Self-catalyzed inactivation of hepatic cytochrome P-450 by ethynyl substrates

Improved methods for the detection of heme and protoporphyrin IX adducts and quantification of heme B from cytochrome P450 containing systems

Heme B is a critical prosthetic group for the function of numerous proteins including the cytochrome P450 (CYP) family of enzymes. CYP enzymes are involved in the metabolism of endogenous and xenobiotic molecules that are of central interest in drug development. Formation of reactive metabolites by CYPs can lead to heme modification and destruction of the enzyme. The structure of the adducted heme can provide key information on the mechanism of inactivation, which is of great interest during preclinical drug discovery. Historically, techniques to extract the modified heme or protoporphyrin IX species involved harsh extraction conditions and esterification of propionate groups to aid chromatography. We have developed a simplified extraction method and LC/MS chromatography system that does not require derivatization to quantify heme B and identify modified heme B species from multiple CYP-containing matrices. The method uses mass defect filter triggered data dependent MS2 scans to rapidly identify heme and protoporphyrin IX adducts. These methods may also be useful for the analysis of other heme variants and hemoproteins.

Structure of the Green Heme Isolated from Allylbenzene-Modified Chloroperoxidase: A Novel Heme Architecture Implicating the Mechanisms of CPO Inactivation and Epoxidation

The chemical identification of the modified heme (the green heme) during chloroperoxidase catalyzed epoxidation of allylbenzene remains unestablished due to its high instability within the protein matrix, the absence of paramagnetically shifted signals, and the difficulty in obtaining crystals of the modified enzyme. We have established the unambiguous structure of the modified prosthetic heme group, which was extracted from the protein matrix using ²D NMR spectroscopy and LC-MS spectrometry. The modified heme was isolated as a µ-oxo dimer that can be quantitatively converted to the corresponding monomer. The depolymerized green heme displayed characteristic NMR signatures of iron porphyrin complexes, but no Nuclear Overhauser Effect was observable to assist in signal assignment. An alternative strategy was applied by removing the iron center of the green heme, resulting in a stable demetallated green porphyrin species. Complete assignment of all the NMR resonances in the demetallated green heme allowed us to establish the molecular architecture of the modified species as a novel N-alkylated heme. Decisive space correlations between the propyl protons of allylbenzene and the γ meso proton coupled with clear dipolar connectivities between the propyl-2H of the substrate and the β proton in the side chain of the propionic acid at carbon-6 of the porphyrin ring, clearly indicate that allylbenzene was covalently attached to the nitrogen atom of the pyrrole ring III of the prosthetic heme. In this study, the mechanism of green CPO formation and its relation to CPO catalyzed chiral transformations are also discussed. It is concluded that the double-phenyl clamp formed by two phenylalanine residues at the distal heme pocket plays a critical role in fine-tuning substrate orientation that determines the outcome of CPO catalyzed epoxidation of substituted styrenes.

Discovery of a Novel Potent and Selective HSD17B13 Inhibitor, BI-3231, a Well-Characterized Chemical Probe Available for Open Science

Genome-wide association studies in patients revealed HSD17B13 as a potential new target for the treatment of nonalcoholic steatohepatitis (NASH) and other liver diseases. However, the physiological function and the disease-relevant substrate of HSD17B13 remain unknown. In addition, no suitable chemical probe for HSD17B13 has been published yet. Herein, we report the identification of the novel potent and selective HSD17B13 inhibitor BI-3231. Through high-throughput screening (HTS), using estradiol as substrate, compound 1 was identified and selected for subsequent optimization resulting in compound 45 (BI-3231). In addition to the characterization of compound 45 for its functional, physicochemical, and drug metabolism and pharmacokinetic (DMPK) properties, NAD⁺ dependency was investigated. To support Open Science, the chemical HSD17B13 probe BI-3231 will be available to the scientific community for free via the opnMe platform, and thus can help to elucidate the pharmacology of HSD17B13.

Acetylenes: cytochrome P450 oxidation and mechanism-based enzyme inactivation

The oxidation of carbon-carbon triple bonds by cytochrome P450 produces ketene metabolites that are hydrolyzed to acetic acid derivatives or are trapped by nucleophiles. In the special case of 17α-ethynyl sterols, D-ring expansion and de-ethynylation have been observed as competing pathways. The oxidation of acetylenic groups is also associated with mechanism-based inactivation of cytochrome P450 enzymes. One mechanism for this inactivation is reaction of the ketene metabolite with cytochrome P450 residues essential for substrate binding or catalysis. However, in the case of monosubstituted acetylenes, inactivation can also occur by addition of the oxidized acetylenic function to a nitrogen of the heme prosthetic group. This addition reaction is not mediated by the ketene metabolite, but rather occurs during oxygen transfer to the triple bond. In some instances, a detectable intermediate is formed that is most consistent with a ketocarbene-iron heme complex. This complex can progress to the N-alkylated heme or revert back to the unmodified enzyme. The ketocarbene complex may intervene in the formation of all the N-alkyl heme adducts, but is normally too unstable to be detected.

Estrogen sulfotransferase in the metabolism of estrogenic drugs and in the pathogenesis of diseases

INTRODUCTION

Biotransformation is important in the metabolism of endobiotics and xenobiotics. This process comprises the activity of phase I and phase II enzymes. Estrogen sulfotransferase (SULT1E1 or EST) is a phase II conjugating enzyme that belongs to the family of cytosolic sulfotransferases. The expression of SULT1E1 can be detected in many tissues, including the liver. SULT1E1 catalyzes the transfer of a sulfate group from 3'-phosphoadenosine-5'-phosphosulfate (PAPS) to any available hydroxyl group in estrogenic molecules. The substrates of SULT1E1 include the endogenous and synthetic estrogens. Upon SULT1E1-mediated sulfation, the hydrosolubility of estrogens increases, preventing the binding between the sulfated estrogens and the estrogen receptor (ER). This sulfated state of the estrogens is not irreversible, as the steroid sulfatase (STS) can convert sulfoconjugated estrogens to free estrogens. The expression of SULT1E1 is inducible by several diseases that involve tissue inflammation, such as type 2 diabetes, sepsis, and ischemia-reperfusion injury. Areas covered: This systematic literature review aims to summarize the role of SULT1E1 in the metabolism of estrogenic drugs and xenobiotics, and the role of SULT1E1 in the pathogenesis of several diseases, including cancer, metabolic disease, sepsis, liver injury, and cystic fibrosis. Meanwhile, ablation or pharmacological inhibition of SULT1E1 can affect the outcomes of the aforementioned diseases. Expert opinion: In addition to its role in metabolizing estrogenic drugs, SULT1E1 is unexpectedly being unveiled as a mediator for the disease effect on estrogen metabolism and homeostasis. Meanwhile, because the expression and activity of SULT1E1 can affect the outcome of diseases, the same sulfotransferase and the reversing enzymes STS can be potential therapeutic targets to prevent or manage diseases. Accumulating evidence suggest that the physiological and pathophysiological effects of SULT1E1 can be estrogen-independent and it is necessary to elucidate what other possible substrates may be recognized by the enzyme. Moreover, human studies are paramount to confirm the human relevance of the animal studies.

