Drug Information for SULFAMETHOXAZOLE AND TRIMETHOPRIM ORAL SUSPENSION, USP (CHERRY FLAVOR) and SULFAMETHOXAZOLE AND TRIMETHOPRIM ORAL SUSPENSION, USP (GRAPE FLAVOR) 200 mg/40 mg per 5 mL (Qualitest Pharmaceuticals): CLINICAL PHARMACOLOGY

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  • Sulfamethoxazole and trimethoprim are rapidly absorbed following oral administration. Both sulfamethoxazole and trimethoprim exist in the blood as unbound, protein-bound, and metabolized forms; sulfamethoxazole also exists as the conjugated form. The metabolism of sulfamethoxazole occurs predominately by N4-acetylation, although the glucuronide conjugate has been identified. The principal metabolites of trimethoprim are the 1- and 3-oxides and the 3'- and 4'-hydroxy derivatives. The free forms of sulfamethoxazole and trimethoprim are considered to be the therapeutically active forms. 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.

    Peak blood levels for the individual components occur 1 to 4 hours after oral administration. The mean serum half-lives of sulfamethoxazole and trimethoprim are 10 and 8 to 10 hours, respectively. However, patients with severely impaired renal function exhibit an increase in the half-lives of both components, requiring dosage regimen adjustment (see DOSAGE AND ADMINISTRATION). Detectable amounts of trimethoprim and sulfamethoxazole are present in the blood 24 hours after drug administration. During administration of 160 mg trimethoprim and 800 mg sulfamethoxazole b.i.d., the mean steady-state plasma concentration of trimethoprim was 1.72 mcg/mL. The steady-state minimal plasma levels of free and total sulfamethoxazole were 57.4 mcg/mL and 68 mcg/mL, respectively. These steady-state levels were achieved after 3 days of drug administration.1

    Excretion of sulfamethoxazole and trimethoprim is primarily by the kidneys through both glomerular filtration and tubular secretion. Urine concentrations of both sulfamethoxazole and trimethoprim are considerably higher than are the concentrations in the blood. The average percentage of the dose recovered in urine from 0 to 72 hours after a single oral dose is 84.5% for total sulfonamide and 66.8% for free trimethoprim. Thirty percent of the total sulfonamide is excreted as free sulfamethoxazole, with the remaining as N4-acetylated metabolite.2 When administered together as sulfamethoxazole and trimethoprim, neither sulfamethoxazole nor trimethoprim affects the urinary excretion pattern of the other.

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

  • Geriatric Pharmacokinetics:

  • The pharmacokinetics of sulfamethoxazole 800 mg and trimethoprim 160 mg were studied in 6 geriatric subjects (mean age: 78.6 years) and 6 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 an average 19% lower in geriatric subjects compared with young adult subjects.3

  • Microbiology:

  • 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 block two consecutive steps in the biosynthesis of nucleic acids and proteins essential to many bacteria.

    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.

    In vitro serial dilution tests have shown that the spectrum of antibacterial activity of sulfamethoxazole and trimethoprim includes the common urinary tract pathogens with the exception of Pseudomonas aeruginosa. The following organisms are usually susceptible: Escherichia coli, Klebsiella species, Enterobacter species, Morganella morganii, Proteus mirabilis, and indole-positive Proteus species including Proteus vulgaris.

    The usual spectrum of antimicrobial activity of sulfamethoxazole and trimethoprim includes bacterial pathogens isolated from middle ear exudate and from bronchial secretions (Haemophilus influenzae, including ampicillin-resistant strains, and Streptococcus pneumoniae), and enterotoxigenic strains of Escherichia coli (ETEC) causing bacterial gastroenteritis. Shigella flexneri and Shigella sonnei are also usually susceptible.

    REPRESENTATIVE MINIMUM INHIBITORY CONCENTRATION VALUES FOR ORGANISMS SUSCEPTIBLE TO SULFAMETHOXAZOLE AND TRIMETHOPRIM (MICmcg/mL)

    TMP=trimethoprim SMX=sulfamethoxazole

    *Rudoy RC, Nelson JD, Haltalin KC. Antimicrobial Agents and Chemotherapy. 1974;5:439–443.

    TMP/SMX (1:19)
    TMPSMX
    BacteriaAloneAloneTMPSMX
    Escherichia coli0.05–1.51.0–2450.05–0.50.95–9.5
    Escherichia coli(enterotoxigenic strains)0.015–0.150.285–>9500.005–0.150.095–2.85
    Proteus species(indole positive)0.5–5.07.35–3000.05–1.50.95–28.5
    TMP/SMX (1:19)
    TMPSMX
    BacteriaAloneAloneTMPSMX
    Morganella morganii0.5–5.07.35–3000.05–1.50.95–28.5
    Proteus mirabilis0.5–1.57.35–300.05–0.150.95–2.85
    Klebsiella species0.15–5.02.45–2450.05–1.50.95–28.5
    Enterobacter species0.15–5.02.45–2450.05–1.50.95–28.5
    Haemophilusinfluenzae0.15–1.52.85–950.015–0.150.285–2.85
    TMP/SMX (1:19)
    TMPSMX
    BacteriaAloneAloneTMPSMX
    Streptococcuspneumoniae0.15–1.57.35–24.50.05–0.150.95–2.85
    Shigella flexneri*<0.01–0.04<0.16–>320<0.002–0.030.04–0.625
    Shigella sonnei*0.02–0.080.625–>3200.004–0.060.08–1.25
  • Susceptibility Testing:

  • The recommended quantitative disc susceptibility method may be used for estimating the susceptibility of bacteria to sulfamethoxazole and trimethoprim.4,5 With this procedure, a report from the laboratory of "Susceptible to trimethoprim and sulfamethoxazole" indicates that the infection is likely to respond to therapy with sulfamethoxazole and trimethoprim. If the infection is confined to the urine, a report of "Intermediate susceptibility to trimethoprim and sulfamethoxazole" also indicates that the infection is likely to respond. A report of "Resistant to trimethoprim and sulfamethoxazole" indicates that the infection is unlikely to respond to therapy with sulfamethoxazole and trimethoprim.

  • Drug Information Provided by National Library of Medicine (NLM).
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