isolated a cDNA clone specific to the altered TS and have determined its sequence. Two point mutations distinguish the normal from the altered TS mRNAs. One, a (A----G) change, is located within the 3'-untranslated region; the other, a T----C change within the amino acid-coding region, predicts replacement of tyrosine by histidine at residue 33 of the polypeptide. This sequence change was confirmed by direct analysis of cDNA amplified by the polymerase chain reaction and was further verified using allele-specific oligonucleotides as probes in Northern blots. These results, along with studies by other laboratories showing Tyr33 to be evolutionarily conserved, suggest that this residue plays an important role in TS function.
...
PMID:Single amino acid substitution defines a naturally occurring genetic variant of human thymidylate synthase. 232 36

We examined the in vitro activity of 2-desamino-5,8-dideazafolate and 2-desamino-N10-propargyl-5,8-dideazafolate (desamino-CB3717), the more water-soluble 2-desamino analogues of 5,8-dideazafolate and N10-propargyl-5,8-dideazafolic acid (CB3717). We report Ki values for the inhibition of L1210 thymidylate synthase (TS) of 2 and 0.027 microM for 2-desamino-5,8-dideazafolate and desamino-CB3717, respectively, indicating a 30- and 10-fold loss in TS-inhibitory activity compared with the corresponding 2-NH2 compounds. The synthetic tri- and tetrapolyglutamate derivatives of desamino-CB3717 were 66- and 101-fold more potent than the monoglutamate form as inhibitors of TS. Both desamino compounds were more potent as inhibitors of L1210 and W1L2 cell growth than were their 2-amino counterparts. 2-Desamino-5,8-dideazafolate retains quite good activity against both the TS-overproducing W1L2:C1 line and the L1210 cell line grown in the presence of thymidine, suggesting that a secondary locus of action may be involved. This other target is a folate-dependent enzyme as evidenced by the protection of the inhibition of cell growth by the addition of hypoxanthine or folinic acid together with thymidine. The methotrexate-resistant, dihydrofolate reductase-overproducing L1210:R7A cell line is cross-resistant to 2-desamino-5,8-dideazafolate, which suggests that dihydrofolate reductase is the other target. An L1210 subline (1565) unable to transport reduced folates is 10-fold resistant to desamino-CB3717 and 2-desamino-5,8-dideazafolate but is not cross-resistant to CB3717 or 5,8-dideazafolate. The removal of the 2-amino function of CB3717 did not affect folylpolyglutamate synthetase substrate activity (CB3717 Km = 48 microM, desamino-CB3717 Km = 40 microM). However, both 5,8-dideazafolate and its desamino analogue were about 10-fold better substrates for folylpolyglutamate synthase than were the N10-propargyl compounds, and this may contribute to their good growth-inhibitory properties. In vivo, desamino-CB3717 cured approximately 75% of mice bearing the L1210:ICR tumor at doses of 50 mg/kg daily for 5 days and above (maximum tolerated dose greater than 1000 mg/kg daily for 5 days).(ABSTRACT TRUNCATED AT 400 WORDS)
...
PMID:Activity of the thymidylate synthase inhibitor 2-desamino-N10-propargyl-5,8-dideazafolic acid and related compounds in murine (L1210) and human (W1L2) systems in vitro and in L1210 in vivo. 238 29

Evidence indicating that modifications at the 5- and 10-positions of classical folic acid antimetabolites lead to compounds with favorable differential membrane transport in tumor vs. normal proliferative tissue prompted an investigation of 5-alkyl-5-deaza analogues. 2-Amino-4-methyl-3,5-pyridinedicarbonitrile, prepared by hydrogenolysis of its known 6-chloro precursor, was treated with guanidine to give 2,4-diamino-5-methylpyrido[2,3-d]pyrimidine-6-carbonitrile which was converted via the corresponding aldehyde and hydroxymethyl compound to 6-(bromomethyl)-2,4-diamino-5-methylpyrido[2,3-d]pyrimidine. Reductive condensation of the nitrile 8 with diethyl N-(4-amino-benzoyl)-L-glutamate followed by ester hydrolysis gave 5-methyl-5-deazaaminopterin. Treatment of 12 with formaldehyde and Na(CN)BH3 afforded 5-methyl-5-deazamethotrexate, which was also prepared from 15 and dimethyl N-[(4-methylamino)benzoyl]-L-glutamate followed by ester hydrolysis. 5-Methyl-10-ethyl-5-deazaaminopterin was similarly prepared from 15. Biological evaluation of the 5-methyl-5-deaza analogues together with previously reported 5-deazaaminopterin and 5-deazamethotrexate for inhibition of dihydrofolate reductase (DHFR) isolated from L1210 cells and for their effect on cell growth inhibition, transport characteristics, and net accumulation of polyglutamate forms in L1210 cells revealed the analogues to have essentially the same properties as the appropriate parent compound, aminopterin or methotrexate (MTX), except that 20 and 21 were approximately 10 times more growth inhibitory than MTX. In in vivo tests against P388/0 and P388/MTX leukemia in mice, the analogues showed activity comparable to that of MTX, with the more potent 20 producing the same response in the P388/0 test as MTX but at one-fourth the dose; none showed activity against P388/MTX. Hydrolytic deamination of 12 and 20 produced 5-methyl-5-deazafolic acid and 5,10-dimethyl-5-deazafolic acid, respectively. In bacterial studies on the 2-amino-4-oxo analogues, 5-deazafolic acid proved to be a potent inhibitor of Lactobacillus casei DHFR and also the growth of both L. casei ATCC 7469 and Streptococcus faecium ATCC 8043. Its 5-methyl congener 22 is also inhibitory toward L. casei, but its IC50 for growth inhibition is much lower than its IC50 values for inhibition of DHFR or thymidylate synthase from L. casei, suggesting an alternate site of action.
...
PMID:Syntheses and antifolate activity of 5-methyl-5-deaza analogues of aminopterin, methotrexate, folic acid, and N10-methylfolic acid. 242 90

