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Tobramycin Injection, USP
SPL UNCLASSIFIED SECTION
Rx only
To reduce the development of drug-resistant bacteria and maintain the effectiveness of Tobramycin Injection, USP and other antibacterial drugs, Tobramycin Injection, USP should be used only to treat or prevent infections that are proven or strongly suspected to be caused by bacteria.
DESCRIPTION
Tobramycin sulfate, a water-soluble antibiotic of the aminoglycoside group, is derived from the actinomycete Streptomyces tenebrarius. Tobramycin Injection, USP is a clear and colorless sterile aqueous solution for parenteral administration.
Tobramycin sulfate is O-3-amino-3-deoxy-α-D-glucopyranosyl-(1→4)-O-[2,6-diamino-2,3,6-trideoxy-α-D-ribo-hexopyranosyl-(1→6)]-2-deoxy-L-streptamine, sulfate (2:5)(salt). The structural formula for tobramycin is as follows:
(C18H37N5O9)2•5H2SO4 M.W. 1425.45
Each mL contains tobramycin sulfate equivalent to 10 mg (pediatric) or 40 mg tobramycin; phenol 5 mg; sodium metabisulfite 3.2 mg; 0.1 mg edetate disodium and water for injection, q.s. Sulfuric acid and/or sodium hydroxide may have been added to adjust the pH (3.0 to 6.5).
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 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
Mechanism of Action
Tobramycin acts by inhibiting protein synthesis in bacterial cells. In vitro tests demonstrate that tobramycin is bactericidal.
Resistance
Aminoglycosides are generally not active against most gram-positive organisms, including Streptococcus pyogenes, Streptococcus pneumoniae, and enterococci. Cross-resistance between aminoglycosides may occur.
Interactions with Other Antibacterial Drugs
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. Species-level identification of enterococci alone cannot be used to predict susceptibility. Susceptibility testing and tests for antibiotic synergism must be performed.
Tobramycin has been shown to be active against most strains of the following bacteria both in vitro and in clinical infections as described in INDICATIONS AND USAGE section (1):
Aerobic Gram-positive bacteria
Staphylococcus aureus
Aerobic Gram-negative bacteria
Citrobacter species
Enterobacter species
Escherichia coli
Klebsiella species
Morganella morganii
Pseudomonas aeruginosa
Proteus mirabilis
Proteus vulgaris
Providencia species
Serratia species
Aminoglycosides are generally not active against most gram-positive bacteria, including Streptococcus pyogenes, Streptococcus pneumoniae, and enterococci. For Salmonella spp. and Shigella spp., aminoglycosides may appear active in vitro but are not effective clinically and should not be reported as susceptible.
Susceptibility Test Methods
When available, the clinical microbiology laboratory should provide the results of in vitro susceptibility test results for antimicrobial drugs used in local hospitals and practice areas to the physician as periodic reports that describe the susceptibility profile of nosocomial and community-acquired pathogens. These reports should aid the physician in selecting an antibacterial drug product for treatment.
Dilution Techniques
Quantitative methods are used to determine antimicrobial minimum inhibitory concentrations (MICs). These MICs provide estimates of the susceptibility of bacteria to antibacterial compounds. The MICs should be determined using a standardized test method1,3. Standardized procedures are based on a dilution method (broth or agar) or equivalent with standardized inoculum concentrations and standardized concentrations of tobramycin powder. The MIC values should be interpreted according to the criteria in Table 1.
Diffusion Techniques
Quantitative methods that require measurement of zone diameters also provide reproducible estimates of the susceptibility of bacteria to antibacterial compounds. One such standardized procedure requires the use of standardized inoculum concentrations and paper disks impregnated with 10 mcg of tobramycin2,3. The disk diffusion values should be interpreted according to the criteria provided in Table 1.
Table 1: Susceptibility Interpretive Criteria for Tobramycin
S = susceptible, I = intermediate, R = resistant
a For Salmonella spp. and Shigella spp., tobramycin may appear active in vitro but is not effective clinically and should not be reported as susceptible.
A report of "Susceptible" indicates that the antimicrobial is likely to inhibit growth of the pathogen if the antimicrobial compound reaches the concentration at the infection site necessary to inhibit growth of the pathogen. A report of "Intermediate" indicates that the result should be considered equivocal, and if the microorganism is not fully susceptible to alternative clinically feasible drugs, the test should be repeated. This category implies possible clinical applicability in body sites where the drug is physiologically concentrated. This category also provides a buffer zone that prevents small uncontrolled technical factors from causing major discrepancies in interpretation. A report of "Resistant" indicates that the antimicrobial is not likely to inhibit growth of the pathogen if the antimicrobial compound reaches the concentrations usually achievable at the infection site; other therapy should be selected.
