Microbiology

Penicillinase-resistant penicillins exert a bactericidal action against penicillin-susceptible microorganisms during the state of active multiplication. All penicillins inhibit the biosynthesis of the bacterial cell wall.

Dicloxacillin sodium has been shown to be active against most strains of the following microorganisms, both in vitro and in clinical infections as described in the INDICATIONS AND USAGE section.

Susceptibility Testing

Quality Control

Standardized susceptibility test procedures require the use of laboratory control microorganisms.1,2,3 The 1 mcg oxacillin disk and the standard oxacillin powder should provide respectively the following zone diameters and MIC values in these laboratory test quality control strains:

Pharmacokinetics

Dicloxacillin sodium, USP is resistant to destruction by acid. Absorption of dicloxacillin sodium after oral administration is rapid but incomplete. Food in the gastrointestinal tract decreases the absorption of dicloxacillin. Studies with an oral dose of 125 mg gave average serum levels at 60 minutes of 4.74 mcg/mL. At four hours, average levels were 0.62 mcg/mL. In one study, single oral doses of dicloxacillin sodium 500 mg produced peak serum concentrations of 10 to 17 mcg/mL at 1 to 1.5 hours.

Once absorbed, dicloxacillin sodium is 95% to 99% bound to serum proteins, mainly albumin.

Dicloxacillin sodium, with normal doses, has insignificant concentrations in the cerebrospinal and ascitic fluids. It is found in therapeutic concentrations in the pleural, bile, and amniotic fluids. Dicloxacillin sodium is rapidly excreted as unchanged drug in the urine by glomerular filtration and active tubular secretion. The elimination half-life for dicloxacillin sodium is about 0.7 hours.

Dicloxacillin is not dialyzable. Only minimal amounts are removed by hemodialysis or peritoneal dialysis.

General

Penicillinase-resistant penicillins should generally not be administered to patients with a history of sensitivity to any penicillin.

Penicillin should be used with caution in individuals with histories of significant allergies and/or asthma. There is clinical and laboratory evidence of partial cross-allergenicity among penicillins and other β-lactam antibiotics including cephalosporins, cephamycins, and other 1-oxa-β-lactams. Whenever allergic reactions occur, penicillin should be withdrawn unless, in the opinion of the physician, the condition being treated is life-threatening and amenable only to penicillin therapy.

The oral route of administration should not be relied upon in patients with severe illness, or with nausea, vomiting, gastric dilation, cardiospasm, or intestinal hypermotility. Occasionally patients will not absorb therapeutic amounts of orally administered penicillin.

Prescribing dicloxacillin sodium capsules 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.

Information for the Patient

Patients receiving penicillins should be given the following information and instructions by the physician:

• 1.Patients should be told that penicillin is an antibacterial agent which will work with the body's natural defenses to control certain types of infections. They should be told that the drug should not be taken if they have had an allergic reaction to any form of penicillin previously, and to inform the physician of any allergies or previous allergic reactions to any drugs they may have had (see WARNINGS).
• 2.Patients who have previously experienced an anaphylactic reaction to penicillin should be instructed to wear a medical identification tag or bracelet.
• 3.Because most antibacterial drugs taken by mouth are best absorbed on an empty stomach, patients should be directed, unless circumstances warrant otherwise, to take penicillin one hour before meals or two hours after eating (see CLINICAL PHARMACOLOGY – Pharmacokinetics).
• 4.Patients should be told to take the entire course of therapy prescribed, even if fever and other symptoms have stopped (see PRECAUTIONS – General).
• 5.If any of the following reactions occur, stop taking your prescription and notify the physician: shortness of breath, wheezing, skin rash, mouth irritation, black tongue, sore throat, nausea, vomiting, diarrhea, fever, swollen joints, or any unusual bleeding or bruising (see ADVERSE REACTIONS).
• 6.Do not take any additional medications without physician approval, including non-prescription drugs such as antacids, laxatives, or vitamins.

