Metabolic Intermediate Complex Research Articles

Time-Dependent Inhibition of CYP1A2 by Stiripentol and Structurally Related Methylenedioxyphenyl Compounds via Metabolic Intermediate Complex Formation

Time-Dependent Inhibition of CYP1A2 by Stiripentol and Structurally Related Methylenedioxyphenyl Compounds via Metabolic Intermediate Complex Formation

Impact of Heterotropic Allosteric Modulation on the Time-Dependent Inhibition of Cytochrome P450 3A4.

The current study was designed to investigate the influence of allosteric effectors on the metabolism of the prototypical cytochrome P450 (CYP) 3A4 substrate midazolam (MDZ), and on the determination in vitro time-dependent inhibition (TDI) of CYP3A4 using human liver microsomes (HLM). As the concentration of midazolam increased to 250 µM in HLMs, homotropic cooperativity resulted in a decrease in the 1'-hydroxymidazolam to 4-hydroxymidazolam ratio to a maximum of 1.1. The presence of varying concentrations of testosterone, progesterone (PGS), or carbamazepine (CBZ) in HLMs with MDZ could recapitulate the effect of homotropic cooperativity such that the formation rates of the 1'hydroxymidazolam and 4-hydroxymidazolam were equal even at low concentrations of MDZ. The presence of PGS (10 or 100 µM) and CBZ (100 or 1000 µM) in in vitro TDI determination of four known CYP3A4 time-dependent inactivators (clarithromycin, troleandomycin, mibefradil, raloxifene) simultaneously decreased potency and inactivation rate constant, resulting in fold changes in inactivation efficiency on average of 1.6-fold and 13-fold for the low and high concentrations of allosteric modulator tested, respectively. The formation of a metabolic-intermediate complex (MIC) for clarithromycin and troleandomycin decreased in the presence of the allosteric modulators in a concentration-dependent manner, reaching a new steady state formation that could not be overcome with increased incubation time. Maximum reduction of the MIC formed by clarithromycin was up to ∼91%, while troleandomycin MIC decreased up to ∼31%. These findings suggest that the absence of endogenous allosteric modulators may contribute to the poor translation of HLM-based drug-drug interaction predictions. SIGNIFICANCE STATEMENT: The reported overprediction of in vitro human liver microsome time-dependent inhibition of CYP3A4 and observed drug interactions in vivo remains an issue in drug development. We provide characterization of allosteric modulators on the CYP3A4 metabolism of the prototypical substrate midazolam, demonstrating the ability of the modulators to recapitulate the homotropic cooperativity of midazolam. Furthermore, we demonstrate that allosteric heterotropic cooperativity of CYP3A4 can impact the time-dependent inhibition kinetics of known mechanisms-based inhibitors, providing a potential mechanism to explain the overprediction.

High-throughput cytochrome P450 loss and metabolic intermediate complex assays to aid in designing out of CYP3A inactivation.

Time-dependent inactivation (TDI) of cytochrome P450 (CYP) enzymes may result in clinical drug-drug interactions (DDIs). Therefore, designing out of CYP TDI prior to advancing a compound to clinical development is highly desirable. As TDI of CYP3A is a common occurrence in small molecule drug discovery, high-throughput methods are sought to help identify the mechanism of inactivation and enable design strategies to mitigate CYP3A TDI. CYP inactivation via modification or destruction of the prosthetic heme group results in loss of the ability of the enzyme to bind carbon monoxide. Additionally, formation of a tight binding complex with the heme iron, referred to as a metabolic intermediate (MI) complex, also results in enzyme inactivation. The methods described herein provide a high-throughput means of identifying and comparing compounds for their ability to inactivate via destruction/modification of the heme via loss of the ability to bind carbon monooxide, as well as via formation of an MI complex.

The Mechanism-Based Inactivation of CYP3A4 by Ritonavir: What Mechanism?

