Synthesis of novel derivatives of sclareol bearing an adamantane moiety was achieved aiming to improve cytotoxicity of sclareol. Novel compounds exhibited up to 100 times more pronounced effects compared to the sclareol against non-small cell lung carcinoma cell lines NCI-H460 and its multidrug resistant variant NCI-H460/R. Nine derivatives (11b, 11c, 11i, 12a, 12b, 12c, 12e, 12f, and 12 g) caused cell growth inhibition below 50 % at 1 μM concentration in NCI-H460 cells, and four derivatives (11c, 11i, 12b and 12e) showed similar activity against NCI-H460/R cells. Notably, compound 12b exhibited selectivity towards cancer cells, as well as collateral sensitivity being more potent against MDR cells. Sclareol, as well as novel derivatives 12a, 12b, 12c, 12e, 12f, and 12 g increased accumulation of rhodamine 123 which suggested their potential to modulate P-glycoprotein (P-gp) activity. Compound 12b emerged as inhibitor of P-gp and 12e and 12f as P-gp substrates. Furthermore, 12b, 12e and 12f induced reversal of doxorubicin resistance. Cell death analysis indicated necrosis as a primary type of cell death in NCI-H460 with significant increase in late apoptosis in MDR cells. We discovered for the first time that sclareol can spontaneously form negatively charged nanoparticles by being dissolved in ultrapure water. Adamantyl derivatives 12a-c and 12e-g and parental diamine derivatives 8c-f formed positively charged nanoparticles, implying that they can bind to the negatively charged cellular membrane and penetrate cancer cells. The unique nanoparticle characteristics combined with the significant cytotoxicity of the novel adamantane-sclareol derivatives underscore their potential as promising candidates for advanced anticancer nanotherapeutic applications.
