Bioactivation of tetrachloroethylene. Role of glutathione S-transferase-catalyzed conjugation versus cytochrome P-450-dependent phospholipid alkylation

The metabolism of [14C]tetrachloroethylene (Tetra) and its metabolite S-(1,2,2-trichlorovinyl)-L-cysteine (TCVC) was investigated with in vitro systems to substantiate metabolic pathways of Tetra deduced from in vivo experiments. In the presence of NADPH, rat hepatic microsomal fractions metabolized Tetra to soluble metabolites, which were identified as trichloroacetic acid and oxalic acid by gas chromatography/mass spectroscopy and a metabolite largely bound to microsomal macromolecules. The majority of the alkylated macromolecules were identified as N-trichloroacetylated phospholipids by high performance liquid chromatography and GC/MS. When Tetra was incubated with hepatic microsomes and cytosol in the presence of 10 mM glutathione, but in the absence of NADPH, the formation of a polar metabolite other than trichloroacetic acid and oxalic acid was observed. This metabolite was identified, after hydrolysis to the corresponding cysteine conjugate, as S-(1,2,2-trichlorovinyl)-glutathione (TCVG). Microsomal GSH S-transferases catalyzed TCVG formation more efficiently than cytosolic GSH S-transferases; the competitive substrate 1-chloro-2,4-dinitrobenzene inhibited TCVG formation. In the presence of both NADPH and GSH, TCVG formation in microsomes was decreased, indicating that oxidative metabolism and GSH conjugation of Tetra are competitive reactions. The Tetra metabolite TCVC was cleaved by bacterial cysteine conjugate b-lyase to dichloroacetic acid and pyruvate. The obtained results substantiate the postulated pathways of Tetra biotransformation and demonstrate that both oxidative and conjugative reactions occur in hepatic Tetra metabolism. Phospholipid alkylation, which occurs during oxidative metabolism, may be a deactivation reaction, whereas TCVG formation, renal metabolism to TCVC, and cleavage of TCVC by b-lyase under formation of mutagenic intermediates may contribute to the nephrocarcinogenic effect of Tetra.     

Solubility studies of silver sulfonamides.

The solubilities of silver sulfapyridine, silver sulfamethazine, and silver sulfamethizole as a function of pH were determined in nitric acid-potassium nitrate, acetate, and sulfonic acid buffers. All silver sulfonamides showed an increase in solubility with increasing hydrogen-ion concentration, a behavior which closely paralleled the protonation of the p-amino function of the sulfonamide. A silver-ion selective electrode was used to measure silver-ion concentration in solution and the methods of known subtraction and known addition were used to measure total silver. Both silver sulfamethizole and silver sulfamethazine were ionized completely in solution. Silver sulfapyridine was ionized completely only in the more acidic pH 2-3 range. A comparison of the physical properties of the silver salts for which mortality studies were available revealed a unique set of properties for silver sulfadiazine.     

CB1 cannabinoid antagonists: structure-activity relationships and potential therapeutic applications

During the last decade there has been a growing interest towards the modulation of the cannabinoid CB1 receptor. The identification of CB1 cannabinoid receptor antagonists has been one of the major advances in cannabinoid research. Thus, the development of these ligands has opened new therapeutic applications. Since the discovery of the first cannabinoid receptor antagonist, rimonabant, by Sanofi in 1994, a large number of structural variations within this chemical series of 1,5-diarylpyrazoles have been described. So far, all attempts to identify novel structures for CB1 antagonists have been based on one or more pharmacophoric elements of the rimonabant structure. The advanced clinical trials of rimonabant confirm the therapeutic potential value of CB1 antagonists for the treatment of obesity. In addition, the results of pharmacological and clinical studies reveal other effective pharmacotherapeutic applications. The current review will mainly focus on the structure-activity relationships that have been established for antagonists/inverse agonists that bind to the CB1 cannabinoid receptors and on their therapeutic applications.     

Inhibition of glutathione reductase by flavonoids. A structure-activity study.

