Dihydrofolate reductase inhibition sulphonamides

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... 

Trimethoprim (TMP) and ormethoprim (OMP) are synergists of SAs that operate by a mechanism of competitive inhibition of dihydrofolate reductase. Sulphonamides (and their synergists) are widely used in farm animal feedstuff and fish cultures furthermore, they act as growth promoters at subtherapeutic concentrations. 

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). 

Considering the drugs in relation to modes of action, dapsone and the sulphonamides block the biosynthesis of tetrahydrofolate by inhibiting di-hydropteroate synthetase, while the 2,4-diamino-pyrimidines (proguanil and pyrimethamine) block the same pathway but at a later step catalysed by dihydrofolate reductase. 

Inhibition of folic acid synthesis in susceptible microorganisms and ultimately the synthesis of nucleic acids. By competing with para-aminobenzoic acid (PABA) for the enzyme dihydropteroate synthetase, sulphonamides prevent the incorporation of PABA into dihydrofolate, while trimethoprin, by selectively inhibiting dihydrofolate reductase, prevents the reduction of dihydrofolate to tetrahydrofolate (folic acid). Animal cells, unlike bacteria, utilize exogenous sources of folic acid. Pyrimethamine inhibits protozoal dihydrofolate reductase, but is less selective for the microbial enzyme and therefore more toxic than trimethoprim to mammalian species. 

A most interesting and useful development concerning DHR inhibitors was the selectivity of inhibition observed between different classes of compounds against dihydrofolate reductases from mammals, protozoa and bacteria, which was found to be due to marked differences in binding affinity to the enzyme methotrexate binds very tightly to all reductases tested and is lethal to any cell it can enter, while trimethoprim and pyrimethamine have selectively strong affinity for bacterial and plasmodial reductases, respectively. This helped to rationalise the clinical use of DHFR inhibitors alone or in combination with sulphonamides and sulphones while trimethoprim is used mainly for bacterial infections, pyrimethamine is used for protozoal infections [58a]. 

Trimethoprim inhibits another enzyme, dihydrofolate reductase, in the same folic acid metabolic pathway. Folic acid is converted into folate, which then has to be converted into an activated form by dihydrofolate reductase. In this way, trimethoprim interferes with the conversion of folate into its activated form, which is a cofactor in the synthesis of bacterial DNA. Dihydrofolate reductase also occurs in host cells, but it is less sensitive to trimethoprim. Trimethoprim is used to treat urinary tract infections. It is also formulated in combination with a sulphonamide, when it is known as co-trimoxazole, to treat pneumonia in patients with HIV (see page 170). Due to the synergistic effect of the two drugs, this combination is more effective than either drug alone. 

The importance of the sulphonamides has decreased as a result of increased bacterial resistance and they have been replaced by compounds with higher therapeutic indices and activity. The bios)mthetic pathway for tetrahy-drofolate provides an additional point for selective attack on bacteria. The conversion of dihydrofolic add to tetra-hydrofolate (coenzyme F) requires dihydrofolate reductase. Trimetoprim (Fig. 22.43), a sulfone, is stmcturally similar to dihydrofolate (see Fig. 22.35) and therefore competitively inhibits the formation of tetrahydrofolate, ultimately affecting the biosynthesis of proteins and nudeic adds. 

Due to a mutation in the human genome, trimetoprim inhibits bacterial dihydrofolate reductase but not mammalian dihydrofolate reductase. Trimetoprim has therefore been used in combination with the sulphonamides to provide synergism in activity while reducing the toxidty potential of the individual compounds. A usefiil dinical example is co-trimoxazole, which is a compound formulation of trimetoprim and sulfamethoxazole. 

The enzyme dihydrofolic acid (DHF) S5mthase (see below) converts p-aminobenzoic acid (PABA) to DHF which is subsequently converted to tetrahydric folic acid (THF), purines and DNA. The sulphonamides are structurally similar to PABA, successfully compete with it for DHF s)mthase and thus ultimately impair DNA formation. Most bacteria do not use preformed folate, but humans derive DHF from dietary folate which protects their cells from the metabolic effect of sulphonamides. Trimethoprim acts at the subsequent step by inhibiting DHF reductase, which converts DHF to THF. The drug is relatively safe because bacterial DHF reductase is much more sensitive to trimethoprim than is the human form of the enzyme. Both sulphonamides and trimethoprim are bacteriostatic. 

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