Investigation of chalcones and benzochalcones as inhibitors of breast cancer resistance protein

Breast cancer resistance protein (BCRP/ABCG2) belongs to the ATP binding cassette family of transport proteins. BCRP has been found to confer multidrug resistance in cancer cells. A strategy to overcome resistance due to BCRP overexpression is the investigation of potent and specific BCRP inhibitors. The aim of the current study was to investigate different multi-substituted chalcones for their BCRP inhibition. We synthesized chalcones and benzochalcones with different substituents (viz. OH, OCH(3), Cl) on ring A and B of the chalcone structure. All synthesized compounds were tested by Hoechst 33342 accumulation assay to determine inhibitory activity in MCF-7 MX and MDCK cells expressing BCRP. The compounds were also screened for their P-glycoprotein (P-gp) and Multidrug resistance-associated protein 1 (MRP1) inhibitory activity in the calcein AM accumulation assay and were found to be selective towards inhibition of BCRP. Substituents at position 2' and 4' on chalcone ring A were found to be essential for activity; additionally there was a great influence of substituents on ring B. Presence of 3,4-dimethoxy substitution on ring B was found to be optimal, while presence of 2- and 4-chloro substitution also showed a positive effect on BCRP inhibition.     

Importance of the B ring and its substitution on the alpha-glucosidase inhibitory activity of baicalein, 5,6,7-trihydroxyflavone

Hydroxychromones and B-ring-substituted 5,6,7-trihydroxyflavones were prepared to evaluate the contribution of the B ring of baicalein (5,6,7-trihydroxyflavone, 1) to its potent alpha-glucosidase inhibitory activity. Hydroxychromones, which lack 6-hydroxyl substitution, did not show any inhibitory activity, while 5,6,7-trihydroxy-2-methylchromone (5) showed high activity. Among the tested B-ring-substituted 5,6,7-trihydroxyflavones, the 4'-hydroxy-, 3',4'-dihydroxy-, and 3',4',5'-trihydroxy-substituted derivatives were found to give more activity than that of 1. The methoxy-substituted derivatives, however, showed less activity than 1. The results suggest that the B ring of 1 was not essential, although advantageous to the activity; hydroxyl substitution on the B ring of 5,6,7-trihydroxyflavones was favorable to the activity, whereas methoxyl substitution was unfavorable; at least 4'-hydroxyl substitution of 5,6,7-trihydroxyflavones was required for enhanced activity, in which the number of hydroxyl groups did not take part.     

Correlation of hydrogen-bonding propensity and anticancer profile of tetrazole-tethered combretastatin analogues

A series of 1,5-disubstituted tetrazole-tethered combretastatin analogues with extended hydrogen-bond donors at the ortho-positions of the aryl A and B rings were developed and evaluated for their antitubulin and antiproliferative activity. We wanted to test whether intramolecular hydrogen-bonding used as a conformational locking element in these analogues would improve their activity. The correlation of crystal structures with the antitubulin and antiproliferative profiles of the modified analogues suggested that hydrogen-bond-mediated conformational control of the A ring is deleterious to the bioactivity. In contrast, although there was no clear evidence that intramolecular hydrogen bonding to the B ring enhanced activity, we found that increased substitution on the B ring had a positive effect on antitubulin and antiproliferative activity. Among the various analogues synthesized, compounds 5d and 5e, having hydrogen-bonding donor groups at the ortho and meta-positions on the 4-methoxy phenyl B ring, are strong inhibitors of tubulin polymerization and antiproliferative agents having IC₅₀ value in micromolar concentrations.     