Ritonavir is the most potent cytochrome P450 (CYP) 3A4 inhibitor in clinical use and is often applied as a booster for drugs with low oral bioavailability due to CYP3A4-mediated biotransformation, as in the treatment of HIV (e.g., lopinavir/ritonavir) and more recently COVID-19 (Paxlovid or nirmatrelvir/ritonavir). Despite its clinical importance, the exact mechanism of ritonavir-mediated CYP3A4 inactivation is still not fully understood. Nonetheless, ritonavir is clearly a potent mechanism-based inactivator, which irreversibly blocks CYP3A4. Here, we discuss four fundamentally different mechanisms proposed for this irreversible inactivation/inhibition, namely the (I) formation of a metabolic-intermediate complex (MIC), tightly coordinating to the heme group; (II) strong ligation of unmodified ritonavir to the heme iron; (III) heme destruction; and (IV) covalent attachment of a reactive ritonavir intermediate to the CYP3A4 apoprotein. Ritonavir further appears to inactivate CYP3A4 and CYP3A5 with similar potency, which is important since ritonavir is applied in patients of all ethnicities. Although it is currently not possible to conclude what the primary mechanism of action in vivo is, it is unlikely that any of the proposed mechanisms are fundamentally wrong. We, therefore, propose that ritonavir markedly inactivates CYP3A through a mixed set of mechanisms. This functional redundancy may well contribute to its overall inhibitory efficacy.

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.

Design and Optimization of an Acyclic Amine Series of TRPV4 Antagonists by Electronic Modulation of Hydrogen Bond Interactions

Investigation of TRPV4 as a potential target for the treatment of pulmonary edema associated with heart failure generated a novel series of acyclic amine inhibitors displaying exceptional potency and PK properties. The series arose through a scaffold hopping approach, which relied on use of an internal H-bond to replace a saturated heterocyclic ring. Optimization of the lead through investigation of both aryl regions revealed approaches to increase potency through substituents believed to enhance separate intramolecular and intermolecular H-bond interactions. A proposed internal H-bond between the amine and neighboring benzenesulfonamide was stabilized by electronically modulating the benzenesulfonamide. In the aryl ether moiety, substituents para to the nitrile demonstrated an electronic effect on TRPV4 recognition. Finally, the acyclic amines inactivated CYP3A4 and this liability was addressed by modifications that sterically preclude formation of a putative metabolic intermediate complex to deliver advanced TRPV4 antagonists as leads for discovery of novel medicines.

Modulation of Major Human Liver Microsomal Cytochromes P450 by Component Alkaloids of Goldenseal: Time-Dependent Inhibition and Allosteric Effects.

