Clinical pharmacokinetics of clarithromycin

Clarithromycin is a macrolide antibacterial that differs in chemical structure from erythromycin by the methylation of the hydroxyl group at position 6 on the lactone ring. The pharmacokinetic advantages that clarithromycin has over erythromycin include increased oral bioavailability (52 to 55%), increased plasma concentrations (mean maximum concentrations ranged from 1.01 to 1.52 mg/L and 2.41 to 2.85 mg/L after multiple 250 and 500mg doses, respectively), and a longer elimination half-life (3.3 to 4.9 hours) to allow twice daily administration. In addition, clarithromycin has extensive diffusion into saliva, sputum, lung tissue, epithelial lining fluid, alveolar macrophages, neutrophils, tonsils, nasal mucosa and middle ear fluid. Clarithromycin is primarily metabolised by cytochrome P450 (CYP) 3A isozymes and has an active metabolite, 14-hydroxyclarithromycin. The reported mean values of total body clearance and renal clearance in adults have ranged from 29.2 to 58.1 L/h and 6.7 to 12.8 L/h, respectively. In patients with severe renal impairment, increased plasma concentrations and a prolonged elimination half-life for clarithromycin and its metabolite have been reported. A dosage adjustment for clarithromycin should be considered in patients with a creatinine clearance <1.8 L/h. The recommended goal for dosage regimens of clarithromycin is to ensure that the time that unbound drug concentrations in the blood remains above the minimum inhibitory concentration is at least 40 to 60% of the dosage interval. However, the concentrations and in vitro activity of 14-hydroxyclarithromycin must be considered for pathogens such as Haemophilus influenzae. In addition, clarithromycin achieves significantly higher drug concentrations in the epithelial lining fluid and alveolar macrophages, the potential sites of extracellular and intracellular: respiratory tract pathogens, respectively. Further studies are needed to determine the importance of these concentrations of clarithromycin at the site of infection. Clarithromycin can increase the steady-state concentrations of drugs that are primarily depend upon CYP3A metabolism (e.g., astemidole, cisapride, pimozide, midazolam and triazolam). This can be clinically important for drugs that have a narrow therapeutic index, such as carbamazepine, cyclosporin, digoxin, theophylline and warfarin. Potent inhibitors of CYP3A (e.g., omeprazole and ritonavir) may also alter the metabolism of clarithromycin and its metabolites. Rifampicin (rifampin) and rifabutin are potent enzyme inducers and several small studies have suggested that these agents may significantly decrease serum clarithromycin concentrations. Overall, the pharmacokinetic and pharmacodynamic studies suggest that fewer serious drug interactions occur with clarithromycin compared with older macrolides such as erythromycin and troleandomycin.