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Orally administered erythromycin ethylsuccinate suspensions are readily and reliably absorbed. Erythromycin ethylsuccinate products have demonstrated rapid and consistent absorption in both fasting and nonfasting conditions. However, higher serum concentrations are obtained when these products are given with food. Bioavailability data are available from Ross Products Division. Erythromycin is largely bound to plasma proteins. After absorption, erythromycin diffuses readily into most body fluids. In the absence of meningeal inflammation, low concentrations are normally achieved in the spinal fluid, but the passage of the drug across the blood-brain barrier increases in meningitis. Erythromycin crosses the placental barrier and is excreted in human milk. Erythromycin is not removed by peritoneal dialysis or hemodialysis.
In the presence of normal hepatic function, erythromycin is concentrated in the liver and is excreted in the bile; the effect of hepatic dysfunction on biliary excretion of erythromycin is not known. After oral administration, less than 5% of the administered dose can be recovered in the active form in the urine.
Wide variation in blood levels may result following identical doses of a sulfonamide. Blood levels should be measured in patients receiving these drugs for serious infections. Free sulfonamide blood levels of 50 to 150 mcg/mL may be considered therapeutically effective for most infections, with blood levels of 120 to 150 mcg/mL being optimal for serious infections. The maximum sulfonamide level should be 200 mcg/mL, because adverse reactions occur more frequently above this concentration.
Following oral administration, sulfisoxazole is rapidly and completely absorbed; the small intestine is the major site of absorption, but some of the drug is absorbed from the stomach. Sulfonamides are present in the blood as free, conjugated (acetylated and possibly other forms), and protein-bound forms. The amount present as "free" drug is considered to be the therapeutically active form. Approximately 85% of a dose of sulfisoxazole is bound to plasma proteins, primarily to albumin; 65% to 72% of the unbound portion is in the nonacetylated form.
Maximum plasma concentrations of intact sulfisoxazole following a single 2-g oral dose of sulfisoxazole to healthy adult volunteers ranged from 127 to 211 mcg/mL (mean, 169 mcg/mL), and the time of peak plasma concentration ranged from 1 to 4 hours (mean, 2.5 hours). The elimination half-life of sulfisoxazole ranged from 4.6 to 7.8 hours after oral administration. The elimination of sulfisoxazole has been shown to be slower in elderly subjects (63 to 75 years) with diminished renal function (creatine clearance 37 to 68 mL/min). 1 After multiple-dose oral administration of 500 mg q.i.d. to healthy volunteers, the average steady-state plasma concentrations of intact sulfisoxazole ranged from 49.9 to 88.8 mcg/mL (mean, 63.4 mcg/mL). 2
Sulfisoxazole and its acetylated metabolites are excreted primarily by the kidneys through glomerular filtration. Concentrations of sulfisoxazole are considerably higher in the urine than in the blood. The mean urinary recovery following oral administration of sulfisoxazole is 97% within 48 hours; 52% of this is intact drug, and the remainder is the N 4 -acetylated metabolite.
Sulfisoxazole is distributed only in extracellular body fluids. It is excreted in human milk. It readily crosses the placental barrier. In healthy subjects, cerebrospinal fluid concentrations of sulfisoxazole vary; in patients with meningitis, however, concentrations of free drug in cerebrospinal fluid as high as 94 mcg/mL have been reported.
Pediazole (erythromycin and sulfisoxazole) has been formulated to contain sulfisoxazole for concomitant use with erythromycin.
Erythromycin acts by inhibition of protein synthesis by binding 50 S ribosomal subunits of susceptible organisms. It does not affect nucleic acid synthesis. Antagonism has been demonstrated in vitro between erythromycin and clindamycin, lincomycin, and chloramphenicol.
The sulfonamides are bacteriostatic agents, and the spectrum of activity is similar for all. Sulfonamides inhibit bacterial synthesis of dihydrofolic acid by preventing the condensation of the pteridine with para -aminobenzoic acid through competitive inhibition of the enzyme dihydropteroate synthetase. Resistant strains have altered dihydropteroate synthetase with reduced affinity for sulfonamides or produce increased quantities of para -aminobenzoic acid.
Quantitative methods that require measurement of zone diameter give the most precise estimates of the susceptibility of bacteria to antimicrobial agents. One such standardized single-disc procedure 3 has been recommended for use with discs to test susceptibility to erythromycin and sulfisoxazole. Interpretation involves correlation of the zone diameters obtained in the disc test with minimal inhibitory concentration (MIC) values for erythromycin and sulfisoxazole.
If the standardized procedure of disc susceptibility is used, a 15-mcg erythromycin disc should give a zone diameter of at least 18 mm when tested against an erythromycin-susceptible bacterial strain, and a 250-300 mcg sulfisoxazole disc should give a zone diameter of at least 17 mm when tested against a sulfisoxazole-susceptible bacterial strain.
In vitro sulfonamide susceptibility tests are not always reliable because media containing excessive amounts of thymidine are capable of reversing the inhibitory effect of sulfonamides, which may result in false resistant reports. The tests must be carefully coordinated with bacteriological and clinical responses. When the patient is already taking sulfonamides, follow-up cultures should have aminobenzoic acid added to the isolation media but not to subsequent susceptibility test media.
Last reviewed on RxList: 12/8/2004
This monograph has been modified to include the generic and brand name in many instances.
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