High-affinity stereoselective reduction of the enantiomers of ketotifen and of ketonic nortriptyline metabolites by aldo-keto reductases from human liver

Cytochrome P450 and Other Drug-Metabolizing Enzymes As Therapeutic Targets

Cytochrome P450 and other families of drug-metabolizing enzymes are commonly thought of and studied for their ability to metabolize xenobiotics and other foreign entities as they are eliminated from the body. Equally as important, however, is the homeostatic role that many of these enzymes play in maintaining the proper levels of endogenous signaling molecules such as lipids, steroids, and eicosanoids as well as their ability to modulate protein-protein interactions involved in downstream signaling cascades. Throughout the years, many of these endogenous ligands or protein partners of drug-metabolizing enzymes have been associated with a wide range of disease states from cancer to various cardiovascular, neurologic, or inflammatory diseases, prompting an interest in whether modulation of drug-metabolizing enzyme activity could have a subsequent pharmacological impact or lessening of disease severity. Beyond direct regulation of endogenous pathways, drug-metabolizing enzymes have also been proactively targeted for their ability to activate prodrugs with subsequent pharmacological activity or enhance the efficacy of a coadministered drug by inhibiting the metabolism of that drug through a rationally designed drug-drug interaction (i.e., ritonavir and human immunodeficiency virus antiretroviral therapy). The focus of this minireview will be to highlight research aimed at characterizing cytochrome P450 and other drug-metabolizing enzymes as therapeutic targets. Examples of successfully marketed drugs as well as early research efforts will be discussed. Finally, emerging areas of research utilizing typical drug-metabolizing enzymes to impact clinical outcomes will be discussed. SIGNIFICANCE STATEMENT: Although generally thought of for their drug-metabolizing capabilities, enzymes such as the cytochromes P450, glutathione S-transferases, soluble epoxide hydrolases, and others play a significant role in regulating key endogenous pathways, making them potential drug targets. This minireview will cover various efforts over the years to modulate drug-metabolizing enzyme activity toward pharmacological outcomes.

Quantitative Evaluation of the Contribution of Each Aldo-Keto Reductase and Short-Chain Dehydrogenase/Reductase Isoform to Reduction Reactions of Compounds Containing a Ketone Group in the Human Liver

Enzymes of the aldo-keto reductase (AKR) and short-chain dehydrogenase/reductase superfamilies are involved in the reduction of compounds containing a ketone group. In most cases, multiple isoforms appear to be involved in the reduction of a compound, and the enzyme(s) that are responsible for the reaction in the human liver have not been elucidated. The purpose of this study was to quantitatively evaluate the contribution of each isoform to reduction reactions in the human liver. Recombinant cytosolic isoforms were constructed, i.e., AKR1C1, AKR1C2, AKR1C3, AKR1C4, and carbonyl reductase 1 (CBR1), and a microsomal isoform, 11β-hydroxysteroid dehydrogenase type 1 (HSD11B1), and their contributions to the reduction of 10 compounds were examined by extrapolating the relative expression of each reductase protein in human liver preparations to recombinant systems quantified by liquid chromatography-mass spectrometry. The reductase activities for acetohexamide, doxorubicin, haloperidol, loxoprofen, naloxone, oxcarbazepine, and pentoxifylline were predominantly catalyzed by cytosolic isoforms, and the sum of the contributions of individual cytosolic reductases was almost 100%. Interestingly, AKR1C3 showed the highest contribution to acetohexamide and loxoprofen reduction, although previous studies have revealed that CBR1 mainly metabolizes them. The reductase activities of bupropion, ketoprofen, and tolperisone were catalyzed by microsomal isoform(s), and the contributions of HSD11B1 were calculated to be 41%, 32%, and 104%, respectively. To our knowledge, this is the first study to quantitatively evaluate the contribution of each reductase to the reduction of drugs in the human liver. SIGNIFICANCE STATEMENT: To our knowledge, this is the first study to determine the contribution of aldo-keto reductase (AKR)-1C1, AKR1C2, AKR1C3, AKR1C4, carbonyl reductase 1, and 11β-hydroxysteroid dehydrogenase type 1 to drug reductions in the human liver by utilizing the relative expression factor approach. This study found that AKR1C3 contributes to the reduction of compounds at higher-than-expected rates.

Cell-based, animal and H1 receptor binding studies relative to the sedative effects of ketotifen and norketotifen atropisomers

OBJECTIVES

Ketotifen (K) and its active metabolite norketotifen (N) exist as optically active atropisomers. They both have antihistaminic and anti-inflammatory properties but the S-atropisomer of N (SN) causes less sedation than K and RN in rodents. This study investigated whether this could be related to a lower concentration of SN in brain or a lower affinity of SN for rat brain H₁ receptors.

