Antimalarial activity of tetrahydrohomopteroic acid.

Tetrahydrohomopteroic acid, an analog of tetrahydrofolic acid, shows antimalarial activity against Plasmodium cynomolgi and a pyrimethamine-resistant variant of this organism.

Quantitation of folic acid enhancement of antifolate synergism.

Trimetrexate (TMTX), 5,10-dideazatetrahydrofolate (DDATHF), and 10-propargyl-5,8-dideazafolate (PDDF, CB3717) are antifolates whose primary intracellular targets are dihydrofolate reductase, glycinamide ribonucleotide formyltransferase, and thymidylate synthase, respectively. Varying the medium folic acid (PteGlu) concentration over the range of 0.5 to 100 microM increasingly blocks the growth inhibitory effects of the individual antifolates in Manca human lymphoma cells, but increasingly enhances the synergistic interaction of both TMTX + DDATHF and TMTX+ PDDF combinations. Drug interactions were quantitated using the universal response surface approach, which consists of fitting a concentration-effect surface to experimental data with weighted nonlinear regression, enabling the estimation of the synergism parameter, alpha. Estimates for alpha are larger (more intense synergism) for the TMTX + DDATHF combination (alpha = 4.68 +/- 0.66 at 2 microM PteGlu; alpha = 53.6 +/- 3.7 at 40 microM PteGlu) than for the TMTX + PDDF combination (alpha = 0.690 +/- 0.25 at 2 microM PteGlu; alpha = 7.20 +/- 1.8 at 40 microM PteGlu). However, the relative increase brought about by increasing the PteGlu concentration from 2 microM to 40 microM is similar in each instance, 11- and 10-fold, respectively. These experiments suggest that the enhanced cytotoxic interaction brought about by increased PteGlu concentration with the TMTX + DDATHF combination and the TMTX + PDDF combination may share a common mechanism. The dramatic intensity of the synergism between DDATHF and TMTX at 40 microM PteGlu, along with the concept of modulation of antifolate synergism by folates, suggests future in vivo and/or clinical applications of combinations of these (or similar) compounds.

Interaction of polyglutamyl derivatives of methotrexate, 10-deazaaminopterin, and dihydrofolate with dihydrofolate reductase.

Polyglutamyl derivatives of methotrexate (MTX) and 10-deazaaminopterin (10-DAM) containing a total of one through six glutamate residues (Glu residues) were tested as inhibitors of dihydrofolate reductase (DHFR) derived from sheep, chicken, and beef liver. The ability of dihydropteroylpentaglutamate to antagonize the inhibitory activity of these analogues was also studied. The most striking effects were seen with sheep liver DHFR, where polyglutamylation of MTX causes stepwise decreases in the concentration required for 50% inhibition (IC50) with each additional Glu residue until MTX with a total of six Glu residues has an IC50 value 1/3 that of MTX. With 10-DAM the pattern is more complex. The IC50 values increase with addition of Glu residues until a maximum is reached with 10-DAM having a total of three Glu residues which has a value twice that of 10-DAM. 10-DAM with a total of four Glu residues and 10-DAM with a total of five Glu residues have progressively lower IC50 values, the latter being equipotent with 10-DAM. With dihydropteroylpentaglutamate as substrate instead of dihydrofolate, the IC50 values are increased 2- to 5-fold for both MTX and 10-DAM derivatives. The results obtained with chicken liver and beef liver DHFR are generally similar to those described for the sheep liver enzyme, but the effects of polyglutamylation are less pronounced. The addition of 0.2 M KCl to the assay system reduces the differences in inhibitory potency of the polyglutamyl derivatives with all three enzymes tested. We conclude that polyglutamylation can alter the interaction of folate analogues and dihydrofolate with DHFR.

Inhibition of glycinamide ribonucleotide formyltransferase and other folate enzymes by homofolate polyglutamates in human lymphoma and murine leukemia cell extracts.

