CLINICAL PHARMACOLOGY
After intramuscular administration of gentamicin (gentamicin injection pediatric) sulfate, peak serum concentrations
usually occur between 30 and 60 minutes and serum levels are measurable for
6 to 8 hours. In infants, a single dose of 2.5 mg/kg usually provides a peak
serum level in the range of 3 to 5 mcg/mL.When gentamicin (gentamicin injection pediatric) is administered by
intravenous infusion over a two-hour period, the serum concentrations are similar
to those obtained by intramuscular administration. Age markedly affects the
peak concentrations: in one report, a 1 mg/kg dose produced mean peak concentrations
of 1.58, 2.03, and 2.81 mcg/mL in patients six months to five years old, 5to
10 years old, and over10 years old, respectively.
In infants one week to six months of age, the half-life is 3 to 3½ hours.
In full-term and large premature infants less than one week old, the approximate
serum half-life of gentamicin (gentamicin injection pediatric) is 5½ hours. In small premature infants,
the half-life is inversely related to birth weight. In premature infants weighing
less than 1500 grams, the half-life is 11½ hours; in those weighing 1500
to 2000 grams, the half-life is eight hours; in those weighing over 2000grams,
the half-life is approximately five hours.While some variation is to be expected
due to a number of variables such as age, body temperature, surface area and
physiologic differences, the individual patient given the same dose tends to
have similar levels in repeated determinations.
Gentamicin (gentamicin injection pediatric) , like all aminoglycosides, may accumulate in the serum and tissues
of patients treated with higher doses and/or for prolonged periods, particularly
in the presence of impaired or immature renal function. In patients with immature
or impaired renal function, gentamicin (gentamicin injection pediatric) is cleared from the body more slowly
than in patients with normal renal function.The more severe the impairment,
the slower the clearance. (Dosage must be adjusted.)
Since gentamicin (gentamicin injection pediatric) is distributed in extracellular fluid, peak serum concentrations
may be lower than usual in patients who have a large volume of this fluid. Serum
concentrations of gentamicin (gentamicin injection pediatric) in febrile patients may be lower than those in
afebrile patients given the same dose. When body temperature returns to normal,
serum concentrations of the drug may rise. Febrile and anemic states may be
associated with a shorter than usual serum half-life. (Dosage adjustment is
usually not necessary.) In severely burned patients, the half-life may be significantly
decreased and resultingserumconcentrationsmaybelowerthan anticipated from the
mg/kg dose.
Protein-binding studies have indicated that the degree of gentamicin (gentamicin injection pediatric) binding
is low, depending upon the methods used for testing, this may be between 0 and
30%.
In neonates less than three days old, approximately 10% of the administered
dose is excreted in 12 hours; in infants 5 to 40 days old, approximately 40%
is excreted over the same period. Excretion of gentamicin (gentamicin injection pediatric) correlates with postnatal
age and creatinine clearance. Thus, with increasing postnatal age and concomitant
increase in renal maturity, gentamicin (gentamicin injection pediatric) is excreted more rapidly. Little, ifany,
metabolic transformation occurs; the drug is excreted principally by glomerular
filtration. After several days of treatment, the amount of gentamicin (gentamicin injection pediatric) excreted
in the urine approaches, but does not equal, the daily dose administered. As
with other aminoglycosides, a small amount of the gentamicin (gentamicin injection pediatric) dose may be retained
in the tissues, especially in the kidneys. Minute quantities of aminoglycosides
have been detected in the urine of some patients weeks after drug administration
was discontinued. Renal clearance of gentamicin (gentamicin injection pediatric) is similar to that of endogenous
creatinine.
In patients with marked impairment of renal function, there is a decrease in
the concentration of aminoglycosides in urine and in their penetration into
defective renal parenchyma. This decreased drug excretion, together with the
potential nephrotoxicity of aminoglycosides, should be considered when treating
such patients who have urinary tract infections.