METHODS

Ketotifen and norketotifen atropisomers were quantified using a validated chiral HPLC assay. RBE4 and Caco-2 cell monolayers were used in uptake and permeability studies, respectively. Free and total brain-to-plasma (B/P) ratios were determined after injecting racemic K and N into rat tail veins. Affinity for rat brain H₁ receptors (K_I ) was determined using the [³ H]mepyramine binding assay.

KEY FINDINGS

Uptake and permeation studies indicate no stereoselective transport for K or N. B/P ratios reveal the brain concentration of N is lower than K with no stereoselective transport into brain. Finally, the [³ H]mepyramine binding assay shows SN has the lowest affinity for rat brain H₁ receptors.

CONCLUSION

The lower sedative effect of SN in rodents is probably due to a combination of a lower uptake of N than K into the brain and less affinity of SN for CNS H₁ receptors.

Identification and Characterization of a Selective Human Carbonyl Reductase 1 Substrate

During drug discovery efforts targeting inhibition of cytochrome P450 11B2 (CYP11B2)-mediated production of aldosterone as a therapeutic approach for the treatment of chronic kidney disease and hypertension, (S)-6-(5-fluoro-4-(1-hydroxyethyl)pyridin-3-yl)-3,4-dihydro-1,8-naphthyridine-1(2H)-carboxamide (1) was identified as a potent and selective inhibitor of CYP11B2. Preclinical studies characterized 1 as low clearance in both in vitro test systems and in vivo in preclinical species. Despite low metabolic conversion, an active ketone metabolite (2) was identified from in vitro metabolite-identification studies. Due to the inhibitory activity of 2 against CYP11B2 as well as the potential for it to undergo reductive metabolism back to 1, the formation and elimination of 2 were characterized and are the focus of this manuscript. A series of in vitro investigations determined that 1 was slowly oxidized to 2 by cytochrome P450s 2D6, 3A4, and 3A5, followed by stereoselective reduction back to 1 and not its enantiomer (3). Importantly, reduction of 2 was mediated by an NADPH-dependent cytosolic enzyme. Studies with human cytosolic fractions from multiple tissues, selective inhibitors, and recombinantly expressed enzymes indicated that carbonyl reductase 1 (CBR1) is responsible for this transformation in humans. Carbonyl reduction is emerging as an important pathway for endogenous and xenobiotic metabolism. With a lack of selective substrates and inhibitors to enable characterization of the involvement of CBR1, 2 could be a useful probe to assess CBR1 activity in vitro in both subcellular fractions and in cell-based systems.

Carbonyl reduction of NNK by recombinant human lung enzymes: identification of HSD17β12 as the reductase important in (R)-NNAL formation in human lung

4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is the most abundant and carcinogenic tobacco-specific nitrosamine in tobacco and tobacco smoke. The major metabolic pathway for NNK is carbonyl reduction to form the (R) and (S) enantiomers of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) which, like NNK, is a potent lung carcinogen. The goal of this study was to characterize NNAL enantiomer formation in human lung and identify the enzymes responsible for this activity. While (S)-NNAL was the major enantiomer of NNAL formed in incubations with NNK in lung cytosolic fractions, (R)-NNAL comprised ~60 and ~95% of the total NNAL formed in lung whole cell lysates and microsomes, respectively. In studies examining the role of individual recombinant cytosolic reductase enzymes in lung NNAL enantiomer formation, AKR1C1, AKR1C2, AKR1C3, AKR1C4 and CBR1 all exhibited (S)-NNAL-formation activity. To identify the microsomal enzymes responsible for (R)-NNAL formation, 28 microsomal reductase enzymes were screened for expression by real-time PCR in normal human lung. HSD17β6, HSD17β12, KDSR, NSDHL, RDH10, RDH11 and SDR16C5 were all expressed at levels ≥HSD11β1, the only previously reported microsomal reductase enzyme with NNK-reducing activity, with HSD17β12 the most highly expressed. Of these lung-expressing enzymes, only HSD17β12 exhibited activity against NNK, forming primarily (>95%) (R)-NNAL, a pattern consistent with that observed in lung microsomes. siRNA knock-down of HSD17β12 resulted in significant decreases in (R)-NNAL-formation activity in HEK293 cells. These data suggest that both cytosolic and microsomal enzymes are active against NNK and that HSD17β12 is the major active microsomal reductase that contributes to (R)-NNAL formation in human lung.