Botanical and other natural products (NPs) are often coconsumed with prescription medications, presenting a risk for cytochrome P450 (P450)-mediated NP-drug interactions. The NP goldenseal (Hydrastis canadensis) has exhibited antimicrobial activities in vitro attributed to isoquinoline alkaloids contained in the plant, primarily berberine, (-)-β-hydrastine, and to a lesser extent, hydrastinine. These alkaloids contain methylenedioxyphenyl rings, structural alerts with potential to inactivate P450s through formation of metabolic intermediate complexes. Time-dependent inhibition experiments were conducted to evaluate their ability to inhibit major P450 activities in human liver microsomes by using a cocktail of isozyme-specific substrate probes. Berberine inhibited CYP2D6 (dextromethorphan O-demethylation; K I = 2.7 μM, kinact = 0.065 minute-1) and CYP3A4/5 (midazolam 1'-hydroxylation; K I = 14.8 μM, kinact = 0.019 minute-1); (-)-β-hydrastine inhibited CYP2C9 (diclofenac 4'-hydroxylation; K I = 49 μM, kinact = 0.036 minute-1), CYP2D6 (K I > 250 μM, kinact > 0.06 minute-1), and CYP3A4/5 (K I = 28 μM, kinact = 0.056 minute-1); and hydrastinine inhibited CYP2D6 (K I = 37 μM, kinact = 0.049 minute-1) activity. Berberine additionally exhibited allosteric effects on midazolam hydroxylation, showing both positive and negative heterotropic cooperativity. Experiments with recombinant isozymes showed that berberine activated midazolam 1'-hydroxylation by CYP3A5, lowering K m(app), but showed mixed inhibition and negative cooperativity toward this reaction when catalyzed by CYP3A4. Berberine inactivated CYP3A4 at a much faster rate than CYP3A5 and was a noncompetitive inhibitor of midazolam 4-hydroxylation by CYP3A4 but a strong mixed inhibitor of the CYP3A5 catalyzed reaction. These complex kinetics should be considered when extrapolating the risk for NP-drug interactions involving goldenseal. SIGNIFICANCE STATEMENT: Robust kinetic parameters were determined for the reversible and time-dependent inhibition of CYP2C9, CYP2D6, and CYP3A4/5 activities in human liver microsomes by major component isoquinoline alkaloids contained in the botanical natural product goldenseal. The alkaloid berberine also exhibited opposing, isozyme-specific allosteric effects on midazolam hydroxylation mediated by recombinant CYP3A4 (inhibition) and CYP3A5 (activation). These data will inform the development of a physiologically based pharmacokinetic model that can be used to predict potential clinically relevant goldenseal-drug interactions.

Evaluation of Strategies for the Assessment of Drug-Drug Interactions Involving Cytochrome P450 Enzymes.

Drug-drug interactions (DDIs) can occur when one drug alters the metabolism of another drug. Drug metabolism mediated by cytochrome P450 enzymes (CYPs) is responsible for the majority of metabolism of known drugs and inhibition of CYP enzymes is a well-known cause of DDIs. In the current study, the use of various human liver microsomes (HLM)-based methods to determine occurrence of CYP-mediated metabolism-dependent inhibition (MDI) and possible follow-up studies were evaluated. Human CYP inhibition was studied using the following methodologies: direct inhibition and (non-diluted) IC50-shift assays, a ferricyanide-based reversibility assay, a spectrophotometric metabolic intermediate complex (MIC) assay, and recording of reduced carbon monoxide (CO)-difference spectra. HLM incubations in the presence and absence of NADPH and glutathione (GSH) were performed to study the possible formation of CYP-dependent GSH adducts. HLM incubations with the radiolabeled inhibitors mifepristone and paroxetine were performed to study CYP-mediated covalent binding. Dihydralazine and furafylline displayed irreversible MDI of CYP1A2. Paroxetine displayed both quasi-irreversible and irreversible MDI of CYP2D6, formation of CYP-dependent GSH adducts was observed, while CYP-mediated covalent binding occurred which was decreased in the presence of GSH. Mifepristone displayed irreversible MDI of CYP3A4, formation of CYP-dependent GSH adducts was observed, while CYP-mediated covalent binding occurred which was decreased in the presence of GSH. Troleandomycin and verapamil displayed quasi-irreversible MDI of CYP3A4; MIC formation was observed, while no formation of CYP-dependent GSH adducts occurred. This study gives a representative overview of current methodologies that can be used to study CYP inhibition. The here presented strategy can be applied as a tool during risk evaluation of CYP-mediated DDIs.

Influence of sesamin on CYP2C-mediated diclofenac metabolism: invitro and invivo analysis.

