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Five serial exposures of mucoid Pseudomonas aeruginosa from patients with cystic fibrosis to subinhibitory concentrations of ciprofloxacin resulted in stepwise increases in the MIC, with a mean proportional increase of 10. MICs were significantly lower in an iron-limited chemically defined medium than in Iso-Sensitest broth. The mucoid phenotype was maintained in chemically defined medium. Acquired resistance was retained either partially or completely in 85% of the isolates following 10 transfers in drug-free media. In cases in which susceptibility was regained, an increase in the MIC was observed on one further exposure to ciprofloxacin.

An in-vitro pharmacokinetic model was used to simulate ciprofloxacin concentrations in serum observed in vivo following oral doses of 250, 500 and 750 mg, in a culture of Pseudomonas aeruginosa isolated from the sputum of a cystic fibrosis patient. In the 12 h interval following each dose, a period of bactericidal activity was observed, succeeded by bacteriostasis and subsequent bacterial regrowth. In all cases bacterial regrowth was first observed when the ciprofloxacin concentration in the culture flask of the dynamic model fell below the MIC. For the three doses investigated, the surviving bacteria at the end of each 12-h dosing interval showed an increase in MIC compared with that of the culture before ciprofloxacin exposure. The post-antibiotic effect of ciprofloxacin against P. aeruginosa was observed for 1-5 h, and was dose related.

Twelve cystic fibrosis patients, aged over 18, who had developed an acute respiratory exacerbation and who had Pseudomonas species isolated from their sputum, were entered into a clinical trial involving ciprofloxacin. The dosage regimen was 100 mg iv followed by 500 mg twice daily orally if less than 40 kg in weight and 200 mg iv followed by 750 mg twice daily orally if greater than 40 kg. Ciprofloxacin was well tolerated with no major side effects, except in one patient who withdrew after onset of headaches and generalized aches and pains. Eleven of the 12 patients showed clinical improvement at the end of the treatment period as determined by weight gain, Shwachman Score, Chrispin Norman Score and pulmonary function tests. MICs of Pseudomonas species isolated from the sputum at the start of the trial were in the range 0.25-4 mg/l. During therapy, sensitivity of isolates decreased and did not return to starting levels at the end of a four week follow-up period. Pharmacokinetic parameters were similar to those reported for fasting healthy volunteers by other workers except for bioavailability which was reduced in the non-fasting patients.