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tobramycin injection, USP Clinical Pharmacology

CLINICAL PHARMACOLOGY

Tobramycin is rapidly absorbed following intramuscular administration. Peak serum concentrations of tobramycin occur between 30 and 90 minutes after intramuscular administration.

Following an intramuscular dose of 1 mg/kg of body weight, maximum serum concentrations reach about 4 mcg/mL, and measurable levels persist for as long as 8 hours. Therapeutic serum levels are generally considered to range from 4 to 6 mcg/mL. When tobramycin injection is administered by intravenous infusion over a 1-hour period, the serum concentrations are similar to those obtained by intramuscular administration. Tobramycin is poorly absorbed from the gastrointestinal tract.

In patients with normal renal function, except neonates, tobramycin administered every 8 hours does not accumulate in the serum. However, in those patients with reduced renal function and in neonates, the serum concentration of the antibiotic is usually higher and can be measured for longer periods of time than in normal adults. Dosage for such patients must, therefore, be adjusted accordingly (see DOSAGE AND ADMINISTRATION).

Following parenteral administration, little, if any, metabolic transformation occurs, and tobramycin is eliminated almost exclusively by glomerular filtration. Renal clearance is similar to that of endogenous creatinine. Ultrafiltration studies demonstrate that practically no serum protein binding occurs. In patients with normal renal function, up to 84% of the dose is recoverable from the urine in 8 hours and up to 93% in 24 hours.

Peak urine concentrations ranging from 75 to 100 mcg/mL have been observed following the intramuscular injection of a single dose of 1 mg/kg. After several days of treatment, the amount of tobramycin excreted in the urine approaches the daily dose administered. When renal function is impaired, excretion of tobramycin is slowed, and accumulation of the drug may cause toxic blood levels.

The serum half-life in normal individuals is 2 hours. An inverse relationship exists between serum half-life and creatinine clearance, and the dosage schedule should be adjusted according to the degree of renal impairment (see DOSAGE AND ADMINISTRATION). In patients undergoing dialysis, 25% to 70% of the administered dose may be removed, depending on the duration and type of dialysis.

Tobramycin can be detected in tissues and body fluids after parenteral administration. Concentrations in bile and stools ordinarily have been low, which suggests minimum biliary excretion. Tobramycin has appeared in low concentration in the cerebrospinal fluid following parenteral administration, and concentrations are dependent on dose, rate of penetration, and degree of meningeal inflammation. It has also been found in sputum, peritoneal fluid, synovial fluid, and abscess fluids, and it crosses the placental membranes. Concentrations in the renal cortex are several times higher than the usual serum levels.

Probenecid does not affect the renal tubular transport of tobramycin.

Microbiology

Tobramycin is an aminoglycoside antibiotic with activity against Gram-positive and Gram-negative bacteria.

Mechanism of Action

Tobramycin acts by inhibiting synthesis of protein in bacterial cells. In vitro tests demonstrate that tobramycin is bactericidal.

Resistance

Cross-resistance between aminoglycosides may occur.

Interactions With Other Antimicrobials

Although most strains of enterococci demonstrate in vitro resistance, some strains in this group are susceptible. In vitro studies have shown that an aminoglycoside combined with an antibiotic that interferes with cell-wall synthesis affects some enterococcal strains synergistically. The combination of penicillin G and tobramycin results in a synergistic bactericidal effect in vitro against certain strains of Enterococcus faecalis. However, this combination is not synergistic against other closely related organisms, e.g. Enterococcus faecium. Speciation of enterococci alone cannot be used to predict susceptibility. Susceptibility testing and tests for antibiotic synergism are emphasized.

Antimicrobial Activity

Tobramycin has been shown to be active against most strains of the following organisms both in vitro and in clinical infections: (see INDICATIONS AND USAGE)

Aerobic and facultative Gram-positive microorganisms

Staphylococcus aureus

Aerobic and facultative Gram-negative microorganisms

Citrobacter sp                       Pseudomonas aeruginosa

Enterobacter sp                    Proteus mirabilis

Escherichia coli                    Proteus vulgaris

Klebsiella sp                         Providencia sp

Morganella morganii           Serratia sp

Aminoglycosides have a low order of activity against most Gram-positive organisms, including Streptococcus pyogenes, Streptococcus pneumoniae, and enterococci.

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.

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