Internalization of biomimetic microcapsules for intracellular delivery of active agents

In drug delivery there has been great interest in the development of nano- and microcarriers of active agents to control their release kinetics and their efficacy by delivering them to where they are most needed. A possible method of delivery is that of carrier internalization by cells. By delivering an active agent directly to the cells, high drug concentrations can be made available to the target cells while circumventing undesirable side effects to surrounding tissues due to premature drug leakage.

In this work, microcapsules prepared using layer-by-layer were conceived using chitosan and biomimetic elastin-like recombinamers (ELRs) as constituents assembled onto templates of calcium carbonate microparticles. Two types of ELRs were used: one containing the bioactive aminoacid sequence RGD and the other a scrambled nonfunctional RDG. Scanning electron microscopy (SEM) showed no morphological differences among both types of microcapsules, being spherical with around 4 μm in diameter. Cell viability studies were performed using human mesenchymal stem cells (hMSCs) and microcapsule/cell ratios from 5:1 to 100:1. After 3 and 72 hours of incubation, no significant cytotoxicity was observed in respect to a positive control of hMSCs co-incubated with microcapsules. The cells were kept in culture for another 7 days in absence of microcapsules. Live/dead assays confirmed that cells retained their cellular integrity, thus the contact of hMSCs with either functionalized microcapsule type does not result in cellular death.

The internalization efficacy of microcapsules was assessed by flow cytometry and microscopy analysis. To our knowledge, this is the first time that the internalization effectiveness of RGD-functionalized LbL microcapsules is compared with a nonfunctional analogue microcapsule type. Loading them with DQ-ovalbumin permitted to follow the intracellular traffic and degradation by monitoring fluorescence changes. The data indicated that 63% of the hMSCs have internalized RGD-functionalized microcapsules, while their nonfunctional analogue triggered internalization in around 53% of the cells. No statistical differences were found between both cases, suggesting that macropinocytosis should be the major endocytosis mechanism involved in the cellular uptake of this class of carrier devices and that the exhibition of the RGD/RDG motifs does not influence significantly the incorporation of the microcapsules by the hMSCs. Intracellular processing was assessed by qualitative fluorescence variations showing that this phenomenon was faster for the RGD-functionalized microcapsules. The developed multilayer microcapsules using biomimetic ingredients for intracellular delivery let foresee new strategies to increase the availability of molecules of interest in cells and for targeted biomedical applications.