In order to determine the biochemical basis for the cytotoxicity of homofolates, poly-gamma-glutamyl derivatives of homofolate (HPteGlu) and tetrahydrohomofolate (H4HPteGlu) were synthesized and tested as inhibitors of glycinamide ribonucleotide formyltransferase (GARFT), aminoimidazolecarboxamide ribonucleotide formyltransferase (AICARFT), thymidylate synthase, and serine hydroxymethyltransferase (SHMT) in extracts of Manca human lymphoma and L1210 murine leukemia cells. The most striking inhibitions are that of GARFT by (6R,S)-H4HPteGlu4-6 with IC50 values from 1.3 to 0.3 microM. Both diastereomers, (6R)-H4HPteGlu6 and (6S)-H4HPteGlu6, inhibit GARFT activity. In Manca cell extracts, the (6S) form is more potent than the (6R) form whereas in the murine system the reverse is true. The (6R,S)-H4HPteGlu polyglutamates are weak inhibitors of human AICARFT (IC50, 6-10 microM). Polyglutamates of HPteGlu, however, are more inhibitory to AICARFT, with HPteGlu4-6 having IC50 values close to 2 microM. Polyglutamates of HPteGlu and of H4HPteGlu are weaker inhibitors of thymidylate synthase (IC50, 8 microM for HPteGlu5-6 and greater than 20 microM for H4HPteGlu1-5). Polyglutamates of HPteGlu and of H4HPteGlu are poor inhibitors of SHMT (IC50, greater than 20 microM). Manca cell growth is inhibited 50% by HPteGlu and (6R,S)-5-methyl-H4HPteGlu at 6 and 8 microM, respectively. Both of these effects are reversed by 0.1 mM inosine. Trimetrexate at a subinhibitory concentration, 10 nM, antagonizes growth inhibition by HPteGlu, raising the IC50 from 6 to 64 microM, but enhances inhibition by (6R,S)-5-methyl-H4HPteGlu, lowering the IC50 from 8 to 5 microM. Our results support the view that homofolates become toxic after conversion to H4HPteGlu polyglutamates which block GARFT, a step in purine biosynthesis.

31P NMR studies on the interaction of deoxyuridylate with thymidylate synthase.

The 31P nuclear magnetic resonance signal of deoxyuridylate was studied in the presence and absence of thymidlate synthase. In the absence of enzyme the chemical shift of deoxyuridylate is pH dependent with a pKa of 6.25. In the presence of enzyme, a peak corresponding to the dianioinc form of deoxyuridylate is observed which is independent of pH between pH 5.7 and pH 7.4. The pKa of the phosphate in the deoxyuridylate-thymidylate synthase complex is therefore less than 5. The release of inorganic phosphate from deoxyuridylate catalyzed by contaminating phosphatase was also observed.

Study of thymidylate synthetase-function by laser Raman spectroscopy.

The Laser-Raman spectra of thymidylate synthetase have been obtained with 488 nm excitation from an argon ion laser. Raman bands observed in the range 600-800 cm-minus-1 have been assigned to functional groups of constituent amino acids. The band positions and intensities in the Amide I (1600-1700 cm-minus-1) and Amide III (1200-1300 cm-minus-1) regions, suggest that the enzyme is a mixture of alpha-helical and unordered conformations. Low levels of beta-structure cannot be excluded. The spectra of the ternary complex formed by reacting thymidylate synthetase with (+)-L-methylenetetrahydrofolate and fluorodeoxyuridylate reveals a new band at 1618 cm-minus-1 assigned to the C=N stretching vibration. This band may be due to formation of dihydrofolate or an iminium ion. The overall secondary structure of thymidylate synthetase does not change on formation of the ternary complex. However, the spectrum of the complex indicates local changes in groups such as ionized carboxyl (1400 cm-minus-1), tryptophan (1003 cm-minus-1) and CH-3, CH-2 deformation modes (1440-1470 cm-minus-1).

Synergistic interactions among antifolates.