Our previous studies revealed that sesamin caused a mechanism-based inhibition (MBI) of CYP2C9 in human liver microsomes. Additionally, we observed a similar MBI of CYP2C by sesamin in the rat liver microsomes. Sesamin-induced difference spectra of rat or human liver microsomes in the presence of NADPH showed a peak at 459 nm, suggesting the formation of a metabolic–intermediate (MI) complex of cytochrome P450 and the methylenedioxyphenyl group of sesamin. However, the peak disappeared in both liver microsomes within 30 min after the termination of the metabolism. These results suggest that the MI complex of cytochrome P450 and sesamin is unstable, and the effects of sesamin on human CYP2C9- or rat CYP2C-mediated drug metabolism may be small. To confirm this, in vivo studies using rats were performed. The pharmacokinetics of diclofenac, which is mainly metabolized by CYP2C11 in male rats, were investigated after a 3-days administration of sesamin (0, 10, and 100 mg/kg bw). No significant differences were observed among the three groups in the pharmacokinetic parameters, Cmax, Tmax, and AUC. Furthermore, administration of sesamin to rats for 7 days had no significant effects on diclofenac hydroxylation activity in rat liver microsomes. These results demonstrate that no significant interaction occurs between diclofenac and sesamin in rats. Moreover, the results of these in vitro and in vivo studies suggest that no significant interaction may occur between sesamin and diclofenac when sesamin is administered to humans as a supplement, since the standard sesamin dose in humans is much lower than that administered to rats in this study.

P450-Based Drug-Drug Interactions of Amiodarone and its Metabolites: Diversity of Inhibitory Mechanisms.

In this study, IC50 shift and time-dependent inhibition (TDI) experiments were carried out to measure the ability of amiodarone (AMIO), and its circulating human metabolites, to reversibly and irreversibly inhibit CYP1A2, CYP2C9, CYP2D6, and CYP3A4 activities in human liver microsomes. The [I]u/Ki,u values were calculated and used to predict in vivo AMIO drug-drug interactions (DDIs) for pharmaceuticals metabolized by these four enzymes. Based on these values, the minor metabolite N,N-didesethylamiodarone (DDEA) is predicted to be the major cause of DDIs with xenobiotics primarily metabolized by CYP1A2, CYP2C9, or CYP3A4, while AMIO and its N-monodesethylamiodarone (MDEA) derivative are the most likely cause of interactions involving inhibition of CYP2D6 metabolism. AMIO drug interactions predicted from the reversible inhibition of the four P450 activities were found to be in good agreement with the magnitude of reported clinical DDIs with lidocaine, warfarin, metoprolol, and simvastatin. The TDI experiments showed DDEA to be a potent inactivator of CYP1A2 (KI = 0.46 μM, kinact = 0.030 minute(-1)), while MDEA was a moderate inactivator of both CYP2D6 (KI = 2.7 μM, kinact = 0.018 minute(-1)) and CYP3A4 (KI = 2.6 μM, kinact = 0.016 minute(-1)). For DDEA and MDEA, mechanism-based inactivation appears to occur through formation of a metabolic intermediate complex. Additional metabolic studies strongly suggest that CYP3A4 is the primary microsomal enzyme involved in the metabolism of AMIO to both MDEA and DDEA. In summary, these studies demonstrate both the diversity of inhibitory mechanisms with AMIO and the need to consider metabolites as the culprit in inhibitory P450-based DDIs.

Characterization of ritonavir-mediated inactivation of cytochrome P450 3A4.

