The recognition of a diarylimine as a metabonate produced during incubation of N-benzyl-4-chloroaniline with hepatic microsomal preparations

Reactive Metabolites from Thiazole-Containing Drugs: Quantum Chemical Insights into Biotransformation and Toxicity

Drugs containing thiazole and aminothiazole groups are known to generate reactive metabolites (RMs) catalyzed by cytochrome P450s (CYPs). These RMs can covalently modify essential cellular macromolecules and lead to toxicity and induce idiosyncratic adverse drug reactions. Molecular docking and quantum chemical hybrid DFT study were carried out to explore the molecular mechanisms involved in the biotransformation of thiazole (TZ) and aminothiazole (ATZ) groups leading to RM epoxide, S-oxide, N-oxide, and oxaziridine. The energy barrier required for the epoxidation is 13.63 kcal/mol, that is lower than that of S-oxidation, N-oxidation, and oxaziridine formation (14.56, 17.90, and 20.20, kcal/mol respectively). The presence of the amino group in ATZ further facilitates all the metabolic pathways, for example, the barrier for the epoxidation reaction is reduced by ∼2.5 kcal/mol. Some of the RMs/their isomers are highly electrophilic and tend to form covalent bonds with nucleophilic amino acids, finally leading to the formation of metabolic intermediate complexes (MICs). The energy profiles of these competitive pathways have also been explored.

Steroidal Aglycones from Stems of Marsdenia tenacissima that Inhibited the Hedgehog Signaling Pathway

Two novel steroidal aglycones, together with four known ones, were isolated from the hydrolysis extract of the CHCl3 soluble extract of the stems of Marsdenia tenacissima. Their structures were determined on the basis of chemical evidence and extensive spectroscopic methods, including 1D and 2D NMR spectroscopy. These compounds displayed inhibition of the Hedgehog signaling pathway in vitro.

High-resolution mass spectrometry elucidates metabonate (false metabolite) formation from alkylamine drugs during in vitro metabolite profiling

In vitro metabolite profiling and characterization experiments are widely employed in early drug development to support safety studies. Samples from incubations of investigational drugs with liver microsomes or hepatocytes are commonly analyzed by liquid chromatography/mass spectrometry for detection and structural elucidation of metabolites. Advanced mass spectrometers with accurate mass capabilities are becoming increasingly popular for characterization of drugs and metabolites, spurring changes in the routine workflows applied. In the present study, using a generic full-scan high-resolution data acquisition approach with a time-of-flight mass spectrometer combined with postacquisition data mining, we detected and characterized metabonates (false metabolites) in microsomal incubations of several alkylamine drugs. If a targeted approach to mass spectrometric detection (without full-scan acquisition and appropriate data mining) were employed, the metabonates may not have been detected, hence their formation underappreciated. In the absence of accurate mass data, the metabonate formation would have been incorrectly characterized because the detected metabonates manifested as direct cyanide-trapped conjugates or as cyanide-trapped metabolites formed from the parent drugs by the addition of 14 Da, the mass shift commonly associated with oxidation to yield a carbonyl. This study demonstrates that high-resolution mass spectrometry and the associated workflow is very useful for the detection and characterization of unpredicted sample components and that accurate mass data were critical to assignment of the correct metabonate structures. In addition, for drugs containing an alkylamine moiety, the results suggest that multiple negative controls and chemical trapping agents may be necessary to correctly interpret the results of in vitro experiments.

IDENTIFICATION OF A NOVEL IN VITRO METABONATE FROM LIVER MICROSOMAL INCUBATIONS

In vitro drug metabolism studies during the early drug discovery stage are becoming increasingly important. With the increasing demand for high throughput and quick turnaround time for in vitro metabolism studies, however, careful examination of the results and proper design of the experiments are still crucial. In this communication, we report the identification and mechanism of formation of a novel metabonate from incubations of a diamine-containing compound with liver microsomes. The metabonate appeared to be the major product, and its formation was NADPH- and microsomal protein-dependent. Liquid chromatography/mass spectrometry and NMR analysis of the metabonate indicated an extra carbon and unusual formation of an imidazolidine ring. Further studies revealed that this metabonate was not a true biotransformation product from the diamine compound itself in the microsomal incubation, but rather a product resulting from a condensation reaction between the compound and a metabolite of the solvent (alcohol) used in the incubation. When the microsomal incubations contained a small amount of methanol or ethanol as solvent, the alcohols were metabolized to formaldehyde or acetaldehyde, which then condensed with the diamine compound through an imine intermediate to form the metabonate. The compound itself was metabolically stable in vitro when acetonitrile or dimethyl sulfoxide was used as solvent. During the study of in vitro microsomal stability and metabolite identification of amine-containing compounds, the use of alcohol as solvent should be avoided.

