Design, synthesis, and biological evaluation of 10-methanesulfonyl-DDACTHF, 10-methanesulfonyl-5-DACTHF, and 10-methylthio-DDACTHF as potent inhibitors of GAR Tfase and the de novo purine biosynthetic pathway

The synthesis and evaluation of 10-methanesulfonyl-DDACTHF (1), 10-methanesulfonyl-5-DACTHF (2), and 10-methylthio-DDACTHF (3) as potential inhibitors of glycinamide ribonucleotide transformylase (GAR Tfase) and aminoimidazole carboxamide ribonucleotide transformylase (AICAR Tfase) are reported. The compounds 10-methanesulfonyl-DDACTHF (1, K(i) = 0.23 microM), 10-methanesulfonyl-5-DACTHF (2, K(i) = 0.58 microM), and 10-methylthio-DDACTHF (3, K(i) = 0.25 microM) were found to be selective and potent inhibitors of recombinant human GAR Tfase. Of these, 3 exhibited exceptionally potent, purine sensitive growth inhibition activity (3, IC50 = 100 nM) against the CCRF-CEM cell line being 3-fold more potent than Lometrexol and 30-fold more potent than the parent, unsubstituted DDACTHF, whereas 1 and 2 exhibited more modest growth inhibition activity (1, IC50 = 1.0 microM and 2, IC50 = 2.0 microM).

Synthesis and biological evaluation of alpha- and gamma-carboxamide derivatives of 10-CF3CO-DDACTHF

Structurally-related, but non-polyglutamylatable, derivatives of 10-CF3CO-DDACTHF (1), which incorporate L-glutamine (2) and L-isoglutamine (3) in place of L-glutamate, were prepared and evaluated as inhibitors of recombinant human (rh) GAR Tfase. While the L-glutamate alpha-carboxamide derivative 3 was much less effective as a rhGAR Tfase inhibitor (K(i) = 4.8 microM) and inactive in cellular functional assays, the gamma-carboxamide derivative 2 was found to be a potent and selective rhGAR Tfase inhibitor (K(i) = 0.056 microM) being only 4-fold less potent than 1 (K(i) = 0.015 microM). Moreover, 2 was effective in cellular functional assays exhibiting purine sensitive cytotoxic activity (IC50 = 300 nM, CCRF-CEM) only 20-fold less potent than 1 (IC50 = 16 nM), consistent with inhibition of de novo purine biosynthesis via selective inhibition of GAR Tfase. Like 1, 2 is transported into the cell by the reduced folate carrier. Unlike 1, the functional activity of 2 is not dependent upon FPGS polyglutamylation.

Microscale synthesis of isotopically labeled R-[6-xH]N5,N10-methylene-5,6,7,8-tetrahydrofolate as a cofactor for thymidylate synthase

A one-pot synthesis of isotopically labeled R-[6-xH]N5,N10-methylene-5,6,7,8-tetrahydrofolate (CH2H4F) is presented, where x=1, 2, or 3 represents hydrogen, deuterium, or tritium, respectively. The current procedure offers high-yield, high-purity, and microscale-quantity synthesis. In this procedure, two enzymes were used simultaneously in the reaction mixture. The first was Thermoanaerobium brockii alcohol dehydrogenase, which stereospecifically catalyzed a hydride transfer from C-2-labeled isopropanol to the re face of oxidized nicotinamide adenine dinucleotide phosphate to form R-[4-xH]-labeled reduced nicotinamide adenine dinucleotide phosphate. The second enzyme, Escherichia coli dihydrofolate reductase, used the xH to reduce 7,8-dihydrofolate (H2F) to form S-[6-xH]5,6,7,8-tetrahydrofolate (S-[6-xH]H4F). The enzymatic reactions were followed by chemical trapping of S-[6-xH]H4F with formaldehyde to form the final product. Product purification was carried out in a single step by reverse phase high-pressure liquid chromatography separation followed by lyophilization. Two analytical methods were developed to follow the reaction progress. Finally, the utility of the labeled cofactor in mechanistic studies of thymidylate synthase is demonstrated by measuring the tritium kinetic isotope effect on the enzyme's second order rate constant.

Rational design, synthesis, evaluation, and crystal structure of a potent inhibitor of human GAR Tfase: 10-(trifluoroacetyl)-5,10-dideazaacyclic-5,6,7,8-tetrahydrofolic acid

Glycinamide ribonucleotide transformylase (GAR Tfase) has been the target of anti-neoplastic intervention for almost two decades. Here, we use a structure-based approach to design a novel folate analogue, 10-(trifluoroacetyl)-5,10-dideazaacyclic-5,6,7,8-tetrahydrofolic acid (10-CF(3)CO-DDACTHF, 1), which specifically inhibits recombinant human GAR Tfase (K(i) = 15 nM), but is inactive (K(i) > 100 microM) against other folate-dependent enzymes that have been examined. Moreover, compound 1 is a potent inhibitor of tumor cell proliferation (IC(50) = 16 nM, CCRF-CEM), which represents a 10-fold improvement over Lometrexol, a GAR Tfase inhibitor that has been in clinical trials. Thus, this folate analogue 1 is among the most potent and selective inhibitors known toward GAR Tfase. Contributing to its efficacious activity, compound 1 is effectively transported into the cell by the reduced folate carrier and intracellularly sequestered by polyglutamation. The crystal structure of human GAR Tfase with folate analogue 1 at 1.98 A resolution represents the first structure of any GAR Tfase to be determined with a cofactor or cofactor analogue without the presence of substrate. The folate-binding loop of residues 141-146, which is highly flexible in both Escherichia coli and unliganded human GAR Tfase structures, becomes highly ordered upon binding 1 in the folate-binding site. Computational docking of the natural cofactor into this and other apo or complexed structures provides a rational basis for modeling how the natural cofactor 10-formyltetrahydrofolic acid interacts with GAR Tfase, and suggests that this folate analogue-bound conformation represents the best template to date for inhibitor design.

