Folate synthesis

Folate metabolism Sulphonamides (also ) Trimethoprim Pyrimethamine Trimetrexate / Inhibit folate synthesis Inhibits dihydrofolate reductase Inhibits dihydrofolate reductase Inhibits dihydrofolate reductase Not present in mammalian cells Mammalian enzyme not inhibited Mammalian enzyme not inhibited Toxicity overcome with leucovorin... 

Mouillon, J.M. et al., Folate synthesis in higher-plant mithocondria coupling between the dihydropterin pyrophosphokinase and the dihydropteroate synthase activities, Biochem. J., 363, 313, 2002. 

Diaz de la Garza R, Quinlivan EP, Klaus SM, Basset GJ, Gregory JFR and Hanson AD. 2004. Folate biofortification in tomatoes by engineering the pteridine branch of folate synthesis. Proc Natl Acad Sci USA 101 13720-13725. 

Para-aminosalicylic Acid (PAS), like the sulfonamides (see Chapter 44), is a structural analogue of p-aminobenzoic acid (PABA). It is a folate synthesis antagonist that interferes with the incorporation of PABA into folic acid. PAS is bacteriostatic, and in vitro, most strains of M. tuberculosis are sensitive to a concentra-... 

The other major class of antimalarials are the folate synthesis antagonists. There is a considerable difference in the drug sensitivity and affinity of dihydrofolate reductase enzyme (DHFR) between humans and the Plasmodium parasite. The parasite can therefore be eliminated successfully without excessive toxic effects to the human host. DHFR inhibitors block the reaction that transforms deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP) at the end of the pyrimidine-synthetic pathway. This reaction, a methylation, requires N °-methylene-tetrahydrofolate as a carbon carrier, which is oxidized to dihydrofolate. If the dihydrofolate cannot then be reduced back to tetrahydrofolate (THF), this essential step in DNA synthesis will come to a standstill. 

PABA) incorporation into folic acid (inhibition of folate synthesis). In large proportion of Mycobacterium leprae infections e.g. in lepromatous leprosy, resistance can develop, so combination of dapsone, rifampicin and clofazimine is used in initial therapy. 

Combination of a sulfonamide with an inhibitor of dihydrofolate reductase (trimethoprim or pyrimethamine) provides synergistic activity because of sequential inhibition of folate synthesis (Figure 46-2). 

Mammalian cells (and some bacteria) lack the enzymes required for folate synthesis from PABA and depend on exogenous sources of folate therefore, they are not susceptible to sulfonamides. Sulfonamide resistance may occur as a result of mutations that (1) cause overproduction of PABA, (2) cause production of a folic acid-synthesizing enzyme that has low affinity for sulfonamides, or (3) impair permeability to the sulfonamide. Dihydropteroate synthase with low sulfonamide affinity is often encoded on a plasmid that is transmissible and can disseminate rapidly and widely. Sulfonamide-resistant dihydropteroate synthase mutants also can emerge under selective pressure. 

Trimethoprim, a trimethoxybenzylpyrimidine, selectively inhibits bacterial dihydrofolic acid reductase, which converts dihydrofolic acid to tetrahydrofolic acid, a step leading to the synthesis of purines and ultimately to DNA (Figure 46-2). Trimethoprim is about 50,000 times less efficient in inhibition of mammalian dihydrofolic acid reductase. Pyrimethamine, another benzylpyrimidine, selectively inhibits dihydrofolic acid reductase of protozoa compared with that of mammalian cells. As noted above, trimethoprim or pyrimethamine in combination with a sulfonamide blocks sequential steps in folate synthesis, resulting in marked enhancement (synergism) of the activity of both drugs. The combination often is bactericidal, compared with the bacteriostatic activity of a sulfonamide alone. 

Several drugs closely related to the sulfonamides have been used effectively in the long-term treatment of leprosy. The most widely used is dapsone (diaminodiphenylsulfone). Like the sulfonamides, it inhibits folate synthesis. Resistance can emerge in large populations of M leprae, eg, in lepromatous leprosy, if very low doses are given. Therefore, the combination of dapsone, rifampin, and clofazimine is recommended for initial therapy. Dapsone may also be used to prevent and treat Pneumocystis jiroveci pneumonia in AIDS patients. 

