Sulphadiazine 200 mg, Trimethoprim 40 mg Oral suspension

The systemic sulfonamides are bacteriostatic agents having a similar spectrum of activity. Sulfonamides competitively inhibit bacterial synthesis of folic acid (pteroylglutamic acid) from aminobenzoic acid. Resistant strains are capable of utilizing folic acid precursors or preformed folic acid.

Sulfonamides exist in the blood in 3 forms – free, conjugated (acetylated and possibly others) and protein bound. The free form is considered to be the therapeutically active one.

Sulfadiazine given orally is readily absorbed from the gastrointestinal tract. After a single 2 g oral dose, a peak of 6.04 mg/100 mL is reached in 4 hours; of this, 4.65 mg/100 mL is free drug.

When a dose of 100 mg/kg of body weight is given initially and followed by 50 mg/kg every 6 hours, blood levels of free sulfadiazine are about 7 mg/100mL. Protein binding is 38% to 48%. Sulfadiazine diffuses into the cerebrospinal fluid; free drug reaches 32% to 65% of blood levels and total drug 40% to 60%.

Sulfadiazine is excreted largely in the urine, where concentrations are 10 to 25 times greater than serum levels. Approximately 10% of a single oral dose is excreted in the first 6 hours, 50% within 24 hours and 60% to 85% in 48 to 72 hours. Of the amount excreted in the urine, 15% to 40% is in the acetyl form.

Trimethoprim is rapidly absorbed following oral administration. It exists in the blood as unbound, protein-bound, and metabolized forms. Ten to twenty percent of trimethoprim is metabolized, primarily in the liver; the remainder is excreted unchanged in the urine. The principal metabolites of trimethoprim are the 1- and 3-oxides and the 3'- and 4'-hydroxy derivatives. The free form is considered to be the therapeutically active form. Approximately 44% of trimethoprim is bound to plasma proteins.

Mean peak serum concentrations of approximately 1.0 mcg/mL occur 1 to 4 hours after oral administration of a single 100 mg dose. A single 200 mg dose will result in serum levels approximately twice as high. The half-life of trimethoprim ranges from 8 to 10 hours. However, patients with severely impaired renal function exhibit an increase in the half-life of trimethoprim, which requires either dosage regimen adjustment or not using the drug in such patients (see DOSAGE AND ADMINISTRATION). During a 13 week study of trimethoprim administered at a daily dosage of 200 mg (50 mg q.i.d.), the mean minimum steady-state concentration of the drug was 1.1 mcg/mL.

Steady-state concentrations were achieved within 2 to 3 days of chronic administration and were maintained throughout the experimental period.

Excretion of trimethoprim is primarily by the kidneys through glomerular filtration and tubular secretion. Urine concentrations of trimethoprim are considerably higher than are the concentrations in the blood. After a single oral dose of 100 mg, urine concentrations of trimethoprim ranged from 30 to 160 mcg/mL during the 0 to 4 hour period and declined to approximately 18 to 91 mcg/mL during the 8 to 24 hour period. A 200 mg single oral dose will result in trimethoprim urine levels approximately twice as high. After oral administration, 50% to 60% of trimethoprim is excreted in the urine within 24 hours, approximately 80% of this being unmetabolized trimethoprim.

Since normal vaginal and fecal flora are the source of most pathogens causing urinary tract infections, it is relevant to consider the distribution of trimethoprim into these sites. Concentrations of trimethoprim in vaginal secretions are consistently greater than those found simultaneously in the serum, being typically 1.6 times the concentrations of simultaneously obtained serum samples. Sufficient trimethoprim is excreted in the feces to markedly reduce or eliminate trimethoprim-susceptible organisms from the fecal flora.

Trimethoprim also passes the placental barrier and is excreted in human milk.

Mechanism of Action — Trimethoprim blocks the production of tetrahydrofolic acid from dihydrofolic acid by binding to and reversibly inhibiting the required enzyme, dihydrofolate reductase. This binding is very much stronger for the bacterial enzyme than for the corresponding mammalian enzyme. Thus, trimethoprim selectively interferes with bacterial biosynthesis of nucleic acids and proteins.

Resistance — Resistance to trimethoprim may be conferred by a variety of mechanisms including cell wall impermeability, overproduction of the chromosomal dihydrofolate reductase (DHFR) enzyme, production of a resistant chromosomal DHFR enzyme or production of a plasmid-mediated trimethoprim-resistant DHFR enzyme. Acinetobacter baumannii/Acinetobacter calcoaceticus complex, Burkholderia cepacia complex, Pseudomonas aeruginosa, Stenotrophomonas maltophilia are intrinsically resistant to trimethoprim. Non-Enterobacteriaceae fecal organisms, Bacteroides spp. and Lactobacillus spp. are not susceptible to trimethoprim at the concentrations obtained with the recommended dosage. Enterococcus spp, (E. faecalis, E. faecium, E. gallinarum/E. casseliflavus) may appear active in vitro to trimethoprim but are not effective clinically and should not be reported as susceptible. Moraxella catarrhalis isolates were found consistently resistant to trimethoprim.

Antimicrobial Activity — Trimethoprim has been shown to be active against most strains of the following microorganisms, both in vitro and in clinical infections as described in the INDICATIONS AND USAGE section.

Aerobic gram-positive bacteria — Staphylococcus species (coagulase-negative strains, including S. saprophyticus)