SULFAMETHOXAZOLE AND TRIMETHOPRIM Concentrate for solution for injection Ref.[108388] Active ingredients:

Following a 1-hour intravenous infusion of a single dose of 160 mg trimethoprim and 800 mg sulfamethoxazole to 11 patients whose weight ranged from 105 lbs to 165 lbs (mean, 143 lbs), the peak plasma concentrations of trimethoprim and sulfamethoxazole were 3.4 ± 0.3 μg/mL and 46.3 ± 2.7 μg/mL, respectively. Following repeated intravenous administration of the same dose at 8-hour intervals, the mean plasma concentrations just prior to and immediately after each infusion at steady state were 5.6 ± 0.6 μg/mL and 8.8 ± 0.9 μg/mL for trimethoprim and 70.6 ± 7.3 μg/mL and 105.6 ± 10.9 μg/mL for sulfamethoxazole. The mean plasma half-life was 11.3 ± 0.7 hours for trimethoprim and 12.8 ± 1.8 hours for sulfamethoxazole. All of these 11 patients had normal renal function, and their ages ranged from 17 to 78 years (median, 60 years).¹¹

Pharmacokinetic studies in children and adults suggest an age-dependent half-life of trimethoprim, as indicated in Table 5.¹²

Table 5. Half-life of Trimethoprim (TMP) in Pediatric Patients and Adults:

Patients with severely impaired renal function exhibit an increase in the half-lives of both components, requiring dosage regimen adjustment [see Dosage and Administration (2.2)].

Distribution

Both trimethoprim and sulfamethoxazole exist in the blood as unbound, protein-bound and metabolized forms; sulfamethoxazole also exists as the conjugated form.

Approximately 44% of trimethoprim and 70% of sulfamethoxazole are bound to plasma proteins. The presence of 10 mg percent sulfamethoxazole in plasma decreases the protein binding of trimethoprim by an insignificant degree; trimethoprim does not influence the protein binding of sulfamethoxazole.

Both trimethoprim and sulfamethoxazole distribute to sputum and vaginal fluid; trimethoprim also distributes to bronchial secretions, and both pass the placental barrier and are excreted in breast milk.

Elimination

Metabolism

Sulfamethoxazole is metabolized in humans to at least 5 metabolites: the N₄acetyl, N4-hydroxy-, 5methylhydroxy-, N₄-acetyl-5-methylhydroxy- sulfamethoxazole metabolites, and an N-glucuronide conjugate. The formation of N₄-hydroxy metabolite is mediated via CYP2C9.

Trimethoprim is metabolized in vitro to 11 different metabolites, of which, five are glutathione adducts and six are oxidative metabolites, including the major metabolites, 1- and 3-oxides and the 3- and 4-hydroxy derivatives.

The free forms of trimethoprim and sulfamethoxazole are considered to be the therapeutically active forms. In vitro studies suggest that trimethoprim is a substrate of P-glycoprotein, OCT1 and OCT2, and that sulfamethoxazole is not a substrate of P-glycoprotein.

Excretion

Excretion of trimethoprim and sulfamethoxazole is primarily by the kidneys through both glomerular filtration and tubular secretion. Urine concentrations of both trimethoprim and sulfamethoxazole are considerably higher than are the concentrations in the blood. The percent of dose excreted in urine over a 12-hour period following the intravenous administration of the first dose of 240 mg of trimethoprim and 1200 mg of sulfamethoxazole on day 1 ranged from 17% to 42.4% as free trimethoprim; 7% to 12.7% as free sulfamethoxazole; and 36.7% to 56% as total (free plus the N4-acetylated metabolite) sulfamethoxazole. When administered together as Sulfamethoxazole and trimethoprim injection, neither trimethoprim nor sulfamethoxazole affects the urinary excretion pattern of the other.