Syntheses of labeled vitamers of folic acid to be used as internal standards in stable isotope dilution assays

[2H4]Folic acid was synthesized by deuterating p-aminobenzoic acid, which was then coupled to glutamic acid and 6-formylpterin. Using [2H4]folic acid as starting component enabled the preparation of labeled vitamers tetrahydrofolate, 5-formyltetrahydrofolate, 5-methyltetrahydrofolate, and 10-formylfolate which were characterized by electrospray mass spectrometry and collision-induced dissociation. The mass spectrometric studies confirmed that the compounds could be used as internal standards in stable isotope dilution assays.

10-Formyl-5,10-dideaza-acyclic-5,6,7,8-tetrahydrofolic acid (10-formyl-DDACTHF): a potent cytotoxic agent acting by selective inhibition of human GAR Tfase and the de novo purine biosynthetic pathway

The synthesis of 10-formyl-DDACTHF (3) as a potential inhibitor of glycinamide ribonucleotide transformylase (GAR Tfase) and aminoimidazole carboxamide ribonucleotide transformylase (AICAR Tfase) is reported. Aldehyde 3, the corresponding gamma- and alpha-pentaglutamates 21 and 25 and related agents were evaluated for inhibition of folate-dependent enzymes including GAR Tfase and AICAR Tfase. The inhibitors were found to exhibit potent cytotoxic activity (CCRF-CEM IC(50) for 3=60nM) that exceeded their enzyme inhibition potency [K(i) (3)=6 and 1 microM for Escherichia coli GAR and human AICAR Tfase, respectively]. Cytotoxicity rescue by medium purines, but not pyrimidines, indicated that the potent cytotoxic activity is derived from selective purine biosynthesis inhibition and rescue by AICAR monophosphate established that the activity is derived preferentially from GAR versus AICAR Tfase inhibition. The potent cytotoxic compounds including aldehyde 3 lost activity against CCRF-CEM cell lines deficient in the reduced folate carrier (CCRF-CEM/MTX) or folylpolyglutamate synthase (CCRF-CEM/FPGS(-)) establishing that their potent activity requires both reduced folate carrier transport and polyglutamation. Unexpectedly, the pentaglutamates displayed surprisingly similar K(i)'s versus E. coli GAR Tfase and only modestly enhanced K(i)'s versus human AICAR Tfase. On the surface this initially suggested that the potent cytotoxic activity of 3 and related compounds might be due simply to preferential intracellular accumulation of the inhibitors derived from effective transport and polyglutamation (i.e., ca. 100-fold higher intracellular concentrations). However, a subsequent examination of the inhibitors against recombinant human GAR Tfase revealed they and the corresponding gamma-pentaglutamates were unexpectedly much more potent against the human versus E. coli enzyme (K(i) for 3, 14nM against rhGAR Tfase versus 6 microM against E. coli GAR Tfase) which also accounts for their exceptional cytotoxic potency.

Synthesis and biological activity of 7-oxo substituted analogues of 5-deaza-5,6,7,8-tetrahydrofolic acid (5-DATHF) and 5,10-dideaza-5,6,7,8-tetrahydrofolic acid (DDATHF)

We recently described the syntheses of 12a-c, 4-amino-7-oxo substituted analogues of 5-deaza-5,6,7,8-tetrahydrofolic acid (5-DATHF), and 5,10-dideaza-5,6,7,8-tetrahydrofolic acid (DDATHF), in six steps from commercially available p-substituted methyl benzoates in 20-27% overall yields. Such analogues were tested in vitro against CCRF-CEM leukemia cells and showed that they are completely devoid of any activity, the IC(50) being higher than 20 microg/mL for all cases. To clarify if the presence of the carbonyl group in position C7, the distinctive feature of our synthetic methodology, is the reason for this lack of activity, we have now obtained the 7-oxo substituted analogues of 5-DATHF and DDATHF, 18a-c, in 10-30% overall yield. Testing of 18a-c in vitro against CCRF-CEM leukemia cells revealed that these compounds are totally inactive. A molecular modeling study of 18b inside the active site of the complex E. coliGARTFase-5-DATHF-GAR pointed to an electronic repulsion between the atoms of the 7-oxo group and the carbonyl group of Arg90 as a possible explanation for the inactivity of 18a-c.

Radioenzymatic assay for reductive catalysis of N(5)N(10)-methylenetetrahydrofolate by methylenetetrahydrofolate reductase

Methylenetetrahydrofolate reductase catalyzes the reduction of N(5), N(10)-methylenetetrahydrofolate to N(5)-methyltetrahydrofolate. Because this substrate is unstable and dissociates spontaneously into formaldehyde and tetrahydrofolate, the customary method to assay the catalytic activity of this enzyme has been to measure the oxidation of [14C]N(5)-methyltetrahydrofolate to N(5), N(10)-methylenetetrahydrofolate and quantify the [14C]formaldehyde that dissociates from this product. This report describes a very sensitive radioenzymatic assay that measures directly the reductive catalysis of N(5),N(10)-methylenetetrahydrofolate. The radio-labeled substrate, [14C]N(5),N(10)-methylenetetrahydrofolate, is prepared by condensation of [C(14)]formaldehyde with tetrahydrofolate and the stability of this substrate is maintained for several months by storage at -80 degrees C in a pH 9.5 buffer. Partially purified methylenetetrahydrofolate reductase from rat liver, incubated with the radio-labeled substrate and the cofactors, NADPH and FAD at pH 7. 5, generates [14C]N(5)-methyltetrahydrofolate, which is stable and partitions into the aqueous phase after the assay is terminated with dimedone and toluene. A K(m) value of 8.2 microM was obtained under conditions of increasing substrate concentration to ensure saturation kinetics. This method is simple, very sensitive and measures directly the reduction of N(5), N(10)-methylenetetrahydrofolate to N(5)-methyltetrahydrofolate, which is the physiologic catalytic pathway for methylenetetrahydrofolate reductase.