The cytogenetic study of colorectal carcinomas is consistent with the following sequence in the tumor evolution: rearrangement of chromosome 17 with loss of 17p and often gain of 17q, loss of chromosome 18, frequent del(5q), frequent del(1p) correlated with the gain of an early replicating X. At least one gene directly involved in nucleotide synthesis, especially in the de novo pathways for thymidine is located on each of these chromosomes or chromosomes segments. A model established on the gene dosage effect, which likely results of these chromosome imbalances, may be proposed: (1) increase of thymidine kinase activity (chromosome 17q) and thus of the salvage pathway of thymidine synthesis (2) decrease of thymidine de novo pathways by decreased of thymidylate synthase (chromosome 18) and of dihydrofolate reductase (chromosome 5q) and thus accumulation of 2'-deoxyuridine-5'-P, which saves 2'-deoxycytidine 5'-P (3) decrease of cytidylate (or uridylate) kinase (chromosome 1p) and thus accumulation of 2-deoxycytidine-5-PP and of uridine-5-P (UMP) decreasing the metabolisation of orotidine-5'-P, precursor of 2-deoxycytidine-5-PP, which (4) saves -D-5-ribosyl-PP (PRPP) or even conversion of orotidine-5'-P in PRPP. The later is the immediate precursor of nucleotides in their major salvage pathways synthesis: PRPP + base----nucleotide + PPi. This reaction which would be much activated needs hypoxanthine phosphorybosyl transferase (HPRT). Its gene is carried by chromosome X which is here duplicated in its active early replicating form.(ABSTRACT TRUNCATED AT 250 WORDS)
...
PMID:Induction of increased salvage pathways of nucleotide synthesis by dosage effect due to chromosome imbalances may be fundamental in carcinogenesis: the example of colorectal carcinoma. 242 58

A series of "stretched" methotrexate (MTX) analogues containing up to five 4-aminobutyryl (Gab) spacers between the 4-amino-4-deoxy-N10-methylpteroyl (MeAPA) moiety and the glutamate (Glu) side chain was prepared. Interest in these compounds stemmed from their relationship to MTX gamma-polyglutamates, from which they differ only in lacking "internal" alpha-carboxyl groups. The ability of the MeAPA-Gabn-Glu derivatives to inhibit dihydrofolate reductase (DHFR) and thymidylate synthase (TS) in vitro and to inhibit the growth of tumor cells in culture was evaluated. The IC50 for DHFR inhibition increased progressively from 0.082 to 0.84 microM as the number of Gab spacers was varied from one to five. At the same time the introduction of Gab spacers was found to produce substantial TS inhibition (Ki 0.1-0.4 microM) similar to that reported for MTX polyglutamates. Despite the activity of the MeAPA-Gabn-Glu derivatives as combined inhibitors of TS and DHFR, there was a steep loss of cell growth inhibitory potency as the number of Gab spacers was increased. This most likely reflects low cell uptake and the fact that when n greater than 1 there is almost total abolition of substrate activity for folylpolyglutamate synthetase, which had previously been observed with n = 1.
...
PMID:Methotrexate analogues. 29. Effect of gamma-aminobutyric acid spacers between the pteroyl and glutamate moieties on enzyme binding and cell growth inhibition. 242 79