Ritonavir is a human immunodeficiency virus (HIV) protease inhibitor and an inhibitor of cytochrome P450 3A4, the major human hepatic drug-metabolizing enzyme. Given the potent inhibition of CYP3A4 by ritonavir, subtherapeutic doses of ritonavir are used to increase plasma concentrations of other HIV drugs oxidized by CYP3A4, thereby extending their clinical efficacy. However, the mechanism of inhibition of CYP3A4 by ritonavir remains unclear. To date, data suggests multiple types of inhibition by ritonavir, including mechanism-based inactivation by metabolic-intermediate complex formation, competitive inhibition, irreversible type II coordination to the heme iron, and more recently heme destruction. The results presented here demonstrate that inhibition of CYP3A4 by ritonavir occurs by CYP3A4-mediated activation and subsequent formation of a covalent bond to the apoprotein. Incubations of [(3)H]ritonavir with reconstituted CYP3A4 and human liver microsomes resulted in a covalent binding stoichiometry equal to 0.93 ± 0.04 moles of ritonavir bound per mole of inactivated CYP3A4. The metabolism of [(3)H]ritonavir by CYP3A4 leads to the formation of a covalent adduct specifically to CYP3A4, confirmed by radiometric liquid chromatography-trace and whole-protein mass spectrometry. Tryptic digestion of the CYP3A4-[(3)H]ritonavir incubations exhibited an adducted peptide (255-RM K: ESRLEDTQKHR-268) associated with a radiochromatic peak and a mass consistent with ritonavir plus 16 Da, in agreement with the whole-protein mass spectrometry. Additionally, nucleophilic trapping agents and scavengers of free oxygen species did not prevent inactivation of CYP3A4 by ritonavir. In conclusion, ritonavir exhibited potent time-dependent inactivation of CYP3A, with the mechanism of inactivation occurring though a covalent bond to Lys257 of the CYP3A4 apoprotein.

Importance of H-Abstraction in the Final Step of Nitrosoalkane Formation in the Mechanism-Based Inactivation of Cytochrome P450 by Amine-Containing Drugs

The metabolism of amine-containing drugs by cytochrome P450 enzymes (P450s) is prone to form a nitrosoalkane metabolic intermediate (MI), which subsequently coordinates to the heme iron of a P450, to produce a metabolic-intermediate complex (MIC). This type of P450 inhibition, referred to as mechanism-based inactivation (MBI), presents a serious concern in drug discovery processes. We applied density functional theory (DFT) to the reaction between N-methylhydroxylamine (NMH) and the compound I reactive species of P450, in an effort to elucidate the mechanism of the putative final step of the MI formation in the alkylamine metabolism. Our DFT calculations show that H-abstraction from the hydroxyl group of NMH is the most favorable pathway via which the nitrosoalkane intermediate is produced spontaneously. H-abstraction from the N–H bond was slightly less favorable. In contrast, N-oxidation and H-abstraction from the C–H bond of the methyl group had much higher energy barriers. Hence, if the conversion of NMH to nitrosoalkane is catalyzed by a P450, the reaction should proceed preferentially via H-abstraction, either from the O–H bond or from the N–H bond. Our theoretical analysis of the interaction between the MI and pentacoordinate heme moieties provided further insights into the coordination bond in the MIC.

Time‐dependent inhibition of CYP3A4 by sertraline, a selective serotonin reuptake inhibitor

Drug-drug interactions associated with selective serotonin reuptake inhibitors (SSRIs) are widely known. A major interaction by SSRIs is the inhibition of cytochrome P450 (P450)-mediated hepatic drug metabolism. The SSRI, sertraline, is also reported to increase the blood concentration of co-administered drugs. The potency of sertraline directly to inhibit hepatic drug metabolism is relatively weak compared with the other SSRIs, implying that additional mechanisms are involved in the interactions. The study examined whether sertraline produces time-dependent inhibition of CYP3A4 and/or other P450 enzymes. Incubation of human liver microsomes with sertraline in the presence of NADPH resulted in marked decreases in testosterone 6β-hydroxylation activities, indicating that sertraline metabolism leads to CYP3A4 inactivation. This inactivation required NADPH and was not protected by glutathione. No significant inactivation was observed for other P450 enzymes. Spectroscopic evaluation revealed that microsomes with and without sertraline in the presence of NADPH gave a Soret peak at 455 nm, suggesting the formation of metabolic intermediate (MI) complexes of sertraline metabolite(s) with the reduced form of P450. This is the first report indicating that sertraline produced time-dependent inhibition of CYP3A4, which may be associated with MI complex formation.

