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.