Regulation of the balance of one-carbon metabolism in Saccharomyces cerevisiae

One-carbon metabolism in yeast is an essential process that relies on at least one of three one-carbon donor molecules: serine, glycine, or formate. By a combination of genetics and biochemistry we have shown how cells regulate the balance of one-carbon flow between the donors by regulating cytoplasmic serine hydroxymethyltransferase activity in a side reaction occurring in the presence of excess glycine. This control governs the level of 5,10-methylene tetrahydrofolate (5,10-CH(2)-H(4)folate) in the cytoplasm, which has a direct role in signaling transcriptional control of the expression of key genes, particularly those encoding the unique components of the glycine decarboxylase complex (GCV1, GCV2, and GCV3). Based on these and other observations, we propose a model for how cells balance the need to supplement their one-carbon pools when charged folates are limiting or when glycine is in excess. We also propose that under normal conditions, cytoplasmic 5,10-CH(2)-H(4)folate is mainly directed to generating methyl groups via methionine, whereas one-carbon units generated from glycine in mitochondria are more directed to purine biosynthesis. When glycine is in excess, 5, 10-CH(2)-H(4)folate is decreased, and the regulation loop shifts the balance of generation of one-carbon units into the mitochondrion.

Synthesis and biological activity of 4-amino-7-oxo-substituted analogues of 5-deaza-5,6,7,8-tetrahydrofolic acid and 5,10-dideaza-5, 6,7,8-tetrahydrofolic acid

The 4-amino-7-oxo-substituted analogues of 5-deaza-5,6,7, 8-tetrahydrofolic acid (5-DATHF) and 5,10-dideaza-5,6,7, 8-tetrahydrofolic acid (DDATHF) were synthesized as potential antifolates. Treatment of the alpha,beta-unsaturated esters 11a-c, obtained in one synthetic step from commercially available para-substituted methyl benzoates (9a-c) and methyl 2-(bromomethyl)acrylate (10), with malononitrile in NaOMe/MeOH afforded the corresponding pyridones 12a-c. Formation of the pyrido[2,3-d]pyrimidines 13a-c was accomplished upon treatment of 12a-c with guanidine in methanol. After the hydrolysis of the ester group present in 13a-c, the resulting carboxylic acids 14a-c were treated with diethyl cyanophosphonate in Et3N/DMF and coupled with L-glutamic acid dimethyl ester to give 15a-c. Finally, the basic hydrolysis of 15a-c yielded the desired 4-amino-7-oxo-substituted analogues 16a-c in 20-27% overall yield. Compounds 16a-c were tested in vitro against CCRF-CEM leukemia cells. The results obtained indicated that our 4-amino-7-oxo analogues are completely devoid of any activity, the IC50 being higher than 20 microg/mL for all cases except 14c for which a value of 6.7 microg/mL was obtained. These results seem to indicate that 16a-c are inactive precisely due to the presence of the carbonyl group in position C7, the distinctive feature of our synthetic methodology.

A simplified and efficient synthesis of 5,10-dideaza-5,6,7,8-tetrahydrofolic acid (DDATHF)

A new and extremely efficient synthesis of DDATHF from 4-vinylbenzoic acid and bromomalondialdehyde as precursors has been developed which proceeds in 48% overall yield.

Thienyl and thiazolyl acyclic analogues of 5-deazatetrahydrofolic acid

Analogues of N-[4-[[3-(2,4-diamino-1,6-dihydro-6-oxo-5-pyrimidinyl)propyl]amino] benzoyl]-L-glutamic acid (5-DACTHF), in which the phenylene group is replaced by either a thienoyl or a thiazolyl group were synthesized. These compounds were prepared by reductive amination of suitably protected pyrimidinylpropionaldehyde with the aminoaroyl glutamates. These glutamates were in turn synthesized from the corresponding nitroaroyl carboxylic acids by condensation with protected glutamic acid followed by catalytic reduction. The compounds were tested as inhibitors of methotrexate uptake as a measure of binding to the reduced folate transport system, as inhibitors of glycinamide ribonucleotide transformylase, as substrates for folylpolyglutamate synthetase, and as inhibitors of tumor cell growth in cell culture. The thiophene analogue was found to be equal in activity to 5-DACTHF in the MCF-7 cell growth inhibition assay while the thiazole analogue was 9-fold more active. Indeed this thiazole was over 4 times more active in the MCF-7 cell line than the clinically investigated compound 5,10-dideaza-5,6,7,8-tetrahydrofolic acid (DDATHF).

5,10-Methylenetetrahydro-5-deazafolic acid and analogues: synthesis and biological activities

The synthesis of 5,10-methylene-5-deazatetrahydrofolic acid (2), a stable, rigid analogue of 5,10-methylenetetrahydrofolate (1), is reported as a potential inhibitor of thymidylate synthase. The target compound was obtained by a Fisher-indole type cyclization of the hydrazone 16 from 2-amino-6-hydrazino-4-oxopyrimidine (10) and diethyl N-[4-(3-formyl-1-pyrrolyl)benzoyl]-L-glutamate (15) followed by catalytic reduction of the product 17. Similarly, modification of the Fisher-indole type cyclization of the appropriate hydrazone precursors 11 and 12 afforded the nonclassical analogues 3-amino-7,8,9-trimethyl-2H-pyrrolo[3',4':4,5]pyrido[2,3-d]pyrimidin-1- one (4) and 3-amino-8-benzyl-7,9-dimethyl-2H-pyrrolo[3',4':4,5]pyrido [2,3-d]pyrimidin-1-one (5), respectively. The target compound 2, its aromatic precursor 18, and the nonclassical analogue 4 were evaluated as inhibitors of the growth of Manca human lymphoma cells and also as inhibitors of human dihydrofolate reductase, human thymidylate synthase, glycinamide ribonucleotide formyltransferase, and aminoimidazole carboxamide ribonucleotide formyltransferase. Compound 18 showed weak inhibition of lymphoma cell growth (IC50 = 42 microM) and of AICAR formylTF (IC50 = 17 microM). Compounds 2 and 4 did not inhibit lymphoma cell growth or thymidylate synthase. The inactivity of 2 was attributed to its lack of flexibility leading to its inability to bind to thymidylate synthase.

