Metabolism of bergamottin by cytochromes P450 2B6 and 3A5

Physiologically Based Pharmacokinetic Modeling of Bergamottin and 6,7-Dihydroxybergamottin to Describe CYP3A4 Mediated Grapefruit-Drug Interactions

Grapefruit is a moderate to strong inactivator of CYP3A4, which metabolizes up to 50% of marketed drugs. The inhibitory effect is mainly attributed to furanocoumarins present in the fruit, irreversibly inhibiting preferably intestinal CYP3A4 as suicide inhibitors. Effects on CYP3A4 victim drugs can still be measured up to 24 hours after grapefruit juice (GFJ) consumption. The current study aimed to establish a physiologically-based pharmacokinetic (PBPK) grapefruit-drug interaction model by modeling the relevant CYP3A4 inhibiting ingredients of the fruit to simulate and predict the effect of GFJ consumption on plasma concentration-time profiles of various CYP3A4 victim drugs. The grapefruit model was developed in PK-Sim and coupled with previously developed PBPK models of CYP3A4 substrates that were publicly available and already evaluated for CYP3A4-mediated drug-drug interactions. Overall, 43 clinical studies were used for model development. Models of bergamottin (BGT) and 6,7-dihydroxybergamottin (DHB) as relevant active ingredients in GFJ were established. Both models include: (i) CYP3A4 inactivation informed by in vitro parameters, (ii) a CYP3A4 mediated clearance estimated during model development, as well as (iii) passive glomerular filtration. The final model successfully describes interactions of GFJ ingredients with 10 different CYP3A4 victim drugs, simulating the effect of the CYP3A4 inactivation on the victims' pharmacokinetics as well as their main metabolites. Furthermore, the model sufficiently captures the time-dependent effect of CYP3A4 inactivation as well as the effect of grapefruit ingestion on intestinal and hepatic CYP3A4 concentrations.

Multidisciplinary Insights into the Structure-Function Relationship of the CYP2B6 Active Site

Cytochrome P450 2B6 (CYP2B6) is a highly polymorphic human enzyme involved in the metabolism of many clinically relevant drugs, environmental toxins, and endogenous molecules with disparate structures. Over the last 20-plus years, in silico and in vitro studies of CYP2B6 using various ligands have provided foundational information regarding the substrate specificity and structure-function relationship of this enzyme. Approaches such as homology modeling, X-ray crystallography, molecular docking, and kinetic activity assays coupled with CYP2B6 mutagenesis have done much to characterize this originally neglected monooxygenase. However, a complete understanding of the structural details that make new chemical entities substrates of CYP2B6 is still lacking. Surprisingly little in vitro data has been obtained about the structure-function relationship of amino acids identified to be in the CYP2B6 active site. Since much attention has already been devoted to elucidating the function of CYP2B6 allelic variants, here we review the salient findings of in silico and in vitro studies of the CYP2B6 structure-function relationship with a deliberate focus on the active site. In addition to summarizing these complementary approaches to studying structure-function relationships, we note gaps/challenges in existing data such as the need for more CYP2B6 crystal structures, molecular docking results with various ligands, and data coupling CYP2B6 active site mutagenesis with kinetic parameter measurement under standard expression conditions. Harnessing in silico and in vitro techniques in tandem to understand the CYP2B6 structure-function relationship will likely offer further insights into CYP2B6-mediated metabolism. SIGNIFICANCE STATEMENT: The apparent importance of cytochrome P450 2B6 (CYP2B6) in the metabolism of various xenobiotics and endogenous molecules has grown since its discovery with many in silico and in vitro studies offering a partial description of its structure-function relationship. Determining the structure-function relationship of CYP2B6 is difficult but may be aided by thorough biochemical investigations of the CYP2B6 active site that provide a more complete pharmacological understanding of this important enzyme.