Carbonyl-reducing enzymes as targets of a drug-immobilised affinity carrier

Proteins, peptides and nucleic acids are commonly isolated and purified in almost all bioscience laboratories. Methods based on molecular recognition are currently the most powerful tool in separation processes due to their selectivity and recovery. The aim of this study was to prove the versatility and the ability of an affinity carrier containing the immobilised ligand oracin (previously developed by our workgroup) to selectively bind carbonyl-reducing enzymes. These enzymes play an important role in metabolic pathways of various endogenic compounds and xenobiotics. Many important drugs, such as doxorubicin, daunorubicin, haloperidol and the model anticancer drug oracin, are metabolised by carbonyl-reducing enzymes. The functionality of the presented carrier was demonstrated with pure recombinant enzymes (AKR1A1, AKR1B1, AKR1B10, AKR1C1, AKR1C2, AKR1C3, AKR1C4, CBR1 and CBR3) as well as with two model biological samples (cell extract from genetically modified Escherichia coli and pre-purified human liver cytosol). Enzymes that show an affinity toward oracin were efficiently captured, gently eluted using 150 mM ammonium hydroxide and subsequently identified by MS. The method is highly selective and robust and may be applied to the purification and identification of various carbonyl-reducing enzymes from any biological sample.

AKR1C3 as a potential target for the inhibitory effect of dietary flavonoids

AKR1C3 (also known as 17beta-hydroxysteroid dehydrogenase type 5 or 3alpha-hydroxysteroid dehydrogenase type 2) functions as a 3-keto, 17-keto and 20-ketosteroid reductase and as a 3alpha-, 17beta- and 20alpha-hydroxysteroid oxidase. Relatively high mRNA expression of AKR1C3 was found in human prostate and mammary gland where it is implicated in regulating ligand access to the androgen and estrogen receptor, respectively. AKR1C3 is an interesting target for the development of agents for treating hormone-dependent forms of cancer like prostate cancer, breast cancer, and endometrial cancer. However, only a few clinically promising and selective inhibitors have been reported so far. Very potent inhibitors of AKR1C3 are the non-steroidal anti-inflammatory drugs, e.g. indomethacin or flufenamic acid. Also dietary phytoestrogens such as coumestrol, quercetin, and biochanin were reported to inhibit the enzyme in low micromolar concentrations. In this study, some dietary flavonoids and other phenolic compounds were tested for their ability to specifically inhibit AKR1C3. Carbonyl reduction of the anticancer drug oracin, which is a very good substrate for AKR1C3 and which could be well monitored by a sensitive HPLC system with fluorescence detection, was employed to determine the inhibitory potency of the compounds. Our results reveal that AKR1C3 could be potentially un-competitively inhibited by 2'-hydroxyflavanone, whose IC(50) value of 300nM is clinically promising. Moreover, since the inhibition is selective towards AKR1C3, 2'-hydroxyflavanone could be useful for treating or preventing hormone-dependent malignancies like prostate and breast cancer.

Carbonyl Reductases and Pluripotent Hydroxysteroid Dehydrogenases of the Short-chain Dehydrogenase/reductase Superfamily

Carbonyl reduction of aldehydes, ketones, and quinones to their corresponding hydroxy derivatives plays an important role in the phase I metabolism of many endogenous (biogenic aldehydes, steroids, prostaglandins, reactive lipid peroxidation products) and xenobiotic (pharmacologic drugs, carcinogens, toxicants) compounds. Carbonyl-reducing enzymes are grouped into two large protein superfamilies: the aldo-keto reductases (AKR) and the short-chain dehydrogenases/reductases (SDR). Whereas aldehyde reductase and aldose reductase are AKRs, several forms of carbonyl reductase belong to the SDRs. In addition, there exist a variety of pluripotent hydroxysteroid dehydrogenases (HSDs) of both superfamilies that specifically catalyze the oxidoreduction at different positions of the steroid nucleus and also catalyze, rather nonspecifically, the reductive metabolism of a great number of nonsteroidal carbonyl compounds. The present review summarizes recent findings on carbonyl reductases and pluripotent HSDs of the SDR protein superfamily.

CBR1 and CBR3 pharmacogenetics and their influence on doxorubicin disposition in Asian breast cancer patients

