Nitrosoalkane complexes of iron-porphyrins: analogy between the bonding properties of nitrosoalkanes and dioxygen

Role of cytochrome P450 enzymes in drug-drug interactions

Many adverse drug-drug interactions are attributable to pharmacokinetic problems and can be understood in terms of alterations of P450-catalyzed reactions. Much is now known about the human P450 enzymes and what they do, and it has been possible to apply this information to issues related to practical problems. A relatively small subset of the total number of human P450s appears to be responsible for a large fraction of the oxidation of drugs. The three major reasons for drug-drug interactions involving the P450s are induction, inhibition, and possibly stimulation, with inhibition appearing to be the most important in terms of known clinical problems. With the available knowledge of human P450s and reagents, it is possible to do in vitro experiments with drugs and make useful predictions. The results can be tested in vivo, again using assays based on our knowledge of human P450s. This approach has the capability of not only improving predictions about which drugs might show serious interaction problems, but also decreasing the number of in vivo interaction studies that must be performed. These approaches should improve with further refinement and technical advances.

The substrate binding site of human liver cytochrome P450 2C9: an NMR study

Purified recombinant human liver cytochrome P450 2C9 was produced, from expression of the corresponding cDNA in yeast, in quantities large enough for UV-visible and 1H NMR experiments. Its interaction with several substrates (tienilic acid and two derivatives, lauric acid and diclofenac) and with a specific inhibitor, sulfaphenazole, was studied by UV-visible and 1H NMR spectroscopy. At 27 degrees C, all those substrates led to an almost complete conversion of CYP 2C9 to high-spin (S = 5/2) CYP 2C9-substrate complexes characterized by a Soret peak at 390 nm; their KD values varied between 1 and 42 microM. On the contrary, sulfaphenazole led to a low-spin (S = 1/2) CYP 2C9 complex upon binding of its NH2 group to CYP 2C9 iron. Interactions of the five substrates with the enzyme were studied by paramagnetic relaxation effects of CYP 2C9-iron(III) on the 1H NMR spectrum of each substrate. Distances between the heme iron atom and substrate protons were calculated from the NMR data, and the orientation of the substrate relative to iron was determined from those distances. Finally, a model for substrate positioning in the CYP 2C9 active site was constructed by molecular modeling studies under the constraint of the iron-proton distances. It points out two structural characteristics for a compound to be selectively recognized by CYP 2C9: (i) the presence of an anionic site able to establish an ionic bond with a putative cationic residue of the protein and (ii) the presence of an hydrophobic zone between the substrate hydroxylation site and the anionic site. Sulfaphenazole was easily included in that model; its very high affinity for CYP 2C9 is due to a third structural feature, the presence of its NH2 function which binds to CYP 2C9 iron.

Interaction of sulfaphenazole derivatives with human liver cytochromes P450 2C: molecular origin of the specific inhibitory effects of sulfaphenazole on CYP 2C9 and consequences for the substrate binding site topology of CYP 2C9

The effects of sulfaphenazole, 1, on typical activities catalyzed by human cytochromes P450 of the 1A, 3A, and 2C subfamilies expressed in yeast were studied. 1 acts as a strong, competitive inhibitor of CYP 2C9 (K(i) = 0.3 +/- 0.1 microM); it is much less potent toward CYP 2C8 and 2C18 (K(i) = 63 and 29 microM, respectively) and fails to inhibit CYP 1A1, 1A2, 3A4, and 2C19. From difference visible spectroscopy experiments using microsomes of yeast expressing various human P450s, 1 selectively interacts only with CYP 2C9 with the appearance of a peak at 429 nm as expected for the formation of a P450 Fe(III)-nitrogenous ligand complex (Ks = 0.4 +/- 0.1 microM). Comparative studies of the spectral interaction and inhibitory effects of twelve compounds related to 1 with CYP 2C9 showed that the aniline function of 1 is responsible for the formation of the iron-nitrogen bond of the 429 nm-absorbing complex and is necessary for the inhibitory effects of 1. The study of two new compounds synthesized during this work, in which the N-phenyl group of 1 was replaced with either an ethyl group or a 3,4-dichlorophenyl group, showed that the presence of an hydrophobic substituent at position 1 of the pyrazole function of 1 is required for a strong interaction with CYP 2C9. A model for the binding of 1 in the CYP 2C9 active site is proposed; that takes into account three major interactions that should be at the origin of the high-affinity and specific inhibitory effects of 1 toward CYP 2C9: (i) the binding of its nitrogen atom to CYP 2C9 iron, (ii) an ionic interaction of its SO2N- anionic site with a cationic residue of CYP 2C9, and (iii) an interaction of its N-phenyl group with an hydrophobic part of the protein active site.