Phytochemicals That Interfere With Drug Metabolism and Transport, Modifying Plasma Concentration in Humans and Animals

Phytochemicals (Pch) present in fruits, vegetables and other foods, are known to inhibit or induce drug metabolism and transport. An exhaustive search was performed in five databases covering from 2000 to 2021. Twenty-one compounds from plants were found to modulate CYP3A and/or P-gp activities and modified the pharmacokinetics and the therapeutic effect of 27 different drugs. Flavonols, flavanones, flavones, stilbenes, diferuloylmethanes, tannins, protoalkaloids, flavans, hyperforin and terpenes, reduce plasma concentration of cyclosporine, simvastatin, celiprolol, midazolam, saquinavir, buspirone, everolimus, nadolol, tamoxifen, alprazolam, verapamil, quazepam, digoxin, fexofenadine, theophylline, indinavir, clopidogrel. Anthocyanins, flavonols, flavones, flavanones, flavonoid glycosides, stilbenes, diferuloylmethanes, catechin, hyperforin, alkaloids, terpenes, tannins and protoalkaloids increase of plasma concentration of buspirone, losartan, diltiazem, felodipine, midazolam, cyclosporine, triazolam, verapamil, carbamazepine, diltiazem, aripiprazole, tamoxifen, doxorubicin, paclitaxel, nicardipine. Interactions between Pchs and drugs affect the gene expression and enzymatic activity of CYP3A and P-gp transporter, which has an impact on their bioavailability; such that co-administration of drugs with food, beverages and food supplements can cause a subtherapeutic effect or overdose. Therefore, it is important for the clinician to consider these interactions to obtain a better therapeutic effect.

Bergamottin a CYP3A inhibitor found in grapefruit juice inhibits prostate cancer cell growth by downregulating androgen receptor signaling and promoting G0/G1 cell cycle block and apoptosis

Prostate cancer is the second leading cause of cancer related death in American men. Several therapies have been developed to treat advanced prostate cancer, but these therapies often have severe side effects. To improve the outcome with fewer side effects we focused on the furanocoumarin bergamottin, a natural product found in grapefruit juice and a potent CYP3A inhibitor. Our recent studies have shown that CYP3A5 inhibition can block androgen receptor (AR) signaling, critical for prostate cancer growth. We observed that bergamottin reduces prostate cancer (PC) cell growth by decreasing both total and nuclear AR (AR activation) reducing downstream AR signaling. Bergamottin’s role in reducing AR activation was confirmed by confocal microscopy studies and reduction in prostate specific antigen (PSA) levels, which is a marker for prostate cancer. Further studies revealed that bergamottin promotes cell cycle block and accumulates G0/G1 cells. The cell cycle block was accompanied with reduction in cyclin D, cyclin B, CDK4, P-cdc2 (Y15) and P-wee1 (S642). We also observed that bergamottin triggers apoptosis in prostate cancer cell lines as evident by TUNEL staining and PARP cleavage. Our data suggests that bergamottin may suppress prostate cancer growth, especially in African American (AA) patients carrying wild type CYP3A5 often presenting aggressive disease.

Mechanistic study of bergamottin-induced inactivation of CYP2C9

Bergamottin (BGM) is a major furanocoumarin constituent of grapefruit and is reported to have inhibitory effects on cytochrome P450 enzymes. This study investigated the chemical interactions between BGM and the enzyme CYP2C9. BGM exhibited time-, concentration-, and NADPH-dependent inhibition of CYP2C9. Co-incubation with diclofenac, a reversible inhibitor of CYP2C9, attenuated the time-dependent enzyme inhibition. Exhaustive dialysis did not restore enzyme activity post-inhibition. Glutathione (GSH) and catalase/superoxide dismutase failed to reverse BGM-induced CYP2C9 inactivation. A GSH trapping study suggested that BGM was metabolized to an epoxide and/or γ-ketoenal that may have been responsible for the enzyme inactivation. In conclusion, BGM can be characterized as a mechanism-based inactivator of CYP2C9 acting via the formation of an epoxide and/or γ-ketoenal.