Norrish-Prins reaction as a key step in the synthesis of 14β-hydroxy-5α (or 5β or Δ(5,6))-pregnane derivatives

Numerous bioactive glycosteroids are characterized by aglycones bearing a 14β-hydroxy pregnane skeleton like boucerin and isoramanone. In general, the syntheses of the latter are achieved by acidic hydrolysis of the corresponding glycosteroids. These aglycones were also obtained by a combined Norrish type I-Prins reaction starting from the corresponding 12-keto-pregnane derivatives. However, for the Norrish-Prins reaction, no reports describe the influence of the A/B ring junction (cis or trans or Δ(5,6) double bond) or the influence of the substitution pattern at position 20. Herein, we describe the use of Norrish type I-Prins reactions to synthesize isoramanone and boucerin derivatives and their A/B cis and trans analogs. The influence of the parameters mentioned above is also presented. These studies showed that the A/B ring junction has little influence on the Norrish type I-Prins reaction but that the substitution pattern at position 20 is important. The presence of a dioxolane group induced not only the formation of the desired 14β-hydroxy pregnane derivatives in the highest yields but also the formation of new spiro derivatives.     

Enzymes of the last steps of chlorophyll biosynthesis: modification of the substrate structure helps to understand the topology of the active centers

Enzymes catalyzing two of the late steps of chlorophyll biosynthesis are NADPH:protochlorophyllide oxidoreductase (POR), responsible for the light-dependent reduction of protochlorophyllide to chlorophyllide, and chlorophyll synthase that catalyses the esterification of chlorophyllide to chlorophyll. Inhibitors of these enzymes are of interest as potential herbicides. Both enzymes presumably form a complex, and the question arose whether chlorophyll synthase can react with chlorophyllide while it is still bound to POR. Here, we describe the chemical modification of protochlorophyllides and chlorophyllides with space-filling substituents at rings A, B, and E of the tetrapyrrole macrocycle and the reactivity of the modified substrates. Both enzymes tolerate the large and flexible phenylamino substituent at ring B, indicating that ring B points toward the enzyme surface while the substrate is bound. On the basis of the standard compound zinc protopheophorbide a (100% activity), the 7(1)-phenylamino derivative shows a comparable activity (83%) with POR that is higher than that of the parent formyl derivative zinc protopheophorbide b (58% activity). In contrast, the 3(1)-phenylamino derivative is less active (12%) than the parent formyl compound zinc protopheophorbide d (49% activity), indicating that the binding pocket leaves less space around ring A than around ring B. Almost no space must be left around ring E because substitution of the 13(2)-carboxymethyl ester (100% activity) by the 13(2)-carboxyethyl ester reduces the activity to 0.2%. Chlorophyll synthase leaves somewhat more space around ring E on the A side of the tetrapyrrole in the binding pocket; substitution of the 13(2)-proton (100% activity) by a methoxy group (53% activity) and an ethoxy group (11% activity) is tolerated to a certain extent, while the carbomethoxy group in this position is not accepted. Opening of ring E to a chlorin e6 dimethylester is tolerated (39% activity), while the large benzylamide residue at this site leads to the loss of activity. We conclude that the tetrapyrroles bind to both enzymes in the same direction: rings C, D, and E are oriented to the interior of the binding cleft, and rings A and B are oriented to the surface of the enzyme; this excludes simultaneous binding to both enzymes.     

Studies on the mechanism of MPTP oxidation by human liver monoamine oxidase B

The neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and its deuterated analogues were oxidized to their corresponding dihydropyridinium species (MPDP+) by preparations of pure human liver MAO B:monoclonal antibody complex to investigate the mechanism of MPTP activation. Lineweaver-Burk plots of initial reaction rates revealed that the Km,app values for the various deuterated MPTP analogues were similar to those of MPTP. In contrast, Vmax,app values were substantially decreased by substitution of deuterium for hydrogen on the tetrahydropyridinium ring, especially at C-6. Deuterium substitution on the N-methyl group alone did not significantly reduce Vmax,app. These studies support the interpretation that oxidation of MPTP at the C-6 position on the tetrahydropyridine ring is a major rate-determining step in its biotransformation by MAO B.     

