Metabolism of 9-(2-chlorobenzyl)-, 9-(2-methylbenzyl)- and 9-(2-methoxybenzyl)-adenines by hamster hepatic microsomes

Comparison of metabolic rates of some 9-aralkyladenines obtained using hamster hepatic microsomes

Previous investigations have revealed that N1-oxidation is a major metabolic pathway in vitro for some 9-aralkyladenines (AAs) such as 9-benzyladenine (BA). However, dealkylation and other metabolic pathways are also involved. In addition, various substituents on the benzyl moiety of BA seem to have a marked effect on the metabolic rate. In order to establish the potential structure-metabolism relationship of this class of compounds, the enzyme kinetics of the substrates, which possess 2'-nitro (2NBA), 3'-nitro (3NBA), 4'nitro (4NBA), 2'-chloro (2CBA), 2'-methyl (2MBA), or 2-methoxy (2MOBA) substitution of the benzyl moiety of BA, were compared using hamster hepatic microsomes. The results show that the formation rates of the N1-oxides are in the order 2NBA > 2CBA > BA > 3NBA and 4NBA > 2MBA and 2MOBA; the formation rates of the total metabolites except N1-oxides are in the order 2MOBA and 2MBA > 2CBA > BA > 4NBA > 3NBA > 2NBA; however, the total biotransformation rates of the substrates are in the order 2MBA and 2MOBA > 2CBA > BA and 2NBA > 4NBA > 3NBA. The results strongly imply that the electronic, steric and other physicochemical properties are potential controlling factors for AA metabolism.

Induction and inhibition of the in vitro N1-oxidation of 9-benzyladenine and isomeric 9-(nitrobenzyl)adenines

The present study investigated some aspects of the enzymology of the in vitro N1-oxidation of 9-benzyladenine (BA) and isomeric 9-(nitrobenzyl)adenines (NBAs) using various potential inducers and inhibitors of cytochrome P-450 (CYP). When incubated with phenobarbital-induced rabbit hepatic microsomes, the N1-oxidation rates of BA and 9-(4-nitrobenzyl)adenine were about 6- and 2-fold higher than that of the control, respectively; while the N1-oxidation of 9-(2-nitrobenzyl)adenine and 9-(3-nitrobenzyl)adenine was not markedly affected. In contrast, beta-naphthoflavone and Arochlor 1254 showed no inductive effects towards the N1-oxidation of any of these substrates. Using 12 typical CYP inhibitors, it was found that nifedipine (CYP3A inhibitor) and haloperidol (CYP2D inhibitor) showed significant inhibition towards the N1-oxidation of BA and NBAs. Therefore, the N1-oxidation of BA and NBAs is probably catalysed by CYP3A and CYP2D subfamilies. Furthermore, when 9-(4-nitrobenzyl)adenine was incubated with compounds which possessed a certain chemical similarity to the adenine substrate, various degrees of inhibition of N1-oxidation of 9-(4-nitrobenzyl)adenine were observed. These observations allowed a preliminary indication as to the structure-metabolism relationship of 9-substituted adenine derivatives.

Studies on the relationship between in vitro metabolism and certain physicochemical characteristics of some 9-alkyl-/9-aralkyl adenines

Previous studies demonstrated that biological N1-oxidation occurred for some 9-alkyl-/9-aralkyladenines, but not for others, when mammalian hepatic microsomal incubates were used as enzyme source. In order to understand the mechanisms controlling the metabolic fate of these compounds, the relationship between N1-oxidation and certain physicochemical characteristics of these substrates was studied. It was found that there was no marked link between N1-oxidation and the computer predicted pKa values of the substances studied. However, a computer predicted LogP value in the range 1.3-4 seems to be the most favourable for N1-oxidation. The 1H-NMR and 13C-NMR spectroscopic characteristics of the substrates, which reflect certain electronic characteristics of the purine moiety, also showed a correlation with their N1-oxidation. The electronic effects of the substrates in relation to their metabolism was investigated using computer modelling techniques; the results showed that different substituents at the 9-position of adenine may modify the electronic characteristics of the purine moiety thus affecting their metabolism. The conformation of the substrates may also be an important controlling factor for their N1-oxidative metabolism.