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. 