How to Turn a Poisonous Plant into Medicine: Non-Polar Extracts of Rhododendron adamsii (Sagan Dalya) Are Free of Grayanotoxins and Inhibit the SARS-CoV-2 Main Protease.

The composition of low-polarity extracts obtained by sequential extraction of the aerial parts of Rhododendron adamsii Rehd. with hexane and methyl tert-butyl ether (MTBE) was investigated using GC-MS. The hexane extract was dominated by non-polar components: squalene, n-alkanes (nonacosane, hentriacontane), sesquiterpenes (trans-nerolidol, spathulenol, β-farnesene), and β-sitosterol. The subsequent MTBE extract was enriched in more polar lipids, primarily free triterpenic acids (ursolic and oleanolic acids). A critical finding was the complete absence of diterpene grayanotoxins in all tested extracts, confirming the safety of the non-polar extraction approach. In bioactivity assays, the total hexane extract demonstrated potent inhibitory activity against the SARS-CoV-2 main protease (3CLpro) with IC50 values of 0.0125-0.025 mg/mL, only one order of magnitude higher than the reference inhibitor disulfiram. Fractionation revealed that the activity was distributed among free acids, bound acids, and the unsaponifiable residue, indicating a multicomponent mechanism. Importantly, none of the samples inhibited HIV-1 protease (IC50 > 0.1 mg/mL), demonstrating selectivity for the cysteine protease 3CLpro over the aspartyl protease of HIV-1. These results highlight that sequential non-polar extraction of R. adamsii provides a grayanotoxin-free lipophilic complex with selective anti-SARS-CoV-2 protease activity, paving the way for bioactivity-guided identification of individual inhibitors.