Polyglutamate Metabolites Research Articles

Studies on the mechanism of antitumor action of 2-desamino-2-methyl-5,8-dideazaisofolic acid

The new folate analogue, 2-desamino-2-methyl-5,8-dideazaisofolic acid, 2c, was synthesized and evaluated using a variety of biochemical and antitumor assays. For purposes of comparison, its 2-desamino, 2b, and 2-amino, 2a, counterparts, as well as N 10-propargyl-5,8-dideazafolic acid, 1a, and the corresponding 2-desamino, 1b, and 2-desamino-2-methyl, 1c, modifications were included in these studies. Compound 2c was found to be a potent inhibitor of the growth of L1210 and MCF-7 cells in culture, being only 2-fold and 5-fold less effective than 1c, respectively. However, although analogue 2c was 189-fold less inhibitory toward L1210 thymidylate synthase (TS) than 1c, its cytotoxicity was reversed completely by thymidine alone which suggests that the compound behaves as a TS inhibitor in cells. Enzymatically synthesized polyglutamates of 2c were substantially more inhibitory toward human TS than the parent compound. Compound 2c was the most efficient substrate for mammalian folylpolyglutamate synthetase of the compounds studied having a V max/ K m nearly 12-fold larger than 1c. Both 1c and 2c were effective inhibitors of the uptake of [ 3methotrexate into MOLT-4 cells, implying that each is efficiently transported into tumor cells. These results suggest that a weak inhibitor of TS in vitro can be a potent cytotoxic agent if it can readily gain entry into target cells and be converted to polyglutamated metabolites.

Impairment of Methotrexate (MTX)‐Polyglutamate Formation of MTX‐resistant K562 Cell Lines

We examined the mechanism of methotrexate (MTX) resistance in five K562 cell subclones resistant to MTX. Based on a clonogenic assay, the IC50s of these MTX‐resistant clones were 10 to 40μMMTX, indicating 2,000 to 5,000‐fold resistance as compared to that of the parent cell line. The doubling times of these MTX‐resistant K562 cell lines are longer (27–60 hr) than that of the parent K562 cell line (24 hr). One‐hour MTX accumulation in the resistant cells was 70–80% of that in parent cells. To investigate the formation of MTX‐polyglutamates (MTX‐PGs), resistant cells were incubated with 3H‐MTX (1 or 1OμM) for 24 hr in the presence of thymidine and deoxyinosine to prevent cytotoxicity. MTX (‐Glu1) and the polyglutamate metabolites (MTX‐Glu2, ‐Glu3, ‐Glu4 and ‐Glu5) were analyzed by a high‐pressure liquid chromatography (HPLC) technique. After a 24‐hr incubation with 10μM MTX, the total concentration of intracellular MTX reached 39 to 89 nmol/g protein, only 20 to 40% of the MTX level of the parent K562 cells. The HPLC analysis revealed that less than 2% of intracellular MTX was in the form of high‐molecular MTX‐PGs (MTX‐Glu3, ‐Glu4 and ‐Glu5) in the five MTX‐resistant K562 cell lines, while in the parent cells MTX‐Glu3–5 comprised 46% of the total intracellular MTX. These data indicate the possibility that impairment of MTX‐PG formation, with transport alteration, may be a special mechanism for the high level of resistance to this agent in human leukemic cells.

Formation and retention and biological activity of N 10-propargyl-5, 8-dideazafolic acid (CB3717) polyglutamates in L1210 cells in vitro