Carbene Generation by Cytochromes and Electronic Structure of Heme-Iron-Porphyrin-Carbene Complex: A Quantum Chemical Study

Carbene-heme-iron-porphyrin complexes generated from cytochrome P450 (CYP450)-mediated metabolism of compounds containing methylenedioxyphenyl (MDP) moiety lead to the mechanism-based inhibition (MBI) of CYPs. This coordination complex is termed as the metabolic-intermediate complex (MIC). The bioinorganic chemistry of MDP carbenes has been studied using quantum chemical methods employing density functional theory (B3LYP functional with implicit solvent corrections) to (i) analyze the characteristics of MDP-carbene in terms of singlet-triplet energy difference, protonation, and dimerization energies, etc.; (ii) determine the electronic structure and analyze the Fe-carbene interactions; and (iii) elucidate the potential reaction pathways for the generation of carbene, using Cpd I (iron(IV)-oxo-porphine with SH(-) as the axial ligand) as the model oxidant to mimic the activity of CYP450. The results show that MDP-carbenes are sufficiently stable and nucleophilic, leading to the formation of stable MIC (-40.35 kcal/mol) on the doublet spin state, formed via interaction between σ(LP) of carbene and empty dz(2) orbital of heme-iron. This was aided by the back-bonding between filled d(xz) orbital of heme-iron and the empty p orbital of carbene. The mechanistic pathway proposed in the literature for the generation of MDP-carbene (CH hydroxylation followed by water elimination) was studied, and observed to be unfavorable, owing to the formation of highly stable hydroxylated product (-57.12 kcal/mol). An intriguing pathway involving hydride ion abstraction and proton transfer followed by water elimination step was observed to be the most probable pathway.

Metabolism-Dependent Inhibition of CYP3A4 by Lapatinib: Evidence for Formation of a Metabolic Intermediate Complex with a Nitroso/Oxime Metabolite Formed via a Nitrone Intermediate

Metabolism-dependent inhibition (MDI) of cytochrome P450 (P450) enzymes has the potential to cause clinically relevant drug-drug interactions. In the case of several alkylamine drugs, MDI of P450 involves formation of a metabolite that binds quasi-irreversibly to the ferrous heme iron to form a metabolic intermediate (MI) complex. The specific metabolites coordinately bound to ferrous iron and the pathways leading to MI complex formation are the subject of debate. We describe an approach combining heme iron oxidation with potassium ferricyanide and metabolite profiling to probe the mechanism of MI complex-based CYP3A4 inactivation by the secondary alkylamine drug lapatinib. Ten metabolites formed from lapatinib by CYP3A4-mediated heteroatom dealkylation, C-hydroxylation, N-oxygenation with or without further oxidation, or a combination thereof, were detected by accurate mass spectrometry. The abundance of one metabolite, the N-dealkylated nitroso/oxime lapatinib metabolite (M9), correlated directly with the prevalence or the disruption of the MI complex with CYP3A4. Nitroso/oxime metabolite formation from secondary alkylamines has been proposed to occur through two possible pathways: (1) sequential N-dealkylation, N-hydroxylation, and dehydrogenation (primary hydroxylamine pathway) or (2) N-hydroxylation with dehydrogenation to yield a nitrone followed by N-dealkylation (secondary hydroxylamine pathway). All intermediates for the secondary hydroxylamine pathway were detected but the primary N-hydroxylamine intermediate of the primary hydroxylamine pathway was not. Our findings support the mechanism of lapatinib CYP3A4 inactivation as MI complex formation with the nitroso metabolite formed through the secondary hydroxylamine and nitrone pathway, rather than by N-dealkylation to the primary amine followed by N-hydroxylation and dehydrogenation as is usually assumed.