Phytochemical characteristics of bergamot oranges from the Ionian islands of Greece: A multi-analytical approach with emphasis in the distribution of neohesperidose flavanones

The present study describes the peculiar phytochemical characteristics of bergamots cultivated in distinct islands of the Ionian Sea. Ultrahigh-performance liquid chromatography high-resolution mass spectrometry (UHPLC-HRMS) supported by 1 and 2D NMR spectroscopy was used for unambiguous metabolic profiling of albedo, flavedo and juice samples. Profile differences were determined using a multi-analytical clustering approach based on high-performance thin-layer chromatography fingerprints and UHPLC-HRMS data. Finally, a validated HPLC method offering good precision (0.12-0.94%) and accuracy (95.25-103.93%) was proposed for the quantification of the major flavanones. A total of 37 secondary metabolites were characterized belonging to flavonoids, limonoids and coumarins. Their distribution was tissue-dependent and varied significantly from bergamots of other geographical locations. Surprisingly, neoeriocitrin was the major flavanone, reaching 1.69 ± 0.05 g/L in the juice and 5.24 ± 0.12 mg/g in albedo. This is the highest reported amount among Citrus species, rendering Ionian bergamots a promising candidate for novel functional products.

Mechanism-based inactivation of cytochrome P450 enzymes by natural products based on metabolic activation

Cytochrome P450 enzymes (P450 enzymes) are the most common and important phase I metabolic enzymes and are responsible for the majority of the metabolism of clinical drugs and other xenobiotics. Drug-drug interactions (DDIs) can occur when the activities of P450 enzymes are inhibited. In particular, irreversible inhibition of P450 enzymes may lead to severe adverse interactions, compared to reversible inhibition. Many natural products have been shown to be irreversible inhibitors of P450 enzymes. The risks for intake of naturally occurring irreversible P450 enzyme inhibitors have been rising due to the rapid growth of the global consumption of natural products. Irreversible inhibition is usually called mechanism-based inactivation, which is time-, concentration- and NADPH- dependent. Generally, the formation of electrophilic intermediates is fundamental for the inactivation of P450 enzymes. This review comprehensively classifies natural P450 enzyme inactivators, including terpenoids, phenylpropanoids, flavonoids, alkaloids, and quinones obtained from herbs or foods. Moreover, the structure - activity correlations according to the IC₅₀ (or K_i) values reported in the literature as well as the underlying mechanisms based on metabolic activation are highlighted in depth.

Anti-Melanogenic Effects of Bergamottin via Mitogen-Activated Protein Kinases and Protein Kinase B Signaling Pathways

In this study, we examined the inhibitory effects of bergamottin on melanogenesis in B16F10 murine melanoma cells, together with its effects on the mechanism of melanin synthesis. α-Melanocyte stimulating hormone-stimulated B16F10 cells were treated with various concentrations of bergamottin, with arbutin as a positive control. Bergamottin significantly decreased the melanin content and tyrosinase activity without showing any cytotoxicity. In addition, bergamottin treatment significantly downregulated the expression of tyrosinase-related protein-1,2 and tyrosinase by suppressing the expression of microphthalmia-associated transcription factor. The phosphorylation status of mitogen-activated protein kinases (MAPKs) and protein kinase B (AKT) was examined to determine the mechanism underlying the antimelanogenic effects of bergamottin. Bergamottin treatment increased the phosphorylation of extracellular signal-regulated kinase (ERK) and AKT, but decreased the phosphorylation of p38 and c-Jun N-terminal kinase in the B16F10 cells. Moreover, the use of PD98059 (ERK inhibitor) and LY294002 (AKT inhibitor) corroborated these findings, indicating that bergamottin inhibits melanogenesis via the MAPKase and AKT signaling pathway. Thus, bergamottin has potential for treating hyperpigmentation disorders and can be a promising chemical for skin-whitening in the cosmetic industry.

Biomolecular Targets of Oxyprenylated Phenylpropanoids and Polyketides

Oxyprenylated secondary metabolites (e.g. phenylpropanoids and polyketides) represent a rare class of natural compounds. Over the past two decades, this group of phytochemicals has become a topic of intense research activity by several teams worldwide due to their in vitro and in vivo pharmacological activities, and to their great therapeutic and nutraceutical potential for the chemoprevention of acute and chronic diseases affecting humans. Such investigations have provided evidence that oxyprenylated secondary metabolites are able to interact with several biological targets at different levels accounting for their observed anticarcinogenic, anti-inflammatory, neuroprotective, immunomodulatory, antihypertensive, and metabolic effects. The aim of the present contribution is to provide a detailed survey of the so far reported data on the capacities of selected oxyprenylated phenylpropanoids and polyketides to trigger receptors, enzymes, and other types of cellular factors for which they exhibit a high degree of affinity and therefore evoke specific responses. With respect to the rather small amounts of these compounds available from natural sources, their chemical synthesis is also highlighted.

