Synthesis of vinyl sulfide analogs of 2,3-oxidosqualene and their inhibition of 2,3-oxidosqualene lanosterol-cyclases

Virtual screening and drug repositioning as strategies for the discovery of new antifungal inhibitors of oxidosqualene cyclase

Candidiasis is the most common fungal infection in immunocompromised patients, and Candida albicans is the fourth leading agent of nosocomial infections. Mortality from this infection is significant; however, the therapeutic treatment is limited, which demands the search for new drugs and new targets. In this context, oxidosqualene cyclase (OSC) catalyzes the cyclization of the 2,3-oxidosqualene to form lanosterol, an intermediate of ergosterol biosynthesis. Therefore, this enzyme constitutes an attractive therapeutic target. Thus, the aim of this study is to identify potential inhibitors of C. albicans OSC (CaOSC) from a marketed drugs database in order to discover new antifungal agents. The CaOSC 3D model was constructed using the Swiss-Model server and important features for CaOSC inhibition were identified by molecular docking of known inhibitors using Autodock Vina 1.1.2. Subsequently, virtual screening helped to identify calcitriol, the active form of vitamin D, and other four drugs, as potential inhibitors of CaOSC. The selected drugs presented an interesting pattern of interactions with this enzyme, including hydrogen bond with Asp450, a key residue in the active site. Thus, the antifungal activity of calcitriol was evaluated in vitro against Candida spp strains. Calcitriol showed antifungal activity against C. albicans and C. tropicalis, which reinforces the potential of this compound as candidate of CaOSC inhibitor. In short, the present study provides important insights for the development of new oxidosqualene cyclase inhibitors as antifungals.

Design strategies of oxidosqualene cyclase inhibitors: Targeting the sterol biosynthetic pathway

Targeting the sterol biosynthesis pathway has been explored for the development of new bioactive compounds. Among the enzymes of this pathway, oxidosqualene cyclase (OSC) which catalyzes lanosterol cyclization from 2,3-oxidosqualene has emerged as an attractive target. In this work, we reviewed the most promising OSC inhibitors from different organisms and their potential for the development of new antiparasitic, antifungal, hypocholesterolemic and anticancer drugs. Different strategies have been adopted for the discovery of new OSC inhibitors, such as structural modifications of the natural substrate or the reaction intermediates, the use of the enzyme's structural information to discover compounds with novel chemotypes, modifications of known inhibitors and the use of molecular modeling techniques such as docking and virtual screening to search for new inhibitors. This review brings new perspectives on structural insights of OSC from different organisms and reveals the broad structural diversity of OSC inhibitors which may help evidence lead compounds for further investigations with various therapeutic applications.

Hydrogen-bond-assisted epoxidation of homoallylic and allylic alcohols with hydrogen peroxide catalyzed by selenium-containing dinuclear peroxotungstate

The reaction of peroxotungstates (H(2)WO(4) + H(2)O(2)) with H(2)SeO(4) gave the novel selenium-containing dinuclear tungsten species, (TBA)(2)[SeO(4){WO(O(2))(2)}(2)] (I, TBA = [(n-C(4)H(9))(4)N](+)), which was characterized by elemental analysis, IR, Raman, UV-vis, (77)Se NMR, (183)W NMR, and CSI-MS. Various kinds of homoallylic and allylic alcohols were efficiently epoxidized to the corresponding epoxy alcohols in high yields with 1 equiv. H(2)O(2) with respect to the substrates. Compound I showed the highest catalytic activity for H(2)O(2)-based epoxidation of homoallylic and allylic alcohols among selenium and tungsten complexes. The turnover frequency reached up to 150 h(-1) in a 10 mmol-scale epoxidation of cis-3-hexen-1-ol and this value was the highest among those reported for the transition-metal catalyzed epoxidation of homoallylic alcohols with H(2)O(2). The kinetic, mechanistic, computational studies showed that the stabilization of the transition-state by the hydrogen bonding between I and the substrates results in the high reactivity for the I-catalyzed epoxidation of homoallylic and allylic alcohols. The nature of the hetero atoms in the di- and tetranuclear peroxotungstates with XO(4)(n-) ligands (X = As(V), P(V), S(VI), Si(IV), etc.) was crucial in controlling the Lewis acidity of the peroxotungstates, which significantly affects their electrophilic oxygen transfer reactivity. All the data of the structural, kinetic, spectroscopic, and computational comparison show that the dimeric peroxotungstate unit, {WO(O(2))(2)}(2), in I is activated by the SeO(4)(2-) ligand.