Bioorthogonal Strategies for Engineering Extracellular Matrices

Hydrogels are commonly used as engineered extracellular matrix (ECM) mimics in applications ranging from tissue engineering to in vitro disease models. Ideal mechanisms used to crosslink ECM-mimicking hydrogels do not interfere with the biology of the system. However, most common hydrogel crosslinking chemistries exhibit some form of cross-reactivity. The field of bio-orthogonal chemistry has arisen to address the need for highly specific and robust reactions in biological contexts. Accordingly, bio-orthogonal crosslinking strategies have been incorporated into hydrogel design, allowing for gentle and efficient encapsulation of cells in various hydrogel materials. Furthermore, the selective nature of bio-orthogonal chemistries can permit dynamic modification of hydrogel materials in the presence of live cells and other biomolecules to alter matrix mechanical properties and biochemistry on demand. In this review, we provide an overview of bio-orthogonal strategies used to prepare cell-encapsulating hydrogels and highlight the potential applications of bio-orthogonal chemistries in the design of dynamic engineered ECMs.

Ethynilestradiol 20 mcg plus Levonorgestrel 100 mcg: Clinical Pharmacology

Estroprogestins (EPs) are combinations of estrogen and progestin with several actions on women's health. The different pharmacological composition of EPs is responsible for different clinical effects. One of the most used low-dose EP associations is ethinylestradiol 20 mcg plus levonorgestrel 100 mcg in monophasic regimen (EE20/LNG100). This review summarizes clinical pharmacology, cycle control, and effects on lipid and glucose metabolism, coagulation, body weight/body composition, acne, and sexuality of EE20/LNG100. Overall, EE20/LNG100 combination is safe and well tolerated, and in several studies the incidence of adverse events in the treated group was comparable to that of the placebo group. Cycle control was effective and body weight/body composition did not vary among treated and untreated groups in most studies. The EE20/LNG100 combination shows mild or no effect on lipid and glucose metabolism. Lastly, EE20/LNG100 is associated with a low risk of venous thromboembolism (VTE). In conclusion, in the process of decision making for the individualization of EPs choice, EE20/LNG100 should be considered for its favorable clinical profile.

Cytochrome P450 family 1 inhibitors and structure-activity relationships

With the widespread use of O-alkoxyresorufin dealkylation assays since the 1990s, thousands of inhibitors of cytochrome P450 family 1 enzymes (P450s 1A1, 1A2, and 1B1) have been identified and studied. Generally, planar polycyclic molecules such as polycyclic aromatic hydrocarbons, stilbenoids, and flavonoids are considered to potentially be effective inhibitors of these enzymes, however, the details of the structure-activity relationships and selectivity of these inhibitors are still ambiguous. In this review, we thoroughly discuss the selectivity of many representative P450 family 1 inhibitors reported in the past 20 years through a meta-analysis.

Inhibition of cytochrome P450 enzymes and biochemical aspects of mechanism-based inactivation (MBI)

Mechanism-based inactivation (MBI) often involves metabolic bioactivation of the xenobiotic by cytochrome P450s (CYPs) to an electrophilic reactive intermediate and results in quasi-irreversible or irreversible inactivation. Such reactive intermediate can cause quasi-irreversible inhibition through coordination to the ferrous state, Fe(II), of the P450 enzyme forming a tight noncovalent bond leading to the formation of metabolic-intermediate complex (MIC). By contrast, irreversible inactivation is one of the most common causes for the observed drug–drug interaction (DDI) and usually implies the formation of a covalent bond between the metabolite and the enzyme via alkylation of either the heme or the P450 apoprotein. Here we illustrate the important points of the current literature understanding of the mechanisms of inhibition of CYP enzymes with emphasis on general mechanistic aspects of MBI for some drugs/moieties associated with the phenomenon. Additionally, the utility of computational and in silico approaches to address bioactivation issues will be briefly discussed.

Pivotal role of water in terminating enzymatic function: a density functional theory study of the mechanism-based inactivation of cytochromes P450

The importance of the mechanism-based inactivation (MBI) of enzymes, which has a variety of physiological effects and therapeutic implications, has been garnering appreciation. Density functional theory calculations were undertaken to gain a clear understanding of the MBI of a cytochrome P450 enzyme (CYP2B4) by tert-butylphenylacetylene (tBPA). The results of calculations suggest that, in accordance with previous proposals, the reaction proceeds via a ketene-type metabolic intermediate. Once an oxoiron(IV) porphyryn π-cation radical intermediate (compound I) of P450 is generated at the heme reaction site, ketene formation is facile, as the terminal acetylene of tBPA can form a C-O bond with the oxo unit of compound I with a relatively low reaction barrier (14.1 kcal/mol). Unexpectedly, it was found that the ketene-type intermediate was not very reactive. Its reaction with the hydroxyl group of a threonine (Thr302) to form an ester bond required a substantial barrier (38.2 kcal/mol). The high barrier disfavored the mechanism by which these species react directly. However, the introduction of a water molecule in the reaction center led to its active participation in the reaction. The water was capable of donating its proton to the tBPA molecule, while accepting the proton of threonine. This water-mediated mechanism lowered the reaction barrier for the formation of an ester bond by about 20 kcal/mol. Therefore, our study suggests that a water molecule, which can easily gain access to the threonine residue through the proton-relay channel, plays a critical role in enhancing the covalent modification of threonine by terminal acetylene compounds. Another type of MBI by acetylenes, N-alkylation of the heme prosthetic group, was less favorable than the threonine modification pathway.

Chemistry and pharmacological characterization of novel nitrogen analogues of AMOP-H-OH (Sazetidine-A, 6-[5-(azetidin-2-ylmethoxy)pyridin-3-yl]hex-5-yn-1-ol) as α4β2-nicotinic acetylcholine receptor-selective partial agonists

In order to advance therapeutic applications of nicotinic ligands, continuing research efforts are being directed toward the identification and characterization of novel nicotinic acetylcholine receptor (nAChR) ligands that are both potent and subtype selective. Herein we report the synthesis and pharmacological evaluation of members of a new series of 3-alkoxy-5-aminopyridine derivatives that display good selectivity for the α4β2-nAChR subtype based on ligand binding and functional evaluations. The most potent ligand in this series, compound 64, showed high radioligand binding affinity and selectivity for rat α4β2-nAChR with a K(i) value of 1.2 nM and 4700-fold selectivity for α4β2- over α3β4-nAChR, and ∼100-fold selectivity for functional, high-sensitivity, human α4β2-nAChR over α3β4*-nAChR. In the mouse forced swim test, compound 64 exhibited antidepressant-like effects. Structure-activity relationship (SAR) analyses suggest that the introduction of additional substituents to the amino group present on the pyridine ring of the N-demethylated analogue of compound 17 can provide potent α4β2-nAChR-selective ligands for possible use in treatment of neurological and psychiatric disorders including depression.