Pharmacological Utilization of Bergamottin, Derived from Grapefruits, in Cancer Prevention and Therapy

Cancer still remains one of the leading causes of death worldwide. In spite of significant advances in treatment options and the advent of novel targeted therapies, there still remains an unmet need for the identification of novel pharmacological agents for cancer therapy. This has led to several studies evaluating the possible application of natural agents found in vegetables, fruits, or plant-derived products that may be useful for cancer treatment. Bergamottin is a furanocoumarin derived from grapefruits and is also a well-known cytochrome P450 inhibitor. Recent studies have demonstrated potent anti-oxidative, anti-inflammatory, and anti-cancer properties of grapefruit furanocoumarin both in vitro and in vivo. The present review focuses on the potential anti-neoplastic effects of bergamottin in different tumor models and briefly describes the molecular targets affected by this agent.

Structures of human cytochrome P450 1A1 with bergamottin and erlotinib reveal active-site modifications for binding of diverse ligands

Human cytochrome P450 1A1 (CYP1A1) is an extrahepatic enzyme involved in the monooxygenation of structurally diverse compounds ranging from natural products to drugs and protoxins. Because CYP1A1 has a role in human carcinogenesis, inhibiting its activity may potentially aid in cancer chemoprevention, whereas utilizing CYP1A1's oxidative activity could help selectively activate anticancer prodrugs. Such potential therapeutic purposes require detailed knowledge of CYP1A1's interactions with potential ligands. Known CYP1A1 ligands also vary substantially in size, and it has not been apparent from a single existing CYP1A1 structure how larger, structurally diverse ligands are accommodated within the enclosed active site. Here, two new X-ray structures with the natural product furanocoumarin bergamottin (at 2.85 Å resolution) and the lung cancer drug erlotinib (3.0 Å) revealed binding orientations consistent with the formation of innocuous metabolites and of toxic metabolites, respectively. They also disclosed local changes in the roof of the active site that enlarge the active site and ultimately form a channel to the protein exterior. Although further structural modifications would be required to accommodate the largest CYP1A1 ligands, knowing which components of the active site are malleable provides powerful information for those attempting to use computational approaches to predict compound binding and substrate metabolism by this clinically relevant monooxygenase.

Characterization and Purification of Bergamottin from Citrus grandis (L.) Osbeck cv. Yongjiazaoxiangyou and Its Antiproliferative Activity and Effect on Glucose Consumption in HepG2 cells

Bergamottin is a natural furanocoumarin compound with weak polarity. Characterization and quantification of bergamottin were carried out in different fruit tissues of various citrus cultivars. Among the four citrus tissues tested, i.e., flavedo, albedo, segment membrane (SM), and juice sacs (JS) in eight citrus cultivars, the highest bergamottin content was found in the flavedo of Citrus grandis (L.) Osbeck cv. Yongjiazaoxiangyou (YJZXY, 666.54 μg·g-1 DW). A combination of silica gel column chromatography and high-speed counter-current chromatography (HSCCC) was established to efficiently purify bergamottin from the flavedo of YJZXY. Bergamottin showed significant antiproliferative activity on three cancer cell lines, i.e., human liver cancer HepG2, promyelocytic leukemia HL-60, and gastric cancer BGC-823 cells, which showed a marked inhibition effect on these cell lines in a dose-dependent manner. In addition, bergamottin significantly increased glucose consumption in HepG2 cells also in a dose-dependent manner, which is the first report of its potential in anti-diabetes applications.

