Folic Acid Analogues

Dihydrofolate reductase acts as an auxiliary enzyme for thymidylate synthase. It is involved in the regeneration of the coenzyme N, N -methylene-THF, initially reducing DHF to THF with NADPH as the reductant (see p. 418). The folic acid analogue methotrexate, a frequently used cytostatic agent, is an extremely effective competitive inhibitor of dihydrofolate reductase. It leads to the depletion of N, N -methylene-THF in the cells and thus to cessation of DNA synthesis. 

Methotrexate is a folic acid analogue. Its mechanism of action is based on the inhibition of dihydrofolate reductase. Inhibition of dihydrofolate reductase leads to depletion of the tetrahydrofolate cofactors that are required for the synthesis of purines and thymidylate (see Fig. 2). Enzymes that are required for purine and thymidylate synthesis are also directly inhibited by the polyglutamates of methotrexate which accumulate with dihydrofolate reductase inhibition. The mechanisms that can cause resistance include decreased transport of methotrexate into the tumor cells, a decreased affinity of the antifolate for dihydrofolate reductase, increased concentrations of intracellular dihydrofolate reductase and decreased thymidylate synthetase activity. 

Raltitrexed is a folic acid analogue which inhibits thymidylate synthetase. Intracellularly formed raltitrexed polyglutamates are even stronger inhibitors of thymidylate synthetase than the parent compound. Similar to methotrexate polyglutamates... 

Aldehyde 82 was extremely reactive and was best isolated as the hydrate 84a. Indeed, recrystallization of the aldehyde 82 from ethanol gave 3-(l-ethoxy-l-hydroxymethyl)fervenulin 84b, while reaction with ethylene glycol gave the cyclic acetal 76a. The reactivity of the aldehyde 82 was exploited by easy Schiff base formation upon reaction with /i-aminobenzoylglutamic acid, a process that was followed by reduction to give the fervenulin-based folic acid analogue 85 <1996JHC949>. 

Dihydrofolate reductase is competitively inhibited by methotrexate, a folic acid analogue used to effect the remission of acute leukemia in children. 

II.1 Folic Acid Derivatives 7.18.11.2 Folic Acid Analogues... 

Pteridines carrying a polyhydroxyalkyl sidechain are valuable synthons for the synthesis of functionalized derivatives. Various 6-(l,2,3-trihydroxypropyl)pteridines (216, 217, 232) have been oxidized by periodate to the corresponding pteridine-6-carbaldehydes (218, 225, 233) which are further substrates for the interconversion into oximes (219, 220, 225, 226, 234, 235), carbonitrile (221, 228, 236), hydroxymethyl (222, 229, 237) and carboxy derivatives (223, 230, 238) as well as folic acid analogues (224, 231, 239) (Scheme 34) <90Ml 718-08). 

During the 1980s and 1990s the role of folic acid analogues, especially methotrexate (MTX) (422), in cancer chemotherapy has been intensively studied enzyme dihydrofolate reductase (DHFR) has been the primary target of this effort. The introduction of 10-ethyl-10-deazaaminopterin (10-EDAM) (423), piritrexim (PTX) (424) and trimetrexate (TMX) (425) into clinical trials attests to the continued interest in this field <87MI 718-06). 

Pyrazino[2,3-c][l,2,6]thiadiazines are 2-thiapteridines, and their 2,2-dioxide derivatives (276) have been used in the preparation of 7-folic acid analogues (279). The latter had no biological activity, however. In the synthesis, the 7-methyl group of derivatives (276) is first brominated with pyridinium... 

Reaction of 5-bromo-6-chloroisocytosine 115 (Scheme 41) with 2-amino-ethanethiol derivatives has been extensively studied by Maki and coworkers [62,63] aimed at the synthesis of folic acid analogues. Treatment of 115 with amino-thiol 116 (R = R = H) in ethanol at reflux temperature in the presence of phosphate buffer at pH 7 provided bicyclic product 121 (R = R = H) in 50% yield. An analogous reaction of aminothiols 116 (R = CH2OH, R = H) and 116 (R = H, R = CH2OH) provided products in 37 and 67% yield, respectively. The mechanism of the reaction is indicated in the scheme. 

One might ask why the enzyme does not join the sulfonamide to the other two components of folic acid to give a folic acid analogue containing the sulfonamide skeleton. This can in fact occur, but it does the cell no good at all since the analogue is not accepted by the next enzyme in the biosynthetic pathway. 

Using radioactive bicarbonate (l4C-NaHCC>3), we found it to be incorporated by infected red cells and free parasites into pyrimidines suggesting that P. lophurae synthesized cytosine, uracil and thymine de novo (Walsh and Sherman, 1968b). Further, evidence was found for the pyrimidine-pathway enzymes orotidine-5 -monophosphate pyropho-sphorylase (OMPDC also named orotidine-5 -decarboxylase) and thymi-dylate synthase (TS) in parasites but not in the red cells. The presence of TS made it possible for us to interpret the action of folic acid analogues in the same way as other microbes inhibition of DHFR would lead to a... 

A different type of modification in Region E was achieved [223, 224] in the form of the MTX analogue (VI.8) and the folic acid analogue (VI. 10), in which NH is inserted between the phenyl and CONH moiety. For the synthesis of (VI.8), 4-amino-4-deoxy-V °-methylpteroic acid was treated with diphenyl-phosphoryl azide in DMSO, the resultant azide ((VI. 12), 87% yield) was photolyzed in DMF solution in the presence of dimethyl L-glutamate to obtain dimethyl ester (VI.9) (51% yield), and (VI.9) was hydrolyzed with 0.1 M... 

Compound (VIII.98), a folic acid analogue with a y-fluoro substituent in the side-chain was described first by Alekseeva e/ al. [281] and several years later by Bergmann and Chun-Hsu [282]. The synthesis of (VIII.98), as mixture of threo and erythro isomers, was achieved via the Waller method from V-(4-ami-nobenzoyl)-y-fluoroglutamic acid, 2,4,5-triamino-6(lif)-pyrimidinone, and 2,3-dibromopropionaldehyde, but the yield was low (5.6%). y-Fluoroglutamic acid, as a mixture of D- and L-enantiomers, was prepared from diethyl 2-fluoromalonate by condensation with ethyl 2-acetamidoacrylate followed by hydrolysis and decarboxylation in refluxing 12 M HCl, or from ethyl 3-chloro-2-hydroxypropanoate by a sequence consisting of (i) 0-t-butylation with CH2 = C(CH3)2, (ii) condensation with diethyl 2-acetamidomalonate, (hi)... 

The folic acid analogues, methotrexate (amethopterin) and aminop-terin, prevent the utilization of folic acid and of one-carbon units, and produce in animals symptoms of deficiency of this vitamin. 

It was found in early studies that the folic acid analogues, methotrexate (amethopterin) and aminopterin very effectively blocked the incorporation of labeled deoxyuridine and of labeled formate into DNA thymine however, the incorporation of thymidine was not blocked. It was apparent, therefore, that the analogues interfered with the introduction of the methyl group into thymine, a process known to involve H -folate. When it became established that the antifolic agents were exceedingly potent inhibitors of tetrahydrofolate dehydrogenase (see Chapter 5), the mechanism of their inhibition of DNA synthesis was apparent. 

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