The present study aimed to identify polymorphic genes encoding carbonyl reductases (CBR1, CBR3) and investigate their influence on doxorubicin disposition in Asian breast cancer patients (n = 62). Doxorubicin (60 mg/m(2)) was administered every 3 weeks for four to six cycles and the pharmacokinetic parameters were estimated using non-compartmental analysis (WinNonlin). The Mann-Whitney U-test was used to assess genotypic-phenotypic correlations. Five CBR1 (-48G>A, c.219G>C, c.627C>T, c.693G>A, +967G>A) and CBR3 (c.11G>A, c.255C>T, c.279C>T, c.606G>A, c.730G>A) polymorphisms were identified. The CBR1 D2 diplotypes were characterized by the presence of at least one variant allele at the c.627C>T and +967G>A loci. Patients in the CBR1 D1 diplotype group had significantly higher clearance (CL) normalized to body surface area (BSA) (CL/BSA[L/h/m(2)]: median 25.09; range 16.44-55.66) and significantly lower exposure levels; area under curve (AUC(0-infinity)/dose/BSA [h/m(5)]; median 15.08; range 6.18-38.03) of doxorubicin compared with patients belonging to the CBR1 D2 diplotype group (CL/BSA[L/h/m(2)]; median 20.88; range 8.68-31.79, P = 0.014; and AUC(0-infinity)/dose/BSA[h/m(5)]; median 21.35; range 9.82-67.17, P = 0.007 respectively). No significant influence of CBR3 polymorphisms on the pharmacokinetics of doxorubicin were observed in Asian cancer patients. The present exploratory study shows that CBR1 D2 diplotypes correlate with significantly higher exposure levels of doxorubicin, suggesting the possibility of lowered intracellular conversion to doxorubicinol in these patients. Further evaluation of carbonyl reductase polymorphisms in influencing the treatment efficacy of doxorubicin-based chemotherapy in Asian cancer patients are warranted.

Ketotifen controlled release from cellulose acetate propionate and cellulose acetate butyrate membranes

Ketotifen was immobilised in cellulose acetate propionate (CAP) membranes and in cellulose acetate butyrate (CAB) membranes. The characteristics of each system were evaluated under a range of experimental conditions. The topography and uniformity of the membranes was assessed using scanning electron microscopy. The release characteristics associated with Ketotifen were monitored spectrophotometrically. The swelling capacity of the membranes was evaluated and attributed to the combined effects of diffusion and of complex dissociation, during swelling. The materials produced were able to provide controlled release of Ketotifen due to their controlled swelling behaviour and adequate release properties. The results showed that the release of Ketotifen from the CAB membranes is higher but the release from the CAP membranes is more uniform.

The aldo-keto reductase superfamily and its role in drug metabolism and detoxification

The aldo-keto reductase (AKR) superfamily comprises enzymes that catalyze redox transformations involved in biosynthesis, intermediary metabolism, and detoxification. Substrates of AKRs include glucose, steroids, glycosylation end-products, lipid peroxidation products, and environmental pollutants. These proteins adopt a (beta/alpha)(8) barrel structural motif interrupted by a number of extraneous loops and helixes that vary between proteins and bring structural identity to individual families. The human AKR family differs from the rodent families. Due to their broad substrate specificity, AKRs play an important role in the phase II detoxification of a large number of pharmaceuticals, drugs, and xenobiotics.

Aldo-keto reductases and bioactivation/detoxication

Aldo-keto reductases (AKRs) are soluble NAD(P)(H) oxidoreductases that primarily reduce aldehydes and ketones to primary and secondary alcohols, respectively. The ten known human AKR enzymes can turnover a vast range of substrates, including drugs, carcinogens, and reactive aldehydes. They play central roles in the metabolism of these agents, and this can lead to either their bioactivation or detoxication. AKRs are Phase I drug metabolizing enzymes for a variety of carbonyl-containing drugs and are implicated in cancer chemotherapeutic drug resistance. They are involved in tobacco-carcinogenesis because they activate polycyclic aromatic trans-dihydrodiols to yield reactive and redox active o-quinones, but they also catalyze the detoxication of nicotine derived nitrosamino ketones. They also detoxify reactive aldehydes formed from exogenous toxicants, e.g., aflatoxin, endogenous toxicants, and those formed from the breakdown of lipid peroxides. AKRs are stress-regulated genes and play a central role in the cellular response to osmotic, electrophilic, and oxidative stress.

Multiplicity of mammalian reductases for xenobiotic carbonyl compounds

A variety of carbonyl compounds are present in foods, environmental pollutants, and drugs. These xenobiotic carbonyl compounds are metabolized into the corresponding alcohols by many mammalian NAD(P)H-dependent reductases, which belong to the short-chain dehydrogenase/reductase (SDR) and aldo-keto reductase superfamilies. Recent genomic analysis, cDNA isolation and characterization of the recombinant enzymes suggested that, in humans, the six members of each of the two superfamilies, i.e., total of 12 enzymes, are involved in the reductive metabolism of xenobiotic carbonyl compounds. They comprise three types of carbonyl reductase, dehydrogenase/reductase (SDR family) member 4, 11beta-hydroxysteroid dehydrogenase type 1, L-xylulose reductase, two types of aflatoxin B1 aldehyde reductase, 20alpha-hydroxysteroid dehydrogenase, and three types of 3alpha-hydroxysteroid dehydrogenase. Accumulating data on the human enzymes provide new insights into their roles in cellular and molecular reactions including xenobiotic metabolism. On the other hand, mice and rats lack the gene for a protein corresponding to human 3alpha-hydroxysteroid dehydrogenase type 3, but instead possess additional five or six genes encoding proteins that are structurally related to human hydroxysteroid dehydrogenases. Characterization of the additional enzymes suggested their involvement in species-specific biological events and species differences in the metabolism of xenobiotic carbonyl compounds.