Metabolic and chemical studies on N-(4-chlorobenzyl)-N'-benzoylhydrazine

The in vitro hepatic microsomal metabolism of N-(4-chlorobenzyl)-N'-benzoylhydrazine (CBBAH), a model compound representing N-alkyl substituted hydrazides, was studied using hepatic washed rat microsomal preparations fortified with NADPH to identify the possible N-oxidative, N-dealkylated and hydrolytic metabolites. CBBAH and its potential metabolites were prepared, characterized using spectroscopic techniques and then separated using a reversed phase HPLC system with UV detection at 254 nm. CBBAH was chemically converted to the corresponding hydrazone by m-chloroperbenzoic acid (m-CPBA) oxidation. CBBAH was incubated with rat microsomal preparations in the presence of NADPH, extracted into dichloromethane and evaporated finally under nitrogen. The TLC and HPLC results from the metabolic experiments showed that CBBAH produced the corresponding hydrolytic and N-dealkylated metabolites together with the corresponding hydrazone.

The in vitro hepatic microsomal metabolism of N-benzyladamantanamine in rats

The metabolism of N-benzyladamantanamine (NBAD) was studied in vitro using rat hepatic microsomal preparations. The substrate and proposed metabolites were synthesized and characterized using spectroscopic techniques and separated using a reverse phase HPLC system. NBAD was incubated with rat microsomal preparations, extracted into DCM in the presence of NaCl and evaporated under a stream of nitrogen. The results from HPLC studies showed that NBAD produced the corresponding nitrone and hydroxylamine. This experiment also revealed that dealkylation occurred. No metabolites were observed which corresponded to authentic amide or oxaziridine. The reactions required a microsomal enzyme source and NADPH as a cofactor. The results indicate that the nitrone observed as a metabolite of NBAD is not an intermediate leading to the formation of an oxaziridine and hence an amide, under careful experimental conditions excluding light.

Studies on the in vitro hepatic microsomal formation of amides during the metabolism of certain secondary and tertiary benzylic amines

Part of our interest during the last few years has been to investigate the possible intermediate(s) and mechanism(s) involved in the formation of amides from N-benzylic amines. A number of benzylic amines with different aryl and alkyl moieties introduced onto the constituent nitrogen were prepared, thus creating a wide variety of secondary, tertiary and heterocyclic benzylic amines with different logP and pKa characteristics (Tables I & II). In some experiments, the possible intermediates of this reaction, i.e. nitrones (Table III), imines (Table IV) and amides themselves (Table V), were used as substrates in our metabolic studies. Their in vitro hepatic microsomal metabolism was studied in order to obtain a structure/metabolic activity relationship for the formation of amides from benzylic amines. This communication reviews these studies and reports our conclusions as to the mechanism of formation of amides from N-benzylic amines.

In vitro microsomal metabolism of nuclear chloro substituted secondary amines and imines

The metabolism of N-(4-chlorobenzyl)-4-chloroaniline (CBCA), N-(4-chlorobenzyl)-4-chlorobenzylamine (CBCBA), and N-(4-chlorobenzylidene)-4-chlorobenzylamine (CBDCBA) were studied in vitro using rat liver microsomal preparations. The secondary amines produced the corresponding N-oxidation products (hydroxylamines and nitrones) and dealkylation products (4-chlorobenzaldehyde and primary amines). Both secondary amines failed to produce the corresponding amides, whilst the parent imine was detected as a metabonate. CBDCBA, the intermediate imine of CBCBA metabolism, was also incubated under similar conditions. However, no oxaziridine was detected.

Chemical and metabolic studies on N-benzyl-tert-butylamine and its potential metabolites

The metabolism of N-benzyl-tert-butylamine was studied in vitro using male hamster hepatic microsomal preparations. This substrate produced the corresponding nitrone, benzaldehyde and an uncharacterised metabolite. No metabolites were detected which corresponded to either authentic amide or oxaziridine. The results indicate that the nitrone observed as a metabolite in this experiment is not an intermediate leading to the formation of an oxaziridine and hence an amide, under careful experimental conditions excluding light.

Structure-activity relationships in the formation of amides from substituted N-benzylanilines

1. The in vitro hepatic microsomal metabolism of certain substituted N-benzylanilines was studied in the male hamster to establish the mechanism(s) and process(es) involved in the formation of the corresponding amides. 2. N-Benzyl-2,4,6-trihalogeno, N-benzyl-4-cyano- and N-benzyl-4-nitroanilines were only metabolized by N-debenzylation. However, N-benzyl-4-methyl- and N-benzyl-2,4,6-trimethylanilines gave rise to both the corresponding amide and nitrone metabolites together with dealkylation products. These latter two substrates also produced hydroxymethyl metabolites as major products. Metabolism of N-(2,4,6,-trimethylbenzyl)aniline also led to the formation of an amide metabolite. The dealkylation products, the corresponding imine and an unknown metabolite, probably an hydroxylated product were also detected with this substrate. 3. N-(2,4-Dichlorobenzyl) and N-(2,6-dichlorobenzyl) anilines yielded the corresponding nitrone metabolites; but no amide metabolite was detected. Oxidative dealkylation leading to the formation of the corresponding primary anilines and aldehydes, together with para hydroxylation of aniline rings, were established as major routes of metabolism for both compounds. Similarly, neither N-(2,4,6-trifluorobenzyl) nor N-(4-nitrobenzyl) anilines produced any amide metabolite although dealkylation products were detected. 4. The pattern of amide formation observed for these N-benzylsubstituted anilines is discussed in terms of the steric and electronic effects of their aromatic substituents.