Many cultured human cell lines show large synergistic cytotoxicity when an inhibitor of dihydrofolate reductase (EC 1.5.1.3) is combined with an antifolate inhibitor of thymidylate synthase (EC 2. 1.1.45) or with an antifolate inhibitor of glycinamide ribonucleotide formyltransferase (EC 2.1.2.2). These synergistic interactions are dependent on medium folic acid concentration and are greatly enhanced by increasing folic acid levels. Synergism is seen only when the thymidylate synthase or glycinamide ribonucleotide formyltransferase inhibitor is polyglutamylatable. Here we will briefly outline the rigorous method used to quantitate synergistic interactions by measuring alpha, a response surface-based parameter; give examples of synergistic interactions from the current literature; and evaluate proposals offered to explain the metabolic basis of the synergism.

Deaza analogs of folic acid as antitumor agents.

Derivatives of the vitamin folic acid function in the body for the synthesis of thymidylate, purines and amino acids and are necessary for normal metabolism and growth. Methotrexate (MTX), an inhibitor of dihydrofolate reductase (DHFR) is the outstanding example of an antitumor antifolate. MTX is clinically useful in the treatment of childhood leukemia, choriocarcinoma and psoriasis, where it corrects abnormal growth, and in rheumatoid arthritis and other autoimmune diseases where it corrects abnormal immune function. Since 1949, when the chemical synthesis of MTX was reported by workers at the Lederle Laboratories of the American Cyanamid Company, much has been learned about the basis of antifolate cytotoxicity and selectivity. This review will focus on deaza antifolates which are: 1). presently under clinical development and 2). less developed compounds which represent novel approaches. Compounds will be grouped according to their enzyme targets; DHFR, thymidylate synthase (TS) and glycinamide ribonucleotide formyltransferase (GARFT). In addition to inhibition of target enzymes, antifolate membrane transport into cells and conversion to poly-L-gamma-glutamate forms are important considerations in drug design along with the reverse processes, cellular hydrolysis of antifolate poly-L-gamma-glutamates to monoglutamates and the extrusion of the monoglutamates through the cell membrane. These processes can be modulated by competition with folates.

Folate analogues. 20. Synthesis and antifolate activity of 1',2',3',4',5',6'-hexahydrohomofolic acid.

The synthesis of 1',2',3',4',5',6'-hexahydrohomofolic acid (3), a close analogue of homofolic acid (2), has been carried out by replacement of the benzene ring of 2 with a cyclohexane ring. The synthetic methods employed here were based on the Boon-Leigh strategy to obtain products with unambiguous structures. Based on a number of chemical and spectral observations, a tentative cis stereochemistry was assigned to the 1,4-substituents of the cyclohexane ring of both the homopteroate analogue 13 and the target compound 3. We investigated hexahydrohomopteroic acid (13), hexahydrohomofolic acid (3), and their 7,8-dihydro and d,l-5,6,7,8-tetrahydro derivatives for antifolate activity employing several biological test systems. The dihydro and tetrahydro derivatives of both 13 and 3 were active against Streptococcus faecium, whereas they were inactive against Lactobacillus casei. These compounds were neither substrates nor inhibitors of L. casei dihydrofolate reductase or thymidylate synthase.

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.

Synthesis and biological activity of 10-thia-10-deaza analogs of folic acid, pteroic acid, and related compounds.

The 10-thia analogs of pteroic acid, folic acid, their esters, and their 4-amino analogs were synthesized through a reaction sequence involving, as a key step, the condensation of 2-amino-3-cyano-5-chloromethylpyrazine with appropriately substituted thiols. The abilities of the products to inhibit the growth of methotrexate (MTX)-sensitive and MTX-resistant microorganisms were investigated as were their abilities to inhibit dihydrofolic acid reductase and thymidylic acid synthetase. Several compounds had high activity.

Effect of bridge region variation on antifolate and antitumor activity of classical 5-substituted 2,4-diaminofuro[2,3-d]pyrimidines.