Biotransformation of 6,7-epoxygeraniol by fungi

The biotransformation of 6,7-epoxygeraniol by resting cells of selected fungi was investigated. The main product obtained from the transformation in Rhodotorula glutinis and R. marina cultures was 6,7-epoxynerol (5-48% of chloroform extracts), whereas Saccharomyces cerevisiae, Candida parapsilosis and C. kefyr reduced this substrate to 6,7-epoxycitronellol (30-33% of chloroform extracts). Cultures of Yarrowia lipolytica, Botrytis cinerea and S. cerevisiae promoted the cyclisation of 6,7-epoxygeraniol to 2-methyl-2-(2-hydroxyethyl)-5-(2-hydroxyprop-2-yl)tetrahydrofuran (11-99% of chloroform extracts). The biotransformation of 6,7-epoxynerol was also investigated. However, none of the tested micro-organisms converted this compound.

Conjugated methyl sulfide and phenyl sulfide derivatives of oxidosqualene as inhibitors of oxidosqualene and squalene-hopene cyclases

Various (1E,3E)- and (1Z,3E)-conjugated methylthio derivatives of oxidosqualene (OS) and conjugated and non-conjugated phenylthio derivatives of OS were obtained. These compounds, designed as inhibitors of pig liver and Saccharomyces cerevisiae 2,3-oxidosqualene-lanosterol cyclases (OSC) (EC 5.4.99.7) and of Alicyclobacillus acidocaldarius squalene-hopene cyclase (SHC) (EC 5.4.99.-), contain the reactive function adjacent to carbons involved in the formation of the third and the fourth cycle during OS cyclization. All the new compounds are inhibitors of OSC and SHC, with various degrees of selectivity. The conjugated methylthio derivatives behaved as potent inhibitors of S. cerevisiae OSC, whereas most of the phenylthio derivatives were especially active toward SHC.

Vinyl sulfide derivatives of truncated oxidosqualene as selective inhibitors of oxidosqualene and squalene-hopene cyclases

Various vinyl sulfide and ketene dithioacetal derivatives of truncated 2,3-oxidosqualene were developed. These compounds, having the reactive functions at positions C-2, C-15 and C-19 of the squalene skeleton, were studied as inhibitors of pig liver and Saccharomyces cerevisiae oxidosqualene cyclases (OSC) (EC 5.4.99.7) and of Alicyclobacillus acidocaldarius squalene hopene cyclase (SHC) (EC 5.4.99.-). They contain one or two sulfur atoms in alpha-skeletal position to carbons considered to be cationic during enzymatic cyclization of the substrate and should strongly interact with enzyme nucleophiles of the active site. Most of the new compounds are inhibitors of the OSC and of SHC, with various degrees of selectivity. The methylthiovinyl derivative, having the reactive group at position 19, was the most potent and selective inhibitor of the series toward S. cerevisiae OSC, with a concentration inhibiting 500% of the activity of 50 nM, while toward the animal enzyme it was 20 times less potent. These results could offer new insight for the design of antifungal drugs.

Stereospecific syntheses of trans-vinyldioxidosqualene and 3-hydroxysulfide derivatives, as potent and time-dependent 2,3-oxidosqualene cyclase inhibitors

trans-Vinyldioxidosqualene and beta-hydroxysulfide derivatives were synthesized stereospecifically and evaluated as inhibitors of animal and yeast oxidosqualene cyclases. Only trans-vinyldioxidosqualene and 2,3-epoxy-vinyl-beta-hydroxysulfides, having the reactive function at crucial positions 14,15 and 18,19, were active as inhibitors of animal and yeast cyclases. (14-trans)-28-Methylidene-2,3: 14,15-dioxidoundecanorsqualene 27 was the most potent inhibitor of the series of pig liver cyclase, with an IC50 of 0.4 microM, and it behaved also as the most active time-dependent inhibitor of the animal enzyme.

19-Azasqualene-2,3-epoxide and its N-oxide: metabolic fate and inhibitory effect on sterol biosynthesis in Saccharomyces cerevisiae

19-Azasqualene-2,3-epoxide was more inhibitory than the corresponding N-oxide against 2,3-oxidosqualene cyclase (OSC) solubilized from Saccharomyces cerevisiae (IC50 7+/-2 and 25+/-5 microM, respectively). Both compounds showed a reversible, noncompetitive-type inhibition on solubilized OSC. Different inhibitory properties between the compounds were especially evident when measuring [14C]acetate incorporation into nonsaponifiable lipids extracted from treated cells. In cells treated with 19-azasqualene-2,3-epoxide at 30 microM, the radioactivity associated with the oxidosqualene fraction, which was negligible in the controls, rose to over 40% of the nonsaponifiable lipids, whereas it remained at a slightly appreciable level in cells treated with the N-oxide derivative under the same conditions. 19-Azasqualene-2,3-epoxide was also more effective than the N-oxide as a cell growth inhibitor (minimal concentration of compound needed to inhibit yeast growth: 45 and >100 microM, respectively). The two inhibitors underwent different metabolic fates in the yeast: while 19-azasqualene-2,3-epoxide did not undergo any transformation, its N-oxide was actively reduced to the corresponding amine in whole and in "ultrasonically stimulated" cells. The N-oxide reductases responsible for this transformation appear to be largely confined within the microsomal fractions and require NADPH for their activity. A possible relationship between the inhibitory properties of the two compounds and their metabolic fates is discussed.