Purification and characterization of akr1b10 from human liver: role in carbonyl reduction of xenobiotics

Members of the aldo-keto reductase (AKR) superfamily have a broad substrate specificity in catalyzing the reduction of carbonyl group-containing xenobiotics. In the present investigation, a member of the aldose reductase subfamily, AKR1B10, was purified from human liver cytosol. This is the first time AKR1B10 has been purified in its native form. AKR1B10 showed a molecular mass of 35 kDa upon gel filtration and SDS-polyacrylamide gel electrophoresis. Kinetic parameters for the NADPH-dependent reduction of the antiemetic 5-HT3 receptor antagonist dolasetron, the antitumor drugs daunorubicin and oracin, and the carcinogen 4-methylnitrosamino-1-(3-pyridyl)-1-butanone (NNK) to the corresponding alcohols have been determined by HPLC. Km values ranged between 0.06 mM for dolasetron and 1.1 mM for daunorubicin. Enzymatic efficiencies calculated as kcat/Km were more than 100 mM-1 min-1 for dolasetron and 1.3, 0.43, and 0.47 mM-1 min-1 for daunorubicin, oracin, and NNK, respectively. Thus, AKR1B10 is one of the most significant reductases in the activation of dolasetron. In addition to its reducing activity, AKR1B10 catalyzed the NADP+-dependent oxidation of the secondary alcohol (S)-1-indanol to 1-indanone with high enzymatic efficiency (kcat/Km=112 mM-1 min-1). The gene encoding AKR1B10 was cloned from a human liver cDNA library and the recombinant enzyme was purified. Kinetic studies revealed lower activity of the recombinant compared with the native form. Immunoblot studies indicated large interindividual variations in the expression of AKR1B10 in human liver. Since carbonyl reduction of xenobiotics often leads to their inactivation, AKR1B10 may play a role in the occurrence of chemoresistance of tumors toward carbonyl group-bearing cytostatic drugs.

Carbonyl reduction of naltrexone and dolasetron by oxidoreductases isolated from human liver cytosol

The opioid receptor antagonist naltrexone and the antiemetic 5-HT(3) receptor antagonist dolasetron are ketonic drugs that are efficiently reduced to their corresponding alcohols in-vivo. These experiments aimed at characterizing the role in these reactions of individual oxidoreductases present in human liver cytosol. Aldo-keto reductases (AKRs) and carbonyl reductase (CR, EC 1.1.1.184) purified from human liver cytosol were incubated with varying substrate concentrations and 6beta-naltrexol or reduced dolasetron were analysed by HPLC. AKR1C1, AKR1C2, and AKR1C4 were able to reduce both substrates. On the basis of k(cat)/K(m) values, AKR1C4 was nearly 1000-fold more efficient in reducing naltrexone than was AKR1C1, while AKR1C2 was of intermediate efficiency. Substrate inhibition was observed on incubating AKR1C2 or AKR1C4 with naltrexone. In contrast, dolasetron was also a substrate of CR. AKR1C1 and AKR1C4 were the most efficient enzymes in producing reduced dolasetron. We concluded that the efficient reduction of naltrexone by AKR1C4 probably causes the high 6beta-naltrexol/naltrexone ratio in man. The rapid disappearance from human plasma of dolasetron given intravenously and its virtual absence after oral dosage are explained by its liability to reduction by several enzymes, including CR which shows widespread expression in human tissues.

Measurement of xenobiotic carbonyl reduction in human liver fractions

Carbonyl reducing enzymes are involved in the metabolism of endogenous as well as xenobiotic molecules. Enzymes that catalyze the reversible oxidoreduction of aldehyde and ketone moieties include alcohol dehydrogenases, aldo-keto reductases, quinone reductases, and short-chain dehydrogenases/reductases. These enzymes differ with respect to subcellular location, cofactor dependence, and susceptibility to chemical inhibitors. Thus, it is possible to assess the relative contributions of these enzyme systems in the hepatic metabolism of a particular xenobiotic through simple in vitro experiments with commercially available reagents. The approaches described in this unit assume the availability of analytical procedures for measuring the parent compound and metabolites, such as HPLC with radiochemical, UV, or MS detection. Thus, the purpose of this unit is to outline methods for the study of the enzymatic carbonyl reduction of a drug development candidate or other xenobiotic molecule of interest.

