The occurrence of idiosyncratic adverse drug reactions during late clinical trials or after a drug has been released can lead to a severe restriction in its use and even in its withdrawal. Metabolic activation of relatively inert functional groups to reactive electrophilic intermediates is considered to be an obligatory event in the etiology of many drug-induced adverse reactions. Therefore, a thorough examination of the biochemical reactivity of functional groups/structural motifs in all new drug candidates is essential from a safety standpoint. A major theme attempted in this review is the comprehensive cataloging of all of the known bioactivation pathways of functional groups or structural motifs commonly utilized in drug design efforts. Potential strategies in the detection of reactive intermediates in biochemical systems are also discussed. The intention of this review is not to "black list" functional groups or to immediately discard compounds based on their potential to form reactive metabolites, but rather to serve as a resource describing the structural diversity of these functionalities as well as experimental approaches that could be taken to evaluate whether a "structural alert" in a new drug candidate undergoes bioactivation to reactive metabolites.

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The role of single electron transfer (SET) in P450-catalyzed N-dealkylation reactions has been studied using the probe substrates N-cyclopropyl-N-methylaniline (2a) and N-(1'-methylcyclopropyl)-N-methylaniline (2b). In earlier work, we showed that SET oxidation of 2a by horseadish peroxidase leads exclusively to products arising via fragmentation of the cyclopropane ring [Shaffer, C. L.; Morton, M. D.; Hanzlik, R. P. J. Am. Chem. Soc. 2001, 123, 8502-8508]. In the present study, we found that liver microsomes from phenobarbital pretreated rats (which contain CYP2B1 as the predominant isozyme) oxidize [1'-(13)C, 1'-(14)C]-2a efficiently (80% consumption in 90 min). Disappearance of 2a follows first-order kinetics throughout, indicating a lack of P450 inactivation by 2a. HPLC examination of incubation mixtures revealed three UV-absorbing metabolites: N-methylaniline (4), N-cyclopropylaniline (6a), and a metabolite (M1) tentatively identified as p-hydroxy-2a, in a 2:5:2 mole ratio, respectively. 2,4-Dinitrophenylhydrazine trapping indicated formation of formaldehyde equimolar with 6a; 3-hydroxypropionaldehyde and acrolein were not detected. Examination of incubations of 2a by (13)C NMR revealed four (13)C-enriched signals, three of which were identified by comparison to authentic standards as N-cyclopropylaniline (6a, 33.6 ppm), cyclopropanone hydrate (11, 79.2 ppm), and propionic acid (12, 179.9 ppm); the fourth signal (42.2 ppm) was tentatively determined to be p-hydroxy-2a. Incubation of 2a with purified reconstituted CYP2B1 also afforded 4, 6a, and M1 in a 2:5:2 mole ratio (by HPLC), indicating that all metabolites are formed at a single active site. Incubation of 2b with PB microsomes resulted in p-hydroxylation and N-demethylation only; no loss or ring-opening of the cyclopropyl group occurred. These results effectively rule out the participation of a SET mechanism in the P450-catalyzed N-dealkylation of cyclopropylamines 2a and 2b, and argue strongly for the N-dealkylation of 2a via a carbinolamine intermediate formed by a conventional C-hydroxylation mechanism.

A comprehensive analysis on the prediction of human clearance based on intravenous pharmacokinetic data from rat, dog, and monkey for approximately 400 compounds was undertaken. This data set has been carefully compiled from literature reports and expanded with some in-house determinations for plasma protein binding and rat clearance. To the authors- knowledge, this is the largest publicly available data set. The present examination offers a comparison of 37 different methods for prediction of human clearance across compounds of diverse physicochemical properties. Furthermore, this work demonstrates the application of each prediction method to each charge class of the compounds, thus presenting an additional dimension to prediction of human pharmacokinetics. In general, the observations suggest that methods employing monkey clearance values and a method incorporating differences in plasma protein binding between rat and human yield the best overall predictions as suggested by approximately 60% compounds within 2-fold geometric mean-fold error. Other single-species scaling or proportionality methods incorporating the fraction unbound in the corresponding preclinical species for prediction of free clearance in human were generally unsuccessful.