Studies on the metabolites difference of psoralen/isopsoralen in human and six mammalian liver microsomes in vitro by UHPLC-MS/MS

Psoralen and isopsoralen are found in many fruits, vegetables and traditional Chinese medicines (TCM), such as Ficus carica L., Celery, Fructus Psoraleae etc. Modern pharmacological studies found that psoralen and isopsoralen can show estrogen-like activity, antitumor, and antibacterial activities etc. However, some research results also show some liver damage associated with the use of psoralen/isopsoralen or related medicines in human. Many studies focus on the pharmacological activities of psoralen/isopsoralen, while it is important to choose the suitable pharmacological models which are relevant to human in drug metabolism and pharmacokinetic process. The aim of this study is to identify the metabolites of psoralen/isopsoralen by human and six mammalian liver microsomes, and compare the metabolites difference of different species. Psoralen/isopsoralen are metabolized by liver microsomes of different animals to form five and seven metabolites, respectively. The metabolism of psoralen/isopsoralen undergoes hydroxylation, hydrogenation and hydrolysis, and oxidation of the furan ring to generate a furanoepoxide or γ-ketoenal intermediate. Furanoepoxide then forms a dihydrodiol, while γ-ketoenal forms 6-(7-hydroxycoumaryl)-acetic acid (in psoralen)/8-(7-hydroxycoumaryl)-acetic acid (in isopsoralen). By comparing the types of metabolites in the seven liver microsomes, it shows that the metabolic behaviors of psoralen by Beagle dog is most relevant to human, while the metabolic behaviors of isopsoralen by Sprague-Dawley rat is most similar to human. By comparing the relative amounts of the main metabolites, it shows that the metabolic capabilities of Sprague-Dawley rat and Rhesus monkey for psoralen are most similar to human, while the metabolic capabilities of Mouse, Dunkin-Hartley guinea pig, Sprague-Dawley rat, and human for isopsoralen are similar. Furthermore, the results show that the metabolic capability of human for psoralen and isopsoralen are weaker than other mammal species. The results of this work are useful for studying the metabolism mechanism of psoralen/isopsolaren, and choosing the most relevant animal species for investigation of psoralen/isopsolaren from experimental animals to human.

Roles of Residues F206 and V367 in Human CYP2B6: Effects of Mutations on Androgen Hydroxylation, Mechanism-Based Inactivation, and Reversible Inhibition

The crystal structures of human CYP2B6 indicate that Phe206 and Val367 are in close proximity to the substrate binding site and suggest that both residues may play important roles in substrate metabolism and inhibitor binding. To test this hypothesis, we investigated the effects of mutating these residues to Ala on the regiospecificity of CYP2B6 for the metabolism of testosterone and androstenedione. For testosterone metabolism, 16β-OH-testosterone formation by the F206A mutant was <5% of the wild type (WT), whereas the V367A mutant exhibited a doubling of 16α-OH-testosterone formation with a 50% decrease in 16β-OH-testosterone formation compared with the WT. Significant alterations in the regiospecificity for androstenedione metabolism were also observed. To investigate the roles of these two residues in the metabolic activation of mechanism-based inactivators, tert-butylphenylacetylene (BPA) and bergamottin (BG) were used to test the susceptibility to inactivation. Although the rates of inactivation of both mutants by BG were not significantly decreased compared with the WT, the efficiency of inactivation by BPA of both mutants was more than an order of magnitude lower. Our results demonstrate that Phe206 plays a crucial role in determining the specificity of CYP2B6 for the 16β-hydroxylation of testosterone and androstenedione and that it also plays an important role in BG binding and mechanism-based inactivation by BPA. In addition, Val367 dramatically enhances the catalytic activity of CYP2B6 toward androstenedione and plays an important role in mechanism-based inactivation by BPA. The results presented here show the important roles of Phe206 and Val367 in interactions of CYP2B6 with substrates and inactivators/inhibitors and are consistent with the crystal structures.

Development of a comprehensive analytical method for furanocoumarins in grapefruit and their metabolites in plasma and urine using UPLC-MS/MS: a preliminary study

To develop a comprehensive analytical method for photoactive furanocoumarins, grapefruit (whole, flesh, peel and juice) was extracted using QuEChERS (Quick, Easy, Cheap, Effective, Rugged and Safe) method. Seven furanocoumarins: bergaptol, psoralen, 8-methoxypsoralen, bergapten, 6',7'-dihydroxybergamottin (6',7'-DHB), epoxybergamottin and bergamottin were determined in grapefruit using UPLC-MS/MS. The concentrations of furanocoumarins in the plasma and urine of six healthy young adults before and after ingestion of grapefruit or grapefruit juice were also determined. Recovery rates of furanocoumarins by QuEChERS method from matrix spike sample and laboratory calibrate sample were 125.7 ± 25.4% and 105.7 ± 6.3%, respectively. Bergamottin and 6',7'-DHB were predominant compounds in grapefruit flesh, juice and plasma, while bergaptol and 6',7'-DHB were major compounds detected in the urine. The results demonstrated that bergamottin and 6',7'-DHB were metabolized to bergaptol. Overall, the analytical methods developed in the present study can be applied to the analysis of various furanocoumarins in plant sources and biological samples.