A structure-activity study of fourteen chemically related flavonoids was conducted to evaluate their abilities to inhibit glutathione reductase (GR). By comparing the I50 values of flavonoids from different classes possessing an identical hydroxyl configuration, we determined the following order of potency for inhibition of GR: anthocyanidin > dihydroflavonol = chalcone > flavonol > catechin. Enzyme inhibition by delphinidin chloride and myricetin was partially prevented in a N2 atmosphere which implicates a role for oxygen in the mechanism of inhibition. To determine the role of oxygen species in enzyme inhibition, GR was preincubated with either mannitol, diethylenetriaminepenta-acetic acid (DETAPAC), superoxide dismutase (SOD), catalase (CAT), or SOD and CAT prior to assays for enzyme inhibition by flavonoids. Enzyme inhibition by delphinidin chloride and myricetin was suppressed by the addition of SOD, suggesting that superoxide (O2-.) is involved. However, inhibition by quercetin and morin was not sensitive to antioxidants. To further investigate the role of O2-. in GR inhibition, a superoxide generating system was utilized in the presence and absence of flavonoid. The O2-. generating system failed to inhibit GR in the absence of flavonoid but enhanced the inhibition by myricetin, indicating that the O2-. did not directly inhibit GR but reacted directly with certain flavonoids to form a reactive intermediate which, in turn, inhibited GR. These findings suggest that the mechanism of inhibition of GR by flavonoids is complex and may have oxygen-dependent and oxygen-independent components.     

Phenylbenzopyrones structure-activity studies identify betuletol derivatives as potential antitumoral agents

We have analyzed the cytotoxicity of 22 compounds with a phenylbenzo-gamma-pirone core structure, most of them obtained from natural sources, in five human tumor cell lines (HL-60, A431, SK-OV-3, HeLa and HOS). Betuletol 3-methyl ether and its diacetate were the most cytotoxic compounds. The HL-60 cell line was especially sensitive to these compounds, with IC50 values of approximately 1 microM. Treatment of HL-60 cells with betuletol 3-methyl ether was associated with apoptosis induction which was prevented by a non-specific caspase inhibitor (z-VAD-fmk) and also by a specific inhibitor of caspase-8 (z-IETD-fmk) indicating activation of the extrinsic apoptotic pathway. The results suggest that betuletol 3-methyl ether has potential as new anticancer agent.     

The role of anionic, imidic, and amidic forms in structure-activity relationships. Correlation of electronic indices and bacteriostatic activity in sulfonamides.

The problem of structure-activity relationships in sulfonamide type compounds is tackled on the ground that both bacteriostatic activities and structural indices must be referred to the specific individual forms assumed by sulfa drugs in the active solutions. The frequency value of the symmetric stretching mode of the sulfonyl group upsilons (SO2) is chosen as a suitable electronic index and measured for the individual active forms in aqueous and Me2SO solutions. The linear correlation that exists between bacteriostatic parameter and vibration frequency (over the complete range of data at present available) proves a strict relationship between electronic structure and bacteriostatic activity in this class of drugs. Furthermore, it justifies the assumption used for the calculation of the bacteriostatic activity of the anionic form; i.e., in equilibrium with a very active species (the anion) a less active species (the neutral form) gives a negligible contribution or does not contribute at all to the total activity. The results can be summarized as follows: the lower the frequency of the symmetric stretching mode of the SO2 group of any active species of sulfonamide type compounds, the higher its bacteriostatic activity. The existence of a clear structure-activity correlation demonstrates that the whole class of compounds, whatever their form, has a single mechanism of action, while incontrovertible deviations from the general trend indicate differences or complications in the mechanism itself, but does not demonstrate that the group on which the structural index is localized plays a dominant role in the biological process. The usefulness of pKa and NH2 proton chemical shift of precursor amine as indirect indices of the electronic structure of the anionic forms is explored on extensive sets of available data.     

Propofol in anesthesia. Mechanism of action, structure-activity relationships, and drug delivery

Propofol (2,6-diisopropylphenol) is becoming the intravenous anesthetic of choice for ambulatory surgery in outpatients. It is extensively metabolized, with most of the administered dose appearing in the urine as glucuronide conjugates. Favorable operating conditions and rapid recovery are claimed as the main advantages in using propofol, whereas disadvantages include a relatively high incidence of apnea, and blood pressure reductions. Besides a literature summary of the pharmacodynamics, pharmacokinetics, toxicology, and clinical use, this review provides a deeper discussion on the current understanding of mechanism of action and structure-activity relationships, and recent findings on drug delivery technologies as applied to the improvement of propofol formulations. The action of propofol involves a positive modulation of the inhibitory function of the neurotransmitter gama-aminobutyric acid (GABA) through GABAA receptors. Recent results from recombinant human GABAA receptor experiments and findings from the work exploring the effects at other receptors (e.g., glycine, nicotinic, and M1 muscarinic receptors) are reviewed. Studies showing its antiepileptic and anxiolytic properties are also discussed. The structure-activity relationships (SAR) of series of alkylphenols and p-X-substituted congeners have been reinvestigated. Interestingly, unlike the other congeners tested sofar, p-iodo-2,6-diisopropylphenol displayed anticonvulsant and anticonflict effects, but not sedative-hypnotic and anesthetic properties. Due to its high lipid-solubility, propofol was initially formulated as a solution with the surfactant Cremophor EL, but the occurrence of pain on injection and anaphylactoid reactions prompted to search for alternative formulations. Results from using cyclodextrins, water-soluble prodrugs, and adopting Bodor's approach to the site-specific chemical delivery system (CDS), as well as the advantages provided by computer-controlled infusion systems, are examined in some detail.     