Side chain modified 5-deazafolate and 5-deazatetrahydrofolate analogues as mammalian folylpolyglutamate synthetase and glycinamide ribonucleotide formyltransferase inhibitors: synthesis and in vitro biological evaluation

5-Deazafolate and 5-deazatetrahydrofolate (DATHF) analogues with the glutamic acid side chain replaced by homocysteic acid (HCysA), 2-amino-4-phosphonobutanoic acid (APBA), and ornithine (Orn) were synthesized as part of a larger program directed toward inhibitors of folylpolyglutamate synthetase (FPGS) as probes of the FPGS active site and as potential therapeutic agents. The tetrahydro compounds were also of interest as non-polyglutamatable inhibitors of the purine biosynthetic enzyme glycinamide ribonucleotide formyltransferase (GARFT). Reductive coupling of N2-acetamido-6-formylpyrido[2,3-d]pyrimidin-4(3H)-one with 4-aminobenzoic acid, followed by N10-formylation, mixed anhydride condensation of the resultant N2-acetyl-N10-formyl-5- deazapteroic acid with L-homocysteic acid, and removal of the N2-acetyl and N10-formyl groups with NaOH, afforded N-(5-deazapteroyl)-L-homocysteic acid (5-dPteHCysA). Mixed anhydride condensation of N2-acetyl-N10-formyl- 5-deazapteroic acid with methyl D,L-2-amino-4-(diethoxyphosphinyl)butanoic acid, followed by consecutive treatment with Me3SiBr and NaOH, yielded D,L-2-[(5-deazapteroyl)amino]-4-phosphonobutanoic acid (5-dPteAPBA). Treatment with NaOH alone led to retention of one ethyl ester group on the phosphonate moiety. Catalytic hydrogenation of N2-acetyl-N10-formyl-5-deazapteroic acid followed by mixed anhydride condensation with methyl L-homocysteate and deprotection with NaOH afforded N-(5,6,7,8-tetrahydro-5-deazapteroyl)-L-homocysteic acid (5-dH4PteHCysA). Similar chemistry starting from methyl D,L-2-amino-4-(diethoxyphosphinyl)butanoic acid and methyl N delta-(benzyloxycarbonyl)-L-ornithinate yielded D,L-2-[(5-deaza-5,6,7,8-tetrahydropteroyl)amino]-4-phosphonobut ano ic acid (5-dH4Pte-APBA) and N alpha-(5-deaza-5,6,7,8-tetrahydropteroyl)-L-ornithine (5-dH4PteOrn), respectively. The 5-deazafolate analogues were inhibitors of mouse liver FPGS, and the DATHF analogues inhibited both mouse FPGS and mouse leukemic cell GARFT. Analogues with HCysA and monoethyl APBA side chains were less active as FPGS inhibitors than those containing an unesterified gamma-PO(OH)2 group, and their interaction with the enzyme was noncompetitive against variable folyl substrate. In contrast, Orn and APBA analogues obeyed competitive inhibition kinetics and were more potent, with Ki values as low as 30 nM. Comparison of the DATHF analogues as GARFT inhibitors indicated that the Orn side chain diminished activity relative to DATHF, but that the compounds with gamma-sulfonate or gamma-phosphonate substitution retained activity, with Ki values in the submicromolar range. The best GARFT inhibitor was the 5-dH4PteAPBA diastereomer mixture, with a Ki of 47 nM versus 65 nM for DATHF. None of the compounds showed activity against cultured WI-L2 or CEM human leukemic lymphoblasts at concentrations of up to 100 microM.(ABSTRACT TRUNCATED AT 400 WORDS)

Synthesis and biological activity of open-chain analogues of 5,6,7,8-tetrahydrofolic acid--potential antitumor agents

This study describes the synthesis and in vitro antitumor activity of inhibitors of purine de novo biosynthesis that are analogues of N-[4-[[3-(2,4-diamino-1,6-dihydro-6-oxo-5-pyrimidinyl) propyl]amino]benzoyl-L-glutamic acid (5-DACTHF). Benzene ring substituted analogues were synthesized from a protected pyrimidinyl propionaldehyde and a substituted benzoyl glutamate moiety by a key reductive amination step. Pyrimidine and linking chain substituted analogues were built up stepwise from p-aminobenzoic acid or analogues. The compounds were tested as inhibitors of methotrexate uptake as a measure of binding to the reduced folate transport system, as inhibitors of glycinamide ribonucleotide transformylase, as substrates for folylpolyglutamate synthetase, and as inhibitors of tumor cell growth in cell culture. With the exception of 2'-F substituent, the ring-substituted analogues are less active than the parent compound. Replacement of the 10-nitrogen by carbon, sulfur, or oxygen produced less than 2-fold changes to biological activity in vitro. A four-atom linking chain and an amino group at the 2-position on the pyrimidine ring are important for good activity.

Synthesis and biological activity of acyclic analogues of 5,10-dideaza-5,6,7,8-tetrahydrofolic acid

The synthesis and biological evaluation of a number of analogues of N-[4-[4-(2,4-diamino-1,6-dihydro-6-oxo-5-pyrimidyl) butyl]benzoyl]-L-glutamic acid (2) (7-DM-DDATHF), an acyclic modification of the novel folate antimetabolite 5,10-dideazatetrahydrofolic acid (DDATHF), are described. The synthetic procedure utilized previously for the synthesis of 2, 15, and 16 was extended to the preparation of analogues modified in the benzoyl region with thiophene and methylene groups replacing the benzene ring (compounds 27a-c) and in the glutamate region with aspartic acid and phenylalanine replacing L-glutamic acid (compounds 36, 37). The 2-amino-4,6-dioxo derivative 33 was obtained from intermediate 30 via a palladium-catalyzed carbon-carbon coupling reaction with diethyl (4-iodobenzoyl)-L-glutamate, followed by reduction and removal of protecting groups with base. Cell culture cytotoxicity studies of all of the above acyclic analogues of DDATHF against CCRF-CEM human lymphoblastic leukemic cells gave IC50s ranging from 0.042 greater than 48 microM. Inhibition and cell culture reversal studies against isolated enzymes suggest the mode of action of these compounds. Compound 2 was only 3-fold less inhibitory toward glycinamide ribonucleotide formyltransferase (GARFT, isolated from L1210 leukemic cells) than DDATHF itself. These acyclic analogues were less efficient substrates for the enzyme folylpolyglutamate synthetase (FPGS) compared with their bicyclic counterparts. Moderate antitumor activity was observed for compound 2 against 6C3HED lymphosarcoma and C3H mammary adenocarcinoma in vivo.