The formation, retention and biological activity of the polyglutamate metabolites of the thymidylate synthase (TS) inhibitor N 10-propargyl-5,8-dideazafolic acid (CB3717) has been investigated in L1210 murine leukaemia cells grown in vitro. CB3717 polyglutamates were measured by HPLC using high specific activity 3H-CB3717. Following the exposure of cells to 50 μM CB3717 for 6, 12 and 24 hr total cellular radioactivity corresponded to 4.5 ± 1.5, 6.8 ± 3.6 and 5.9 ± 3.4 μM drug derived material, respectively. Of this material, > 70%, 57 ± 3% and 51 ± 5% was in the form of unchanged CB3717 at 6, 12 and 24 hr respectively. The remaining radioactivity was associated with polyglutamate metabolites of CB3717, predominantly the tetra and pentaglutamate forms. Following the removal of extracellular drug after incubation for 24 hr and resuspension in drug free medium, unchanged CB3717 was lost rapidly from the cells such that after 6 hr it accounted for only 5% of total cellular radioactivity. In contrast, levels of CB3717 tetra and pentaglutamates declined solely due to dilution during cell division. Measurement of the whole cell TS activity by 3H-deoxyuridine incorporation into DNA indicated that, despite the loss of unchanged CB3717 from the cell, enzyme activity remained suppressed (< 10% of control) for at least 24 hr after resuspension in drug free medium. The TS inhibitory activity of the polyglutamated metabolites of CB3717 was investigated using enzyme purified from L1210 cells. As inhibitors, the metabolites were 26-, 87-, 119- and 114-fold more potent than CB3717 as the di-, tri-, tetra- and pentaglutamate forms, respectively. However, as inhibitors of dihydrofolate reductase prepared from rat liver, CB3717 polyglutamates were no more than 5-fold more potent than the parent compound. This study has shown that CB3717 can undergo polyglutamation in tumour cells and that the metabolites are preferentially retained giving rise to prolonged TS inhibition. By virtue of their potent TS inhibitory activity these metabolites are, therefore, most probably the intracellular effectors of CB3717 cytotoxicity.

Development of an assay for the estimation of N10-propargyl-5,8-dideazafolic acid polyglutamates in tumor cells

A method is described herein for the isolation and quantitation of polyglutamates of the thymidylate synthase (TS) inhibitor N 10-propargyl-5,8-dideazafolic acid (CB3717) in tumor cells exposed to the drug in vitro. Cells were incubated with 50 μ m 3H-CB3717 for 12 h and then disrupted by sonication. CB3717 and its polyglutamates were extracted by boiling in 0.01 m Tris-HCl pH 10. The extract was concentrated by lyophilization and analyzed by reverse phase HPLC (10 × 0.46-cm Polygosil 5-μm C 18 column) using linear gradient elution (5–16% acetonitrile in 0.1 m sodium acetate, pH 5, over 15 min, 2 ml/min). Recovery of radioactivity at each stage of the method was >70%. CB3717 and its polyglutamates were identified by co-chromatography with synthetic standards and by inhibition of partially purified TS. Quantitation was by means of radiochemical analysis. The 3H-CB3717 used in these studies was prepared by catalytic tritiation of diethyl-(2-chloro-4-nitrobenzoyl)- l-glutamate followed by consecutive alkylation with propargyl bromide and 2-amino-6-bromomethyl-3,4-dihydro-4-oxoquinazoline hydrobromide. The free diacid was prepared as required by hydrolysis in sodium hydroxide and purified by HPLC. Tritiation in only one position was confirmed by 3H NMR. Following the exposure of L1210 leukemia cells to 50 μ m 3H-CB3717 for 12 h the total cellular radioactivity level was approximately 7 μ m, of which 27% was present as polyglutamated metabolites with four and five glutamate residues.

Methotrexate and its polyglutamate derivatives in erythrocytes during and after weekly low-dose oral methotrexate therapy of children with acute lymphoblastic leukemia