Metabolic‐intermediate complex formation with cytochrome P450: Theoretical studies in elucidating the reaction pathway for the generation of reactive nitroso intermediate

Mechanism-based inhibition (MBI) of cytochrome P450 (CYP) can lead to drug-drug interactions and often to toxicity. Some aliphatic and aromatic amines can undergo biotransformation reactions to form reactive metabolites such as nitrosoalkanes, leading to MBI of CYPs. It has been proposed that the nitrosoalkanes coordinate with the heme iron, forming metabolic-intermediate complex (MIC), resulting in the quasi-irreversible inhibition of CYPs. Limited mechanistic details regarding the formation of reactive nitroso intermediate and its coordination with heme-iron have been reported. A quantum chemical analysis was performed to elucidate potential reaction pathways for the generation of nitroso intermediate and the formation of MIC. Elucidation of the energy profile along the reaction path, identification of three-dimensional structures of reactive intermediates and transition states, as well as charge and spin density analyses, were performed using the density functional B3LYP method. The study was performed using Cpd I [iron (IV-oxo] heme porphine with SH(-) as the axial ligand) to represent the catalytic domain of CYP, simulating the biotransformation process. Three pathways: (i) N-oxidation followed by proton shuttle, (ii) N-oxidation followed by 1,2-H shift, and (iii) H-abstraction followed by rebound mechanism, were studied. It was observed that the proton shuttle pathway was more favorable over the whole reaction leading to reactive nitroso intermediate. This study revealed that the MIC formation from a primary amine is a favorable exothermic process, involving eight different steps and preferably takes place on the doublet spin surface of Cpd I. The rate-determining step was identified to be the first N-oxidation of primary amine.

Cytochrome P450 metabolic intermediate complex of nefopam.

NADPH-catalysed biotransformation of nefopam in liver microsomes obtained from phenobarbitone-pretreated rats leads to the formation of an inactive cytochrome P450 metabolic intermediate (MI) complex. This complex can be detected spectrophotometrically by an absorbance maximum at 459 nm. The extent of the in-vitro MI complexation of 33 microM nefopam, a cyclic analogue of orphenadrine, was almost equal to the extent of the in-vitro MI complexation of 33 microM tofenacine, the mono-N-demethylated metabolite of orphenadrine. The time course of the MI complexation of nefopam and studies with two of its major metabolites suggest an initial biotransformation, which has to occur before MI complexation can take place. Maximal MI complexation of nefopam occurred at approximately 25 microM, whereas the MI complexation could not be detected at 100 microM nefopam.

Metabolic Intermediate Complex Formation of Human Cytochrome P450 3A4 by Lapatinib

Lapatinib, an oral breast cancer drug, has recently been reported to be a mechanism-based inactivator of cytochrome P450 (P450) 3A4 and also an idiosyncratic hepatotoxicant. It was suggested that formation of a reactive quinoneimine metabolite was involved in mechanism-based inactivation (MBI) and/or hepatotoxicity. We investigated the mechanism of MBI of P450 3A4 by lapatinib. Liquid chromatography-mass spectrometry analysis of P450 3A4 after incubation with lapatinib did not show any peak corresponding to irreversible modifications. The enzymatic activity inactivated by lapatinib was completely restored by the addition of potassium ferricyanide. These results indicate that the mechanism of MBI by lapatinib is quasi-irreversible and mediated via metabolic intermediate complex (MI complex) formation. This finding was verified by the increase in a signature Soret absorbance at approximately 455 nm. Two amine oxidation products of the metabolism of lapatinib by P450 3A4 were characterized: N-hydroxy lapatinib (M3) and the oxime form of N-dealkylated lapatinib (M2), suggesting that a nitroso or another related intermediate generated from M3 is involved in MI complex formation. In contrast, P450 3A5 was much less susceptible to MBI by lapatinib via MI complex formation than P450 3A4. In addition, P450 3A5 had a significantly lower ability than 3A4 to generate M3, consistent with N-hydroxylation as the initial step in the pathway to MI complex formation. In conclusion, our results demonstrate that the primary mechanism for MBI of P450 3A4 by lapatinib is not irreversible modification by the quinoneimine metabolite, but quasi-irreversible MI complex formation mediated via oxidation of the secondary amine group of lapatinib.