Synthesis and biological activity of an acyclic analogue of 5,6,7,8-tetrahydrofolic acid, N-[4-[[3-(2,4-diamino-1,6-dihydro-6-oxo-5- pyrimidinyl)propyl]amino]-benzoyl]-L-glutamic acid

The synthesis and biological evaluation of N-[4-[[3-(2,4-diamino-1,6-dihydro-6-oxo-5-pyrimidinyl)propyl]amino]- benzoyl]-L-glutamic acid (1) (5-DACTHF, 543U76), an acyclic analogue of 5,6,7,8-tetrahydrofolic acid (THFA), are described. The key intermediate, hemiaminal 8, was prepared in four stages from 3-chloropropionaldehyde diethyl acetal. Reaction of 8 with dimethyl N-(4-aminobenzoyl)-L-glutamate gave the 2,4-bis(acetylamino) derivative 11, which was hydrolyzed with 1 N sodium hydroxide to give 1; the glycine analogue 16 was prepared in a similar manner. The N-methyl analogue 2 and N-formyl analogue 3 were prepared from 11 and 1, respectively. Compounds 1-3 inhibited growth of Detroit 98 and L cells in cell culture, with IC50s ranging from 2 to 0.018 microM. Cell culture toxicity reversal studies and enzyme inhibition tests showed that 1 was cytotoxic but not by the mechanism of the dihydrofolate reductase inhibitor aminopterin. Compound 1 and its polyglutamylated homologues inhibited glycinamide ribonucleotide transformylase (GAR-TFase) and aminoimidazole ribonucleotide transformylase (AICAR-TFase), the folate-dependent enzymes in de novo purine biosynthesis; and 1 was an effective substrate for mammalian folyl-polyglutamate synthetase. The compound inhibited (IC50 = 20 nM) the conversion of [14C]formate to [14C]-formylglycinamide ribonucleotide by MOLT-4 cells in culture. These data suggest that the site of action of 1 is inhibition of purine de novo biosynthesis. Moderate activity was observed against P388 leukemia in vivo.

Synthesis and antitumor activity of 5-deaza-5,6,7,8-tetrahydrofolic acid and its N10-substituted analogues

Syntheses of 5-deaza-5,6,7,8-tetrahydrofolic acid (7a) and its 10-formyl (7b), 10-acetyl (7c), and 10-methyl (7d) derivatives are described. These compounds, prepared as analogues of 5,10-dideaza-5,6,7,8-tetrahydrofolic acid (DDATHF), the lead compound of a new class of folate antimetabolites, exhibit potent growth inhibition against leukemic cells in culture as well as substantial antitumor activity against transplantable murine solid tumors in vivo.

(6R,6S)-5,8,10-trideaza-5,6,7,8-tetrahydrofolate and 6(R,6S)-5,8,10-trideaza-5,6,7,8-tetrahydropteroyl-L-ornithine as potential antifolates and antitumor agents

(6R,6S)-5,8,10-Trideaza-5,6,7,8-tetrahydropteroic acid was synthesized in several steps from 4,4-(ethylenedioxy)-cyclohexanone and [4-(tert-butyloxycarbonyl)benzyl]triphenylphosphonium bromide and was elaborated to (6R,6S)-5,8,10-trideaza-5,6,7,8-tetrahydropteroyl-L-glutamic acid and (6R,6S)-5,8,10-trideaza-5,6,7,8-tetrahydropteroyl-L-ornithin e. Compound 1 was found to be a good substrate for partially purified mouse liver folypolyglutamate synthetase (FPGS), with a Michaelis constant (Km = 15 microM) comparable to that reported for the reduced folate substrate (6S)-5,6,7,8-tetrahydropteroyl-L-glutamic acid and for (6R,6S)-5,10-dideaza-5,6,7,8-tetrahydropteroyl-L-glutamic acid (DDATHF). However, in striking contrast to DDATHF, which is potently cytotoxic, 1 failed to inhibit tumor cell growth in culture at concentrations of up to 100 microM. These results suggested that the NH at position 8 of DDATHF is important for cytotoxic activity but not for polyglutamylation. Just as 1 was a good substrate for FPGS, the ornithine analogue 2 proved to be among the more potent competitive inhibitors of this enzyme discovered to date, with a Ki,s of 10 microM. While the binding affinity of 2 was lower than that reported for 5,6,7,8-tetrahydropteroyl-L-ornithine (H4PteOrn), very substantial FPGS inhibition was observed even though N5,N8, and N10 in H4PteOrn were replaced by carbon. Binding to FPGS thus appears to be tolerant of bioisosteric replacements made simultaneously in ring B and the bridge region. Neither 1 nor 2 was active in preventing cell growth in culture at concentrations of up 100 microM. The N delta-hemiphthaloyl derivative of 2, synthesized as a potential prodrug, was also inactive.

Synthesis and antifolate activity of 5-methyl-5,10-dideaza analogues of aminopterin and folic acid and an alternative synthesis of 5,10-dideazatetrahydrofolic acid, a potent inhibitor of glycinamide ribonucleotide formyltransferase

