Like other sulfonamides, sulfadimethoxine is a
dihydropteroate synthase inhibitor. Bacteria and some protozoa are unable to obtain
folic acid from the environment, and must instead synthesize it by converting PABA (
para-aminobenzoate) to
dihydropteroate using the enzyme
dihydropteroate synthase. Sulfonamides act as a
competitive inhibitor; being structurally similar to PABA, they are able to bind to the enzyme's active site and prevent the synthesis of folic acid from progressing. Folic acid is necessary for these organisms to produce nucleic acids (i.e.
DNA and
RNA), which are required for cell division. Thus, it has a
microbiostatic effect rather than a
microbiocidal one (it prevents pathogen growth rather than killing them), and has the strongest effect in the beginning stages of an infection, when the pathogen is rapidly dividing. Since it is microbiostatic, sulfadimethoxine still requires the animal to still be able to mount an
immune response to kill the pathogen.
With ormetoprim Sulfadimethoxine can either be given alone (such as under the commercial name Albon) or in combination with ormetoprim to as a "potentiated sulfonamide" to increase antimicrobial activity. Ormetoprim is a
diaminopyridine, inhibiting
dihydrofolate reductase, which is further along the pathway for synthesizing folic acid. Though the optimum ratio of sulfadimethoxine to ormetoprim has been found to be 20:1, it is sold pharmaceutically as a 5:1 mixture. ==Pharmacokinetics==