In silico studies of polyaromatic hydrocarbon inhibitors of cytochrome P450 enzymes 1A1, 1A2, 2A6, and 2B1

A computational study was undertaken to understand the nature of binding and the structural features that play a significant role in the binding of arylacetylene molecules to cytochrome P450 enzymes 1A1, 1A2, 2A6, and 2B1. Nine polycyclic arylacetylenes determined to be mechanism-based P450 enzyme inhibitors were studied. The lack of polar substituents in these compounds causes them to be incapable of hydrogen bonding to the polar protein residues. The four P450 enzymes of interest all have phenylalanine residues in the binding pocket for potential pi-pi interactions with the aromatic rings of the inhibitors. The inhibition potency of these arylacetylenes toward P450s 1A1 and 2B1 showed a dependence on the proximity of the inhibitor's triple bond to the prosthetic heme Fe of the enzyme. In P450 enzyme 1A2, the inhibitor's potency showed more dependence on the pi-pi interactions of the inhibitor's ring systems with the phenylalanine residues of the protein, with the proximity of the inhibitor triple bond to the heme Fe weighing in as the second most important factor. The results suggest that maximizing the pi-pi interactions with phenylalanine residues in the binding pocket and optimum proximity of the acetylene moiety to the heme Fe will provide for a substantial increase in the potency of the polyaromatic hydrocarbon mechanism-based inhibitors. A fine balance of these two aspects of binding coupled with attention to supplementing hydrophobic interactions could address potency and selectivity issues for these inhibitors.

Cytochrome P450-mediated bioactivation of the epidermal growth factor receptor inhibitor erlotinib to a reactive electrophile

The epidermal growth factor receptor tyrosine kinase inhibitor erlotinib (ERL) is approved for treatment of non-small-cell lung cancer. Numerous reports of ERL-associated toxicities are consistent with immune-mediated toxicity, including drug-induced hepatitis, interstitial lung disease, Stevens-Johnson syndrome, and toxic epidermal necrolysis. Although the mechanism of toxicity has not been established, we present evidence that reactive intermediates are formed during the metabolism of ERL, which can covalently conjugate to the cysteine group of the peptide-mimetic GSH. Seven ERL-GSH conjugates were identified in incubations with hepatic microsomes. Cytochrome P450 (P450)-dependent adducts are proposed to be formed via reactive epoxide and electrophilic quinone-imine intermediates. In incubations of human liver microsomes, intestinal microsomes, pulmonary microsomes, and recombinant P450s, CYP3A4 was the primary enzyme responsible for the bioactivation of ERL; however, CYP1A1, CYP1A2, CYP3A5, and CYP2D6 were capable of catalyzing the bioactivation as well. During the metabolism of ERL, CYP3A4 and CYP3A5 are irreversibly inactivated by ERL in a time- and concentration-dependent manner. Inactivation was not dependent on oxidation of the ERL alkyne group to form a reactive oxirene or ketene, as shown by synthesizing analogs where the alkyne was replaced with a cyano group. CYP1A1, CYP1A2, and CYP2D6 were not inactivated despite catalyzing the formation of ERL-GSH adducts.

Copper-free click chemistry in living animals

Chemical reactions that enable selective biomolecule labeling in living organisms offer a means to probe biological processes in vivo. Very few reactions possess the requisite bioorthogonality, and, among these, only the Staudinger ligation between azides and triarylphosphines has been employed for direct covalent modification of biomolecules with probes in the mouse, an important model organism for studies of human disease. Here we explore an alternative bioorthogonal reaction, the 1,3-dipolar cycloaddition of azides and cyclooctynes, also known as "Cu-free click chemistry," for labeling biomolecules in live mice. Mice were administered peracetylated N-azidoacetylmannosamine (Ac(4)ManNAz) to metabolically label cell-surface sialic acids with azides. After subsequent injection with cyclooctyne reagents, glycoconjugate labeling was observed on isolated splenocytes and in a variety of tissues including the intestines, heart, and liver, with no apparent toxicity. The cyclooctynes tested displayed various labeling efficiencies that likely reflect the combined influence of intrinsic reactivity and bioavailability. These studies establish Cu-free click chemistry as a bioorthogonal reaction that can be executed in the physiologically relevant context of a mouse.

Metabolic activation of mifepristone [RU486; 17beta-hydroxy-11beta-(4-dimethylaminophenyl)-17alpha-(1-propynyl)-estra-4,9-dien-3-one] by mammalian cytochromes P450 and the mechanism-based inactivation of human CYP2B6

Mifepristone [RU486; 17beta-hydroxy-11beta-(4-dimethylaminophenyl)-17alpha-(1-propynyl)-estra-4,9-dien-3-one] inactivates CYP2B6 in the reconstituted system in a mechanism-based manner. The loss of 7-ethoxy-4-(trifluoromethyl)-coumarin deethylation activity of CYP2B6 is concentration- and time-dependent. The inactivation requires NADPH and is irreversible. The concentration of inactivator required to give the half-maximal rate of inactivation is 2.8 microM, and the maximal rate constant for inactivation at a saturating concentration of the inactivator is 0.07 min(-1). Incubation of CYP2B6 with 20 microM RU486 for 15 min resulted in 61% loss of catalytic activity, 60% loss of the reduced cytochrome P450 (P450)-CO complex, and a 40% loss of native heme. The partition ratio is approximately 5, and the stoichiometry of binding is approximately 0.6 mol RU486/mol P450 inactivated. SDS-polyacrylamide gel electrophoresis and high-pressure liquid chromatography analysis showed that [(3)H]RU486 was irreversibly bound to CYP2B6 apoprotein. RU486 is metabolized to form three major metabolites and bioactivated to give reactive intermediates by purified P450s in the reconstituted system. After incubation of RU486 with the purified P450s and liver microsomes from rats and humans in the presence of glutathione (GSH) and NADPH, GSH conjugates with MH(+) ions at m/z 769, 753, and 751 were detected by liquid chromatography-tandem mass spectrometry. Two GSH conjugates with MH(+) ions at m/z 753 are formed from the reaction of GSH with RU486. The adducts are formed after addition of an activated oxygen to the carbon-carbon triple bond of the propynyl moiety. This suggests that oxirene intermediates may be involved in the mechanism of inactivation. It seems that the potential for drug-drug interactions of RU486 may not be limited only to CYP3A4 and should also be evaluated for drugs metabolized primarily by CYP2B6, such as bupropion and efavirenz.

Differential inhibition of cytochromes P450 3A4 and 3A5 by the newly synthesized coumarin derivatives 7-coumarin propargyl ether and 7-(4-trifluoromethyl)coumarin propargyl ether

The abilities of 7-coumarin propargyl ether (CPE) and 7-(4-trifluoromethyl)coumarin propargyl ether (TFCPE) to act as mechanism-based inactivators of P450 3A4 and 3A5 in the reconstituted system have been investigated using 7-benzyloxy-4-(trifluoromethyl)coumarin (BFC) and testosterone as probes. CPE inhibited the BFC O-debenzylation activity of P450 3A4 in a time-, concentration-, and NADPH-dependent manner characteristic of a mechanism-based inactivator with a half-maximal inactivation (K(I)) of 112 microM, a maximal rate of inactivation (k(inact)) of 0.05 min(-1), and a t(1/2) of 13.9 min. Similarly, TFCPE inhibited the BFC O-debenzylation activity of P450 3A4 in a time-, concentration-, and NADPH-dependent manner with a K(I) of 14 microM, a k(inact) of 0.04 min(-1), and a t(1/2) of 16.5 min. Parallel losses of P450 3A4 enzymatic activity and heme were observed with both compounds as measured by high-performance liquid chromatography and reduced CO spectra. Interestingly, neither compound inhibited the BFC O-debenzylation activity of P450 3A5. Reactive intermediates of CPE and TFCPE formed by P450 3A4 were trapped with glutathione, and the resulting adducts were identified using tandem mass spectral analysis. Metabolism studies using TFCPE resulted in the identification of a single metabolite that is formed by P450 3A4 but not by P450 3A5 and that may play a role in the mechanism-based inactivation.

