Microbiology
Dicloxacillin exerts a bactericidal action against
penicillin-susceptible microorganisms during the state of active multiplication.
All penicillins inhibit the biosynthesis of the bacterial cell wall.
The drugs in this class are highly resistant to inactivation by
staphylococcal penicillinase and are active against penicillinase-producing and
nonpenicillinase-producing strains of Staphylococcus aureus.
The penicillinase- resistant penicillins are active in vitro against a variety of other bacteria.
Susceptibility Plate TestingQuantitative methods of susceptibility testing that require
measurements of zone diameters or minimal inhibitory concentrations (MICs) give
the most precise estimates of antibiotic susceptibility. One such procedure has
been recommended for use with discs to test susceptibility to this class of
drugs. Interpretations correlate diameters on the disc test with MIC values.
A penicillinase-resistant class disc may be used to determine microbial
susceptibility to dicloxacillin.
TABLE I shows the interpretation of test results for
penicillinase-resistant penicillins using the FDA Standard Disc Test Method
(formerly Bauer-Kirby-Sherris-Turck method) of disc bacteriological
susceptibility testing for staphylococci with a disc containing 5 micrograms of
methicillin sodium.
With this procedure, a report from a laboratory of “susceptible” indicates
that the infecting organism is likely to respond to therapy. A report of
“resistant” indicates that the infecting organism is not likely to respond to
therapy. A report of “intermediate” susceptibility suggests that the organism
might be susceptible if high doses of the antibiotic are used, or if the
infection is confined to tissues and fluids (e.g., urine) in which high
antibiotic levels are attained.
In general, all staphylococci should be tested against the penicillin G disc
and against the methicillin disc. Routine methods of antibiotic susceptibility
testing may fail to detect strains of organisms resistant to the
penicillinase-resistant penicillins. For this reason, the use of large inocula
and 48 hour incubation periods may be necessary to obtain accurate
susceptibility studies with these antibiotics. Bacterial strains which are
resistant to one of the penicillinase-resistant penicillins should be considered
resistant to all of the drugs in the class.
Table I STANDARDIZED DISC TEST METHOD OF BACTERIOLOGICAL SUSCEPTIBILITY
TESTING USING A CLASS DISC CONTAINING 5 MICROGRAMS OF METHICILLIN
SODIUM| Diameter of
Zone | Diameter of Zone | Diameter of Zone |
| Indicating
“Susceptible” | Indicating | Indicating
“Resistant” |
| at least | “Intermediate” | Less than |
| 14 mm | 10 – 13 mm | 10 mm |
PharmacokineticsMethicillin sodium is readily destroyed by gastric acidity and
must be administered by intramuscular or intravenous injection. The isoxazolyl
penicillins (cloxacillin, dicloxacillin and oxacillin) and nafcillin are more
acid-resistant and may be administered orally.
Absorption of the isoxazolyl penicillins after oral administration is rapid
but incomplete; peak blood levels are achieved in 1-1.5 hours. In one study,
after ingestion of a single 500 mg oral dose, peak serum concentrations range
from 5-7 micrograms/milliliter for oxacillin, from 7.5-14.4 mcg/mL for
cloxacillin and from 10-17 mcg/mL for dicloxacillin.
Oral absorption of cloxacillin, dicloxacillin, oxacillin and nafcillin is
delayed when the drugs are administered after meals.
Once absorbed, the penicillinase-resistant penicillins bind to serum protein,
mainly albumin. The degree of protein binding reported varies with the method of
study and the investigator (see TABLE II).
TABLE II PENICILLINASE-RESISTANT PENICILLINS PERCENT PROTEIN BINDING ±
SD| Methicillin | 37.3 ± 7.9 |
| Nafcillin | 89.9 ± 1.5 |
| Oxacillin | 94.2 ± 2.1 |
| Cloxacillin | 95.2 ± 0.5 |
| Dicloxacillin | 97.9 ± 0.6 |
The penicillinase-resistant penicillins vary in the extent to which they are
distributed in the body fluids. With normal doses, insignificant concentrations
are found in the cerebrospinal fluid and aqueous humor. All the drugs in this
class are found in therapeutic concentrations in the pleural, bile and amniotic
fluids.
The penicillinase-resistant penicillins are rapidly excreted, primarily as
unchanged drug in the urine by glomerular filtration and active tubular
secretion. The elimination half-life for dicloxacillin is about 0.7 hour.
Nonrenal elimination includes hepatic inactivation and excretion in bile.