Acaricidal Activity of Bufadienolides Isolated from Drimia pancration against Tetranychus urticae, and Structural Elucidation of Arenobufagin-3-O-α-L-rhamnopyranoside

Chemical characterization of the bulbs of Drimia pancration was conducted to isolate four steroidal saponins (1-4). Earlier, we focused on the structural elucidation of compounds 1-3. Herein, by means of H-1-NMR, C-13-NMR, Nuclear Overhauser Effects (NOE), and 2D-NMR spectra, the full stereochemical structure of 4 is reported, and all the H-1 and C-13 signals are assigned. Compounds 1-4 were tested for their acaricidal properties against the two-spotted spider mite Tetranychus urticae. Our results showed excellent activity of compound 1, with an LD50 (mu g/cm(2)) of 0.29 and a LD90 (mu g/cm(2)) of 0.96, whereas compounds 2, 3, and 4 showed moderate activity. Furthermore, the acaricidal and cytotoxic properties of the crude extract were also investigated. Of note, after 96 h of exposure, the acaricidal activity of compound 1 was higher than that of the positive control, hexythiazox. Indeed, for compound 1, LD50 and LD90 were 0.29 and 0.96 mu g/cm(2), respectively, while hexythiazox LD50(90) was 18.7 (132.5) mu g/cm(2). Additionally, D. pancration extract, after 72 h, induced a high cytotoxic effect in HaCaT and THP-1 cell lines, with an IC50 of 7.37 +/- 0.5 mu g/mL and 3.50 +/- 0.15 mu g/mL, respectively. Overall, D. pancration can be considered as a green source of novel acaricides effective against mites of agricultural importance, such as T. urticae, pending proper field validation and the assessment of non-target effects on other invertebrate species.