Patients should be counseled that antibacterial drugs including dicloxacillin sodium capsules should only be used to treat bacterial infections. They do not treat viral infections (e.g., the common cold). When dicloxacillin sodium capsules are 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 dicloxacillin sodium capsules or other antibacterial drugs in the future.

Diarrhea is a common problem caused by antibiotics which usually ends when the antibiotic is discontinued. Sometimes after starting treatment with antibiotics, patients can develop watery and bloody stools (with or without stomach cramps and fever) even as late as two or more months after having taken the last dose of the antibiotic. If this occurs, patients should contact their physician as soon as possible.

Laboratory Tests

Bacteriologic studies to determine the causative organisms and their susceptibility to the penicillinase-resistant penicillins should be performed (see CLINICAL PHARMACOLOGY – Microbiology). In the treatment of suspected staphylococcal infections, therapy should be changed to another active agent if culture tests fail to demonstrate the presence of staphylococci.

Periodic assessment of organ system function including renal, hepatic, and hematopoietic should be made during prolonged therapy with the penicillinase-resistant penicillins.

Blood cultures, white blood cell, and differential cell counts should be obtained prior to initiation of therapy and at least weekly during therapy with penicillinase-resistant penicillins.

Periodic urinalysis, blood urea nitrogen, and creatinine determinations should be performed during therapy with the penicillinase-resistant penicillins and dosage alterations should be considered if these values become elevated. If any impairment of renal function is suspected or known to exist, a reduction in the total dosage should be considered and blood levels monitored to avoid possible neurotoxic reactions (see DOSAGE AND ADMINISTRATION).

AST (SGOT) and ALT (SGPT) values should be obtained periodically during therapy to monitor for possible liver function abnormalities.

Drug Interactions

Tetracycline, a bacteriostatic antibiotic, may antagonize the bactericidal effect of penicillin and concurrent use of these drugs should be avoided.

Probenecid administered concomitantly with penicillins increases and prolongs serum penicillin levels (see DOSAGE AND ADMINISTRATION).

Penicillinase-resistant penicillins, like other penicillins, are physically and/or chemically incompatible with aminoglycosides and can inactivate the drugs in vitro. In vitro mixing of penicillins and aminoglycosides should be avoided during concomitant therapy and the drugs should be administered separately. Penicillins can inactivate aminoglycosides in vitro in serum samples from patients receiving both drugs, which could produce falsely decreased results in serum aminoglycoside assays of the serum samples.

Dicloxacillin may reduce the anticoagulant response to dicumarol and warfarin. Careful monitoring of prothrombin times is suggested during concomitant therapy, and dosage of the anticoagulant should be adjusted as required. The mechanism of this possible interaction is unclear, but may be due to hepatic enzyme induction.

Carcinogenesis, Mutagenesis, Impairment of Fertility

No long-term animal studies have been conducted with these drugs.

Studies on reproduction (nafcillin) in rats and rabbits reveal no fetal or maternal abnormalities before conception and continuously through weaning (one generation).

Pregnancy Category B

Reproduction studies performed in the mouse, rat, and rabbit have revealed no evidence of impaired fertility or harm to the fetus due to the penicillinase-resistant penicillins. Human experience with the penicillins during pregnancy has not shown any positive evidence of adverse effects on the fetus. There are, however, no adequate or well-controlled studies in pregnant women showing conclusively that harmful effects of these drugs on the fetus can be excluded. Because animal reproduction studies are not always predictive of human response, this drug should be used during pregnancy only if clearly needed.

Nursing Mothers

Penicillins are excreted in breast milk. Caution should be exercised when penicillins are administered to a nursing woman.

Pediatric Use

Because of incompletely developed renal function in newborns, penicillinase-resistant penicillins (especially methicillin) may not be completely excreted, with abnormally high blood levels resulting. Frequent blood levels are advisable in this group with dosage adjustments when necessary. All newborns treated with penicillin should be monitored closely for clinical and laboratory evidence of toxic or adverse effects (see DOSAGE AND ADMINISTRATION). Experience in the neonatal period is limited. Therefore a dose for the newborn is not recommended.