Design and synthesis of novel sulfonamide-containing bradykinin hB2 receptor antagonists. 2. Synthesis and structure-activity relationships of alpha,alpha-cycloalkylglycine sulfonamides

Recently we reported on the design and synthesis of a novel class of selective nonpeptide bradykinin (BK) B2 receptor antagonists (J. Med. Chem. 2006, 3602-3613). This work led to the discovery of MEN 15442, an antagonist with subnanomolar affinity for the human B2 receptor (hB2R), which also displayed significant and prolonged activity in vivo (for up to 210 min) against BK-induced bronchoconstriction in the guinea-pig at a dose of 300 nmol/kg (it), while demonstrating only a slight effect on BK-induced hypotension. Here we describe the further optimization of this series of compounds aimed at maximizing the effect on bronchoconstriction and minimizing the effect on hypotension, with a view to developing topically delivered drugs for airway diseases. The work led to the discovery of MEN 16132, a compound which, after intratracheal or aerosol administration, inhibited, in a dose-dependent manner, BK-induced bronchoconstricton in the airways, while showing minimal systemic activity. This compound was selected as a preclinical candidate for the topical treatment of airway diseases involving kinin B2 receptor stimulation.     

Three-dimensional quantitative structure-activity relationship analysis of cytochromes p450: effect of incorporating higher-affinity ligands and potential new applications.

Recently, two new classes of reversible inhibitors, the benzbromarones (BZBRs) and the N-3 substituted phenobarbitals (PBs), were used to study the active site characteristics of CYP2C9 and 2C19, respectively. Since these ligands are some of the first CYP2C ligands to extend into the low nanomolar K(i) range (K(i) < 100 nM), they were subjected to three-dimensional quantitative structure-activity relationship (3D-QSAR) analysis. Given that BZBRs or the PB ligands bind very tightly, it can be assumed that these structures complement the binding pocket(s) for these enzymes. Thus, the resulting models should output a 3D arrangement of interaction sites predicted to be important for near optimal binding to the CYP2C9 and CYP2C19 enzymes. These predicted interaction regions may then improve the ability to predict drug-drug interactions. The resulting models generated from these new high affinity ligands are discussed, as are novel uses of 3D-QSAR and molecular modeling techniques that may be useful in the study of cytochromes P450 specifically.     

Quantitative structure-activity studies on monoamine oxidase inhibitors.

Quantitative structure-activity studies were carried out on a series of N-isopropylaryl hydrazides which inhibits monoamine oxidase (MAO). The inhibitory potencies of these compounds of MAO were found to correlate with the electron-withdrawing capacity of the aryl ring substituents as estimated by both empirical Hammet sigma constants and electronic indices from molecular orbital calculations. Based on these correlations and previously published data on other classes of MAO inhibitors, a general model for the inhibitor pharmacophore is proposed: potent MAO of an aromatic ring; electron-withdrawing groups on the aromatic ring or replacing the phenyl ring with certain types of heterocyclic rings will tend to increase the potency.     

Quantitative structure-activity relationship studies on cholecystokinin antagonists.

A review is presented on quantitative structure-activity relationship (QSAR) studies on cholecystokinin antagonists. Cholecystokinin (CCK) is a gastrointestinal peptide hormone closely related chemically to gastrin. However, its receptors are found in both peripheral and central nervous systems. Those present in peripheral system have been termed as CCK-A receptors and those present in central nervous system as CCK-B receptors. QSAR studies verify that CCK-B receptors are closely related structurally to gastrin receptors. QSAR studies have been reported on different classes of CCK antagonists, e.g., benzodiazepine derivatives, amino acid derivatives, quinazolinones, and peptides and pseudopeptide analogs. These QSAR studies unravel the mechanisms of interactions of each category of antagonists with the CCK receptors. In the case of benzodiazepines, the hydrophobic interactions and hydrogen bondings are found to be the most important binding force, while in the case of quinazolinones, only the hydrogen bonding is found to be important. The hydrophobic as well as the dispersion interactions are shown to be important for the binding of glutamic acid analogs and steric factors appear to govern the activity of peptides and pseudopeptide analogs.     

Stimulation of topoisomerase II-mediated DNA cleavage by ellipticine derivatives: structure-activity relationship.