Aminosalicylic acid is a folate synthesis antagonist that is active almost exclusively againstM tuberculosis. It is structurally similar to /i-aminobenzoic aid (PABA) and to the sulfonamides (see Chapter 46 Sulfonamides, Trimethoprim, Quinolones). 

Bx Obsolete name for p-aminobenzoic acid (intermediate in folate synthesis) also used at one time for pantothenic acid C2 A postulated antipneumonia factor (also called vitamin J)... 

Folate analogs such as trimethoprim have potent antibacterial and antiprotozoal activity. Trimethoprim binds lO -fold less tightly to mammalian dihydrofolate reductase than it does to reductases of susceptible microorganisms. Small differences in the active-site clefts of these enzymes account for its highly selective antimicrobial action. The combination of trimethoprim and sulfamethoxazole (an inhibitor of folate synthesis) is widely used to treat infections. 

Proguardl (t) 17 h) inhibits dihydrofolate reductase which converts folic to folinic acid, deficiency of which inhibits plasmodial cell division. Plasmodia, like most bacteria and unlike humans, cannot make use of preformed foUc acid. Pyrimethamine and trimethoprim, which share this mode of action, are collectively known as the antifols. Their plasmod-icidal action is markedly enhanced by combination with sulphonamides or sulphones because there is inhibition of sequential steps in folate synthesis (see Sulphonamide combinations, p. 231). 

Trimethoprim is a 2,4-diamino-5-(3, 4, 5 -trimethoxy-benzyl) pyrimidine that inhibits dihydrofolate reductase, the enzyme in folate synthesis after the step that is blocked by sulfonamides (1). Trimethoprim therefore inhibits the conversion of dihydrofolate to tetrahydrofo-late. It has been combined with sulfonamides, including sulfamethoxazole, sulfametrol, sulfadiazine, sulfamoxole, and sulfadimidine (2,3). 

Inhibition of Folate Synthesis Competitive inhibition of a biosynthetic step in folate synthesis accounts for the antimicrobial action of sulfonamides, which are structural analogues of p-aminobenzoic acid (PABA) ... 

The folic acid biosynthesis has been extensively explored to design effective antimalarials. A number of classes of compounds have been developed, which interfere with different steps of the folate synthesis in plasmodia [14]. These antimalarials, which are collectively known as antifolates, may be broadly divided in three groups. 

With respect to mechanism of action, the most extensive kinetic and equilibrium exchange studies have been carried out on monofunctional 10-formyl-H4-folate synthetase from Cl. cylindrosporum [84]. The data support a random sequential mechanism that does not involve the formation of freely dissociable intermediates. The most likely mechanism, however, is not concerted but probably involves the formation of a formyl phosphate intermediate, since the synthetase catalyzes phosphate transfer from carbamyl phosphate but not acetyl phosphate to ADP with H 4-folate serving as an activator. Carbamyl phosphate is an inhibitor of 10-formyl-H 4-folate synthesis - an inhibition that can be eliminated only when both ATP and formate are present in accord with the concept that it spans both sites [85]. It would be of considerable interest to attempt to demonstrate positional isotope exchange employing [, y- 0]ATP for this enzyme in order to further implicate an enzyme-bound formyl phosphate species [86]. 

Another frequently employed combination is that of a sulfonamide and an inhibitor of dihydrofolate reductase, such as trimethoprim this combination is synergistic because the drugs block sequential steps in microbial folate synthesis. 

Trimethoprim plus sulfamethoxazole When the two drugs are used in combination, antimicrobial synergy results from the sequential blockade of folate synthesis (Figure 46-1). The drug combination is bactericidal against susceptible organisms. 

Co-trimoxazole te.g., Bactrim or Septra) Inhibits the folate synthesis pathway at two different sites (Fig. 7.14). This increases the likelihood that bacteria will produce too little folate to effectively reproduce. Additionally, the likelihood of resistance is substantially reduced. Indicated for urinary tract infections, acute otitis, shigellosis, and Pneumocystis carinii pneumonitis in all age groups. In adults, it is also indicated for exacerbations of chronic bronchitis, bacterial prostatitis and traveller s diarrhea. 

Pyrimethamine Inhibits folate synthesis by interfering with dihydrofolate reductase. Malaria prophylaxis. Erythrocytic form of P. falciparum. Used in combination with sulfonamides or sulfones for acute attacks. 

---