The title compounds were prepared in extensions of a general synthetic approach used earlier to prepare 5-alkyl-5-deaza analogues of classical antifolates. Wittig condensation of 2,4-diaminopyrido[2,3-d]pyrimidine-6-carboxaldehyde (2a) and its 5-methyl analogue 2b with [4-(methoxycarbonyl)benzylidene] triphenylphosphorane gave 9,10-ethenyl precursors 3a and 3b. Hydrogenation (DMF, ambient, 5% Pd/C) of the 9,10-ethenyl group of 3b followed by ester hydrolysis led to 4-[2-(2,4-diamino-5-methylpyrido[2,3-d]pyrimidin-6-yl)ethyl]ben zoi c acid (5), which was converted to 5-methyl-5,10-dideazaaminopterin (6) via coupling with dimethyl L-glutamate (mixed-anhydride method using i-BuOCOCl) followed by ester hydrolysis. Standard hydrolytic deamination of 6 gave 5-methyl-5,10-dideazafolic acid (7). Intermediates 3a and 3b were converted through concomitant deamination and ester hydrolysis to 8a and 8b. Peptide coupling of 8a,b (using (EtO)2POCN) with diesters of L-glutamic acid gave intermediate esters 9a and 9b. Hydrogenation of both the 9,10 double bond and the pyrido ring of 9a and 9b (MeOH-0.1 N HCl, 3.5 atm, Pt) was followed by ester hydrolysis to give 5,10-dideaza-5,6,7,8-tetrahydrofolic acid (11a) and the 5-methyl analogue 11b. Biological evaluation of 6, 7, 11a, and 11b for inhibition of dihydrofolate reductase (DHFR) isolated from L1210 cells and for growth inhibition and transport characteristics toward L1210 cells revealed 6 to be less potent than methotrexate in the inhibition of DHFR and cell growth. Compounds 6, 11a, and 11b were transported into cells more efficiently than methotrexate. Growth inhibition IC50 values for 11a and 11b were 57 and 490 nM, respectively; the value for 11a is in good agreement with that previously reported (20-50 nM). In tests against other folate-utilizing enzymes, 11a and 11b were found to be inhibitors of glycinamide ribonucleotide formyltransferase (GAR formyltransferase) from one bacterial (Lactobacillus casei) and two mammalian (Manca and L1210) sources with 11a being decidedly more inhibitory than 11b. Neither 11a nor 11b inhibited aminoimidazolecarboxamide ribonucleotide formyltransferase. These results support reported evidence that 11a owes its observed antitumor activity to interference with the purine de novo pathway with the site of action being GAR formyltransferase.

Synthesis and biological properties of 5,10-dideaza-5,6,7,8-tetrahydrofolic acid

The synthesis of the antifolate 5,10-dideaza-5,6,7,8-tetrahydrofolic acid (DDATHF) has been modified. It is prepared from 2-acetamido-6-formyl-4(3H)-pyrido[2,3-b]pyrimidone and [P-(N-[1,3-bis(ethoxycarbonyl)propan-1-yl]aminocarbonyl)] phenylmethyl]-triphenylphosphonium bromide. The synthesis proceeds via a sodium hydride promoted Wittig condensation in 1-methyl-2-pyrrolidone followed by catalytic reduction, mild base hydrolysis, and acid precipitation of the product. Synthesis of DDATHF is achieved in a total of seven steps from commercially available reagents. DDATHF is transported effectively into CCRF-CEM cells and inhibits growth of both human (CEM) and murine (L1210) cells in culture. Studies reported here support the view that methotrexate and DDATHF are transported via a shared transport mechanism.

Potential anticancer agents: 5-(N-substituted-aminocarbonyl)- and 5-(N-substituted-aminothiocarbonyl)-5,6,7,8-tetrahydrofolic acids

5-[[N-[(Ethoxycarbonyl)alkyl]amino]carbonyl] (6-9) and the corresponding aminothiocarbonyl (12-15) derivatives of 5,6,7,8-tetrahydrofolic acid were prepared as multisubstrate analogues of the substrate--cofactor adduct in the reactions catalyzed by the folate-mediated one-carbon transfer reactions. Evaluation in vitro showed that 7 (alkyl = hexyl) was cytotoxic to H.Ep.-2 cells (ED50, 4 microM) but noncytotoxic to proliferating L1210 cells. No activity was observed for 7 against the P388 leukemia in mice.

Synthesis and biological evaluation of 8-deazahomofolic acid and its tetrahydro derivative

The syntheses of 8-deazahomofolic acid and its tetrahydro derivative, potential inhibitors of thymidylate synthase (TS) and other folate related enzymes, are described. Wittig condensation of 2-acetamido-6-formyl-4-pyrimidinol with the triphenylphosphine ylide 3 derived from N-acetyl-4-(p-carbethoxyanilino)-1-chloro-2-butanone, hydrogenation of the enone intermediate 5, introduction of a 5-amino group via diazonium coupling, and reductive ring closure yielded ethyl N11-acetyl-8-deazahomopteroate (8). Alkaline hydrolysis gave 8-deazahomopteroic acid, which was blocked as the 11-trifluoroacetyl derivative, coupled with diethyl L-glutamate, and the blocking groups saponified to afford 8-deazahomofolic acid (12). Hydrogenation of the glutamate diester intermediate and subsequent saponification yielded the tetrahydro-8-deazahomofolate (14). Growth inhibition of Streptococcus faecium, Lactobacillus casei, and L1210 cells in culture by the target compounds was modest. They were also weak inhibitors of thymidylate synthase, dihydrofolate reductase, glycinamide-ribonucleotide transformylase, and aminoimidazolecarboxamide ribonucleotide transformylase. In contrast, 8-deazafolate showed moderate inhibition of aminoimidazolecarboxamide ribonucleotide transformylase, suggesting that inhibition of this enzyme may be related to its cytotoxic action. Tetrahydro-8-deazahomofolate showed low substrate activity with thymidylate synthase.

Interaction of tetrahydrofolate and other folate derivatives with bacterial sarcosine oxidase

Sarcosine oxidase from Corynebacterium sp. P-1 binds 2 mol of tetrahydrofolate/mol of enzyme (KD = 8.8 microM). The same stoichiometry is observed with tetrahydropteroyltetraglutamate (KD = 15.4 microM). Binding is also observed with pteroyltetraglutamate and with 5-formyltetrahydrofolate. In the case of the pteroylmonoglutamates, binding appears to be sensitive to changes in the pteridine ring since no binding is observed with 5-methyltetrahydrofolate or with folate. Sarcosine oxidase can be specifically adsorbed onto an affinity matrix prepared by coupling 5-formyltetrahydrofolate to AH-Sepharose. Tetrahydrofolate does not affect the rate of sarcosine oxidation but does block the formation of formaldehyde as a final product. In the presence of tetrahydrofolate, sarcosine oxidation is accompanied by the formation of 5,10-methylenetetrahydrofolate at a rate that exceeds the rate at which formaldehyde (or a precursor) can be released into solution and which is also considerably faster than the nonenzymic reaction of free formaldehyde with tetrahydrofolate. It is suggested that tetrahydrofolate may serve primarily to trap formaldehyde as it is formed at the active site during sarcosine oxidation. The existence of a catalytically significant binding site for tetrahydrofolate appears to be a general property of sarcosine oxidizing enzymes since similar results have previously been obtained with mammalian sarcosine dehydrogenase, an enzyme that is structurally and mechanistically very different from bacterial sarcosine oxidase.