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.

Pharmacokinetic Drug Interactions Involving 17??-Ethinylestradiol

17alpha-Ethinylestradiol (EE) is widely used as the estrogenic component of oral contraceptives (OC). In vitro and in vivo metabolism studies indicate that EE is extensively metabolised, primarily via intestinal sulfation and hepatic oxidation, glucuronidation and sulfation. Cytochrome P450 (CYP)3A4-mediated EE 2-hydroxylation is the major pathway of oxidative metabolism of EE. For some time it has been known that inducers of drug-metabolising enzymes (such as the CYP3A4 inducer rifampicin [rifampin]) can lead to breakthrough bleeding and contraceptive failure. Conversely, inhibitors of drug-metabolising enzymes can give rise to elevated EE plasma concentrations and increased risks of vascular disease and hypertension. In vitro studies have also shown that EE inhibits a number of human CYP enzymes, such as CYP2C19, CYP3A4 and CYP2B6. Consequently, there are numerous reports in the literature describing EE-containing OC formulations as perpetrators of pharmacokinetic drug interactions. Because EE may participate in multiple pharmacokinetic drug interactions as either a victim or perpetrator, pharmaceutical companies routinely conduct clinical drug interaction studies with EE-containing OCs when evaluating new chemical entities in development. It is therefore critical to understand the mechanisms underlying these drug interactions. Such an understanding can enable the interpretation of clinical data and lead to a greater appreciation of the profile of the drug by physicians, clinicians and regulators. This article summarises what is known of the drug-metabolising enzymes and transporters governing the metabolism, disposition and excretion of EE. An effort is made to relate this information to known clinical drug-drug interactions. The inhibition and induction of drug-metabolising enzymes by EE is also reviewed.

Pharmacology of estrogens and progestogens: influence of different routes of administration

This review comprises the pharmacokinetics and pharmacodynamics of natural and synthetic estrogens and progestogens used in contraception and therapy, with special consideration of hormone replacement therapy. The paper describes the mechanisms of action, the relation between structure and hormonal activity, differences in hormonal pattern and potency, peculiarities in the properties of certain steroids, tissue-specific effects, and the metabolism of the available estrogens and progestogens. The influence of the route of administration on pharmacokinetics, hormonal activity and metabolism is presented, and the effects of oral and transdermal treatment with estrogens on tissues, clinical and serum parameters are compared. The effects of oral, transdermal (patch and gel), intranasal, sublingual, buccal, vaginal, subcutaneous and intramuscular administration of estrogens, as well as of oral, vaginal, transdermal, intranasal, buccal, intramuscular and intrauterine application of progestogens are discussed. The various types of progestogens, their receptor interaction, hormonal pattern and the hormonal activity of certain metabolites are described in detail. The structural formulae, serum concentrations, binding affinities to steroid receptors and serum binding globulins, and the relative potencies of the available estrogens and progestins are presented. Differences in the tissue-specific effects of the various compounds and regimens and their potential implications with the risks and benefits of hormone replacement therapy are discussed.

Minimising the potential for metabolic activation in drug discovery

Investigations into the role of bioactivation in the pathogenesis of xenobiotic-induced toxicity have been a major area of research since the link between reactive metabolites and carcinogenesis was first reported in the 1930s. Circumstantial evidence suggests that bioactivation of relatively inert functional groups to reactive metabolites may contribute towards certain drug-induced adverse reactions. Reactive metabolites, if not detoxified, can covalently modify essential cellular targets. The identity of the susceptible biomacromolecule(s), and the physiological consequence of its covalent modification, will dictate the resulting toxicological response (e.g., covalent modification of DNA by reactive intermediates derived from procarcinogens that potentially leads to carcinogenesis). The formation of drug-protein adducts often carries a potential risk of clinical toxicities that may not be predicted from preclinical safety studies. Animal models used to reliably predict idiosyncratic drug toxicity are unavailable at present. Furthermore, considering that the frequency of occurrence of idiosyncratic adverse drug reactions (IADRs) is fairly rare (1 in 1000 to 1 in 10,000), it is impossible to detect such phenomena in early clinical trials. Thus, the occurrence of IADRs during late clinical trials or after a drug has been released can lead to an unanticipated restriction in its use and even in its withdrawal. Major themes explored in this review include a comprehensive cataloguing of bioactivation pathways of functional groups commonly utilised in drug design efforts with appropriate strategies towards detection of corresponding reactive intermediates. Several instances wherein replacement of putative structural alerts in drugs associated with IADRs with a latent functionality eliminates the underlying liability are also presented. Examples of where bioactivation phenomenon in drug candidates can be successfully abrogated via iterative chemical interventions are also discussed. Finally, appropriate strategies that aid in potentially mitigating the risk of IADRs are explored, especially in circumstances in which the structural alert is also responsible for the primary pharmacology of the drug candidate and cannot be replaced.

Drug bioactivation, covalent binding to target proteins and toxicity relevance

A number of therapeutic drugs with different structures and mechanisms of action have been reported to undergo metabolic activation by Phase I or Phase II drug-metabolizing enzymes. The bioactivation gives rise to reactive metabolites/intermediates, which readily confer covalent binding to various target proteins by nucleophilic substitution and/or Schiff's base mechanism. These drugs include analgesics (e.g., acetaminophen), antibacterial agents (e.g., sulfonamides and macrolide antibiotics), anticancer drugs (e.g., irinotecan), antiepileptic drugs (e.g., carbamazepine), anti-HIV agents (e.g., ritonavir), antipsychotics (e.g., clozapine), cardiovascular drugs (e.g., procainamide and hydralazine), immunosupressants (e.g., cyclosporine A), inhalational anesthetics (e.g., halothane), nonsteroidal anti-inflammatory drugs (NSAIDSs) (e.g., diclofenac), and steroids and their receptor modulators (e.g., estrogens and tamoxifen). Some herbal and dietary constituents are also bioactivated to reactive metabolites capable of binding covalently and inactivating cytochrome P450s (CYPs). A number of important target proteins of drugs have been identified by mass spectrometric techniques and proteomic approaches. The covalent binding and formation of drug-protein adducts are generally considered to be related to drug toxicity, and selective protein covalent binding by drug metabolites may lead to selective organ toxicity. However, the mechanisms involved in the protein adduct-induced toxicity are largely undefined, although it has been suggested that drug-protein adducts may cause toxicity either through impairing physiological functions of the modified proteins or through immune-mediated mechanisms. In addition, mechanism-based inhibition of CYPs may result in toxic drug-drug interactions. The clinical consequences of drug bioactivation and covalent binding to proteins are unpredictable, depending on many factors that are associated with the administered drugs and patients. Further studies using proteomic and genomic approaches with high throughput capacity are needed to identify the protein targets of reactive drug metabolites, and to elucidate the structure-activity relationships of drug's covalent binding to proteins and their clinical outcomes.