Importance of the B Ring and Its Substitution on the α-Glucosidase Inhibitory Activity of Baicalein, 5,6,7-Trihydroxyflavone

Hydroxychromones and B-ring-substituted 5,6,7-trihydroxyflavones were prepared to evaluate the contribution of the B ring of baicalein (5,6,7-trihydroxyflavone, 1) to its potent α-glucosidase inhibitory activity. Hydroxychromones, which lack 6-hydroxyl substitution, did not show any inhibitory activity, while 5,6,7-trihydroxy-2-methylchromone (5) showed high activity. Among the tested B-ring-substituted 5,6,7-trihydroxyflavones, the 4′-hydroxy-, 3′,4′-dihydroxy-, and 3′,4′,5′-trihydroxy-substituted derivatives were found to give more activity than that of 1. The methoxy-substituted derivatives, however, showed less activity than 1. The results suggest that the B ring of 1 was not essential, although advantageous to the activity; hydroxyl substitution on the B ring of 5,6,7-trihydroxyflavones was favorable to the activity, whereas methoxyl substitution was unfavorable; at least 4′-hydroxyl substitution of 5,6,7-trihydroxyflavones was required for enhanced activity, in which the number of hydroxyl groups did not take part.     

QSAR study on some p-arylthio cinnamides as antagonists of biochemical ICAM-1/LFA-1 interaction and ICAM-1/JY-8 cell adhesion in relation to anti-inflammatory activity

In an attempt to find out the chemical and structural features of some p-arylthio cinnamides 1 as antagonists of biochemical ICAM-1/LFA-1 interaction as well as ICAM-1/JY-8 cell adhesion in relation to anti-inflammatory activity, QSAR study was performed. Steric effect on the arylthio ring and lipophilic substitutions at 2,3-positions, especially 2,3-disubstitution with Cl or CF(3) or both on cinnamides 1 were conducive to the activity, whereas simultaneous presence of methoxy group at arylthio ring and NCOCH(3) group at heterocyclic ring of cinnamides 1 were detrimental to activity in antagonism of biochemical ICAM-1/LFA-1 interaction. When inhibition of ICAM-1/JY-8 cell adhesion was considered, lipophilic substitution on ring B and simultaneous presence of CF(3) groups at 2 and 3 positions of the ring B were advantageous to antagonism. This QSAR study showed that B ring has played the most important role for both types of activities.     

The primary acid product of DPNH

Analysis of the proton magnetic resonance spectra obtained at 220 MHz confirms the axial conformation of the C-6 hydroxyl in the model primary acid product 1-n-(2,6-dichlorobenzyl)-6-hydroxy-1,4,5,6-tetrahydronicotinamide. In the primary acid product of DPNH however the reaction occurs stereospecifically with the substitution at the C-6 position equatorial and on the B-side of the pyridine ring and the C-4A proton axial. A cyclic structure α,O^2′-6B cyclotetrahydronicotinamide is proposed for the primary acid product of DPNH, formed by epimerization of βDPNH to the α configuration followed by protonation at C-5 and subsequent attack of the ribose C-2′-OH on the C-6 position forming a new five membered ring.     

Oxalone and lactone moieties of podophyllotoxin exhibit properties of both the B and C rings of colchicine in its binding with tubulin