Purification and characterization of rat liver enzyme catalyzing stereoselective reduction of acetylpyridines

Acetylpyridines (1-3) are known as aroma components of foods, perfumes, and smoking suppressants, showing several biological activities and constituting part of the structure of some important biologically active compounds. We purified and characterized an enzyme that catalyzes the stereoselective reduction of acetylpyridines so that we could clarify its function. The enzyme participating in the reductive metabolism of 4-acetylpyridine (1) in the rat liver was purified by successively applying ammonium sulfate fractionation, anion-exchange, gel filtration, and affinity chromatography, and it was definitively identified as 3alpha-HSD. It preferentially reduced acetylpyridines (1-3) and acetophenone (7) to their corresponding (S)-alcohols, with high enantioselectivity. Kinetic analyses of the compounds were performed, and the V(max)/K(m) values decreased in the order of 4-, 2-, and 3-acetylpyridine (1, 3, 2), while acetophenone (7) showed almost the same value as 3-acetylpyridine (2). These results suggested that the reduction of the substrates by 3alpha-HSD is affected by the nitrogen atom in the aromatic ring.

The Metabolic Fate of Amitriptyline, Nortriptyline and Amitriptylinoxide in Man

Amitriptyline (AT), the most widely used tricyclic antidepressant, undergoes oxidative metabolism in the side chain with production of the secondary amine nortriptyline (NT), a primary amine, and the N-oxide amitriptylinoxide (AT-NO); in addition, direct conjugation leads to a quaternary ammonium-linked glucuronide. Hydroxylation of AT or NT at the ethylene bridge of the central seven-membered ring results in four isomeric alcohols and occurs with high stereo- and enantioselectivity, the (-)-(E)-10-hydroxy compounds usually being the major products. The disposition of the alcohols is also partially enantioselective, for instance with regard to glucuronidation and reversible oxidation to ketones. Introduction of a second hydroxy group results in isomeric glycols. Oxidative attack at an aromatic ring is a minor pathway leading to dihydrodiols and phenols. Numerous metabolites originate by combinations of reactions in the ring system and the side chain. AT-NO is by about one-third excreted in unchanged form or as 10-hydroxy derivative; the major part is reduced to AT and metabolized further. The review covers current knowledge on the enzymes participating in the individual pathways. Their quantitative importance is inferred from kinetic studies in volunteers and patients and from experiments in vitro. Clinical consequences of biochemical findings mainly derive from the impact of the polymorphic CYP2D6 mediating (-)-(E)-10-hydroxylation and from its potential inhibition by other psychoactive drugs.

Differential gene expression between squamous cell carcinoma of esophageus and its normal epithelium; altered pattern of mal, akr1c2, and rab11a expression

AIM: To identify the altered gene expression patterns in squamous cell carcinoma of esophagus (ESCC) in relation to adjacent normal esophageal epithelium.

METHODS: Total RNA was extracted using SV total RNA isolation kit from snap frozen tissues of ESCC samples and normal esophageal epithelium far from the tumor. Radio-labeled cDNA were synthesized from equal quantities of total RNAs of tumor and normal tissues using combinations of 24 arbitrary 13-mer primers and three different anchoring oligo-dT primers and separated on sequencing gels. cDNA with considerable different amounts of signals in tumor and normal tissue were reamplified and cloned. Using southern blot, the clones of each band were controlled for false positive results caused by probable heterogeneity of cDNA population with the same size. Clones that confirmed differential expression by slot blot selected for sequencing and northern analysis. Corresponding full-length gene sequences was predicted using human genome project data, related transcripts were translated and used for various protein/motif searches to speculate their probable functions.

RESULTS: The 97 genes showed different levels of cDNA in tumor and normal tissues of esophagus. The expression of mal gene was remarkably down regulated in all 10 surveyed tumor tissues. Akr1c2, a member of the aldo-keto reductase 1C family, which is involved in metabolism of sex hormones and xenobiotics, was up-regulated in 8 out of 10 inspected ESCC samples. Rab11a, RPL7, and RPL28 showed moderate levels of differential expression. Many other cDNAs remained to further studies.

CONCLUSION: The mal gene which is switched-off in all ESCC samples can be considered as a tumor suppressor gene that more studies in its regulation may lead to valuable explanations in ESCC development. Akr1c2 which is up-regulated in ESCC probably plays an important role in tumor development of esophagus and may be proposed as a potential molecular target in ESCC treatments. Differential display technique in spite of many disadvantages is still a valuable technique in gene function exploration studies to find new candidates for improved ones like gene chips.