Geriatric Use

Clinical studies of dicloxacillin sodium capsules did not include sufficient numbers of subjects aged 65 and over to determine whether they respond differently from younger subjects. Other reported clinical experience has not identified differences in responses between the elderly and younger patients. In general, dose selection for an elderly patient should be cautious, usually starting at the low end of the dosing range, reflecting the greater frequency of decreased hepatic, renal, or cardiac function, and of concomitant disease or other drug therapy.

Postmarketing Experience

Rare reports have been received during postmarketing surveillance of esophageal burning, esophagitis, and esophageal ulceration, particularly after ingestion of dicloxacillin capsules with an insufficient quantity of water and/or before going to bed. To minimize the risk of developing such events, dicloxacillin should be taken with at least 4 fluid ounces (120 mL) of water and dicloxacillin should NOT be taken in the supine position or immediately before going to bed.

Hypersensitive Reactions

The reported incidence of allergic reactions to penicillin ranges from 0.7 to 10 percent (see WARNINGS). Sensitization is usually the result of treatment but some individuals have had immediate reactions to penicillin when first treated. In such cases, it is thought that the patients may have had prior exposure to the drug via trace amounts present in milk and vaccines.

Two types of allergic reactions to penicillin are noted clinically, immediate and delayed.

Immediate reactions usually occur within 20 minutes of administration and range in severity from urticaria and pruritus to angioneurotic edema, laryngospasm, bronchospasm, hypotension, vascular collapse, and death. Such immediate anaphylactic reactions are very rare (see WARNINGS) and usually occur after parenteral therapy but have occurred in patients receiving oral therapy. Another type of immediate reaction, an accelerated reaction, may occur between 20 minutes and 48 hours after administration and may include urticaria, pruritus, and fever. Although laryngeal edema, laryngospasm, and hypotension occasionally occur, fatality is uncommon.

Delayed allergic reactions to penicillin therapy usually occur after 48 hours and sometimes as late as 2 to 4 weeks after initiation of therapy. Manifestations of this type of reaction include serum sickness-like symptoms (ie, fever, malaise, urticaria, myalgia, arthralgia, abdominal pain) and various skin rashes.

Gastrointestinal Reactions

Nausea, vomiting, diarrhea, stomatitis, black or hairy tongue, and other symptoms of gastrointestinal irritation may occur, especially during oral penicillin therapy.

Pseudomembranous colitis has been reported with the use of dicloxacillin. Therefore, it is important to consider its diagnosis in patients who develop diarrhea in association with dicloxacillin use.

Rare reports have been received during postmarketing surveillance of esophageal burning, esophagitis, and esophageal ulceration, particularly after ingestion of dicloxacillin capsules with an insufficient quantity of water and/or before going to bed (see PRECAUTIONS and DOSAGE AND ADMINISTRATION).

Nervous System Reactions

Neurotoxic reactions similar to those observed with penicillin G (e.g., lethargy, confusion, twitching, multifocal myoclonus, localized or generalized epileptiform seizures) may occur with large intravenous doses of the penicillinase-resistant penicillins especially in patients with renal insufficiency.

Renal Reactions

Renal tubular damage and interstitial nephritis have been associated with the administration of methicillin sodium and infrequently with the administration of nafcillin and oxacillin. Manifestations of this reaction may include rash, fever, eosinophilia, hematuria, proteinuria, and renal insufficiency. Methicillin-induced nephropathy does not appear to be dose-related and is generally reversible upon prompt discontinuation of therapy.

Hematologic Reactions

Eosinophilia, hemolytic anemia, agranulocytosis, neutropenia, leukopenia, granulocytopenia, thrombocytopenia, and bone marrow depression have been associated with the use of penicillinase-resistant penicillins.

Hepatic Reactions

Hepatotoxicity, characterized by fever, nausea, and vomiting associated with abnormal liver function tests, mainly elevated SGOT levels, has been associated with the use of oxacillin and cloxacillin. Cholestatic hepatitis has been reported rarely. Asymptomatic, transient increases in serum concentrations of alkaline phosphatase, AST (SGOT), and ALT (SGPT) have been reported.