Folate analogues. 30. Synthesis and biological evaluation of N10-propargyl-5,8-dideaza-5,6,7,8-tetrahydrofolic acid and related compounds

The 5,6,7,8-tetrahydro derivative (1) of the powerful thymidylate synthase inhibitor N10-propargyl-5,8-dideazafolic acid (PDDF) has been synthesized and evaluated for its antifolate activity. A convenient method for the preparation of the key intermediate 2-amino-6-(bromomethyl)-4-hydroxy-5,6,7,8-tetrahydroquinazoline (18) is described. Two closely related analogues of 1 were also synthesized and evaluated for their antifolate activity and thymidylate synthase inhibition. N10-Propargyl-5,8-dideaza-5,6,7,8-tetrahydrofolate (1) and N10-methyl and N10-hydrogen analogues 2 and 3 were weaker inhibitors of Lactobacillus casei thymidylate synthase compared to PDDF. N10-Methyl-5,8-dideaza-5,6,7,8-tetrahydrofolate (2) exhibited the most potent antifolate activity against L. casei (IC50 = 2.8 nM) and Streptococcus faecium (IC50 = 0.57 nM). In intact and permeabilized murine leukemia L1210 cells, the replacement of the quinazoline moiety with its tetrahydro derivative resulted in a marked decrease in potency and a loss of the contribution of the propargyl substituent to enzyme inhibition, indicating an altered binding mode to thymidylate synthase.

Formyltetrahydrofolate dehydrogenase-hydrolase from pig liver: simultaneous assay of the activities

The bifunctional folate-dependent enzyme, 10-formyltetrahydrofolate dehydrogenase-hydrolase (10-formyltetrahydrofolate: NADP+ oxidoreductase, EC 1.5.1.6), has been purified to homogeneity from pig liver. Its amino acid composition was determined and gave a calculated v of 0.735 ml/g; a molecular weight of 92500 for the protein subunit was determined as well. Spectrophotometric, fluorescence emission and radiochemical methods were devised to assay the activities. Quantitative separation of carbon dioxide and formate produced by the dehydrogenase and the hydrolase reactions, respectively, demonstrated that both activities occur simultaneously. This fact, together with a 5-fold difference in the Km values for the folate substrate, strongly suggests that these two activities are functions of different sites. The possible role of polyglutamate specificity for the preferential selection of one of the activities under physiological conditions was ruled out when both proved to have similar specificities, as determined by sensitivity to inhibition by tetrahydropteroylpolyglutamates.

Preparation of (6R)-tetrahydrofolic acid and (6R)-5-formyltetrahydrofolic acid of high stereochemical purity

Commercially available 5-formyltetrahydrofolate (5-CHO-H4PteGlu) is chemically prepared in a reaction that introduces an asymmetric center at the 6 carbon, and hence is the mixture of diastereomers differing in chirality about this position. (6R)-5-CHO-H4PteGlu, the diastereomer that is not normally found in vivo, was prepared from folic acid. Folic acid was chemically reduced and (6R)-tetrahydrofolate (H4PteGlu) was obtained from the resultant (6R,S)-H4PteGlu by enzymatic consumption of the natural diastereomer of (6R,S)-5,10-CH2-H4PteGlu (reversibly formed from (6R,S)-H4PteGlu in the presence of formaldehyde) with Lactobacillus casei thymidylate synthase. The 5 position of purified (6R)-H4PteGlu was directly formylated in a carbodiimide-catalyzed reaction. The level of contamination of these preparations with the corresponding 6S diastereomers was estimated using the binding of fluorodeoxyuridylate to thymidylate synthase promoted by folate cofactor (for H4PteGlu) and by the growth of folate requiring bacteria (for 5-CHO-H4PteGlu). Purified preparations of (6R)-H4PteGlu promoted the binding of fluorodeoxyuridylate to L. casei thymidylate synthase (in the presence of formaldehyde) only at concentrations greater than 1000-fold higher than equiactive levels of (6S)-H4PteGlu. Likewise, the (6R)-5-CHO-H4PteGlu made by this method was 600 times less active as a growth factor for Pediococcus cerevisiae than was authentic (6S)-5-CHO-H4PteGlu. Hence, the minimum stereochemical purity of these preparations was 99.9% for (6R)-H4PteGlu and 99.8% for (6R)-5-CHO-H4PteGlu.

Synthesis of the antileukemic agents 5,10-dideazaaminopterin and 5,10-dideaza-5,6,7,8-tetrahydroaminopterin

Total syntheses from pyridine precursors of 5,10-dideazaaminopterin (1) and 5,10-dideaza-5,6,7,8-tetrahydroaminopterin (2) are described. These compounds exhibit significant in vivo activity against L1210 leukemia that is comparable to that observed with methotrexate.

Preparation of tritium-labeled tetrahydropteroylpolyglutamates of high specific radioactivity

Tritium-labeled [6S]-tetrahydropteroylpolyglutamates of high radiospecific activity were prepared from the corresponding pteroylpolyglutamates. Malic enzyme and D,L-[2-3H]malate were used as a generating system to produce [4A-3H]NADPH which was coupled to the dihydrofolate reductase-catalyzed reduction of chemically prepared dihydropteroylpolyglutamate derivatives. Passage of the reaction mixtures through a column of immobilized boronate effectively removed NADPH, and the tetrahydropteroylpolyglutamates were subsequently purified by chromatography on DEAE-cellulose. Overall yields of the [6S]-tetrahydro derivatives were 18-48% and the radiospecific activities were 3-4.5 mCi X mumol-1.

