The antimicrobial activity of the trimethoprim/sulfamethoxazole combination results from actions on two steps of the biosynthetic pathway for tetrahydrofolic acid. Trimethoprim prevents the reduction of dihydrofolate to tetrahydrofolate (Figure 43-2). Mammalian cells use preformed folates from the diet and do not synthesize the compound. Trimethoprim is a highly selective inhibitor of dihydrofolate reductase of lower organisms; ~100,000 times more drug is required to inhibit human reductase than the bacterial enzyme. This relative selectivity is vital because the enzyme is essential to all species.
The synergistic interaction between sulfonamide and trimethoprim is predictable from their respective mechanisms. The most effective ratio of these two drugs for the greatest number of microorganisms is 20 parts sulfamethoxazole to 1 part trimethoprim. The combination thus is formulated to achieve a sulfamethoxazole concentration in vivo that is 20 times greater than that of trimethoprim.
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