Etoposide nanocarriers suppress glioma cell growth by intracellular drug delivery and simultaneous P-glycoprotein inhibition

In cancer treatment, efficient therapeutic strategies could be impeded by cellular mechanisms such as the multidrug resistance. Recently, drug-loaded nanoparticles have been reported to be useful, since they allow entering the cancer cell and act as an intracellular anti-cancer drug reservoir. A new approach is proposed here by the use of lipid nanocapsules (LNC) which were hypothesized to reverse multidrug resistance additionally by their P-glycoprotein (P-gp) inhibiting surfactant. LNC (mean diameter 25 to 100 nm) were loaded with etoposide, tested for the drug release and their efficiency to reduce cell growth in cell culture for C6, F98, and 9L glioma cell lines. Sustained etoposide release can be provided over a period of 1 week (t10%: 1.4 ± 0.1 h; t50%: 15.9 ± 2.8 h). The P-gp inhibiting activity in-vitro was found to be independent from the LNC size. In cell culture, an internalization of LNC was observed in all glioma cell types. Etoposide LNC showed a generally higher efficiency than the drug solution while blank LNC were found to be less inhibitory than the pure drug at equivalent concentrations (IC50: C6: etoposide: 25.2 μM; LNC: 2.6–8.9 μM, F98: etoposide: 46.5 μM; LNC: 1.4–14.7 μM, 9L: etoposide: 58.2 μM; LNC: 4.4–12.7 μM). This effect was found to be particle size dependent within a range of an 8- (C6) to 33-fold (F98) increased cytotoxicity for smallest particles. When cells were incubated with etoposide solution in the presence of blank LNC, a slight growth inhibition was observed, however, distinctly lower than the drug-trapping particles. Moreover, cell toxicity on astrocytes was similar for etoposide LNC and etoposide solution. The mechanism of action of etoposide LNC was proposed to be a cell uptake followed by a sustained drug release from the LNC in combination with an intracellular P-gp inhibition ensuring a higher anticancer drug concentration inside the cancer cells.