Quality Control
Standardized susceptibility test procedures require the use of laboratory controls to monitor and ensure the accuracy and precision of supplies and reagents used in the assay, and the techniques of the individuals performing the test1,2,3. Standard tobramycin powder should provide the following range of MIC values provided in Table 2. For the diffusion technique using the 10 mcg tobramycin disk, the criteria provided in Table 2 should be achieved.
Table 2: Acceptable Quality Control Ranges for Susceptibility Testing
INDICATIONS AND USAGE
To reduce the development of drug-resistant bacteria and maintain the effectiveness of Tobramycin Injection, USP and other antibacterial drugs, Tobramycin Injection, USP should be used only to treat or prevent infections that are proven or strongly suspected to be caused by susceptible bacteria. When culture and susceptibility information are available, they should be considered in selecting or modifying antibacterial therapy. In the absence of such data, local epidemiology and susceptibility patterns may contribute to the empiric selection of therapy.
Tobramycin injection is indicated for the treatment of serious bacterial infections caused by susceptible strains of the designated microorganisms in the diseases listed below:
Septicemia in the pediatric patient and adult caused by P. aeruginosa, E. coli, and Klebsiella spp
Lower respiratory tract infections caused by P. aeruginosa, Klebsiella spp, Enterobacter spp, Serratia spp, E. coli, and S. aureus (penicillinase- and non-penicillinase-producing strains)
Serious central-nervous-system infections (meningitis) caused by susceptible organisms
Intra-abdominal infections, including peritonitis, caused by E. coli, Klebsiella spp, and Enterobacter spp
Skin, bone, and skin structure infections caused by P. aeruginosa, Proteus spp, E. coli, Klebsiella spp, Enterobacter spp, Serratia spp and S. aureus
Complicated and recurrent urinary tract infections caused by P. aeruginosa, Proteus spp, (indole-positive and indole-negative), E. coli, Klebsiella spp, Enterobacter spp, Serratia spp, S. aureus, Providencia spp, and Citrobacter spp
Aminoglycosides, including tobramycin, are not indicated in uncomplicated initial episodes of urinary tract infections unless the causative organisms are not susceptible to antibiotics having less potential toxicity. Tobramycin may be considered in serious staphylococcal infections when penicillin or other potentially less toxic drugs are contraindicated and when bacterial susceptibility testing and clinical judgment indicate its use.
Bacterial cultures should be obtained prior to and during treatment to isolate and identify etiologic organisms and to test their susceptibility to tobramycin. If susceptibility tests show that the causative organisms are resistant to tobramycin, other appropriate therapy should be instituted. In patients in whom a serious life-threatening gram-negative infection is suspected, including those in whom concurrent therapy with a penicillin or cephalosporin and an aminoglycoside may be indicated, treatment with tobramycin may be initiated before the results of susceptibility studies are obtained. The decision to continue therapy with tobramycin should be based on the results of susceptibility studies, the severity of the infection, and the important additional concepts discussed in the WARNINGS box.
CONTRAINDICATIONS
A hypersensitivity to any aminoglycoside is a contraindication to the use of tobramycin. A history of hypersensitivity or serious toxic reactions to aminoglycosides may also contraindicate the use of any other aminoglycoside because of the known cross-sensitivity of patients to drugs in this class.
WARNINGS
See WARNINGS box above.
Tobramycin injection contains sodium metabisulfite, a sulfite that may cause allergic-type reactions, including anaphylactic symptoms and life-threatening or less severe asthmatic episodes, in certain susceptible people. The overall preva lence of sulfite sensitivity in the general population is unknown and probably low. Sulfite sensitivity is seen more frequently in asthmatic than in nonasthmatic people.
Serious allergic reactions including anaphylaxis and dermatologic reactions including exfoliative dermatitis, toxic epidermal necrolysis, erythema multiforme, and Stevens-Johnson Syndrome have been reported rarely in patients on tobramycin therapy. Although rare, fatalities have been reported (see CONTRAINDICATIONS).
If an allergic reaction occurs, the drug should be discontinued and appropriate therapy instituted.
PRECAUTIONS
General
Prescribing tobramycin injection in the absence of a proven or strongly suspected bacterial infection or a prophylactic indication is unlikely to provide benefit to the patient and increases the risk of the development of drug-resistant bacteria.
Serum and urine specimens for examination should be collected during therapy, as recommended in the WARNINGS box. Serum calcium, magnesium, and sodium should be monitored.
Peak and trough serum levels should be measured periodically during therapy. Prolonged concentrations above 12 mcg/mL should be avoided. Rising trough levels (above 2 mcg/mL) may indicate tissue accumulation. Such accumulation, advanced age, and cumulative dosage may contribute to ototoxicity and nephrotoxicity. It is particularly important to monitor serum levels closely in patients with known renal impairment.
