Subtherapeutic dicloxacillin levels in a neonate: possible mechanisms

A neonate treated initially with oxacillin intravenously for two weeks and who was receiving phenobarbital for a seizure disorder subsequently failed to achieve therapeutic levels of orally administered dicloxacillin, even when the dosage was as high as 175 mg/kg/day. Intestinal absorption was documented by high serum peak levels. The low trough levels correlated with a high urinary excretion rate. The possibillity that renal tubular transport of dichloxacillin was stimulated by administration of penicillin derivatives (and/or phenobarbital) is suggested, and the need for careful monitoring of serum levels of antibiotics in neonates is emphasized.

Correction of protein binding defect in uremic sera by charcoal treatment

Protein binding of numerous drugs, primarily organic acids, is decreased in sera from uremic patients. The defect in binding is (1) greater than can be accounted for by hypoalbuminemia alone; (2) unchanged by prolonged in vitro dialysis; (3) transferred in the protein but not the ultrafiltrate fraction of uremic serum; and (4) not reproduced by addition of low and middle molecular weight compounds known to accumulate in uremia. However, treatment with activated charcoal at pH3 was found to significantly increase drug protein binding in uremic sera. This effect was studied with six different drugs in sera from groups of 6 normal subjects and 8 patients on chronic hemodialysis. The percentage of sulfamethoxazole, dicloxacillin, diphenylhydantoin, salicylate, and digitoxin bound to protein in normal sera (65.9, 97.1, 93.1, 96.7, and 92.7, respectively) was unchanged by charcoal treatment. In contrast, charcoal treatment significantly (p less than 0.01) increased the percentage of drug bound to protein in uremic sera from 41.7 to 59.0 for sulfamethoxazole, 90.7 to 96.3 for dicloxacillin, 84.3 to 90.8 for diphenyhydantoin, 86.4 to 93.8 for salicylate, and 89.5 to 90.9 for digitoxin. Charcoal treatment significantly (p less than 0.05) reduced penicillin protein binding in normal sera and failed to correct the binding defect for penicillin in uremic sera. The effect of charcoal can be explained by removal of an inhibitor which accumulates in uremia and (1) occupies the binding site of certain drugs, (2) changes the configuration of the albumin molecule, or (3) both. Free fatty acid (FFA) concentrations in uremic patients were similar to those in normal subjects and were not the cause of the binding defect.

Uptake of antibiotics by human erythrocytes

Human erythrocytes have been used as a model system for the study of uptake and release of antibiotics. Penicillin G, dicloxacillin, tetracycline, and minocycline were all taken up by the cells, but each showed a characteristic ratio of distribution between the extracellular and intracellular compartments. Comparison of the penicillin analogues indicated that dicloxacillin, the more lipid-soluble compound, reached higher intracellular concentrations than did penicillin G. Use of human plasma as the incubation medium markedly decreased antibiotic uptake and enhanced antibiotic egress from preloaded cells. These effects were related to the binding of drugs to serum proteins. In vivo studies in which penicillin G was injected intravenously by bolus and was then given by constant infusion showed that intraerythrocytic concentrations of drug after 2 hr approached or exceeded those in plasma. These results can be explained, in part, by the fact that the slower rate of efflux of penicillin G from the cells than of clearance from plasma serves to maintain the high initial levels of drug for a longer period.

Comparative studies of the activity of ciclacillin and dicloxacillin

The penicillins ciclacillin and dicloxacillin demonstrate marked similarities in biological activity but, as far as can be determined, differ substantially in respect to the degree of protein binding, which is relatively low for ciclacillin and relatively high for dicloxacillin. In mice infected with Staphylococcus aureus Smith, ciclacillin is considerably more active than dicloxacillin, although both drugs are similarly effective in vitro and similarly absorbed and eliminated in vivo. The high degree of protein binding exhibited by dicloxacillin could therefore very probably explain its relatively low chemotherapeutic activity. Moreover, the in vitro and in vivo findings of the study are inconsistent with the tenets of the tau/2 thesis.