Reactive metabolites in the biotransformation of molecules containing a furan ring

Many xenobiotics containing a furan ring are toxic and/or carcinogenic. The harmful effects of these compounds require furan ring oxidation. This reaction generates an electrophilic intermediate. Depending on the furan ring substituents, the intermediate is either an epoxide or a cis-enedione with more ring substitution favoring epoxide formation. Either intermediate reacts with cellular nucleophiles such as protein or DNA to trigger toxicities. The reactivity of the metabolite determines which cellular nucleophiles are targeted. The toxicity of a particular furan is also influenced by the presence of competing metabolic pathways or efficient detoxification routes. GSH plays an important role in modulating the harmful effects of this class of compound by reacting with the reactive metabolite. However, this may not represent a detoxification step in all cases.

Identification of the residue in human CYP3A4 that is covalently modified by bergamottin and the reactive intermediate that contributes to the grapefruit juice effect

Previous studies have demonstrated that bergamottin (BG), a component of grapefruit juice, is a mechanism-based inactivator of CYP3A4 and contributes, in part, to the grapefruit juice-drug interaction. Although the covalent binding of [(14)C]BG to the CYP3A4 apoprotein has been demonstrated by SDS-polyacrylamide gel electrophoresis, the identity of the modified amino acid residue and the reactive intermediate species of BG responsible for the inactivation have not been reported. In the present study, we show that inactivation of CYP3A4 by BG results in formation of a modified apoprotein-3A4 and a GSH conjugate, both exhibiting mass increases of 388 Da, which corresponds to the mass of 6',7'-dihydroxybergamottin (DHBG), a metabolite of BG, plus one oxygen atom. To identify the adducted residue, BG-inactivated 3A4 was digested with trypsin, and the digests were then analyzed by liquid chromatography-tandem mass spectrometry (MS/MS). A mass shift of 388 Da was used for the SEQUEST database search, which revealed a mass increase of 388 Da for the peptide with the sequence (272)LQLMIDSQNSK(282), and MS/MS analysis of the adducted peptide demonstrated that Gln273 is the residue modified. Mutagenesis studies showed that the Gln273 to Val mutant was resistant to inactivation by BG and DHBG and did not generate two of the major metabolites of BG formed by 3A4 wild type. In conclusion, we have determined that the reactive intermediate, oxygenated DHBG, covalently binds to Gln273 and thereby contributes to the mechanism-based inactivation of CYP3A4 by BG.

Molecular characterization of CYP2B6 substrates

CYP2B6 has not been as fully characterized at the molecular level as other members of the human cytochrome P450 family. As more widely used in vitro probes for characterizing the involvement of this enzyme in the metabolism of xenobiotics have become available, the number of molecules identified as CYP2B6 substrates has increased. In this study we have analyzed the available kinetic data generated by multiple laboratories with human recombinant expressed CYP2B6 and along with calculated molecular properties derived from the ChemSpider database, we have determined the molecular features that appear to be important for CYP2B6 substrates. In addition we have applied 2D and 3D QSAR methods to generate predictive pharmacophore and 2D models. For 28 molecules with K(m) data, the molecular weight (mean +/- SD) is 253.78+/-74.03, ACD/logP is 2.68+/-1.51, LogD(pH 5.5) is 1.51+/-1.43, LogD(pH 7.4) is 2.02+/-1.25, hydrogen bond donor (HBD) count is 0.57 +/-0.57, hydrogen bond acceptor (HBA) count is 2.57+/-1.37, rotatable bonds is 3.50+/-2.71 and total polar surface area (TPSA) is 27.63+/-19.42. A second set of 15 molecules without K(m) data possessed similar mean molecular property values. These properties are comparable to those of a set of 21 molecules used in a previous pharmacophore modeling study (Ekins et al., J Pharmacol Exp Ther 288 (1), 21-29, 1999). Only the LogD and HBD values were statistically significantly different between these different datasets. We have shown that CYP2B6 substrates are generally small hydrophobic molecules that are frequently central nervous system active, which may be important for drug discovery research.

