Differential inhibition of fungal oxidosqualene cyclase by 6E and 6Z isomers of 2,3-epoxy-10-aza-10,11-dihydrosqualene

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.

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.

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.

Inhibition of 2,3-oxidosqualene cyclase and sterol biosynthesis by 10- and 19-azasqualene derivatives

The inhibition of 2,3-oxidosqualene-lanosterol cyclase (EC 5.4.99.7) (OSC) by new azasqualene derivatives, mimicking the proC-8 and proC-20 carbocationic high-energy intermediates of the cyclization of 2,3-oxidosqualene to lanosterol, was studied using pig liver microsomes, partially purified preparations of OSC, and yeast microsomes. The azasqualene derivatives tested were: 6E- and 6Z-10aza-10,11-dihydrosqualene-2,3-epoxide 17 and 18, 19-aza-18,19,22,23-tetrahydrosqualene-2,3-epoxide 19 and its corresponding N-oxide 20, and 19-aza-18,19,22,23-tetrahydrosqualene 21. The compounds 17 and 19 (i.e. the derivatives bearing the 2,3-epoxide ring and the same geometrical configuration as the OSC substrate) were effective inhibitors, as shown by the Ki obtained using partially purified OSC: 2.67 microM and 2.14 microM, respectively. Compound 18, having an incorrect configuration and the 19-aza derivative 21, lacking the 2,3-epoxide ring, were poor inhibitors, with IC50 of 44 microM and 70 microM, respectively. Compound 21 was a competitive inhibitor of OSC, whereas 17 and 19 were noncompetitive inhibitors, and showed a biphasic time-dependent inactivation of OSC, their apparent binding constants being 250 microM and 213 microM, respectively. The inhibition of sterol biosynthesis was studied using human hepatoma HepG2 cells. The incorporation of [14C] acetate in the C27 sterols was reduced by 50% by 0.55 microM 17, 0.22 microM 19, and 0.45 microM 21, whereas 2 microM 18 did not affect sterol biosynthesis. In the presence of 17, 19 and 21, only the intermediate metabolites 2,3-oxidosqualene and 2,3,22,23-dioxidosqualene accumulated, demonstrating a very specific inhibition of OSC.

Preliminary screening of some squalenoid derivatives for toxicity towards dermatophytes

We report a preliminary study of the in vitro anti-dermatophyte activity of six squalenoid derivatives that inhibit 2,3-oxidosqualene cyclase and squalene epoxidase: 2-aza-2,3-dihydrosqualene, 22,23-epoxy-2-aza-2,3-dihydrosqualene, azasqualene alcohol, 19-aza-18,19,22,23-tetrahydrosqualene, 2,3-epoxy-19-aza-18,19,22,23-tetrahydrosqualene and hexafluorosqualene epoxide. The tests were done by inoculating 10 microliters of Trichophyton mentagrophytes (Robin) Blanchard or Microsporum canis Bodin homogenate into 1 ml of Sabouraud glucose liquid medium containing serial dilutions from 100 to 0.25 micrograms ml-1 of the substance. For each compound the minimum inhibitory concentration (MIC) and the minimum fungicidal concentration (MFC) were determined. The most effective compounds were 22,23-epoxy-2-aza-2,3-dihydrosqualene and azasqualene alcohol, with MICs respectively of 3 and 6.25 micrograms ml-1 for each of the two species of dermatophyte. The first of these compounds was the only one to show fungicidal activity over the range of concentrations tested.

2,3-Oxidosqualene cyclase: from azasqualenes to new site-directed inhibitors

2,3-Oxidosqualene cyclases (OSC) are enzymes which convert 2,3-oxidosqualene (OS) into polycyclic triterpenoids such as lanosterol, cycloartenol, and alpha- and beta-amyrin. Our interest in the study of OSC is the development of new OSC inhibitors for potential use as hypocholesterolemic, antifungal, or phytotoxic drugs. In particular, we describe the biological activity and the mechanism of a series of acyclic azasqualene derivatives mimicking the C-2, C-8, and C-20 carbonium ions formed during OS cyclization. Some of these carbonium ion analogues are very promising as specific hypocholesterolemic agents. The toxicity, the biodistribution, and the pharmacokinetics of different azasqualene derivatives in mice are also presented. In order to obtain new, site-directed irreversible inhibitors of OSC, a series of squalene derivatives containing functional groups that can link covalently to an active-site thiol group was designed. Among these compounds, squalene maleimide was the most active toward mammalian OSC, whereas squalene Ellman behaved as an irreversible inhibitor of OSC from yeast.

Inhibition of sterol biosynthesis in Saccharomyces cerevisiae and Candida albicans by 22,23-epoxy-2-aza-2,3-dihydrosqualene and the corresponding N-oxide

The abilities of 22,23-epoxy-2-aza-2,3-dihydrosqualene and the corresponding N-oxide, 22,23-epoxy-2-aza-2,3-dihydrosqualene-N-oxide, to inhibit sterol biosynthesis were studied in microsomes and cells of Saccharomyces cerevisiae and Candida albicans. 22,23-Epoxy-2-aza-2,3-dihydrosqualene, which differs from the other inhibitor only in lacking oxygen at position 2, exhibited higher inhibitory properties in all preparations tested. The different levels of effectiveness of the two azasqualene derivatives were evident mostly in microsomes from S. cerevisiae (the 50 inhibitory concentrations of the 2-aza derivative and the corresponding N-oxide on oxidosqualene cyclase were 30 and 120 microM respectively) and in cell cultures of the same strain (1 order of magnitude separated the inhibitory activities of the two compounds on sterol biosynthesis). A possible explanation for the differences between 22,23-epoxy-2-aza-2,3-dihydrosqualene and the corresponding N-oxide arose from the study of their metabolic fates in vivo and in vitro. While the 2-aza derivative did not undergo any transformation, the N-oxide compound was actively reduced to the corresponding amine in microsomes and in cells of both yeast strains. 22,23-Epoxy-2-aza-2,3-dihydrosqualene-N-oxide seems to behave as a proinhibitor of sterol biosynthesis, becoming active only after transformation into the active form 22,23-epoxy-2-aza-2,3-dihydrosqualene.