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ObjectiveSeveral reports indicate that fluvoxamine decreases the clearance of cytochrome P4501A2 (CYP1A2) substrates. This study compared in vitro and in vivo inhibition potencies of fluvoxamine toward CYP1A2 with an approach based on inhibition constants (Ki) determined in vitro and in vivo.MethodsIn vitro inhibition constant values were determined with human liver microsomes and complementary deoxyribonucleic acid‐expressed CYP1A2 (supersomes). Fluvoxamine in vivo inhibition constants (Kiiv) for CYP1A2 were obtained from an investigation of single‐dose theophylline (250 mg) disposition in 9 healthy volunteers receiving steady‐state (9 days) fluvoxamine at 3 doses (0, 25, or 75 mg/d) in a randomized crossover design.ResultsIn vitro Ki values based on total inhibitor concentrations were 177 ±56 nmol/L, 121 ±21 nmol/L, and 52 ±13 nmol/L in human liver microsomes with 1 mg/ml protein and 0.5 mg/ml protein and in supersomes with 0.3 mg/ml protein, respectively. The corresponding in vitro Ki values based on unbound fluvoxamine concentrations were 35 nmol/L, 36 nmol/L, and 36 nmol/L. The ratio of 1‐methyluric acid formation clearances (control/inhibited) in 8 subjects was positively correlated with fluvoxamine concentration (r2 = 0.87; P < .001) with an intercept near 1. Mean values for Kiiv based on total and unbound plasma concentrations at steady state were 25.3 nmol/L (range, 14–39 nmol/L) and 3.6 nmol/L (range, 2.4–5.9 nmol/L), respectively.ConclusionComparison of in vitro and in vivo Ki values based on unbound fluvoxamine concentrations suggests that fluvoxamine inhibition potency is approximately 10 times greater in vivo than in vitro.Clinical Pharmacology & Therapeutics (2001) 70, 415–424; doi: 10.1016/S0009‐9236(01)62844‐3

ABSTRACT:A previous study suggested that fluvoxamine inhibition potency toward CYP1A2 is 10 times greater in vivo than in vitro. The present study was designed to determine whether the same gap exists for CYP2C19, another isozyme inhibited by fluvoxamine. In vitro studies examined the effect of nonspecific binding on the determination of inhibition constant (K i ) values of fluvoxamine toward CYP2C19 in human liver microsomes and in a cDNA-expressed microsomal (Supersomes) system using (S)-mephenytoin as a CYP2C19 probe. K i values based on total added fluvoxamine concentration (K i,total ) and unbound fluvoxamine concentration (K i,ub ) were calculated, and interindividual variability in K i values was examined in six nonfatty livers. There has been a growing interest in predicting in vivo metabolic drug-drug interactions from in vitro systems. In the case of inhibitionbased interactions, there is no consensus on the methodology for accurate predictions of the extent of in vivo inhibition based on in vitro data (Schmider et al., 1999;Yamano et al., 1999;Kohl and Steinkellner, 2000;Komatsu et al., 2000;Yao and Levy, 2002). Several issues remain unsolved, such as estimations of inhibition constants in vitro and inhibitor concentration around the enzyme site in vivo. For example, studies on fluvoxamine inhibition of CYP1A2 have shown that in vitro K i values varied with microsomal protein concentration (Yao et al., 2001), suggesting that the concentration of microsomal protein present in the incubation is a factor contributing to the variance in in vitro K i . However, even after correction for nonspecific binding of fluvoxamine in microsomes, there was still a 10-fold difference between the in vitro inhibition constant and the corresponding in vivo inhibition constant based on unbound fluvoxamine concentration in plasma.The 10-fold underprediction of fluvoxamine inhibition potency toward CYP1A2 activity in vivo, based on in vitro data, may be due to a number of factors. These include, but are not limited to, active uptake of fluvoxamine from plasma into hepatocytes, thereby increasing the amount of inhibitor available to the enzyme (partitioning), the presence of inhibitory metabolites of fluvoxamine in plasma, and/or environmental differences that may differentially affect enzyme behavior or affinity in the two systems. Should it be the case that some type of partitioning alone is the dominant factor governing the in vitro-in vivo difference, one might expect that the RK i (the ratio of the in vitro K i to the in vivo K i based on unbound concentration) of any enzyme inhibited by fluvoxamine would be similar to that of CYP1A2. In essence, RK i would be largely enzyme independent. However, if inhibitory metabolites or enzyme environment play important roles in promoting the RK i difference, RK i values might become enzyme-dependent. A suitable candidate enzyme was sought to test the hypothesis that RK i values will be conserved for a single inhibitor among the family of P450 1 enzymes. There is some in vivo evide...