Thermodynamics of podophyllotoxin binding to tubulin and its multiple points of attachment with tubulin has been studied in detail using isothermal titration calorimetry. The calorimetric enthalpy of the association of podophyllotoxin with tubulin is negative and occurs with a negative heat capacity change (DeltaC(p) = -2.47 kJ mol(-)(1) K(-)(1)). The binding is unique with a simultaneous participation of both hydrophobic and hydrogen-bonding forces with unfavorable negative entropic contribution at higher temperature, favored with an enthalpy-entropy compensation. Interestingly, the binding of 2-methoxy-5-(2',3',4'-trimethoxyphenyl)tropone (AC, a colchicine analogue without the B ring) with tubulin is enthalpy-favored. However, the podophyllotoxin-tubulin association depending upon the temperature of the reaction has a favorable entropic and enthalpic component, which resembles both B- and C-ring properties of colchicine. On the basis of the crystal structure of the podophyllotoxin-tubulin complex, distance calculations have indicated a possible interaction between threonine 179 of alpha-tubulin and the hydroxy group on the D ring of podophyllotoxin. To confirm the involvement of the oxalone moiety as well as the lactone ring of podophyllotoxin in tubulin binding, analogues of podophyllotoxin are synthesized with methoxy substitution at the 4' position of ring D along with its isomer and another analogue epimerized at ring E. From these results, involvement of oxalone as well as the lactone ring of the drug in a specific orientation inclusive of ring A is indicated for podophyllotoxin-tubulin binding. Therefore, podophyllotoxin, like colchicine, behaves as a bifunctional ligand having properties of both the B and C rings of colchicine by making more than one point of attachment with the protein tubulin.     

The synthesis of alpha,beta-unsaturated carbonyl derivatives with the ability to inhibit both glutathione S-transferase P1-1 activity and the proliferation of leukemia cells

Ethacrynic acid (EA), an alpha,beta-unsaturated carbonyl compound, is a glutathione S-transferase P1-1 (GSTP1-1) inhibitor. Twenty-one novel EA derivatives have been synthesized. The effects of these compounds on GSTP1-1 activity and on the proliferation of human leukemia HL-60 cells have been determined. Compounds with a halogen substitution at the 3'-position of the aromatic ring have greater inhibitory effects on GSTP1-1 activity than those of compounds with a methyl substitution there. Compounds with substitutions at both the 2'- and 3'-positions of the aromatic ring have more antiproliferative ability than those with one substitution at 3'-position. Esterification of the carboxyl group appears to increase the antiproliferative ability.     

Fluorinated phenylcyclopropylamines. Part 5: Effects of electron-withdrawing or -donating aryl substituents on the inhibition of monoamine oxidases A and B by 2-aryl-2-fluoro-cyclopropylamines

A series of racemic, diastereoisomeric aryl cyclopropylamines substituted with fluorine in the 2-position and electron-donating and electron-withdrawing groups on the aromatic ring have been prepared. These represent analogues of the classic MAO inhibitor tranylcypromine (trans-2-phenylcyclopropylamine, 1). Their activities as inhibitors of recombinant human liver monoamine oxidases A (MAO A) and B (MAO B) were determined. The trans-compounds were low micromolar inhibitors of both MAO A and MAO B with moderate MAO A selectivity while the less active cis-analogues were MAO B selective. In the trans-series, electron-withdrawing para-substituents increased the potency of MAO A inhibition while electron-donating groups such as methyl or methoxy had no influence on this activity. In contrast, aromatic ring substitution in the trans-series had essentially no effect on the inhibition of MAO B. The corresponding cis-compounds were shown to be 10-100 times less active against MAO A, while trans- and cis-compounds were quite similar in terms of inhibition of MAO B. The best MAO A/MAO B selectivity (7:1) in the trans-series was found for trans-2-fluoro-2-(para-trifluoromethylphenyl)cyclopropylamine (7d), while a 1:27 selectivity was found for cis-2-fluoro-2-(para-fluorophenyl)cyclopropylamine (10c). These results are discussed in connection with the pK(a) and logD values, the mechanism of action of tranylcypromines, and the geometry of the active site of the enzymes.     

Indole-2-carboxamides as novel NR2B selective NMDA receptor antagonists

A novel series of indole-2-carboxamide derivatives was prepared and identified as NR2B selective NMDA receptor antagonists. The influence of the number and position of OH groups on the indole skeleton as well as the substitution of the piperidine ring on the biological activity of the compounds was studied.     