Human Carbonyl Reduction Pathways and a Strategy for Their Study In Vitro

Carbonyl reduction plays a significant role in physiological processes throughout the body. Although much is known about endogenous carbonyl metabolism, much less is known about the roles of carbonyl-reducing enzymes in xenobiotic metabolism. Multiple pathways exist in humans for metabolizing carbonyl moieties of xenobiotics to their corresponding alcohols, readying these molecules for subsequent conjugation and/or excretion. When exploring carbonyl reduction clearance pathways for a drug development candidate, it is possible to assess the relative contributions of these enzymes due to their differences in subcellular locations, cofactor dependence, and inhibitor profiles. In addition, the contributions of these enzymes may be explored by varying incubation conditions, such as pH. Presently, individual isoforms of carbonyl-reducing enzymes are not widely available, either in recombinant or purified form. However, it is possible to study carbonyl reduction clearance pathways from simple experiments with commercially available reagents. This article provides an overview of carbonyl-reducing enzymes, including some kinetic data for substrates and inhibitors. In addition, an experimental strategy for the study of these enzymes in vitro is presented.

Enzymology of a carbonyl reduction clearance pathway for the HIV integrase inhibitor, S-1360: role of human liver cytosolic aldo-keto reductases

S-1360, a 1,3-diketone derivative, was the first HIV integrase inhibitor to enter human trials. Clinical data suggested involvement of non-cytochrome P450 clearance pathways, including reduction and glucuronidation. Reduction of S-1360 generates a key metabolite in humans, designated HP1, and constitutes a major clearance pathway. For characterization of subcellular location and cofactor dependence of HP1 formation, [(14)C]-S-1360 was incubated with commercially available pooled human liver fractions, including microsomes, cytosol, and mitochondria, followed by HPLC analysis with radiochemical detection. Incubations were performed in the presence and absence of the cofactors NADH or NADPH. Results showed that the enzyme system responsible for generation of HP1 in vitro is cytosolic and NADPH-dependent, implicating aldo-keto reductases (AKRs) and/or short-chain dehydrogenases/reductases (SDRs). A validated LC/MS/MS method was developed for investigating the reduction of S-1360 in detail. The reduction reaction exhibited sigmoidal kinetics with a K(m,app) of 2 microM and a Hill coefficient of 2. The ratio of V(max)/K(m) was approximately 1 ml/(min mg cytosolic protein). The S-1360 kinetic data were consistent with positive cooperativity and a single enzyme system. The relative contributions of AKRs and SDRs were examined through the use of chemical inhibitors. For these experiments, non-radiolabeled S-1360 was incubated with pooled human liver cytosol and NADPH in the presence of inhibitors, followed by quantitation of HP1 by LC/MS/MS. Quercetin and menadione produced approximately 30% inhibition at a concentration of 100 microM. Enzymes sensitive to these inhibitors include the carbonyl reductases (CRs), a subset of the SDR enzyme family predominantly located in the cytosol. Flufenamic acid and phenolphthalein were the most potent inhibitors, with > 67% inhibition at a concentration of 20 microM, implicating the AKR enzyme family. The cofactor dependence, subcellular location, and chemical inhibitor results implicated the aldo-keto reductase family of enzymes as the most likely pathway for generation of the major metabolite HP1 from S-1360.

Carbonyl reduction of the potential cytostatic drugs benfluron and 3,9-dimethoxybenfluron in human in vitro

Benfluron (B, [5-(2-N-oxo-2-N',N"-dimethylaminoethoxy)-7-oxo-7H-benzo[c]fluorene]) is a potential benzo[c]fluorene antineoplastic agent with high activity against a broad spectrum of experimental tumors in vitro and in vivo. The structure of B has been modified to repress its rapid deactivation through carbonyl reduction on C7. 3,9-Dimethoxybenfluron (D, [3,9-dimethoxy-5-(2-N-oxo-2-N',N"-dimethylaminoethoxy)-7-oxo-7H-benzo[c]fluorene]) is one of the B derivatives developed. The present paper was designed to compare the C7 carbonyl reduction of B and D in microsomes, cytosol and hepatocytes from human liver. Two purified human enzymes, microsomal 11beta-hydroxysteroid dehydrogenase 1 (11beta-HSD 1) and cytosolic carbonyl reductase, were tested if they are responsible for B and D carbonyl reduction in the respective fractions. Indeed, carbonyl reduction of D in comparison to that of B was 4 and 6-10 times less extensive in human liver microsomes and cytosol, respectively. Moreover, about 10-20 times higher amounts of dihydro B than dihydro D were detected in primary culture of human hepatocytes. 11beta-HSD 1 was shown to be able to reduce B and D. For this enzyme, about 10 times higher rates of carbonyl reduction were observed for B than for D. Likewise, CR participates in B and D carbonyl reduction, although smaller amounts of both reduced metabolites were detected. In summary, carbonyl reduction of D was significantly less extensive than that of B in all in vitro experiments. This lower rate of D inactivation was especially pronounced in hepatocytes which represent a close to in vivo situation. Our results clearly demonstrate that dimethoxy substitution protects the carbonyl group of the benzo[c]fluorene moiety against the deactivation by microsomal and cytosolic reductases. Detailed knowledge on the participating enzymes may serve as a basis for the co-application of specific inhibitors in chemotherapy to further improve the pharmacokinetics of benzo[c]fluorene derivatives.