Recent studies have clarified the critical role that polyglutamylation plays in methotrexate (MTX) action. Polyglutamate derivatives of MTX bind to dihydrofolate reductase (DHFR) with affinities comparable to the monoglutamate, but their retention in cells results in a sustained block in tetrahydrofolate (FH4) synthesis. One important element in the selectivity of MTX action is the preferential buildup and retention of these polyglutamyl forms in susceptible tumor cells as compared to host cells of the bone marrow or gastrointestinal mucosa. This selectivity in the accumulation of MTX polyglutamyl forms has now been further shown to play an important role in the selectivity of leucovorin rescue and may provide a unique new approach to nucleoside protection as well. This paper reviews the current understanding of the biochemical basis for leucovorin rescue and its selectivity. Important elements in leucovorin rescue are reactivation of DHFR with depression of cellular dihydrofolate (FH2) and provision of folate substrate to circumvent the block in FH4 synthesis. Selectivity of leucovorin rescue may be attributed to direct inhibition by MTX polyglutamyl forms, as well as FH2 polyglutamates that accumulate in their presence, at the levels of thymidylate synthase and transformylation during purine nucleotide biosynthesis. The presence of cellular MTX polyglutamates impairs reactivation of endogenous DHFR activity by leucovorin metabolites, and the resultant maintenance of high cellular levels of cellular FH2 and the polyglutamyl derivations of MTX impair the utilization of added FH4 in susceptible tumor cells. This paper also develops the concept of "early" nucleoside protection in antifolate therapy. In this approach, nucleosides are administered simultaneously with a pulse of MTX to provide early host protection from the cytotoxic effects of modest doses of MTX. Cessation of protection occurs at a time when extracellular and intracellular monoglutamate has fallen to low levels, and the polyglutamyl forms of the drug are present in susceptible tumors but not in host tissues of the gut and bone marrow. Data are presented to demonstrate that increased doses of MTX can be administered in normal and tumor-bearing animal systems as well as in humans by this technique.
...
PMID:Biochemical factors in the selectivity of leucovorin rescue: selective inhibition of leucovorin reactivation of dihydrofolate reductase and leucovorin utilization in purine and pyrimidine biosynthesis by methotrexate and dihydrofolate polyglutamates. 244 54

Biochemical alterations associated with acquired resistance of tumor cells to antifolates are diverse and multiple in number. These most often have included both quantitative and qualitative alterations at the level of membrane transport and of the primary intracellular target, dihydrofolate reductase (DHFR). More recent studies suggest determining biochemical alterations at the level of thymidylate synthase activity and 4-aminofolate polyglutamylation. Approaches to the circumvention of acquired antifolate resistance at the level of new drug design are described which incorporate a kinetic analysis of the various biochemical phenotypes and a systematic analysis of their structure-activity relationships. A consideration of the relative frequency of occurrence of individual phenotypes during therapy is also included. This introduces the notion of population genetics in an evaluation of resistance phenomenon and of clinically significant approaches for its circumvention.
...
PMID:Determinants of resistance to antifolates: biochemical phenotypes, their frequency of occurrence and circumvention. 244 55

High-dose methotrexate (HDMTX) regimens were initially designed to overcome methotrexate (MTX) resistance due to defective transport of the drug. At high concentrations, enough MTX diffuses into resistant cells to saturate and inhibit the target enzyme, dihydrofolate reductase (DHFR). The high doses of MTX needed to achieve these high drug concentrations must be administered with the reduced folate antidote, leucovorin (LV; 5-formyltetrahydrofolate), to prevent increased toxicity. To increase MTX therapeutic index, LV rescue must be selective, i.e., more effective in normal than in tumor cells. In experimental models, selective rescue can be achieved if strict LV administration guidelines are respected. Since both MTX and LV use the membrane transport system, it was hypothesized that selective rescue occurred because transport-deficient, MTX-resistant tumor cells also transported LV poorly. The unsatisfactory clinical results frequently obtained with HDMTX regimens suggest a need to re-evaluate this underlying rationale, especially in view of recent findings concerning the mechanisms of MTX resistance and LV rescue. Experimentally, cells resistant to MTX because of an increased or altered DHFR, decreased metabolism to polyglutamates, or decreased thymidylate synthase activity are not always significantly more sensitive to higher concentrations of MTX. Furthermore, recent studies on human small cell lung cancer cell lines suggest that decreased MTX polyglutamate metabolism and thymidylate synthase activity might be prevalent MTX-resistant mechanisms in human tumors. Selective LV rescue could also occur through mechanisms other than selective uptake by normal tissues.(ABSTRACT TRUNCATED AT 250 WORDS)
...
PMID:Biochemical and pharmacologic rationale for high-dose methotrexate. 244 56


<< Previous 1 2 3 4 5 6 7 8 9 10 Next >>

---