Structure-function relationships among wild-type variants of Staphylococcus aureus beta-lactamase: importance of amino acids 128 and 216.

beta-Lactamases inactivate penicillin and cephalosporin antibiotics by hydrolysis of the beta-lactam ring and are an important mechanism of resistance for many bacterial pathogens. Four wild-type variants of Staphylococcus aureus beta-lactamase, designated A, B, C, and D, have been identified. Although distinguishable kinetically, they differ in primary structure by only a few amino acids. Using the reported sequences of the A, C, and D enzymes along with crystallographic data about the structure of the type A enzyme to identify amino acid differences located close to the active site, we hypothesized that these differences might explain the kinetic heterogeneity of the wild-type beta-lactamases. To test this hypothesis, genes encoding the type A, C, and D beta-lactamases were modified by site-directed mutagenesis, yielding mutant enzymes with single amino acid substitutions. The substitution of asparagine for serine at residue 216 of type A beta-lactamase resulted in a kinetic profile indistinguishable from that of type C beta-lactamase, whereas the substitution of serine for asparagine at the same site in the type C enzyme produced a kinetic type A mutant. Similar bidirectional substitutions identified the threonine-to-alanine difference at residue 128 as being responsible for the kinetic differences between the type A and D enzymes. Neither residue 216 nor 128 has previously been shown to be kinetically important among serine-active-site beta-lactamases.     

Novel Dmt-Tic dipeptide analogues as selective delta-opioid receptor antagonists

A series of Dmt-Tic analogues with substitution on the Tic aromatic ring has been synthesized and evaluated for opioid receptor affinity and activation. Incorporation of large hydrophobic groups at position 7 of Tic did not greatly alter the delta opioid receptor binding affinities of the dipeptides whereas substitution at position 6 substantially diminished their affinity. These modified Dmt-Tic peptides showed binding affinities as low as 2.5 nM with up to 500-fold selectivity for the delta versus mu opioid receptor and proved to be delta receptor antagonists.     

Chemical synthesis and cytotoxicity of some azinomycin analogues devoid of the 1-azabicyclo[3.1.0]hexane subunit

A series of compounds related to the left-hand domain of the azinomycins have been made and evaluated for cytotoxic activity against a small panel of human tumour cell lines. The epoxide ring is shown to be essential for biological activity. Cytotoxicity is also shown to be sensitive to changes in the substitution pattern on the aromatic ring and the amide group.     

Benzimidazole-2-carboxamides as novel NR2B selective NMDA receptor antagonists

A novel series of benzimidazole-2-carboxamide derivatives was prepared and identified as NR2B selective NMDA receptor antagonists. The influence of some structural elements, like H-bond donor groups placed on the benzimidazole skeleton and the substitution pattern of the piperidine ring, on the biological activity was studied. Compound 6a showed excellent analgetic activity in the mouse formalin test following po administration.     

Synthesis and comparative evaluation of 4-oxa- and 4-aza-podophyllotoxins as antiproliferative microtubule destabilizing agents

A series of novel 4-oxa-podophyllotoxin derivatives 7 featuring the intact lactone ring D and various substituents in rings B and E has been synthesized and evaluated in a phenotypic sea urchin embryo assay along with the representative 4-aza-analogs 5 for their antimitotic and microtubule destabilizing activity. The most active compounds exhibited myristicin-derived or a 3',5'-dimethoxy substitution pattern in the ring E and a 6-methoxy moiety replacing the methylenedioxy ring A. Compounds 5xb, 5xe, 5yb, 7xa, 7xb, and 7xc showed potent antiproliferative effects in the NCI60 cytotoxicity screen. Notably, growth of the multi-drug resistant NCI/ADR-RES cells was more affected by these agents than the parent OVCAR-8 cell line. Although generally 4-oxa-podophyllotoxins were less potent than the respective 4-aza-derivatives in these assays, stability of the former series towards oxidation may prove to be of interest for the development of anticancer agents with in vivo activity.