Pharmacokinetic interpretation of dicloxacillin levels in serum after extravascular administration

Evidence was found for the existence of an episome-specified variant of the enzyme dihydropteroate synthase involved in folic acid formation. Since the plasmid-borne enzyme showed a decreased susceptibility for sulfonamide inhibition and was transferable together with resistance to this drug, it is proposed that diploidy for the target enzyme in some cases could be the mechanism of R-factor-mediated resistance to sulfonamides. Two types of evidence were obtained. One was the rescue from temperature sensitivity of bacterial mutants with a lesion in the chromosomal dihydropteroate synthase by the R factor R1dr19 mediating sulfonamide resistance. The other evidence was found by the determination of dihydropteroate forming activity in extracts from R⁻ and R⁺ bacteria. Cells harboring R1dr19 were found to contain an enzyme activity which was far less susceptible to sulfonamide inhibition than the corresponding activity from R⁻ cells.

Enhanced renal excretion of dicloxacillin in patients with cystic fibrosis

Single oral doses of 6.25 mg/kg of dicloxacillin suspension were given to ten cystic fibrosis (CF) patients and eight normal subjects. Peak serum concentrations and areas under the concentration versus time curves for dicloxacillin were variable and, on average, were 2 1/2 times lower in the CF patients. The time of occurrence of the peak serum concentration was similar in both groups and the total urinary recovery of dicloxacillin was normal or increased in the CF patients, suggesting that the intestinal absorption of the drug was unaffected by the disease. The low serum concentrations in the CF patients were caused by unusually high renal clearances of dicloxacillin which average 282 +/- 135 compared to 95 +/- 28 ml/min/1.73 sq m in the normal subjects. Creatinine clearances were also elevated in the CF patients by 55% on average, while urea clearances were normal. The serum protein binding of dicloxacillin was similar in both groups of subjects. Because the rapid excretion results in low and variable serum concentrations of the antibiotic, treatment of CF patients with dicloxacillin may warrant use of increased or more frequent doses and monitoring of serum antibiotic levels.

Treatment of chronic staphylococcal osteomyelitis with cloxacillin and dicloxacillin--a comparative study in 12 patients

12 patients with chronic staphylococcal osteomyelitis were treated with cloxacillin (6 patients) and dicloxacillin (6 patients) in a prospective, comparative study. The drugs were given perorally in the dose 1 g every 4th hour and serum concentrations were followed. 0.5, 1 and 2 hours after administration the serum level of dicloxacillin was about double that of cloxacillin. The lowest serum level of dicloxacillin at the time of administration of the drugs was nearly 3 times as high as the level of cloxacillin. An evaluation of clinical benefit and side-effects revealed no differences between the drugs. The prolonged serum level of dicloxacillin may permit a longer interval between doses than the administration of cloxacillin.

Flucloxacillin, a new isoxazolyl penicillin, compared with oxacillin, cloxacillin, and dicloxacillin

Flucloxacillin, a new isoxazole penicillin, is active against penicillinase-producing strains of Staphylococcus aureus and is well absorbed in man after oral and intramuscular administration. Compared with isoxazole penicillins in current clinical use-namely, oxacillin, cloxacillin, and dicloxacillin-flucloxacillin has proved as active against Gram-positive cocci, including penicillin-resistant staphylococci. The extent of binding of flucloxacillin to the protein of human serum was similar to that of oxacillin and cloxacillin and less than that of dicloxacillin. In man flucloxacillin given orally produced total and free serum levels higher than those obtained with oxacillin and cloxacillin; total serum levels similar to those of dicloxacillin, and free levels greater than those of dicloxacillin. Similarly, after intramuscular injection the free serum levels of flucloxacillin were higher than those of oxacillin, cloxacillin, and dicloxacillin.