Variation of the bridge linking the heterocyclic ring and p-aminobenzoyl-L-glutamate portions of our previously described classical 2,4-diaminofuro[2,3-d]pyrimidines 1 and 2 are reported as inhibitors of dihydrofolate reductase (DHFR) and thymidylate synthase (TS) and as antitumor agents. Specifically -CH2CH2- and -CH2NHCH2- bridged analogues, N-[4-[2-(2,4-diaminofuro[2,3-d]pyrimidin-5-yl) ethyl]benzoyl]-L-glutamic acid (3) and N-[4-[[N-[(2,4-diaminofuro[2,3-d]pyrimidin-5-yl) methyl]amino]methyl]benzoyl]-L-glutamic acid (4), respectively, were synthesized. Compound 3 was obtained via a Wittig reaction of the tributylphosphonium salt of 2,4-diamino-5-(chloromethyl)furo[2,3-d]pyrimidine (5) and methyl 4-formylbenzoate (6) followed by reduction and coupling with the diethyl ester of L-glutamic acid. Compound 4 was synthesized by the nucleophilic displacement of 5 with diethyl N-[4-(aminomethyl)benzoyl]-L-glutamate (15) and saponification. Both analogues were evaluated in vitro as inhibitors of DHFRs from (recombinant) human, human CCRF-CEM cells, and Lactobacillus casei. Compound 3 showed moderate activity (IC50 10(-6)-10(-7) M). Compound 4 was essentially inactive (IC50 10(-5) M, CCRF-CEM). The compounds were also evaluated against TS from (recombinant) human and L. casei and were of low activity (IC50 10(-5) M). The three-atom-bridged analogue 4 was somewhat more inhibitory to human TS than methotrexate (MTX). Compound 3 inhibited the growth of tumor cells in culture (IC50 10(-7) M) while 4 showed a low level of growth inhibitory activity. The inhibition of the growth of leukemia CCRF-CEM cells by both compounds parallels their inhibition of CCRF-CEM DHFR. Analogue 3 was a good substrate for human folylpolyglutamate synthetase (FPGS) derived from CCRF-CEM cells (Km 8.5 microM). Further evaluation of the growth inhibitory activity of 3 against the MTX-resistant subline of CCRF-CEM cells (R30dm) with decreased FPGS indicated that poly-gamma-glutamylation was important for its action. Protection studies with 3 in the FaDu squamous cell carcinoma cell line indicated that inhibition was completely reversed by leucovorin [(6R,S-5-formyltetrahydrofolate] or by a combination of thymidine and hypoxanthine, suggesting an antifolate effect directed at DHFR.

5-Arylthio-substituted 2-amino-4-oxo-6-methylpyrrolo[2,3-d]pyrimidine antifolates as thymidylate synthase inhibitors and antitumor agents.

Classical antifolate inhibitors of thymidylate synthase (TS) often require the reduced folate uptake system in order to exert their antitumor effects. In addition, these analogues are polyglutamylated via the enzyme folylpoly-gamma-glutamate synthetase (FPGS), which prevents analogue efflux from the cell and usually increases their inhibitory potency against TS. Impaired function of the reduced folate uptake system and that of FPGS are potential sources of resistance to such antifolates. We designed and synthesized a classical 6-5 ring-fused analogue N-[4-[(2-amino-6-methyl-3,4-dihydro-4-oxo-7H-pyrrolo[2,3- d]pyrimidin-5-yl)thio]-benzoyl]-L-glutamic acid (5) and a nonclassical 6-5 ring-fused analogue 2-amino-6-methyl-5-(pyridin-4-ylthio)-3,4-dihydro-4-oxo-7H-pyrrolo [2,3- d]pyrimidine (6) as TS inhibitors and antitumor agents. The syntheses of analogues 5 and 6 were achieved via the oxidative addition of the sodium salt of ethyl 4-mercaptobenzoate or 4-mercaptopyridine to 2-(pivaloylamino)-6-methyl-3,4-dihydro-4-oxo-7H-pyrrolo[2,3-d]pyri midine (17) in the presence of iodine. For the synthesis of 5 the ester obtained from the reaction was deprotected and coupled with diethyl L-glutamate followed by saponification. Compound 5 was a potent inhibitor of human and bacterial TS with IC50 values of 42 and 21 nM, respectively. Compound 6 was 10-fold less potent than 5 against human TS but more than 4700-fold less potent than 5 against Lactobacillus casei TS. The classical analogue 5 was neither a substrate nor an inhibitor of human FPGS derived from CCRF-CEM cells. Compound 5 was cytotoxic to CCRF-CEM and FaDu tumor cell lines as well as to an FPGS-deficient subline of CCRF-CEM. Thymidine protection studies established that TS was the primary target of 5.