Molecular Characterization of Two Monkey Dihydrodiol Dehydrogenases

Japanese monkey liver contains multiple forms of dihydrodiol dehydrogenase with 3(20)alpha-hydroxysteroid dehydrogenase activity. Here we have purified the major and minor forms (DD1 and DD4) of the enzyme from Cynomolgus monkey liver, and isolated cDNA species for the two enzyme forms by reverse transcription-PCR. The cDNAs encoded proteins comprising of 323 amino acids, in which the sequence identity between DD1 and DD4 was 83%. The sequences deduced from the cDNAs for DD1 and DD4 perfectly matched the partial sequences of peptides derived from the respective enzymes. We also isolated the cDNAs for DD1 and DD4 of Japanese monkey liver, which had almost identical amino acid sequences with those of the respective enzymes of Cynomolgus monkey liver. The monkey DD1s and DD4s showed the highest sequence identity (94%) with AKR1C1 and AKR1C4, respectively, of four isoenzymes of human 3(20)alpha-hydroxysteroid dehydrogenase, which belongs to the aldo-keto reductase family. The substrate specificity and inhibitor sensitivity of the purified recombinant Cynomolgu monkey DD1 and Japanese monkey DD4 were also essentially identical to those of the recombinant AKR1C1 and AKR1C4, respectively, indicating that DD1 and DD4 are homologues of human AKR1C1 and AKR1C4, respectively. The mRNA for DD1 was detected only in liver, kidney, intestine and adrenal gland among Japanese monkey tissues, and that for DD4 was expressed in liver and kidney. These tissue distribution patterns differ from those of human AKR1C1 and AKR1C4, which are expressed ubiquitously and liver-specific, respectively. In addition, no mRNA for an enzyme corresponding to another isoenzyme (AKR1C2) of the human enzyme was detected in livers of the two monkey strains. The results suggest a difference in the metabolism of steroids and xenobiotics mediated by 3(20)alpha-hydroxysteroid dehydrogenase isoenzymes between monkeys and humans.

Purification and characterization of oxidoreductases-catalyzing carbonyl reduction of the tobacco-specific nitrosamine 4-methylnitrosamino-1-(3-pyridyl)-1-butanone (NNK) in human liver cytosol

1. Four enzymes were purified to homogeneity from human liver cytosol and were demonstrated to be responsible for carbonyl reduction of the tobacco-specific nitrosamine 4-methylnitrosamino-1-(3-pyridyl)-1-butanone (NNK). 2. Carbonyl reductase (EC 1.1.1.184), a member of the short-chain dehydrogenase/reductase (SDR) superfamily, was compared with three isoenzymes of the aldo-keto reductase (AKR) superfamily in terms of enzyme kinetics, co-substrate dependence and inhibition pattern. 3. AKR1C1, 1C2 and 1C4, previously designated as dihydrodiol dehydrogenases (DD1, DD2 and DD4), showed lower K(m) (0.2, 0.3 and 0.8 mM respectively) than did carbonyl reductase (7 mM), whereas carbonyl reductase exhibited the highest enzyme efficiency (Vmax/K(m)) for NNK. Multiplication of enzyme efficiencies with the relative quantities of individual enzymes in cytosol resulted in a rough estimate of their contributions to total alcohol metabolite formation. These were approximately 60% for carbonyl reductase, 20% each for AKR1C1 and 1C2, and 1% for AKR1C4. 4. Except for AKR1C4, the enzymes had a strong preference for NADPH over NADH, and the highest activities were measured with an NADPH-regenerating system. Carbonyl reductase activity was extensively inhibited by menadione, rutin and quercitrin, whereas medroxyprogesterone acetate, phenolphthalein and flufenamic acid were potent inhibitors of AKR1C1, 1C2 and 1C4. 5. In conclusion, cytosolic members of the SDR and AKR superfamilies contribute to reductive NNK detoxification in human liver, the enzymes responsible being carbonyl reductase and aldoketo reductases of the AKRIC subfamily.