Ellipticines are aromatic compounds that intercalate between DNA base pairs and display significant antitumor activity. The cytotoxicity of these compounds is mediated by DNA topoisomerase II, and the presence of a hydroxy group at position 9 of the pyridocarbazole ring system of ellipticines has been found to be essential for high levels of cytotoxicity. The ability of 13 ellipticine derivatives to stabilize the topoisomerase II-DNA covalent complex in vitro was studied, and the data obtained with five pairs of hydroxylated and nonhydroxylated analogues indicate that the hydroxy group at position 9 plays a crucial role in the stabilization of the complex. The influence, upon the complex stabilization, of various substituents at positions 1, 2, 5, and 6 of the pyridocarbazole ring system was investigated. The interaction with DNA of four ellipticine derivatives was studied in the topoisomerase II standard medium. Results suggest that the degree of unwinding might be a determinant of topoisomerase II-DNA-drug complex stability. In addition, the 5-ethyl derivative was observed to induce covalent complex stabilization by a cooperative mechanism.     

Antiviral and antitumor structure-activity relationship studies on tetracyclic eudistomines.

The in vitro antiviral and antitumor activities of (-)-debromoeudistomin K (1a) and 10 structural analogues (1b-1j and 11) were evaluated. The synthesis was accomplished with an intramolecular Pictet-Spengler condensation reaction as the key step. This examination revealed some structural and stereochemical features that are important for both the antiviral and antitumor activities. The most striking points for activity are the necessity to have the correct natural stereochemistry at both C(1) and C(13b) and the presence of the C(1)-NH2 substituent. As was revealed before with naturally isolated eudistomins a substituent in the indole ring greatly influences the biological activity. The 5-OMe derivative 1h shows high potency in both antiviral and antitumor models.     

Design, synthesis, and structure-activity relationships of haloenol lactones: site-directed and isozyme-selective glutathione S-transferase inhibitors

Overexpression of glutathione S-transferase (GST), particularly the GST-pi isozyme, has been proposed to be one of the biochemical mechanisms responsible for drug resistance in cancer chemotherapy, and inhibition of overexpressed GST has been suggested as an approach to combat GST-induced drug resistance. 3-Cinnamyl-5(E)-bromomethylidenetetrahydro-2-furanone (1a), a lead compound of site-directed GST-pi inactivator, has been shown to potentiate the cytotoxic effect of cisplatin on tumor cells. As an initial step to develop more potent and more selective haloenol lactone inactivators of GST-pi, we examined the relationship between the chemical structures of haloenol lactone derivatives and their GST inhibitory activity. A total of 16 haloenol lactone derivatives were synthesized to probe the effects of (1) halogen electronegativity, (2) electron density of aromatic rings, (3) molecular size and rigidity, (4) lipophilicity, and (5) aromaticity on the potency of GST-pi inactivation. The inhibitory potency of each compound was determined by time-dependent inhibition tests, and recombinant human GST-pi was used to determine their inhibitory activity. Our structure-activity relationship studies demonstrated that (1) reactivity of the halide leaving group plays a weak role in GST inactivation by the haloenol lactones, (2) aromatic electron density may have some influence on the potency of GST inactivation, (3) high rigidity likely disfavors enzyme inhibition, (4) lipophilicity is inversely proportional to enzyme inactivation, and (5) an unsaturated system may be important for enzyme inhibition. This work facilitated understanding of the interaction of GST-pi with haloenol lactone derivatives as site-directed and isozyme-selective inactivators, possibly potentiating cancer chemotherapy.     

Antitumor potential of crown ethers: structure-activity relationships, cell cycle disturbances, and cell death studies of a series of ionophores

The present paper demonstrates the antiproliferative ability and structure-activity relationships (SAR) of 14 crown and aza-crown ether analogues on five tumor-cell types. The most active compounds were di-tert-butyldicyclohexano-18-crown-6 (3), which exhibited cytotoxicity in the submicromolar range, and di-tert-butyldibenzo-18-crown-6 (5) (IC50 values of approximately 2 microM). Also, 3 and 5 induced marked influence on the cell cycle phase distribution--strong G1 arrest, followed by the induction of apoptosis. A computational SAR modeling effort offers insight into possible mechanisms of crown ether biological activity, presumably involving penetration into cell membranes, and points out structural features of molecules important for this activity. The results reveal that crown ethers possess marked tumor-cell growth inhibitory activity, the extent of which depends on the characteristics of the hydrophilic macrocylic cavity and the surrounding hydrophobic ring. Our work supports the hypothesis that crown ether compounds inhibit tumor-cell growth by disrupting potassium ion homeostasis, which in turn leads to cell cycle perturbations and apoptosis.