Identification of amino acid residues involved in the inactivation of cytochrome P450 2B1 by two acetylenic compounds: the role of three residues in nonsubstrate recognition Sites

The homologous rat cytochrome P450s 2B1 and 2B2 differ by 13 amino acids. A chimeric construct of P450 2B1/2B2 was used in conjunction with several site-directed mutants to identify key residues involved in the inactivation of P450 2B1 by two acetylenic compounds, 17alpha-ethynylestradiol (17EE) and tert-butyl 1-methyl-2-propynyl ether (tBMP). 17EE is a mechanism-based inactivator of P450 2B1 but not of P450 2B2. We show here that tBMP is also a mechanism-based inactivator of P450 2B1 and not P450 2B2. Minimal loss in 7-ethoxy-4-(trifluoromethyl)coumarin (7-EFC) activity was observed when P450 2B1 G478A was incubated with either inactivator, suggesting that this residue plays a role in the inactivation. However, P450 2B2 A478G behaved like wild-type P450 2B2, indicating that this residue alone is not sufficient for inactivation. A chimeric construct of P450 2B1/2B2 that is essentially P450 2B1 with five residues of P450 2B2 (including residue 478), was not inactivated by either tBMP or 17EE, suggesting that these five residues are important for inactivation. Sequential mutagenesis of the chimeric construct to quadruple (S407T-N417D-A419T-G478A) and triple (S407T-N417D-A419T) mutants of P450 2B1 did not result in inactivation by either inactivator. However, the triple mutant with mutations only in non-substrate recognition site (SRS) regions still exhibits wild-type P450 2B1 7-EFC O-deethylation activity with a K(m) value of 25 microM and V(max) of 8 nmol/min/nmol P450. These results demonstrate that substitution of three non-SRS residues in P450 2B1 leads to protection against inactivation of 2B enzymes by these two acetylenic compounds.

The mechanism-based inactivation of p450 2B4 by tert-butyl 1-methyl-2-propynyl ether: structural determination of the adducts to the p450 heme

tert-Butyl 1-methyl-2-propynyl ether (tBMP) was analyzed for its ability to act as a mechanism-based inactivator of p450 2B4. tBMP inactivated p450 2B4 in a time-, concentration-, and NADPH-dependent manner. Losses in activity occurred with concurrent losses in the reduced CO spectrum and native p450 heme; however, there was a greater loss in activity than could be accounted for by reduced CO spectra or native heme loss. LC/MS analysis demonstrated that the losses in native heme were accompanied by the appearance of two modified hemes with m/z values of 705Da, consistent with tBMP adducted hemes. Both adducts had identical fragmentation patterns when analyzed by LC/MS/MS. The spectra were consistent with a tBMP molecule and an oxygen atom attached to iron-depleted heme. Proton NMR studies suggest that the two modified hemes in p450 2B1 are N-alkylated on pyrrole rings A and D.

Critical role of oxidative stress in estrogen-induced carcinogenesis

Mechanisms of estrogen-induced tumorigenesis in the target organ are not well understood. It has been suggested that oxidative stress resulting from metabolic activation of carcinogenic estrogens plays a critical role in estrogen-induced carcinogenesis. We tested this hypothesis by using an estrogen-induced hamster renal tumor model, a well established animal model of hormonal carcinogenesis. Hamsters were implanted with 17beta-estradiol (betaE2), 17alpha-estradiol (alphaE2), 17alpha-ethinylestradiol (alphaEE), menadione, a combination of alphaE2 and alphaEE, or a combination of alphaEE and menadione for 7 months. The group treated with betaE2 developed target organ specific kidney tumors. The kidneys of hamsters treated with alphaE2, alphaEE, or menadione alone did not show any gross evidence of tumor. Kidneys of hamsters treated with a combination of alphaE2 and alphaEE showed early signs of proliferation in the interstitial cells. Kidneys of hamsters treated with a combination of menadione and alphaEE showed foci of tumor with congested tubules and atrophic glomeruli. betaE2-treated tumor-bearing kidneys showed >2-fold increase in 8-iso-prostaglandin F(2alpha) (8-iso-PGF(2alpha)) levels compared with untreated controls. Kidneys of hamsters treated with a combination of menadione and alphaEE showed increased 8-iso-PGF(2alpha) levels compared with untreated controls, whereas no increase in 8-iso-PGF(2alpha) was detected in kidneys of alphaEE-treated group. A chemical known to produce oxidative stress or a potent estrogen with poor ability to produce oxidative stress, were nontumorigenic in hamsters, when given as single agents, but induced renal tumors, when given together. Thus, these data provide evidence that oxidant stress plays a crucial role in estrogen-induced carcinogenesis.

Mechanism-based inactivation of cytochrome P450 2B6 by a novel terminal acetylene inhibitor

N-(3,5-Dichloro-4-pyridyl)-3-(cyclopentyloxy)-4-methoxybenzamide (DCMB) is a known marker substrate for cytochrome p450 2B6. Based on the chemical template of DCMB, a novel terminal acetylene compound, N-(3,5-dichloro-4-pyridyl)-4-methoxy-3-(prop-2-ynyloxy)benzamide (TA) was synthesized and evaluated as a mechanism-based inactivator of p450 2B6. The pseudo first-order inactivation of expressed p450 2B6 by TA was both substrate and time-dependent. The kinetics of inhibition resulted in a maximal rate constant (k(inactivation)) of 0.09 min(-1) and an apparent K(I) of 5.1 microM. Incubation of expressed p450 2B6 with TA and NADPH resulted in a 68% loss in enzyme activity and a concurrent 62% loss in the formation of a reduced carbon monoxide complex, suggesting that heme destruction is the primary mode of enzyme inactivation. Enzyme inactivation of p450 2B6 was not reduced by the presence of 10 mM glutathione and was protected by incubation of excess DCMB with TA. The production of the carboxylic acid metabolite, N-(3,5-Dichloro-4-pyridyl)-3-(2-carboxyethoxy)-4-methoxybenzamide (TA-COOH), during the incubation of TA with 2B6 suggests that inactivation proceeds through a ketene intermediate. For 2B6 inactivation, the partition ratio was approximately 1.5 nmol TA-COOH formed/nmol P450 inactivated. Finally, TA was evaluated for mechanism-based inactivation of p450 3A4, 2C9, 2C19, 2D6, and 2E1 using human liver microsomes. In addition to 2B6, p450 2C forms were also found to be sensitive to TA-mediated inactivation, suggesting that subtle changes in the O-alkyl chain of the parent may be critical for the selectivity of enzyme inactivation.

Differential oxidation of mifepristone by cytochromes P450 3A4 and 3A5: selective inactivation of P450 3A4

The principal enzyme involved in the oxidation of mifepristone is cytochrome P450 3A4 (CYP3A4), which undergoes mechanism-based inactivation by the drug. However, no information is available on the interaction with CYP3A5, the second most abundant CYP3A enzyme in adult human liver. Oxidation of mifepristone by recombinant CYP3A4 produced mono- and didemethylated products and one C-hydroxylated metabolite, as reported previously. However, CYP3A5 produced only the demethylated metabolites. The apparent V(max) and K(M) values for formation of the monodemethylated product by CYP3A4 and CYP3A5 were 46 and 30 nmol/min/nmol P450, and 36 and 16 microM, respectively. Unlike CYP3A4, CYP3A5 was not inactivated by mifepristone. The basis of this differential susceptibility was explored using site-directed mutants in which a CYP3A4 residue was converted to its 3A5 counterpart. Surprisingly, none of these replacements caused a significant decrease in CYP3A4 inactivation by mifepristone. Examination of selected CYP3A4 mutants at 20 other positions indicated that the relative formation rate of the C-hydroxylated product could not account for the differential susceptibility of CYP3A4 and 3A5. Together these results indicate that mifepristone fails to orient itself in the CYP3A5 active site in such a way that its propylenic group is accessible for oxidation, thus rendering CYP3A5 unable to produce the C-hydroxylated product or putative ketene that leads to enzyme inactivation. Identification of mifepristone as a selective mechanism-based inactivation of CYP3A4 may be very useful in distinguishing between the two major CYP3A enzymes collectively responsible for the oxidative metabolism of over half of the drugs currently in use.