Cytochrome P-455 nm complex formation in the metabolism of phenylalkylamines. XII. Enantioselectivity and temperature dependence in microsomes and reconstituted cytochrome P-450 systems from rat liver

Formation of metabolic intermediate (MI) complexes was studied with the enantiomers of amphetamine, 1-phenyl-2-pentanamine, N-hydroxyamphetamine, and 2-nitroso-1-phenylpropane (the C-nitroso analogue of amphetamine). Three different enzyme systems were used; liver microsomes from phenobarbital pretreated rats and two reconstituted systems containing the P450 2B1 and P450 2C11 forms of cytochrome P-450. Enantioselective complex formation in microsomes was shown for the amines and the nitroso compound, but not for the hydroxylamine. The highly purified P450 2B1 system formed the MI complex with all substrates tested, and the enantioselectivity observed with the microsomal system was reproduced. In the P450 2C11 system the nitroso compounds were completely inactive, whereas the enantiomers of N-hydroxyamphetamine still produced the complex at a high rate. Changes in temperature were shown to affect (R)-2-nitroso-1-phenylpropane more than its enantiomer. Both enantiomers showed biphasic Arrhenius plots for MI complex formation in microsomes (breaks around 22 degrees C), but the activation energies of the (R)-isomer were about five times higher than those of the (S)-isomer. A theory is presented which suggests different modes of interaction with the active site of P-450 to account for the different behaviour of the various substrates.

Cytochrome P-455 nm complex formation in the metabolism of phenylalkylamines. IX. A comparative study with enantiomeric 2-nitroso-1-phenylpropane in microsomes and a reconstituted cytochrome P-450 system from rat liver

Formation of cytochrome P-455 nm complexes was investigated with enantiomeric 2-nitroso-1-phenylpropane--the C-nitroso analogue of amphetamine--and optically active N-hydroxyamphetamine, in the presence of NADPH. For comparative reasons, three different drug-metabolizing enzyme systems were used, namely microsomes from control and phenobarbital-treated rats, and a reconstituted system containing the main phenobarbital-induced form of cytochrome P-450 from rat liver. In microsomes obtained from phenobarbital-treated rats, pronounced differences in the kinetics of complex formation were observed between the enantiomeric C-nitroso compounds, but not between the isomers of N-hydroxyamphetamine. In the reconstituted enzyme system the S-nitroso compound formed the P-455 nm chromophore at the highest initial rate, while the R analogue was devoid of complexing activity. The rates of complex formation from the N-hydroxylamine enantiomers were high and equal.

Direct formation of complexes between cytochrome P-450 and nitrosoarenes

The mechanism of the formation of the complexes between various nitrosobenzenes and cytochrome P-450 has been investigated. We have observed the formation of these complexes by a new and, as yet, undescribed route. Nitrosobenzene (NOB) itself reacts with cytochrome P-450 in the iron(III) state, in the absence of any exogenous reducing agent, to produce the iron(II)-NOB complex. Apparently, NOB is a ligand that is capable of causing the spontaneous autoreduction of the iron. The reduction of the iron may occur via ligand-induced oxidation of the axially bound thiolate of cytochrome P-450.

Dual effects of macrolide antibiotics on rat liver cytochrome P-450. Induction and formation of metabolite-complexes: a structure-activity relationship

Previous studies have shown that the macrolide antibiotics, troleandomycin and erythromycin, are able to induce their own transformation into a metabolite forming an inactivated complex with rat liver cytochrome P-450. This paper reports the results of a study on the effects of several macrolide antibiotics including oleandomycin, erythromycin derivatives, josamycin, methymycin, tylosin, spiramycin and rifampicin, as well as antibiotics of other series, such as tetracycline and lincomycin, on rat liver cytochromes P-450 in vivo and in vitro. Only the antibiotics containing the desosamine and mycaminose amino sugars were able to give the dual effects already found with troleandomycin: induction of cytochrome p-450 and formation of an inhibitory cytochrome P-450--iron--nitrosoalkane metabolite complex in vivo or in vitro. From these studies, it appears that two structural factors are important for a macrolide antibiotic to lead to such effects: the presence of a non-hindered readily accessible N-dimethylamino group and the hydrophobic character of the molecule. These data are discussed in relation to the adverse effects observed during drug associations involving some of these macrolide antibiotics.