A useful guideline would be to perform serum level assays after 2 or 3 doses, so that the dosage could be adjusted if necessary, and at 3- to 4-day intervals during therapy. In the event of changing renal function, more frequent serum levels should be obtained and the dosage or dosage interval adjusted according to the guidelines provided in DOSAGE AND ADMINISTRATION.
In order to measure the peak level, a serum sample should be drawn about 30 minutes following intravenous infusion or 1 hour after an intramuscular injection. Trough levels are measured by obtaining serum samples at 8 hours or just prior to the next dose of tobramycin. These suggested time intervals are intended only as guidelines and may vary according to institutional practices. It is important, however, that there be consistency within the individual patient program unless computerized pharmacokinetic dosing programs are available in the institution. These serum-level assays may be especially useful for monitoring the treatment of severely ill patients with changing renal function or of those infected with less susceptible organisms or those receiving maximum dosage.
Neuromuscular blockade and respiratory paralysis have been reported in cats receiving very high doses of tobramycin (40 mg/kg). The possibility of prolonged or secondary apnea should be considered if tobramycin is administered to anesthetized patients who are also receiving neuromuscular blocking agents, such as succinylcholine, tubocurarine, or decamethonium, or to patients receiving massive transfusions of citrated blood. If neuromuscular blockade occurs, it may be reversed by the administration of calcium salts.
Cross-allergenicity among aminoglycosides has been demonstrated.
In patients with extensive burns or cystic fibrosis, altered pharmacokinetics may result in reduced serum concentrations of aminoglycosides. In such patients treated with tobramycin measurement of serum concentration is especially important as a basis for determination of appropriate dosage.
Elderly patients may have reduced renal function that may not be evident in the results of routine screening tests, such as BUN or serum creatinine. A creatinine clearance determination may be more useful. Monitoring of renal function during treatment with aminoglycosides is particularly important in such patients.
An increased incidence of nephrotoxicity has been reported following concomitant administration of aminoglycoside antibiotics and cephalosporins.
Aminoglycosides should be used with caution in patients with muscular disorders, such as myasthenia gravis or parkinsonism, since these drugs may aggravate muscle weakness because of their potential curare-like effect on neuromuscular function.
Aminoglycosides may be absorbed in significant quantities from body surfaces after local irrigation or application and may cause neurotoxicity and nephrotoxicity.
Aminoglycosides have not been approved for intraocular and/or subconjunctival use. Physicians are advised that macular necrosis has been reported following administration of aminoglycosides, including tobramycin, by these routes.
See WARNINGS box regarding concurrent use of potent diuretics and concurrent and sequential use of other neurotoxic or nephrotoxic drugs.
The inactivation of tobramycin and other aminoglycosides by ß-lactam-type antibiotics (penicillins or cephalosporins) has been demonstrated in vitro and in patients with severe renal impairment. Such inactivation has not been found in patients with normal renal function who have been given the drugs by separate routes of administration.
Therapy with tobramycin may result in overgrowth of nonsusceptible organisms. If overgrowth of nonsusceptible organisms occurs, appropriate therapy should be initiated.
Information for Patients
Patients should be counseled that antibacterial drugs including tobramycin injection should only be used to treat bacterial infections. They do not treat viral infections (e.g., the common cold). When tobramycin injection is prescribed to treat a bacterial infection, patients should be told that although it is common to feel better early in the course of therapy, the medication should be taken exactly as directed. Skipping doses or not completing the full course of therapy may (1) decrease the effectiveness of the immediate treatment and (2) increase the likelihood that bacteria will develop resistance and will not be treatable by tobramycin injection or other antibacterial drugs in the future.
Pregnancy Category D
Aminoglycosides can cause fetal harm when administered to a pregnant woman. Aminoglycoside antibiotics cross the placenta, and there have been several reports of total irreversible bilateral congenital deafness in children whose mothers received streptomycin during pregnancy. Serious side effects to mother, fetus, or newborn have not been reported in the treatment of pregnant women with other aminoglycosides. If tobramycin is used during pregnancy or if the patient becomes pregnant while taking tobramycin, she should be apprised of the potential hazard to the fetus.
Geriatric Use
Elderly patients may be at a higher risk of developing nephrotoxicity and ototoxicity while receiving tobramycin (see WARNINGS, PRECAUTIONS, and OVERDOSAGE). Other factors that may contribute to nephrotoxicity and ototoxicity are rising trough levels, excessive peak concentrations, dehydration, concomitant use of other neurotoxic or nephrotoxic drugs, and cumulative dose. Peak and trough serum levels should be measured periodically during therapy to assure adequate levels and to avoid potentially toxic levels (see WARNINGS and PRECAUTIONS). Tobramycin is known to be substantially excreted by the kidney, and the risk of toxic reactions to this drug may be greater in patients with impaired renal function. Dose reduction is required for patients with impaired renal function (see DOSAGE AND ADMINISTRATION). Elderly |
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