Effects of unsaturated side-chain analogs of tetrahydrocannabinol on cytochromes P450

The ability of unsaturated side-chain analogs of Delta(8)-tetrahydrocannabinol (THC) to selectively inactivate mouse hepatic cytochromes P450 3A11 and 2C29 was examined. THC side-chain analogs were preincubated with mouse hepatic microsomes and NADPH for various times before dilution and determination of Delta(9)-THC metabolism specific for P450s 3A11 and 2C29. THC-enyl analogs had little or no effect on P450 3A11 but inactivated P450 2C29 in a time-dependent manner, with approximately 50% inactivation observed after a 30-min preincubation. THC-ynyl analogs were less selective in their P450 inactivation but appeared to be more effective than their corresponding enyl analogs. THC-ynyl analogs inactivated P450s 3A11 and 2C29 in a time-dependent manner and could inactive 40-80% of their activities after a 30-min preincubation. The THC-ynyl analogs were nearly as effective as cannabidiol, a well-characterized inactivator of these mouse P450s. Despite their ability to inactivate P450 in vitro, neither the THC-enyl nor the THC-ynyl analogs were very effective after in vivo administration. Unsaturated side-chain THC analogs may be useful in the development of specific P450 inactivators.

Characterization of novel glutathione adducts of a non-nucleoside reverse transcriptase inhibitor, (S)-6-chloro-4-(cyclopropylethynyl)-4-(trifluoromethyl)-3, 4-dihydro-2(1H)-quinazolinone (DPC 961), in rats. Possible formation of an oxirene metabolic intermediate from a disubstituted alkyne

The postulated formation of oxirene-derived metabolites from rats treated with a disubstituted alkyne, (S)-6-chloro-4-(cyclopropylethynyl)-4-(trifluoromethyl)-3, 4-dihydro-2(1H)-quinazolinone (DPC 961), is described. The reactivity of this postulated oxirene intermediate led to the formation of novel glutathione adducts whose structures were confirmed by LC/MS and by two-dimensional NMR experiments. These metabolites were either excreted in rat bile or degraded to mercapturic acid conjugates and eliminated in urine. To demonstrate the oxidation of the triple bond, an analogue of DPC 961 was synthesized, whereby the two carbons of the alkyne moiety were replaced with (13)C stable isotope labels. Rats were orally administered [(13)C]DPC 961 and glutathione adducts isolated from bile. The presence of an oxygen atom on one of the (13)C labels of the alkyne was demonstrated unequivocally by NMR experiments. Administration of (14)C-labeled DPC 961 showed that biliary elimination was the major route of excretion with the 8-OH glucuronide conjugate (M1) accounting for greater than 90% of the eliminated radioactivity. On the basis of radiochemical profiling, the glutathione-derived metabolites were minor in comparison to the glucuronide conjugate. Studies with cDNA-expressed rat enzymes, polyclonal antibodies, and chemical inhibitors pointed to the involvement of P450 3A1 and P450 1A2 in the formation of the postulated oxirene intermediate. The proposed mechanism shown in Scheme 1 begins with P450-catalyzed formation of an oxirene, rearrangement to a reactive cyclobutenyl ketone, and a 1,4-Michael addition with endogenous glutathione to produce two isomeric adducts, GS-1 and GS-2. The glutathione adducts were subsequently catabolized via the mercapturic acid pathway to cysteinylglycine, cysteine, and N-acetylcysteine adducts. The transient existence of the alpha,beta-unsaturated cyclobutenyl ketone was demonstrated by incubating the glutathione adduct in the presence of N-acetylcysteine and monitoring the formation of N-acetylcysteine adducts by LC/MS. Epimerization of GS-1 to GS-2 was also observed when N-acetylcysteine was omitted from the incubation.

New Selective Inhibitors of Cytochrome P450 2B4 and an Activator of Cytochrome P450 3A6 in Rabbit Liver Microsomes

We investigated interactions of adamantane, diamantane and their two substituted derivatives, 2-isopropenyl-2-methyladamantane (2-PMADA,1) and 3-isopropenyl-3-methyldiamantane (3-PMDIA,2), with various isoforms of rabbit cytochrome P450 (CYP). The data of spectroscopic experiments showed that all the substances are bound to the substrate binding site of rabbit CYP2B4 and CYP3A6.1and2are compounds having higher affinities to these CYP isoforms than adamantane and diamantane. All compounds inhibit CYP2B4 specific enzyme activity (the 7-pentoxyresorufinO-depentylase activity). The 50% inhibition of CYP2B4 was due to 3.82, 0.61, 0.66 and 0.37 μM adamantane, diamantane,1and2, respectively. The products formed during the CYP2B4-mediated metabolism of studied substances are less effective inhibitors than parent compounds. An opposite effect of1on CYP3A6 was determined. The specific enzyme activity of CYP3A6 increased to 138% of control when1was used in the presence of 40 μM erythromycin as a substrate. Here, we report the finding of a new activator of CYP3A6 having the structure quite different from that of CYP3A6 activators known to date.

Pharmacokinetic changes of theophylline and amikacin through the menstrual cycle in healthy women

The objective of this open-label, single-dose study was to clarify the influence of the menstrual cycle on the pharmacokinetics of theophylline (n = 10) and amikacin (n = 8) in young healthy Japanese women with regular menstrual cycles. Each subject received an intravenous infusion of theophylline or amikacin sulfate at four different phases--mid-follicular (phase I), peri-ovulatory (phase II), mid-luteal (phase III), and premenstrual days (phase IV). In the theophylline study, there were no significant differences in the pharmacokinetic parameters among the four phases studied. In the amikacin study, CLtot was 15% higher in phase III than in phase I (p < 0.01). Vd beta was 35% higher in phase III than in phase I (p < 0.05). The other pharmacokinetic parameters of amikacin were not significantly altered during the menstrual cycle. Evidence suggests that the phase of the menstrual cycle may be a factor in determining the pharmacokinetics of amikacin.

Selectivity of polycyclic inhibitors for human cytochrome P450s 1A1, 1A2, and 1B1

Human cytochrome P450s 1A1, 1A2, and 1B1 are known to have overlapping substrate specificities. All are regulated in part by the Ah locus; P450 1A2 is expressed essentially only in liver, but P450s 1A1 and 1B1 are both expressed in many extrahepatic tissues. Twenty-five polycyclic hydrocarbons, many containing acetylenic side chains, were examined as inhibitors of the three enzymes using 7-ethoxyresorufin O-deethylation as the enzyme assay in all cases. Several compounds were inhibitory at low nanomolar concentrations. 1-(1-Propynyl)pyrene and 2-(1-propynyl)phenanthrene nearly completely inhibited P450 1A1 at concentrations at which no P450 1B1 inhibition was observed. 2-Ethynylpyrene and alpha-naphthoflavone (7, 8-benzoflavone) nearly completely inhibited P450 1B1 at concentrations at which no P450 1A1 inhibition was noted. All four of the above compounds also inhibited P450 1A2. Several polycyclic hydrocarbons devoid of acetylenic groups were also inhibitory with respect to all three P450s. Some of the acetylenic compounds examined showed enhanced inhibition following preincubation with the P450s in the presence of cofactors NADPH and O2. However, of seven compounds (five acetylenes) tested with P450 1B1, only two [2-ethynylpyrene and 4-(1-propynyl)biphenyl] showed such evidence for mechanism-based inactivation. We conclude that (i) several polycyclic hydrocarbons and their oxidation products are very inhibitory with respect to human P450s 1A1, 1A2, and 1B1; (ii) of these inhibitors only some are mechanism-based inactivators; and (iii) some of the inhibitors are potentially useful for distinguishing between human P450s 1A1 and 1B1.