Intestinal transport of 5-methyltetrahydrofolate

The mechanism of intestinal absorption of 5-methyltetrahydrofolate (5- CH3THF ) has been the topic of some controversy. In the present study, we have used enzymatically prepared 5- CH3THF to characterize transport by rat intestinal loops in vivo and everted jejunal sacs in vitro. Transport of 5- CH3THF is saturable (Km = 5.2 microM) and highly pH dependent, with the rate of maximal transport occurring at pH 5.8. Transport is competitively inhibited by folic acid (Ki = 4.2 microM) and methotrexate (Ki = 4.65 microM). Metabolic poisons and anaerobiosis greatly reduce 5- CH3THF transport. We conclude that 5- CH3THF transport in the rat intestine occurs by the same structure-specific mechanism responsible for the transport of unreduced folic acid and other monoglutamyl folates.

Synthesis of pseudo cofactor analogues as potential inhibitors of the folate enzymes

Reaction of 5,6,7,8-tetrahydrofolic acid (THF,7) with phosgene, thiophosgene, and cyanogen bromide gave the bridged derivatives, 5,10-(CO)-THF (8), 5,10-(CS)-THF (9), and 5,10-(C = NH)-THF (11), respectively. Catalytic hydrogenation of 10-(chloroacetyl)folic acid (2) gave 5,10-(CH2CO)-THF (12). A similar reaction with 10-(3-chloropropionyl)folic acid (3) gave 10-(ClCH2CH2CO)-THF (14) rather than 5,10-(CH2CH2CO)-THF (13). In the catalytic hydrogenation of 10-ethoxalylfolic acid (5), the initial product 10-(EtO2CCO)-THF (22) rearranged readily to give 5-(EtO2CCO)-THF (21). Acylation of THF with chloroacetyl chloride gave a N5,N10-diacylated product (18 or 19), which could not be converted to 5,10-COCH2)-THF (17). Reductive alkylation of THF with glyoxylic acid and 5-hydroxypentanal, respectively, gave 5-(HO2CCH2)-THF (24) and 5-[HO(CH2)5]-THF (25). Reductive dialkylation of THF with formaldehyde gave 5,10-(CH3)2-THF (27), whereas glyoxal gave 5,10-CH2CH2)-THF (10). Also, both folic acid and 5-(CHO)-THF were reductively alkylated with formaldehyde to give 10-methylfolic acid (6) and 5-(CHO)-10-(CH3)-THF (28), respectively. These compounds were tested as inhibitors of the enzymes involved in folate metabolism and for activity against lymphocytic leukemia P388 in mice.

New synthesis of N-[4-[[(2-amino-4(3H)-oxopyrido[3,2-d]pyrimidin-6-yl)methyl]amino]benzoyl]-L-glutamic acid (8-deazafolic acid) and the preparation of some 5,6,7,8-tetrahydro derivatives

Previously, 8-deazafolic acid (17) was shown to be a potent inhibitor of the folate-dependent bacteria, Streptococcus faecium (ATCC 8043) and Lactobacillus casei (ATCC 7469), and to have activity against lymphoid leukemia L1210 in mice. To examine the 5,6,7,8-tetrahydro derivatives, a new synthesis of 17 was developed from 8-deaza-2,4-dichloro-6-methylpteridine. Treatment of the latter with aqueous base gave the corresponding pteridin-4(3H)-one, which was aminated with ammonia to give 8-deaza-6-methylpterin (9). Bromination of 9 gave mainly 8-deaza-6-(tribromomethyl)pterin, which on reaction with p-aminobenzoyl-L-glutamic acid resulted in the formation of the 9-oxo derivative of 17. In contrast, bromination of the 2-acetyl derivative of 9 gave mainly the corresponding 6-(bromomethyl)pterin, which was converted to 17 in 23% yield (from 9). Hydrogenation of 17 at atmospheric pressure and room temperature was unsuccessful either in a basic medium or formic acid. In trifluoroacetic acid, overreduction occurred to give a mixture containing 8-deaza-5,6,7,8-tetrahydro-6-methylpterin and the 5,6,7,8-tetrahydro derivative of 17. The latter was characterized by conversion to the methenyl analogue 21, which was also prepared by hydrogenation of the 10-formyl derivative of 17. Treatment of 21 with hydroxide gave 8-deaza-10-formyl-5,6,7,8-tetrahydrofolic acid. Compound 21 showed cytotoxicity to cultured H.Ep.-2 cells and was tested as an inhibitor of bovine dihydrofolic reductase. Lineweaver-Burk analysis indicated inhibition competitive with dihydrofolate.

A convenient preparation of tetrahydropteridines and tetrahydrofolate analogs. Synthesis of tetrahydro-3',5'-dichloromethotrexate

Tetrahydropteridine enzymatic cofactors and inhibitors may be conveniently prepared by reduction of the aromatic precursors with dimethylamine--borane in glacial acetic acid. A short reaction time at 20-60 degrees C affords tetrahydrofolate analogs in high yield and purity under anhydrous conditions and without protection from air. 3',5'-Dichloromethotrexate is reduced without hydrogenolysis of chlorine.

Preparation and purification of L-(+/-)-5-formyl-5,6,7,8-tetrahydrofolic acid

Reinvestigation of the conversion of folic acid to leucovorin [L-(+/-)-5-CHO-THF] led to improved methods for the synthesis of this drug, which is suitable for clinical use. Also, methods were developed for the chromatographic and nonchromatographic purification of less pure samples of L-(+/-)-5-CHO-THF.

Stereochemistry of methylene transfer involving 5,10-methylenetetrahydrofolate

The stereochemistry of the transfer catalyzed by rabbit liver serine transhydroxymethylase (EC 2.1.2.1) of the prochiral hydroxymethyl group from serine to tetrahydrofolate to form 5,10-methylenetetrahydrofolate was studied. Initial kinetic studies on labeled 5,10-methylenetetrahydrofolate showed that nonenzymatic racemization of the prochiral methylene center was buffer dependent and was slow under the conditions employed. Specifically tritiated (3R)- and (3S)-serines were employed to study the transfer reaction. Reactions were carried out under various conditions and the stereochemistry of the methylene carbon of the 5,10-methylenetetrahydrofolate produced was determined. The enzyme was shown to be partially stereospecific for this transfer reaction, proceeding with a loss of about 50% of the stereochemical purity of the transferred carbon center. Possible mechanistic interpretations of this finding are discussed.