Microbiology: Trimethoprim blocks bacterial production of tetrahydrofolic acid from dihydrofolic acid by binding to and reversibly inhibiting the enzyme dihydrofolate reductase.
Sulfadiazine, in common with other sulfonamides, inhibits bacterial synthesis of dihydrofolic acid by competing with para-aminobenzoic acid.
Trimethoprim and sulfadiazine thus imposes a sequential double blockade on bacterial metabolism. This deprives bacteria of nucleic acids and proteins essential for survival and multiplication, and produces a high level of antibacterial activity which is usually bactericidal.
Although both sulfadiazine and trimethoprim are antifolate, neither affects the folate metabolism of animals. The reasons are: animals do not synthesize folic acid and cannot, therefore, be directly affected by sulfadiazine; and although animals must reduce their dietary folic acid to tetrahydrofolic acid, trimethoprim does not affect this reduction because its affinity for dihydrofolate reductase of mammals is significantly less than for the corresponding bacterial enzyme.
Trimethoprim and sulfadiazine is active against a wide spectrum of bacterial pathogens, both gram-negative and gram-positive. The following in vitro data are available, but their clinical significance is unknown. In general, species of the following genera are sensitive to trimethoprim and sulfadiazine:
| Very Sensitive | Sensitive | Moderately Sensitive | Not Sensitive |
| Escherichia | Staphylococcus | Moraxella | Mycobacterium |
| Streptococcus | Neisseria | Nocardia | Leptospira |
| Proteus | Klebsiella | Brucella | Pseudomonas |
| Salmonella | Fusiformis | | Erysipelothrix |
| Pasteurella | Corynebacterium | | |
| Shigella | Clostridium | | |
| Haemophilus | Bordetella | | |
As a result of the sequential double blockade of the metabolism of susceptible organisms by trimethoprim and sulfadiazine, the minimum inhibitory concentration (MIC) of trimethoprim and sulfadiazine is markedly less than that of either of the components used separately. Many strains of bacteria that are not susceptible to one of the components are susceptible to trimethoprim and sulfadiazine. A synergistic effect between trimethoprim and sulfadiazine in combination has been shown experimentally both in vitro and in vivo (in dogs).
Trimethoprim and sulfadiazine is bactericidal against susceptible strains and is often effective against sulfonamide-resistant organisms. In vitro sulfadiazine is usually only bacteriostatic.
The precise in vitro MIC of the combination varies with the ratio of the drugs present, but action of trimethoprim and sulfadiazine occurs over a wide range of ratios with an increase in the concentration of one of its components compensating for a decrease in the other. It is usual, however, to determine MICs using a constant ratio of 1 part trimethoprim in 20 parts of the combination.
The following table shows MICs, using the above ratio, of bacteria which were susceptible to both trimethoprim (TMP) and sulfadiazine (SDZ). The organisms are those most commonly involved in conditions for which trimethoprim and sulfadiazine is indicated:
AVERAGE MINIMUM INHIBITORY CONCENTRATION (MIC-mcg/mL)| Bacteria | TMP Alone | SDZ Alone | TMP/SDZ |
| TMP | SDZ |
Escherichia coli Proteus species Staphylococcus aureus Pasteurella species Salmonella species
ß Streptococcus
| 0.31 1.30 0.60 0.06 0.15 0.5 | 26.5 24.5 17.6 20.1 61.0 24.5 | 0.07 0.15 0.13 0.03 0.05 0.15 | 1.31 2.85 2.47 0.56 0.95 2.85 |
The following table demonstrates the marked effect of the trimethoprim and sulfadiazine combination against sulfadiazine-resistant strains of normally susceptible organisms:
AVERAGE MINIMUM INHIBITORY CONCENTRATION OF SULFADIAZINE-RESISTANT STRAINS (MIC-mcg/mL)| Bacteria | TMP Alone | SDZ Alone | TMP/SDZ |
| TMP | SDZ |
Escherichia coli Proteus species | 0.32 0.66 | >245 >245 | 0.27 0.32 | 5.0 6.2 |
Susceptibility Testing: In testing susceptibility to trimethoprim and sulfadiazine, it is essential that the medium used does not contain significant amounts of interfering substances which can bypass the metabolic blocking action, e.g., thymidine or thymine.
The standard SxT disc is appropriate for testing by the disc diffusion method.
Pharmacology: Following oral administration, trimethoprim and sulfadiazine is rapidly absorbed and widely distributed throughout body tissues. Concentrations of trimethoprim are usually higher in tissues than in blood. The levels of trimethoprim are high in lungs, kidney, and liver, as would be expected from its physical properties.
Serum trimethoprim concentrations in horses following oral administration indicate rapid absorption of the drug; peak concentrations occur in 1.5 hours. The mean serum elimination half-life is 2 to 2.5 hours. Sulfadiazine absorption is slower, requiring 2.5 to 6 hours to reach peak concentrations. The mean serum elimination half-life for sulfadiazine is about 4 to 5.5 hours.
Usually, the concentration of an antibacterial in the blood and the in vitro MIC of the infecting organism indicate an appropriate period between doses of a drug. This does not hold entirely for trimethoprim and sulfadiazine because trimethoprim, in contrast to sulfadiazine, localizes in tissues and therefore its concentration and ratio to sulfadiazine are higher there than in blood.
The following table shows the average concentration of trimethoprim and sulfadiazine, as measured in either serum or plasma, in twenty-four adult horses observed after a single dose of UNIPRIM POWDER FOR HORSES:
AVERAGE SERUM/PLASMA CONCENTRATION (mcg/mL)| Trimethoprim (5 mg/kg) | Sulfadiazine (25 mg/kg) |
1 hr 0.82 | 3 hr 0.69 | 6 hr 0.36 | 10 hr 0.12 | 24 hr <0.025 | 1 hr 9.9 | 3 hr 18.8 | 6 hr 17.3 | 10 hr 9.0 | 24 hr 1.6 |
Excretion of trimethoprim and sulfadiazine is chiefly by the kidneys, by both glomerular filtration and tubular secretion. Urine concentrations of both trimethoprim and sulfadiazine are several fold higher than blood concentrations. Neither trimethoprim nor sulfadiazine interferes with the excretion pattern of the other.