Synthesis of aza homologues of folic acid.

Folic acid analogues containing an additional nitrogen atom between the phenyl ring and the carbonyl group of the side chain were synthesized. None of the compounds showed significant inhibitory activity against human lymphoblastic leukemia cells (CCRF-CEM) in culture or against Lactobacillus casei (ATCC 7469) growth. Against L1210 leukemia in mice, the aza homologue of folic acid, 4, and the aspartic acid analogue, 14, showed no increase in life span over control animals. These compounds were more toxic in vivo than the corresponding methotrexate analogues. Compound 4 supported the growth of Streptococcus faecium (ATCC 8043), and its tetrahydro derivative supported the growth of Pediococcus cerevisiae (ATCC 8081). These results strongly suggest that 4 can substitute for folate derivatives as cofactors for serine transhydroxymethylase, thymidylate synthetase, and dihydrofolate reductase.

Phosphonate analogue of 2'-deoxy-5-fluorouridylic acid.

The phosphonate analogue (6) of 2'-deoxy-5-fluorouridylic acid has been prepared via a Pfitzner--Moffatt oxidation and Witting reaction. This compound was found to inhibit thymidylate synthetase from three sources and to be cytotoxic to H.Ep.-2 cells in culture.

6-substituted 2,4-diamino-5-methylpyrido[2,3-d]pyrimidines as inhibitors of dihydrofolate reductases from Pneumocystis carinii and Toxoplasma gondii and as antitumor agents.

The synthesis and biological activity of 15 6-substituted 2,4-diamino-5-methylpyrido[2,3-d]-pyrimidines are reported. These compounds were synthesized in improved yields by modifications of procedures previously reported by us. Specifically, dimethoxyphenyl-substituted compounds with H and CH3 at the N-10 position and trimethoxyphenyl-substituted compounds with N-10 ethyl, isopropyl, and propargyl moieties were synthesized. These compounds were evaluated as inhibitors of dihydrofolate reductases (DHFR) from Pneumocystis carinii, Toxoplasma gondii, and rat liver, and selected analogues were evaluated as inhibitors of the growth of T. gondii and tumor cells in culture. All the compounds showed increased selectivity (vs rat liver DHFR) for T. gondii DHFR compared to trimetrexate. In general, for the trimethoxy-substituted analogues, increasing the size of the N-10 substituent from a methyl group to larger groups resulted in a decrease in selectivity and potency for both P. carinii and T. gondii DHFR. For the dimethoxy-substituted analogues, N-10 methylation in general decreased potency but increased selectivity for T. gondii DHFR. In an attempt to improve the cell penetration of these analogues, the N-10 naphthyl-substituted analogues were also synthesized. These analogues displayed excellent cell penetration and inhibition of T. gondii cells in culture. Further, these analogues were potent inhibitors of the growth of tumor cells in the preclinical in-vitro screening program of the National Cancer Institute with IC50s in the nanomolar range.

2,4-diamino-5-deaza-6-substituted pyrido[2,3-d]pyrimidine antifolates as potent and selective nonclassical inhibitors of dihydrofolate reductases.