N omega-propargyl-L-arginine and N omega-hydroxy-N omega-propargyl-L-arginine are inhibitors, but not inactivators, of neuronal and macrophage nitric oxide synthases

N omega-Propargyl-L-arginine (7) was synthesized as a potential mechanism-based inactivator of neuronal nitric oxide synthase (nNOS) and macrophage nitric oxide synthase (iNOS). Compound 7 is a potent reversible competitive inhibitor for both isoforms, having Ki values of 430 +/- 50 nM and 620 +/- 30 nM for nNOS and iNOS, respectively. These values are 12 and 32 times lower than the K(m) for L-arginine with nNOS and iNOS, respectively; however, 7 does not exhibit time-dependent inhibition with either. It also only undergoes oxidation very slowly. N omega-Hydroxy-N omega-propargyl-L-arginine also was synthesized to determine if the initial proposed enzyme-catalyzed hydroxylation of N omega-propargyl-L-arginine was problematic. This compound also is a potent reversible inhibitor of both nNOS and iNOS, but is not a time-dependent inactivator and is oxidized only very slowly. These results are in sharp contrast with the corresponding olefins, N omega-allyl-L-arginine and N omega-allyl-N omega-hydroxy-L-arginine recently reported to be potent time-dependent, irreversible inhibitors of nNOS (Zhang, H. Q.; Dixon, R. P.; Marletta, M. A.; Silverman, R. B., J. Am. Chem. Soc. 1997, 119, in press); N omega-allyl-L-arginine also was reported to be an inactivator of iNOS (Olken, N. M.; Marletta, M. A. J. Med. Chem. 1992, 35, 1137). This suggests that the active site of both isoforms of NOS can accommodate a variety of structures, but binding must have the appropriate juxtaposition for hydroxylation; otherwise, no oxidation occurs.

Drug-mediated inactivation of cytochrome P450

1. Multiple forms of cytochrome P450 (CYP) catalyse the oxidation of chemicals of endogenous and exogenous origin, including drugs, carcinogens, steroids and eicosanoids. However, this unusual low substrate specificity also makes CYP susceptible to inhibition by a wide range of drugs, leading to pharmacokinetic interactions of potential clinical significance. 2. Some drugs are converted by CYP to reactive metabolites that bind covalently to sites within the active centre of the same CYP. Such mechanism-based inhibition leads to CYP inactivation or complexation. These processes give rise to long-term effects on drug pharmacokinetics, as the inactivated or complexed CYP must be replaced by newly synthesized CYP protein. 3. Drugs that inactivate CYP generally possess recognizable functional groups that are oxidized to reactive products. Thus, drugs with side chains containing unsaturated carbon-carbon bonds and furan ring systems are associated with CYP inactivation. Nitrogen-containing systems may also inactivate CYP. 4. Metabolites formed from drugs containing alkylamino and methylenedioxy functionalities can trap CYP as inert complexes without eliciting inactivation. However, the functional effects of inactivation and complexation on drug pharmacokinetics are indistinguishable. Drugs that elicit CYP complexation include the first generation macrolide antibiotics, but newer analogues appear much safer. Some antidepressants, antiepileptics and tuberculostatic agents have been associated with CYP complexation.

Aryl acetylenes as mechanism-based inhibitors of cytochrome P450-dependent monooxygenase enzymes

Aryl acetylenes have been investigated as inhibitors of cytochrome P450 (P450)-dependent alkoxyresorufin dealkylation dealkylation activities in liver microsomes prepared from rats exposed to beta-naphthoflavone, isosafrole, or phenobarbital. Many of the acetylenes investigated produce pseudo-first-order time-dependent and NADPH-dependent losses of the dealkylation activities characteristic of mechanism-based irreversible inactivation (suicide inhibition). Replacing the terminal hydrogen of aryl acetylenes with a methyl group to convert ethynes into propynes enhances the inhibition of P450 1A enzymes; in some instances, this modification converts a reversible inhibitor of P450s into a suicide inhibitor. In contrast, ethynes are more effective suicide inhibitors of P450 2B-dependent dealkylations than the corresponding propynes. Aryl acetylenes with an ethynyl group on the 2 position of naphthalene or on the 9 position of phenanthrene and arylalkyl acetylenes with alkyl chains containing 2, 3, or 4 methylene groups are selective inhibitors of P450 2B1/2B2 in liver microsomes from rats. Aryl acetylenes also act as suicide inhibitors of P450 1A2 in human liver microsomes, of purified P450 1A2 from rabbit or rat liver in reconstituted systems, and of purified recombinant human P450 1A2 and 1A1 in reconstituted systems. 4-(1-Propynyl)biphenyl (4PBi) inactivated P450 1A2-dependent ethoxyresourfin deethylation (EROD) activity in human liver microsomes in an NADPH-dependent process (k(inactivation), 0.23 min-1; KI, 2.3 microM). 4PBi also inactivated purified recombinant human P450 1A2 (k(inactivation), 0.24 min-1; KI, 4.3 microM). In agreement with previous reports [Yun, C.-H., Hammons, G. J., Jones, G., Martin, M. V., Hopkins, N. E., Alworth, W. L., and Guengerich, F. P. (1992) Biochemistry 31, 10556-10563], 2-ethynylnaphthalene (2EN) was not a suicide inhibitor of the P450 1A2 activity in human liver microsomes but did inactivate purified human P450 1A2. Neither 4PBi nor 2EN affected diagnostic activities of human microsomal P450 2E1, 2C9/10, 3A4, or 2C19. In the systems examined, the losses of P450-dependent activity produced by these aryl acetylenes were not accompanied by corresponding decreases in the measured P450 absorption spectra. Thus P450 inactivation by these aryl acetylenes does not involve labeling and destruction of the heme. Incubation of 4PBi with microsomal P450 1A1 or 1A2 from rat liver under conditions that lead to P450-dependent, enzyme inactivations generates a 2-biphenylylpropionic acid product. This suggests that the suicide inhibition of P450s by propynylaryl acetylenes proceeds via a methylaryl ketene formed by a 1,2-methyl rearrangement, analogous to the mechanism of suicide inhibition by ethynyl acetylenes that proceed via ketene intermediates formed by 1,2-hydrogen shifts [Ortiz de Montellano, P. R., and Kunze, K. L. (1981) Arch. Biochem. Biophys. 209, 710-712].