Assessment of Competitive and Mechanism-Based Inhibition by Clarithromycin: Use of Domperidone as a CYP3A Probe-Drug Substrate and Various Enzymatic Sources Including a New Cell-Based Assay with Freshly Isolated Human Hepatocytes

Clarithromycin is involved in a large number of clinically relevant drug-drug interactions. Discrepancies are observed between the magnitude of drug interactions predicted from in vitro competitive inhibition studies and changes observed clinically in the plasma levels of affected CYP3A substrates. The formation of metabolic-intermediate complexes has been proposed to explain these differences. The objectives of our study were: 1) to determine the competitive inhibition potency of clarithromycin on the metabolism of domperidone as a CYP3A probe drug using human recombinant CYP3A4 and CYP3A5 isoenzymes, human liver microsomes and cultured human hepatocytes; 2) to establish the modulatory role of cytochrome b5 on the competitive inhibition potency of clarithromycin; 3) to demonstrate the clarithromycin-induced formation of CYP450 metabolic-intermediate complexes in human liver microsomes; and 4) to determine the extent of CYP3A inhibition due to metabolic-intermediate complex formation using human liver microsomes and cultured human hepatocytes. At high concentrations (100 µM), clarithromycin had weak competitive inhibition potency towards CYP3A4 and CYP3A5. Inhibition potency was further decreased by the addition of cytochrome b5 (9-19%). Clarithromycin-induced metabolic-intermediate complexes were revealed by spectrophotometry analysis using human liver microsomes while time- and concentration-dependent mechanism-based inhibitions were quantified using isolated hepatocytes. These results indicate that mechanism-based but not competitive inhibition of CYP3As is the major underlying mechanism of drug-drug interactions observed clinically with clarithromycin. Drug interactions between clarithromycin and several CYP3A substrates are predicted to be insidious; the risk of severe adverse events should increase over time and persist for a few days after cessation of the drug.

Sequential Metabolism of Secondary Alkyl Amines to Metabolic-Intermediate Complexes: Opposing Roles for the Secondary Hydroxylamine and Primary Amine Metabolites of Desipramine, (S)-Fluoxetine, and N-Desmethyldiltiazem

Three secondary amines desipramine (DES), (S)-fluoxetine [(S)-FLX], and N-desmethyldiltiazem (MA) undergo N-hydroxylation to the corresponding secondary hydroxylamines [N-hydroxydesipramine, (S)-N-hydroxyfluoxetine, and N-hydroxy-N-desmethyldiltiazem] by cytochromes P450 2C11, 2C19, and 3A4, respectively. The expected primary amine products, N-desmethyldesipramine, (S)-norfluoxetine, and N,N-didesmethyldiltiazem, are also observed. The formation of metabolic-intermediate (MI) complexes from these substrates and metabolites was examined. In each example, the initial rates of MI complex accumulation followed the order secondary hydroxylamine > secondary amine >> primary amine, suggesting that the primary amine metabolites do not contribute to formation of MI complexes from these secondary amines. Furthermore, the primary amine metabolites, which accumulate in incubations of the secondary amines, inhibit MI complex formation. Mass balance studies provided estimates of the product ratios of N-dealkylation to N-hydroxylation. The ratios were 2.9 (DES-CYP2C11), 3.6 [(S)-FLX-CYP2C19], and 0.8 (MA-CYP3A4), indicating that secondary hydroxylamines are significant metabolites of the P450-mediated metabolism of secondary alkyl amines. Parallel studies with N-methyl-d(3)-desipramine and CYP2C11 demonstrated significant isotopically sensitive switching from N-demethylation to N-hydroxylation. These findings demonstrate that the major pathway to MI complex formation from these secondary amines arises from N-hydroxylation rather than N-dealkylation and that the primary amines are significant competitive inhibitors of MI complex formation.