Fifteen novel nonclassical and two classical 2,4-diamino-6-(benzylamino)pyrido[2,3-d]pyrimidine antifolates were synthesized as potential inhibitors of Pneumocystis carinii, (pc) Toxoplasma gondii, (tg) rat liver (rl), and human (h) recombinant dihydrofolate reductases (DHFR). These analogues lack a 5-methyl substitution which has been shown to be important for increased hDHFR inhibitory activity. In addition, they contain a reversal of the C9-N10 bridge present in folates and most antifolates. The synthesis of the compounds involved the reaction of 2,4,6-triaminopyrimidine with the sodium salt of nitromalonaldehyde to afford the key intermediate 2,4-diamino-6-nitropyrido[2,3-d]pyrimidine (7), in a single step. Reduction of 7 to the 2,4,6-triaminopyrido[2,3-d]pyrimidine (8), followed by reductive amination with the appropriate benzaldehydes or phenylacetaldehydes afforded the target compounds. N9 methylation of these analogues was carried out using formaldehyde and sodium cyanoborohydride. The analogues demonstrated significant inhibition of pcDHFR and tgDHFR. N9 methylation significantly increased DHFR inhibitory potency. Compound 11, the 3'4'5'-trimethoxy-substituted analogue with a selectivity ratio of 9.4 for tgDHFR (compared to rlDHFR) was the most selective analogue of the nonclassical series. Compound 22, the N9 methyl 2'5'-dimethoxy-substituted analogue was the most potent analogue against tgDHFR (IC 50 = 6.3 nM) and was the second most selective analogue for tgDHFR (compared to rlDHFR) in the nonclassical series. The naphthyl-substituted analogues 23-25 were generally more potent against rlDHFR than against pcDHFR and tgDHFR. Selected analogues were also evaluated against Streptococcus faecium (sf) DHFR, Escherichia coli (ec) DHFR, Lactobacillus casei (lc) DHFR and tgDHFR with hDHFR as the mammalian reference, under slightly different assay conditions than those employed for rlDHFR. Analogues 11 and 22 had selectivity ratios of greater than 100 for tgDHFR (compared to hDHFR). Analogue 22 in particular, was the most selective analogue of the nonclassical series against tgDHFR (selectivity ratio = 303.5) with excellent potency (28 nM). Analogue 11, also displayed significant selectivity for sfDHFR (selectivity ratio = 4902). Compound 22 was evaluated in vivo for the inhibition of the growth of T.gondii trophozoites in mice, where at 50 mg/kg orally, it demonstrated distinct prolongation of survival without toxicity. Compounds 11, 12 and 21-23 were evaluated as antitumor agents in the National Cancer Institutes preclinical in vitro screening program. Compounds 12, 22, and 23 showed GI50s for tumor growth inhibition in the 10 -6 - 10 -7 M range.

Effect of C9-methyl substitution and C8-C9 conformational restriction on antifolate and antitumor activity of classical 5-substituted 2,4-diaminofuro[2,3-d]pyrimidines.

