Insights on profiling of phorbol, deoxyphorbol, ingenol and jatrophane diterpene esters by high performance liquid chromatography coupled to multiple stage mass spectrometry.

This paper reports our effort to develop a comprehensive HPLC-MS(n)-based dereplication strategy for phorbol ester (PE), deoxyphorbol ester (dPE) and ingenol ester (IE) profiling in plant extracts. This strategy is composed of two sequential analysis exploiting specific hybrid triple quadrupole/linear ion trap instrument modes. A first run was performed using a multiple reaction monitoring (MRM) mode targeting fragmentation of PE and dPE/IE coupled with the acquisition of MS(2) spectrum for the ions at m/z 311 and m/z 313, respectively. A second run was then completed based on precursor ion scan mode (PIS) and automatic MS(2) acquisition for each quasimolecular ion. The developed approach was used to investigate ten Euphorbia extracts showing bioactivity against chikungunya virus replication. Experiments allowed partial annotation of three dPE/IE but no PE was detected. Results suggested that other types of diterpene esters displayed PE- and dPE/IE-like fragmentations. The study of jatrophane ester (JE) standards by CID fragmentation using low and high resolution mass spectrometry confirmed this hypothesis, highlighting challenges and difficulties of diterpene esters profiling within plant extracts. Nonetheless, the present LC-MS(n) method can be easily adapted to profile other types of diterpene esters.

Advanced Structural Determination of Diterpene Esters Using Molecular Modeling and NMR Spectroscopy.

Three new jatrophane esters (1-3) were isolated from Euphorbia amygdaloides ssp. semiperfoliata, including an unprecedented macrocyclic jatrophane ester bearing a hemiketal substructure, named jatrohemiketal (3). The chemical structures of compounds 1-3 and their relative configurations were determined by spectroscopic analysis. The absolute configuration of compound 3 was determined unambiguously through an original strategy combining NMR spectroscopy and molecular modeling. Conformational search calculations were performed for the four possible diastereomers 3a-3d differing in their C-6 and C-9 stereocenters, and the lowest energy conformer was used as input structure for geometry optimization. The prediction of NMR parameters ((1)H and (13)C chemical shifts and (1)H-(1)H coupling constants) by density functional theory (DFT) calculations allowed identifying the most plausible diastereomer. Finally, the stereostructure of 3 was solved by comparison of the structural features obtained by molecular modeling for 3a-3d with NMR-derived data (the values of dihedral angles deduced from the vicinal proton-proton coupling constants ((3)JHH) and interproton distances determined by ROESY). The methodology described herein provides an efficient way to solve or confirm structural elucidation of new macrocyclic diterpene esters, in particular when no crystal structure is available.

LC-MS²-Based dereplication of Euphorbia extracts with anti-Chikungunya virus activity.

Recently, phorbol esters from Euphorbiaceae have been shown to elicit potent and selective antiviral activity on the replication of Chikungunya virus (CHIKV) in cell culture. With the objective to found new compounds with anti-CHIKV activities, 45 extracts from various plant parts of 11 Mediterranean Euphorbia and one Mercurialis species were evaluated for selective inhibition of CHIKV replication. All EtOAc extracts, especially those prepared from latex, exhibited significant and selective antiviral activity in a Chikungunya virus-cell-based assay. An LC-MS(2) dereplication method was then developed to investigate whether known diterpenoids with anti-CHIKV activity, such as the potent anti-CHIKV 12-O-tetradecanoylphorbol-13-acetate (TPA), phorbol-12,13-didecanoate, and prostratin as well as 24 other commercially available diterpenoids of tigliane-, ingenane-, and daphnane-type for which the anti-CHIKV activity have been established in advance (Nothias-Scaglia et al. 2015), were present in the Euphorbia extracts. Only ingenol-3-mebutate, 13-O-isobutyryl-12-deoxyphorbol-20-acetate, and ingenol-3,20-dibenzoate, all exhibiting weak anti-CHIKV activities, were detected in the EtOAc extracts of Euphorbia peplus, Euphorbia segetalis ssp. pinea, and Euphorbia pithyusa ssp. pithyusa. Given the potent anti-CHIKV activities of these Euphorbia extracts, the present study suggested that their antiviral activities are probably due to untargeted diterpenoids.

Antiviral Activity of Diterpene Esters on Chikungunya Virus and HIV Replication.

Recently, new daphnane, tigliane, and jatrophane diterpenoids have been isolated from various Euphorbiaceae species, of which some have been shown to be potent inhibitors of chikungunya virus (CHIKV) replication. To further explore this type of compound, the antiviral activity of a series of 29 commercially available natural diterpenoids was evaluated. Phorbol-12,13-didecanoate (11) proved to be the most potent inhibitor, with an EC50 value of 6.0 ± 0.9 nM and a selectivity index (SI) of 686, which is in line with the previously reported anti-CHIKV potency for the structurally related 12-O-tetradecanoylphorbol-13-acetate (13). Most of the other compounds exhibited low to moderate activity, including an ingenane-type diterpene ester, compound 28, with an EC50 value of 1.2 ± 0.1 µM and SI = 6.4. Diterpene compounds are known also to inhibit HIV replication, so the antiviral activities of compounds 1-29 were evaluated also against HIV-1 and HIV-2. Tigliane- (4ß-hydroxyphorbol analogues 10, 11, 13, 15, 16, and 18) and ingenane-type (27 and 28) diterpene esters were shown to inhibit HIV replication in vitro at the nanomolar level. A Pearson analysis performed with the anti-CHIKV and anti-HIV data sets demonstrated a linear relationship, which supported the hypothesis made that PKC may be an important target in CHIKV replication.

Jatrophane diterpenes as inhibitors of chikungunya virus replication: structure-activity relationship and discovery of a potent lead.

Bioassay-guided purification of an EtOAc extract of the whole plant of Euphorbia amygdaloides ssp. semiperfoliata using a chikungunya virus-cell-based assay led to the isolation of six new (1-4, 9, and 10) and six known (5-7, 8, 11, and 12) jatrophane esters. Their planar structures and relative configurations were determined by extensive spectroscopic analysis, and their absolute configurations by X-ray analysis. These compounds were investigated for selective antiviral activity against chikungunya virus (CHIKV), Semliki Forest virus, Sindbis virus, and HIV-1 and HIV-2 viruses. Compound 3 was found to be the most potent and selective inhibitor of the replication of CHIKV and of HIV-1 and HIV-2 (EC50 = 0.76, IC50 = 0.34 and 0.043 µM, respectively). A preliminary structure-activity relationship study demonstrated that potency and selectivity are very sensitive to the substitution pattern on the jatrophane skeleton. Although replication strategies of CHIK and HIV viruses are quite different, the mechanism of action by which these compounds act may involve a similar target for both viruses. The present results provide additional support for a previous hypothesis that the anti-CHIKV activity could involve a PKC-dependent mechanism.