Mechanism-based inactivation of human cytochromes p450s: experimental characterization, reactive intermediates, and clinical implications

The P450 type cytochromes are responsible for the metabolism of a wide variety of xenobiotics and endogenous compounds. Although P450-catalyzed reactions are generally thought to lead to detoxication of xenobiotics, the reactions can also produce reactive intermediates that can react with cellular macromolecules leading to toxicity or that can react with the P450s that form them leading to irreversible (i.e., mechanism-based) inactivation. This perspective describes the fundamentals of mechanism-based inactivation as it pertains to P450 enzymes. The experimental approaches used to characterize mechanism-based inactivators are discussed, and the criteria required for a compound to be classified as a mechanism-based inactivator are outlined. The kinetic scheme for mechanism-based inactivation and the calculation of the relevant kinetic constants that describe a particular inactivation event are presented. The structural aspects and important functional groups of several classes of molecules that have been found to impart mechanism-based inactivation upon metabolism by P450s such as acetylenes, thiol-containing compounds that include isothiocyanates, thiazolidinediones, and thiophenes, arylamines, quinones, furanocoumarins, and cyclic tertiary amines are described. Emphasis throughout this perspective is placed on more recent findings with human P450s where the site of modification, whether it be the apoprotein or the heme moiety, and, at least in part, the identity of the reactive intermediate responsible for the loss in P450 activity are known or inferred. Recent advances in trapping procedures as well as new methods for identification of reactive intermediates are presented. A variety of clinically important drugs that act as mechanism-based inactivators of P450s are discussed. The irreversible inactivation of human P450s by these drugs has the potential for causing serious drug-drug interactions that may have severe toxicological effects. The clinical significance of inactivating human P450s for improving drug efficacy as well as drug safety is discussed along with the potential for exploiting mechanism-based inactivators of P450s for therapeutic benefits.

The inactivation of cytochrome P450 3A5 by 17alpha-ethynylestradiol is cytochrome b5-dependent: metabolic activation of the ethynyl moiety leads to the formation of glutathione conjugates, a heme adduct, and covalent binding to the apoprotein

17Alpha-ethynylestradiol (EE) inactivates cytochrome P450 3A5 (3A5) in the reconstituted system in a mechanism-based manner. The inactivation is dependent on NADPH, and it is irreversible. The inactivation of 3A5 by EE is also dependent on cytochrome b5 (b5). The values for the K(I) and k(inact) of the 7-benzyloxy-4-(trifluoromethyl)coumarin O-debenzylation activity of 3A5 are 26 microM and 0.06 min(-1), respectively. Incubation of 3A5 with EE resulted in a 62% loss of catalytic activity, 60% loss in the reduced CO difference spectrum, and 40% decrease in native heme with the formation of a heme adduct. The partition ratio was approximately 25, and the stoichiometry of binding was approximately 0.3 mol of EE metabolite bound/mol of P450 inactivated. Four major metabolites were formed during the metabolism of EE by 3A5. SDS-polyacrylamide gel electrophoresis analysis demonstrated that [3H]EE was irreversibly bound to 3A5 apoprotein. Liquid chromatography-tandem mass spectrometry analysis (LC-MS/MS) revealed that two glutathione (GSH) conjugates with m/z values of 620 were formed only in the presence of b5. These two conjugates are formed from the reaction of GSH with the ethynyl group with the oxygen being inserted into either the internal or terminal carbon. A heme adduct with the ion at m/z 927 and two dipyrrole adducts with ions at m/z 579 were detected by LC-MS/MS analysis. In conclusion, 3A5 can activate EE to a 17alpha-oxirene-related reactive species that can then partition the oxygen between the internal and terminal carbons of the ethynyl group to form heme and apoprotein adducts, resulting in the inactivation of P450 3A5.