Mechanism-based inhibitors and other active-site targeted inhibitors of oxidosqualene cyclase and squalene cyclase

Enzymatic cyclizations of squalene and oxidosqualene lead to sterols and other triterpenoids in bacteria, fungi, plants, and animals. The cyclases for these reactions catalyze formation and stabilization of polycyclic carbocations and direct the enzyme-specific, templated formation of new carbon-carbon bonds in regio- and stereochemically defined contexts. The development of mechanism-based irreversible inhibitors, photoactivatable inhibitors, and numerous substrate analogs have helped to unravel the stepwise events occurring in the catalytic sites of these enzymes by covalent modification of specific amino acid residues.

29-Methylidene-2,3-oxidosqualene derivatives as stereospecific mechanism-based inhibitors of liver and yeast oxidosqualene cyclase

Two pairs of isomers (18Z)- (8), (18E)-29-methylidene-2,3-oxidohexanorsqualene (21), and (18Z)- (31), (18E)-29-methylidene-2,3-oxidosqualene (34), have been obtained in a fully stereospecific manner, as inhibitors of rat and yeast oxidosqualene cyclase. A new method for the synthesis of C22 squalene aldehyde 2,3-epoxide is reported, as well as that of other 19-modified 2,3-oxidosqualene analogues. We found that the activity is the opposite in the two series: the (E)-hexanormethylidene 21 and the (Z)-methylidene 31 are potent and irreversible inhibitors of oxidosqualene cyclase, while (Z)-hexanormethylidene 8 and (E)-methylidene 34 are almost completely inactive. Reduction of the 18,19-double bond, such as in 39, eliminates the activity, while removal of both of the 19-linked groups such as in heptanor derivative 40 greatly reduces inhibition of the enzyme. (E)-Hexanormethylidene 21 results the first irreversible inhibitor of the series toward the yeast enzyme.

Synthesis and inhibition studies of sulfur-substituted squalene oxide analogues as mechanism-based inhibitors of 2,3-oxidosqualene-lanosterol cyclase

The synthesis and biological evaluation of three new sulfur-substituted oxidosqualene (OS) analogues (1-3) are presented. In these analogues, C-11, C-15, or C-18 in the OS skeleton was replaced by sulfur. The sulfur position in the OS skeleton was chosen to disrupt one or more key processes involved in cyclization: (a) the folding of the B-ring into a boat conformation, (b) the anti-Markovnikov cyclization leading to the C-ring, or (c) the formation of the D-ring during the lanosterol biosynthesis. Enzyme inhibition kinetics using homogeneous mammalian oxidosqualene cyclases (OSC) were also examined for the previously reported S-19 analogue 4. The four analogues were potent inhibitors of mammalian OSCs (IC50 = 0.05-2.3 microM for pig and rat liver OSC) and fungal cell-free Candida albicans OSC (submicromolar IC50 values). In particular, the S-18 analogue 3 showed the most potent inhibition toward the rat liver enzyme (IC50 = 50 nM) and showed potent, selective inhibition against the fungal enzyme (IC50 = 0.22 nM, 10-fold more potent than the S-19 analogue 4). Thus, 3 is the most potent OSC inhibitor known to date. The Ki values ranged from 0.5 to 4.5 microM for pig OSC, with 3 and 4 showing about 10-fold higher potency for rat liver OSC. Interestingly, the S-18 analogue 3 showed time-dependent irreversible inhibition with homogeneous pig liver OSC (kinact = 0.06 min-1) but not with rat OSC.

Inhibition of 2,3-oxidosqualene-lanosterol cyclase in Candida albicans by pyridinium ion-based inhibitors

The N-(4E,8E)-5,9,13-trimethyl-4,8,12-tetradecatrien-1- ylpyridinium and N-(4E,8E)-5,9,13-trimethyl-4,8,12-tetradecatrien-1- ylpicolinium cations were evaluated for their ability to inhibit 2,3-oxidosqualene-lanosterol cyclase activity in Candida albicans. Both compounds inhibited fungal growth, were fungicidal, and resulted in the accumulation of squalene epoxide concurrent with a decrease in ergosterol, monomethyl sterols, and lanosterol, as was expected for the specific inhibition of 2,3-oxidosqualene-lanosterol cyclase activity. These compounds are electron-poor aromatic mimics of a monocyclized transition state or high-energy intermediate formed from oxidosqualene, which may explain their selective action.