Specific Populations

Geriatric Patients

The pharmacokinetics of sulfamethoxazole 800 mg and trimethoprim 160 mg were studied in six geriatric subjects (mean age: 78.6 years) and six young healthy subjects (mean age: 29.3 years) using a non-US approved formulation. Pharmacokinetic values for sulfamethoxazole in geriatric subjects were similar to those observed in young adult subjects. The mean renal clearance of trimethoprim was significantly lower in geriatric subjects compared with young adult subjects (19 mL/h/kg vs. 55 mL/h/kg). However, after normalizing by body weight, the apparent total body clearance of trimethoprim was on average 19% lower in geriatric subjects compared with young adult subjects.

Mechanism of Action

Sulfamethoxazole inhibits bacterial synthesis of dihydrofolic acid by competing with para-aminobenzoic acid (PABA). Trimethoprim blocks the production of tetrahydrofolic acid from dihydrofolic acid by binding to and reversibly inhibiting the required enzyme, dihydrofolate reductase. Thus, sulfamethoxazole and trimethoprim injection blocks two consecutive steps in the biosynthesis of nucleic acids and proteins essential to many bacteria.

Resistance

In vitro studies have shown that bacterial resistance develops more slowly with both sulfamethoxazole and trimethoprim in combination than with either trimethoprim or sulfamethoxazole alone.

Antimicrobial Activity

Sulfamethoxazole and trimethoprim injection has been shown to be active against most isolates of the following microorganisms, both in in vitro and in clinical infections [see Indications and Usage (1)].

Aerobic gram-negative bacteria:

Escherichia coli
Klebsiella species
Enterobacter species
Morganella morganii
Proteus mirabilis
Proteus vulgaris
Shigella flexneri
Shigella sonnei

Other Microorganisms:

Pneumocystis jirovecii

The following in vitro data are available, but their clinical significance is unknown. At least 90 percent of the following bacteria exhibit an in vitro minimum inhibitory concentration (MIC) less than or equal to the susceptible breakpoint for sulfamethoxazole and trimethoprim injection against isolates of similar genus or organism group. However, the efficacy of sulfamethoxazole and trimethoprim injection in treating clinical infections caused by these bacteria has not been established in adequate and well-controlled clinical trials.

Aerobic gram-positive bacteria:

Streptococcus pneumoniae

Aerobic gram-negative bacteria:

Haemophilus influenzae

Susceptibility Testing

For specific information regarding susceptibility test interpretive criteria and associated test methods and quality control standards recognized by FDA for this drug, please see: https://www.fda.gov/STIC.

Carcinogenesis

Sulfamethoxazole was not carcinogenic when assessed in a 26-week tumorigenic mouse (Tg-rasH2) study at doses up to 400 mg/kg/day sulfamethoxazole; equivalent to 2-fold the human systemic exposure (at a daily dose of 800 mg sulfamethoxazole twice a day).

Mutagenesis

In vitro reverse mutation bacterial tests according to the standard protocol have not been performed with sulfamethoxazole and trimethoprim in combination. An in vitro chromosomal aberration test in human lymphocytes with sulfamethoxazole/trimethoprim was negative. In in vitro and in vivo tests in animal species, sulfamethoxazole/trimethoprim did not damage chromosomes. In vivo micronucleus assays were positive following oral administration of sulfamethoxazole/trimethoprim. Observations of leukocytes obtained from patients treated with sulfamethoxazole and trimethoprim revealed no chromosomal abnormalities.

Sulfamethoxazole alone was positive in an in vitro reverse mutation bacterial assay and in in vitro micronucleus assays using cultured human lymphocytes.

Trimethoprim alone was negative in in vitro reverse mutation bacterial assays and in in vitro chromosomal aberration assays with Chinese Hamster ovary or lung cells with or without S9 activation. In in vitro Comet, micronucleus and chromosomal damage assays using cultured human lymphocytes, trimethoprim was positive. In mice following oral administration of trimethoprim, no DNA damage in Comet assays of liver, kidney, lung, spleen, or bone marrow was recorded.

Impairment of Fertility

No adverse effects on fertility or general reproductive performance were observed in rats given oral dosages as high as 350 mg/kg/day sulfamethoxazole plus 70 mg/kg/day trimethoprim, doses roughly two times the recommended human daily dose on a body surface area basis.