Summary:Purpose: To characterize the pharmacokinetics of the new antiepileptic and CNS drug RWJ-333369 following single and multiple oral doses to healthy subjects, including the effect of food on bioavailability.Method: Two studies were conducted. The first study had a randomized, double-blind, placebo-controlled, sequential, ascending-dose crossover design. Subjects were divided into four dose groups (100, 250, 500, and 750 mg) of 10 to 11subjects each. RWJ-333369 or placebo was administered for two 7-day periods, separated by a 14-day washout. In the second study RWJ-333369 (750 mg) was administered to 12 healthy subjects under fasted and fed conditions. Plasma and urine samples were analyzed for RWJ-333369 by liquid chromatography-mass spectroscopy. Safety was assessed throughout the studies.Results: Mean (range) pharmacokinetic parameters in the above studies were: oral clearance (CL/F) 3.4-4.2 L/h, halflife (t 1/2 ) 10.6-12.8 h, and renal clearance (CLr) 0.042-0.094 L/h, indicating that RWJ-333369 is eliminated primarily by metabolism. These parameters were not significantly different (p > 0.05) for the four dose groups and for single and multiple dosing. C max and AUC increased proportionally with dose and decreased with food by 11% and 5%, respectively.Conclusions: Following single and repetitive (q12h) doses of 100-750 mg, RWJ-333369 had linear pharmacokinetics; food did not alter pharmacokinetics to a clinically relevant extent. RWJ-333369 is extensively metabolized and has a low CL/F that equals < 5% of the liver blood flow. Thus, orally administered RWJ-333369 has no hepatic first-pass effect. The 12-h half-life will enable bid dosing with an immediate-release oral formulation. Key words: RWJ-333369-New antiepileptic drugPharmacokinetics-Healthy subjects-Food effect.In the last decade 10 new antiepileptic drugs (AEDs) have been introduced, offering favorable pharmacokinetics, better tolerability and a lower potential for drug interactions (Bialer et al., 2002;Bialer et al., 2004;Perucca, 2000). In addition, the availability of old and new AEDs with varying spectra of activity and tolerability profiles enable clinicians to better tailor drug choice to the characteristics of the individual patients (Perucca, 2000). Despite the large therapeutic arsenal of old and new AEDs, about 30% of patients with epilepsy are still not seizure-free; thus, there is a substantial need to develop new AEDs (Walker and Sander, 1996). In this respect, the new AEDs developed thus far are not completely effective, as they allow achievement of a seizure-free status in no more than 15-20% of previously refractory patients (Perucca, 2000). (Fig. 1) is a new AED and CNS drug that is currently undergoing phase II clinical trials. The drug has a wide spectrum of anticonvulsant activity, with ED 50 ranging from 5 to 60 mg/kg in various rodent models, and there is a substantial safety margin between effective and neurotoxic doses. RWJ-333369 substantially reduced spontaneous seizures in rats with kainite-induced epilepsy to a grea...

Comparison of in vitro and in vivo K(i) values based on unbound fluvoxamine concentrations suggests that fluvoxamine inhibition potency is approximately 10 times greater in vivo than in vitro.