Mechanism-based inhibition of mouse P4502b-10 by selected arylalkynes

Suicide inhibitors of cytochrome P450 families are excellent tools to predict which isoforms mediate the metabolism/activation of a variety of chemical agents. We compared the inhibitory effects of several arylalkynes on mouse cytochromes P450 with published data for the rat model. The inhibition of P4502b specific dealkylation of benzyloxyresorufin by 2-ethynylnaphthalene (2-EN), 5-phenyl-1-pentyne (PPY), 4-phenyl-1-butyne (PBY), and 9-ethynylphenanthrene (9-EPh) was measured in hepatic microsomes from male mice treated with 1,4-bis[2-(3,5-dichloropyridyloxy)]-benzene (TCPOBOP) to induce cytochrome P4502b. Pulmonary microsomes were prepared from untreated mice. 9-EPh, 2-EN, and PPY caused a time-, concentration-, and NADPH-dependent loss in P4502b activity in both tissues. PBY, however, demonstrated this type of inhibition only in liver microsomes. The IC50 was calculated for both liver and lung microsomes and compared with published Ki (concentration required for half-maximal inhibition) or KI (concentration required for half-maximal inactivation) values for the rat. PPY, PBY, and 9-EPh were equally effective inhibitors of mouse P4502b and rat P4502B1. 2-EN was a 5- to 10-fold less potent inhibitor of mouse P4502b, as compared with the rat, even though it was shown to bind to the active site of the mouse isoform as demonstrated by its metabolism to 2-naphthylacetic acid. These data suggest that the active site of the mouse P4502b enzyme is functionally similar to the rat P4502B isoform, with the exception of the disparity in its susceptibility to inactivation by 2-EN as measured by the Ki values.

Enzyme-activated inhibitors of steroidal hydroxylases

Cytochrome P450 monooxygenases (CYP450) of the steroid biosynthetic pathways are highly substrate specific in comparison to the variable specificities of hepatic CYP450 enzymes. Both groups of enzymes catalyze the reductive cleavage of molecular oxygen with transfer of oxygen to the substrate to form hydroxylated derivatives. Those steroids formed in endocrine tissues represent highly specific endocrine/autocrine hormones with enhanced biological potency, while hepatic hydroxylation of steroids reduces their endocrine bioactivities and enhances urinary elimination. Changes of the hormonal milieu of endocrine and peripheral tissues are associated with the development of hyperplastic and/or malignant conditions. Hormone deprivation induces regression of endocrine dependent growth via apoptosis and may also alter growth of hormone insensitive cells by the induction of negative growth factors. Biosynthetic CYP450 enzymes of those steroids that mediate specific disease processes are potential therapeutic targets for selective intervention. This objective can be accomplished by the design of specific pseudo-substrate analogs that will be activated during enzyme-directed catalysis to produce a reactive functional group in the enzyme's active site that will either tightly or irreversibly bind and inactivate the host enzyme. The CYP450 enzymes that hydroxylate the C19 carbon of androgens (aromatase) and the C18 carbon of corticosterone (aldosterone synthase) were selected as target enzymes because they are terminal enzymes of biosynthetic pathways which hydroxylate specific angular methyl groups. Hypersecretion of their respective hormonal products, estrogens and aldosterone, are associated with specific disease conditions. Substrate analogs containing ethynyl, vinyl, or nitrile groups attached to the C19 or C18 methyl groups were enzyme-activated inhibitors. The ethynyl analogs, 19-acetylenic androstenedione (Plomestane) and 18-acetylenic deoxycorticosterone, had nanomolar inhibitory constants (Ki values) and were irreversible inactivators of their target enzymes in animal models.

Cytochrome P450-dependent oxidation of fatty acids

Cytochrome P450-dependent monooxygenases from plants catalyse in-chain and omega hydroxylation as well as epoxidation of medium- and long-chain fatty acids. Recent research efforts have clarified that there are multiple forms of cytochrome P450 involved in these reactions, each of which possesses distinguishable substrate specificity. The biological roles of these distinct P450 forms are poorly understood. However, evidence suggests that some may play an important role in the biosynthesis of plant cuticles. We review current knowledge on the induction and inhibition of activities as well as the regio- and stereo-specificity of the distinct forms so far characterised.

[Inhibition of ecdysone biosynthesis by synthetic molecules]

We studied the putative inhibitory activity of about 50 synthetic molecules on the biosynthesis of ecdysone. Most of these molecules had been synthesized according to the conceptual framework of suicide substrate type inhibitors. They potentially react either with well-known catalytic mechanisms (hydroxylations at C-22 and C-25) or with more hypothetic ones (introduction of the keto group at C-6 and the hydroxylation at C-14). The two hydroxylations which take place on the side chain at C-22 and C-25 in the last steps of the ecdysone biosynthetic pathway, and which are catalysed by cytochrome P-450 dependent monooxygenases, can be effectively affected. The essential chemical arrangement which produced a consequent inhibitory effect included an acteylenic or an allenic inhibitory function, near the hydroxylation to inhibit, with a hydroxyl group, preferably grafted in C-20. In order to increase the inhibitory effect, several characteristics gradually appeared: shortness of the side chain, hydroxyl group in position (R) at C-20 and at C-22, if necessary; in the proximal shortening side chain molecules, hydroxyl group at C-17 in position beta; lowering steric hindrance at C-20. It seemed that a molecule bearing a side chain in a relative position behind the midplan of the steroid nucleus induced a more important inhibitory effect. On the contrary, the form of the steroid nucleus itself (as in cholesterol, 7-dehydrocholesterol, 3-dehydrocholesterol, or in a molecule with a saturated B cycle) did not play a deciding part in the activity of the compound. Only the molecules with a typical ecdysteroid nucleus showed a poor inhibitory effect. Molecules acting as suicide substrate type inhibitors on the ecdysone biosynthesis should produce an irreversible inactivation of the enzyme and show a biosynthetic inhibition specifically linked to ecdysteroid. It was not the case of all the tested molecules. Some of them induced a very important inhibition without possessing the other characteristics of a suicide substrate type compound. Other derived chemicals, which were not synthesized according to the framework of the suicide substrate type molecules, showed all the characteristics of this type of molecules. In the course of this work, it has been possible to point out several molecules showing an important inhibitory effect on ecdysone biosynthesis.

Catalytic properties of the plant cytochrome P450 CYP73 expressed in yeast. Substrate specificity of a cinnamate hydroxylase

The catalytic properties of CYP73, a cinnamate 4-hydroxylase isolated from Helianthus tuberosus tuber [Teutsch, H. G., Hasenfratz, M. P., Lesot, A., Stoltz, C., Garnier, J. M., Jeltsch, J. M., Durst, F. & Werck-Reichhart, D. (1993) Proc. Natl Acad. Sci. USA 90, 4102-4106] and expressed in an optimised yeast system [Urban, P., Werck-Reichart, D., Teutsch, G. H., Durst, F., Regnier, S., Kazmaier, M. & Pompon, D. (1994) Eur. J. Biochem. 222, 843-850] have been investigated. Microsomes from transformed yeast catalysed trans-cinnamate hydroxylation with high efficiency. CYP73 was highly specific for its natural substrate, and did not catalyse oxygenation of p-coumarate, benzoate, ferulate, naringenin or furanocoumarins. No metabolism of terpenoids or fatty acids, known substrates of plant P450s, was observed. CYP73 however demethylated the natural coumarin herniarin into umbelliferone. In addition, it was shown to oxygenate five xenobiotics and mechanism-based inactivators, including the herbicide chlorotoluron. All substrates of CYP73 were small planar aromatic molecules. Comparison of the kinetic parameters of CYP73 for its various substrates showed that, as expected, cinnamate was by far the best substrate of this P450. The physiological and toxicological significance of these observations are discussed.