N-[4-[1-methyl-2-(2,4-diaminofuro[2, 3-d]pyrimidin-5-yl)ethyl]benzoyl]-L-glutamic acid (5) and its C8-C9 conformationally restricted E- and Z-isomers (6 and 7) were designed and synthesized in order to investigate the effect of incorporating a methyl group at the C9 position and of conformational restriction at the C8-C9 bridge of N-[4-[2-(2,4-diaminofuro[2, 3-d]pyrimidin-5-yl)ethyl]benzoyl]-L-glutamic acid (1) with respect to dihydrofolate reductase (DHFR) inhibitory activity as well as antitumor activity. The compounds were synthesized by a Wittig reaction of 2,4-diamino-5-(chloromethyl)furo[2,3-d]pyrimidine with ethyl 4-acetylbenzoate followed by catalytic reduction, hydrolysis, and standard peptide coupling with diethyl L-glutamate. The biological results indicated that the addition of a 9-methyl group to the C8-C9 bridge, as in 5, increased recombinant human (rh) DHFR inhibitory potency (IC(50) = 0.42 microM) as well as the potency against the growth inhibition of tumor cells in culture (CCRF-CEM EC(50) = 29 nM, A253 EC(50) = 28.5 nM, and FaDu EC(50) = 17.5 nM) compared with the 9-desmethyl analogue 1. However, the conformationally restricted 4:1 Z/E mixture of 7 and 6 was less potent than 5 in both assays, and the pure E-isomer 6 was essentially inactive. These three classical analogues were also evaluated as inhibitors of Lactobacillus casei, Escherichia coli, and rat and rh thymidylate synthase (TS) and were found to be weak inhibitors. All three analogues 5-7 were good substrates for human folylpolyglutamate synthetase (FPGS). These data suggested that FPGS is relatively tolerant to different conformations in the bridge region. Further evaluation of the cytotoxicity of 5 and 7 in methotrexate (MTX)-resistant CCRF-CEM cell sublines suggested that polyglutamylation was crucial for their mechanism of action. Metabolite protection studies of 5 implicated DHFR as the primary intracellular target. Compound 5 showed GI(50) values in 10(-9)-10(-7) M range against more than 30 tumor cell lines in culture.

Synthesis of classical and a nonclassical 2-amino-4-oxo-6-methyl-5-substituted pyrrolo[2,3-d]pyrimidine antifolate inhibitors of thymidylate synthase.

Compounds 2-5 were designed as potential antifolate nonpolyglutamatable inhibitors of thymidylate synthase (TS). These analogues are structurally related to 2-amino-4-oxo-5-substituted quinazolines and 2-amino-4-oxo-5-substituted pyrrolo[2, 3-d]pyrimidines which have shown excellent inhibition of TS and, for the quinazoline, significant promise as clinically useful antitumor agents. Compounds 2-4 were synthesized by appropriate amine exchange reactions on pivaloyl-protected 5-dimethylaminomethyl-substituted 6-methyl pyrrolo[2,3-d]pyrimidine 7 which in turn was obtained from the Mannich reaction of pivaloylated-6-methyl pyrrolo[2, 3-d]pyrimidine 6. In instances where the amine exchange reaction was sluggish, the Mannich base was quaternized with methyl iodide which afforded much faster exchange reaction with improved yields. For compound 5, 4-mercaptopyridine was used as the nucleophile and reacted with 7. The analogues 2-4 inhibited Lactobacillus casei (lc) TS and recombinant human (h) TS with IC50 in the 10(-4) to 10(-5) M range. Compound 5 inhibited lcTS and hTS 20% at 26 and 25 microM, respectively. In addition, compound 5 inhibited the growth of Pneumocystis carinii and Toxoplasma gondii cells in culture by 76% at 32 x 10(-6) M and 50% at 831 x 10(-6) M, respectively.

Folate analogues. 25. Synthesis and biological evaluation of N10-propargylfolic acid and its reduced derivatives.

N10-Propargylfolic acid (2), which is the closest pteridine analogue of the thymidylate synthase inhibitor N10-propargyl-5,8-dideazafolic acid (PDDF), was synthesized starting from diethyl [p-(N-propargylamino)benzoyl]-L-glutamate (5) and N-(3-bromo-2-oxopropyl)phthalimide (8). The 7,8-dihydro derivative of propargylfolic acid served as a synthetic substrate of Lactobacillus casei dihydrofolate reductase. Propargylfolic acid and its reduced derivatives were weak inhibitors of L. casei thymidylate synthase compared to PDDF. All derivatives of propargylfolate were active against the growth of Streptococcus faecium, but with the exception of 7,8-dihydropropargylfolic acid, all were inactive against L. casei. Although less potent than PDDF, marked inhibition of thymidylate synthase by 2 was observed in permeabilized L1210 cells.