Probenecid does not affect renal tubular transport of gentamicin (gentamicin injection pediatric) .
The endogenous creatinine clearance rate and the serum creatinine level have
a high correlation with the half-life of gentamicin (gentamicin injection pediatric) in serum. Results of these
tests may serve as guides for adjusting dosage in patients with renalim pairment
(see DOSAGE AND ADMINISTRATION).
Following parenteral administration, gentamicin (gentamicin injection pediatric) can be detected in serum, lymph,
tissues, sputum, andinpleural, synovial, and peritoneal fluids. Concentrations
in renal cortex sometimes may be eight times higher than the usual serum levels.
Concentrations in bile, in general, have been low and have suggested minimal
biliary excretion. Gentamicin (gentamicin injection pediatric) crosses the peritoneal as well as the placental
membranes.Since aminoglycosides diffuse poorly into the subarachnoid space after
parenteral administration, concentrations of gentamicin (gentamicin injection pediatric) in cerebrospinal fluid
are often low and dependent upon dose, rate of penetration, and degree of meningeal
inflammation.There is minimal penetration of gentamicin (gentamicin injection pediatric) into ocular tissues
following intramuscular or intravenous administration.
Microbiology
In vitro tests have demonstrated that gentamicin (gentamicin injection pediatric) is a bactericidal antibiotic
which acts by inhibiting normal protein synthesis in susceptible microorganisms.
It is active against a wide variety of pathogenic bacteria including Escherichia
coli, Proteus species, (indole-positive and indole-negative), Pseudomonas
aeruginosa, species of the Klebsiella-Enterobacter-Serratia group,
Citrobacter species and Staphylococcus species (including penicillin-and
methicillin-resistant strains). Gentamicin (gentamicin injection pediatric) is also active in vitro against
species of Salmonella and Shigella. The following bacteria are
usually resistant to aminoglycosides: Streptococcus pneumoniae, most
species of streptococci, particularly group D and anaerobic organisms, such
as Bacteroides species or Clostridium species.
In vitro studies have shown that an aminoglycoside combined with an
antibiotic that interferes with cell wall synthesis may act synergistically
against some group D streptococcal strains. The combination of gentamicin (gentamicin injection pediatric) and
penicillin G has a synergistic bactericidal effect against virtually all strains
of Streptococcus faecalis and its varieties (S. faecalis var.
liquifaciens, S. faecalis var. zymogenes), S. faecium
and S. durans. An enhanced killing effect against many of these strains
has also been shown in vitro with combinations of gentamicin (gentamicin injection pediatric) and ampicillin,
carbenicillin, nafcillin, or oxacillin.
The combined effect of gentamicin (gentamicin injection pediatric) and carbenicillin is synergistic for many
strains of Pseudomonas aeruginosa. In vitro synergism against
other gram-negative organisms has been shown with combinations of gentamicin (gentamicin injection pediatric)
and cephalosporins. Gentamicin (gentamicin injection pediatric) may be active against clinical isolates of bacteria
resistant to other aminoglycosides. Bacteria resistant to one aminoglycoside
may be resistant to one or more other aminoglycosides. Bacterial resistance
to gentamicin (gentamicin injection pediatric) is generally developed slowly.
Susceptibility Testing
If the disc method of susceptibility testing used is that described by Bauer
et al. (Am J Clin Path 45:493, 1966; Federal Register 37:20525-20529,
1972), a disc containing 10 mcg of gentamicin (gentamicin injection pediatric) should give a zone of inhibition
of 15 mm or more to indicate susceptibility of the infecting organism. Zones
of 12 mm or less indicate that the infecting organism is likely to be resistant.
Zones of greater than 12 mm and less than 15 mm indicate intermediate susceptibility.
In certain conditions it may be desirable to do additional susceptibility testing
by the tube or agar dilution method; gentamicin (gentamicin injection pediatric) substance is available for this
purpose.