Cytochrome P450 1A2 is a major determinant of lidocaine metabolism in vivo: effects of liver function

OBJECTIVES: This study was designed (1) to evaluate the effect of a cytochrome P450 (CYP) 1A2 inhibitor, fluvoxamine, on the pharmacokinetics of intravenous lidocaine and its 2 pharmacologically active metabolites, monoethylglycinexylidide (MEGX) and glycinexylidide (GX), to confirm recent in vitro results indicating that CYP1A2 is the main isoform responsible for lidocaine biotransformation and (2) to assess whether liver function has any influence on the fluvoxamine-lidocaine interaction. METHODS: The study was carried out in 10 healthy volunteers and 20 patients with cirrhosis, 10 with mild (Child grade A) and 10 with severe (Child grade C) liver dysfunction, according to a randomized, double-blind, 2-phase, crossover design. In one phase all participants received placebo for 6 days; in the other phase they received 50 mg fluvoxamine for 2 days and 100 mg fluvoxamine for the next 4 days. On day 6, a 1-mg/kg lidocaine dose was administered intravenously 2 hours after the last dose of fluvoxamine or placebo. Plasma concentrations of lidocaine, MEGX, GX, and fluvoxamine were measured up to 12 hours after lidocaine injection. RESULTS: The effects of fluvoxamine coadministration were dependent on liver function. Lidocaine clearance was decreased on average by 60% (from 12.1 mL/min.kg to 4.85 mL/min.kg, P .05). Analogous effects were observed on MEGX and GX formation kinetics, which were drastically impaired in healthy subjects and patients with mild liver cirrhosis but virtually unaffected in patients with severe cirrhosis. CONCLUSION: CYP1A2 is the enzyme principally responsible for the metabolic disposition of lidocaine in subjects with normal liver function. The extent of fluvoxamine-lidocaine interaction decreases as liver function worsens, most likely because of the concomitant decrease in the hepatic level of CYP1A2. These observations indicate that results obtained in healthy subjects cannot be extended a priori to patients with liver dysfunction, but the clinical consequences of inhibition of drug metabolism must also be assessed in such patients