Methotrexate and methotrexate polyglutamates were quantitatively determined in red blood cells from 12 children with acute lymphoblastic leukemia who were treated with MTX (15-20 mg/m2 per week) and daily 6-mercaptopurine orally during the steady-state period of erythrocyte MTX concentration (ery-MTX). The terminal decline of the ery-MTX and its polyglutamate metabolites were determined for up to 15 weeks after cessation of MTX treatment. Methotrexate polyglutamates with 2-4 extra glutamyl derivatives (MTX-glu2-5) constituted 75% of the MTX in the entire red blood cell population. MTX-glu3 was the principal metabolite; no MTX-glu6-7 was identified. After discontinuation of MTX therapy, the ery-MTX declined in a non-linear manner because of different half-lives for the individual polyglutamates. From about 5 weeks until 13-15 weeks after the last MTX dose, the erythrocyte MTX elimination curve was linear. The approximate half-life of MTX-glu1 was 2-3 days; for MTX-glu2 it was 4-15 days. The concentration of MTX-glu3-5 remained constant in the erythrocyte throughout its life span and declined only with age-dependent destruction of the red blood cell. It was calculated that 80%-90% of MTX in newly formed reticulocytes was MTX-glu1+2, the remainder being MTX-glu3-5. Mature red blood cells did not form methotrexate polyglutamates to any significant degree. There was a significant correlation between the amount of MTX-glu3-5 and the steady -state ery-MTX, which to some extent explained the interindividual variation of the ery-MTX in children with ALL in maintenance therapy.

Determinants of the sensitivity of human small-cell lung cancer cell lines to methotrexate.

We have characterized the determinants of methotrexate (MTX) responsiveness in eight patient-derived cell lines of small-cell lung cancer (SCLC). Clonogenic survival was correlated with factors known to affect sensitivity to drug. NCI-H209 and NCI-H128 were most drug sensitive, with drug concentrations required to inhibit clonogenic survival by 50% with less than 0.1 microM MTX. Six cell lines (NCI-H187, NCI-H345, NCI-H60, NCI-H524, NCI-H146, and NCI-N417D) were relatively drug resistant. In all cell lines studied, higher molecular weight MTX-polyglutamates (MTX-PGs) with 3-5 glutamyl moieties (MTX-Glu3 through MTX-Glu5) were selectively retained. Relative resistance to low (1.0 microM) drug concentrations appeared to be largely due to decreased intracellular metabolism of MTX. Five of the six resistant lines were able to synthesize polyglutamates at higher (10 microM) drug concentrations, although one resistant cell line (NCI-N417D) did not synthesize higher molecular weight MTX-PGs, even after exposure to 10 microM drug. Two cell lines with resistance to 10 microM MTX (NCI-H146 and NCI-H524) synthesized and retained higher molecular weight MTX-PGs in excess of binding capacity after exposure to 10 microM drug. However, the specific activity of thymidylate synthase in these cell lines was low. MTX sensitivity in patient-derived cell lines of SCLC requires the ability of cells to accumulate and retain intracellular drug in the form of polyglutamate metabolites in excess of dihydrofolate reductase, as well as a high basal level of consumption of reduced folates in the synthesis of thymidylate.

Factors controlling the concentrations of methotrexate in cultured hepatic cells

The polyglutamate metabolites of methotrexate are as inhibitory to the target enzyme dihydrofolate reductase as is methotrexate. Because of their greater retention they have a longer half-life within the cells and thus a greater potential for cytotoxicity. These metabolites have been found in numerous cells and tissues and are extensively synthesized in cultured hepatic cells. Uptake of methotrexate by primary cultures of rat hepatocytes occurs by a pathway which is independent of the folate coenzymes but appears to be related in some way to cholic acid and organic anion uptake. The evidence for the commonality of these pathways is (a) an instability of both uptake systems in the absence of hormones in the culture medium, (b) nearly equal inhibition of uptake by PCMS and NEM, and (c) cross competition of cholic acid and methotrexate for entry into the cells. Cholic acid and BSP can also selectively inhibit methotrexate polyglutamate formation in hepatocytes. Methotrexate entry into H35 hepatoma cells is mediated by the transport system which is shared by folate coenzymes and is not inhibited by cholic acid, BSP or sulfhydryl reagents. At concentrations of cholic acid or BSP which inhibit methotrexate polyglutamate formation in hepatocytes there is little or no loss of polyglutamate formation in H35 cells, possibly because BSP and cholic acid are taken up less by H35 cells than by hepatocytes.