Structural requirements for inhibitors of cytochromes P450 2B: Assessment of the enzyme interaction with diamondoids

Cytochrome P450 and flavin-containing monooxygenase enzymes are responsible for differential oxidation of the anti-thyroid-cancer drug vandetanib by human and rat hepatic microsomal systems

We studied the in vitro metabolism of the anti-thyroid-cancer drug vandetanib in a rat animal model and demonstrated that N-desmethylvandetanib and vandetanib N-oxide are formed by NADPH- or NADH-mediated reactions catalyzed by rat hepatic microsomes and pure biotransformation enzymes. In addition to the structural characterization of vandetanib metabolites, individual rat enzymes [cytochrome P450 (CYP) and flavin-containing monooxygenase (FMO)] capable of oxidizing vandetanib were identified. Generation of N-desmethylvandetanib, but not that of vandetanib N-oxide, was attenuated by CYP3A and 2C inhibitors while inhibition of FMO decreased formation of vandetanib N-oxide. These results indicate that liver microsomal CYP2C/3A and FMO1 are major enzymes participating in the formation of N-desmethylvandetanib and vandetanib N-oxide, respectively. Rat recombinant CYP2C11 > >3A1 > 3A2 > 1A1 > 1A2 > 2D1 > 2D2 were effective in catalyzing the formation of N-desmethylvandetanib. Results of the present study explain differences between the CYP- and FMO-catalyzed vandetanib oxidation in rat and human liver reported previously and the enzymatic mechanisms underlying this phenomenon.

Exploring the size of the lipophilic unit of the soluble epoxide hydrolase inhibitors

Soluble epoxide hydrolase (sEH) inhibitors are potential drugs for several diseases. Adamantyl ureas are excellent sEH inhibitors but have limited metabolic stability. Herein, we report the effect of replacing the adamantane group by alternative polycyclic hydrocarbons on sEH inhibition, solubility, permeability and metabolic stability. Compounds bearing smaller or larger polycyclic hydrocarbons than adamantane yielded all good inhibition potency of the human sEH (0.4 ≤ IC₅₀ ≤ 21.7 nM), indicating that sEH is able to accommodate inhibitors of very different size. Human liver microsomal stability of diamantane containing inhibitors is lower than that of their corresponding adamantane counterparts.

NADPH- and NADH-dependent metabolism of and DNA adduct formation by benzo[a]pyrene catalyzed with rat hepatic microsomes and cytochrome P450 1A1

ABSTRACT

Benzo[a]pyrene (BaP) is a human carcinogen that covalently binds to DNA after metabolic activation by cytochrome P450 (CYP) enzymes. Here we investigated the efficiencies of rat hepatic microsomes and rat recombinant CYP1A1 expressed with its reductase, NADPH:CYP oxidoreductase (POR), NADH:cytochrome b5 reductase, epoxide hydrolase and/or cytochrome b5 in Supersomes™ to metabolize this carcinogen. We also studied the effectiveness of coenzymes of two of the microsomal reductases, NADPH as a coenzyme of POR, and NADH as a coenzyme of NADH:cytochrome b5 reductase, to mediate BaP metabolism in these systems. Up to eight BaP metabolites and two DNA adducts were generated by the systems, both in the presence of NADPH and NADH. Among BaP metabolites, BaP-9,10-dihydrodiol, BaP-4,5-dihydrodiol, BaP-7,8-dihydrodiol, BaP-1,6-dione, BaP-3,6-dione, BaP-9-ol, BaP-3-ol, and a metabolite of unknown structure were formed by hepatic microsomes and rat CYP1A1. One of two DNA adducts formed by examined enzymatic systems (rat hepatic microsomes and rat CYP1A1) was characterized to be 10-(deoxyguanosin-N2-yl)-7,8,9-trihydroxy-7,8,9,10-tetrahydrobenzo[a]pyrene (dG-N2-BPDE), while another adduct has similar chromatographic properties on polyethylaneimine-cellulose thin layer chromatography to a guanine adduct derived from reaction with 9-hydroxy-BaP-4,5-oxide. In the presence of either of the reductase cofactors tested, NADPH or NADH, cytochrome b5 stimulated CYP1A1-mediated formation of both BaP-DNA adducts. The results demonstrate that NADH can act as a sole electron donor for both the first and the second reduction of CYP1A1 during its reaction cycle catalyzing oxidation of BaP, and suggest that the NADH:cytochrome b5 reductase as the NADH-dependent reductase might substitute POR in this enzymatic system.

GRAPHICAL ABSTRACT

Role of rat cytochromes P450 in the oxidation of 17α-ethinylestradiol

17α-Ethinylestradiol (EE2) is an endocrine disruptor (ED) used as an ingredient of oral contraceptives. Rat hepatic microsomes metabolize EE2 to three products; two of them are hydroxylated EE2 derivatives. Of the hydroxylation reactions, 2-hydroxylation, is the major reaction. Cytochrome P450 (CYP) plays a major role in EE2 hydroxylation. To resolve which rat CYPs are responsible for EE2 oxidation, three approaches were used: induction of specific CYPs, selective inhibition of CYPs, and recombinant rat CYPs. The results demonstrate that EE2 is hydroxylated by several rat CYPs, among them CYP2C6 and 2C11 are most efficient in 2-hydroxy-EE2 formation, while CYP2A and 3A catalyze EE2 hydroxylation to the second product. EE2 is also an inhibitor of CYP2C- and CYP3A-catalyzed hydroxylation of endogenous EDs progesterone and testosterone. EE2 acts as a reversible inhibitor of CYP3A-mediated progesterone 6β-hydroxylation and inactivates CYP3A- and CYP2C-catalyzed testosterone 6β-hydroxylation and progesterone 21- or 16α-hydroxylation, respectively, in a mechanism-based manner.

Site-specific incorporation of diamondoids on DNA using click chemistry

A simple and robust solid phase synthetic method for the ligation of diamondoids on the phosphate backbone of DNA with "click" chemistry using [Cu(CH(3)CN)(4)]PF(6) without a stabilizing ligand is reported. It was found that as the size of dimondoid increased, a corresponding increase in melting temperature was observed.

Nanodiamond

This chapter discusses various methods of surface modification for the development of functionalized diamond nanoparticles for biomedical applications. To be used in biomedical applications, nanoparticles must be biocompatible, non-toxic, non-detective by immune systems, and should not induce side effects. Size control of particles is a prerequisite for biomedical applications. Carbon nanostructures span the same length scale as bio-compounds, ranging from subnanometer-size nucleotides to tens and hundreds of nanometer-sized organelles and viruses, and up to micron-sized cell sizes. The chapter also summarizes different approaches to the surface functionalization of nanodiamonds (ND) particles—that is, the key in successful biomedical applications followed by a discussion of modification of diamond surfaces with nucleic acids and proteins. Both current and potential applications of diamond films and particles in the area of biosensing are addressed.