Clinical Pharmacokinetics of the Newer Antibacterial 4-Quinolones

Abstract

Structural modification of the so-called first-generation’ or ‘urinary’ quinolones has led to a considerable increase in their intrinsic antibacterial activity, together with marked changes in the pharmacokinetic properties. Tissue penetration is the most notable change, and the newer quinolones are comparable with the newer broad spectrum β-lactams in their clinical spectrum of activity. Marketed compounds in the 4-quinolones group include pefloxacin, ofloxacin, enoxacin, ciprofloxacin and norfloxacin; many more compounds are in various stages of research and development. The 4-quinolones act by inhibition of bacterial DNA gyrase, a process which is pH and concentration dependent. The bactericidal activity can be partly abolished if protein syn thesis is inhibited by chloramphenicol, or if RNA synthesis is inhibited by rifampicin (rifampin). The antibacterial spectrum of activity includes methicillin- and gentamicin-resistant staphylococci, multiresistant non-fermenters, all Enterobacteriaceae, Legionella, Neisseria species, Branhamella and Haemophilus influenzae. With the exception of norfloxacin, which is only 30 to 40% bioavailable from the oral route, the 4-quinolones are 80 to 100% bioavailable, absorption occurring within 1 to 3 hours. Food does not significantly alter C_max, AUC or elimination half-life, although t_max, may be increased. The 4-quinolones are widely distributed throughout the body, with volumes of distribution greater than 1.5 L/kg. Protein binding is less than 30% in most cases. Penetration into most tissues is good. With the exception of ofloxacin and lomefloxacin (NY 198), which are metabolically stable, metabolism of the 4-quinolones occurs primarily at the C7 position in the piperazinyl ring. Biotransformation is extensive (85%) with pefloxacin, medium (25 to 40%) with ciprofloxacin and enoxacin, and low (<20%) with norfloxacin. Elimination half-lives vary between 3 and 5 hours (ciprofloxacin) and 8 to 14 hours (pefloxacin). Biliary concentrations of the 4-quinolones are 2 to 10 times greater than those in serum or plasma, with several compounds undergoing enterohepatic circulation. There is some evidence that ciprofloxacin, norfloxacin, ofloxacin and enoxacin have an active renal tubular excretion pathway. In impaired renal function, reduction of the glomerular filtration rate below 30 ml/min (1.8 L/h) is associated with an increase in elimination half-life and AUC, and a decrease in renal and total clearance of the 4-quinolones, and a decrease in 24-hour urinary recovery. Thus, with the exception of pefloxacin, dosage adjustment of the 4-quinolones is recommended, according to the degree of reduction in creatinine clearance. Further reductions may be necessary for ciprofloxacin in concomitant liver failure because of the extensive non-renal clearance of the drug. Ciprofloxacin and pefloxacin are partly removed by haemodialysis, but ofloxacin is not cleared by this technique. Although dosage adjustment is recommended in impaired renal function, dialysate concentrations should be monitored to ensure that inhibitory concentrations are maintained. Drug interactions between 4-quinolones have been reported between theophylline, cimetidine and non-steroidal anti-inflammatory drugs. However, the clinical significance of these interactions has not been fully elucidated. The good tissue penetration of the 4-quinolones, despite considerable interindividual variation, has been associated with good response rates in a number of types of infection. However, because bacteriological follow-up was less than ideal in many trials, these promising early results require confirmation in larger studies.