Optimisation of carrier systems in terms of cell selectivity and cellular uptake is crucial for efficient drug delivery. Peptide recognizing membrane-located receptors and so called cell-penetrating peptides (CPP), chemically attached or stably adsorbed, mediate efficient uptake of nanoparticles into different cells. We developed a lipopeptide (LP) consisting of a cationic CPP derived from apolipoprotein E and two palmitoyl chains (P2A2) which is able to self assemble into micelles (1) and to anchor into the lipid bilayer of liposomes (2). Depending upon the formulation, the carriers are able to activate different transmembrane transport routes. Small micelles composed of P2A2 are selectively internalized into blood brain barrier (BBB) endothelial cells (2) as revealed by fluorescence-based detection methods. Furthermore, magnetic resonance imaging studies using Xe-Hyper CEST NMR confirmed the distinctly pronounced accumulation of P2A2-contrast agent in brain capillary endothelial cells suggesting the micelles to be highly suitable systems to target the blood brain barrier.
In contrast, larger liposomes with a reduced P2A2 surface density efficiently entered different cell lines. We used such liposomes to develop a delivery system for Transglutaminase 1 (TG 1), an enzyme which is responsible for the proper development of the human skin. TG1-deficiency causes a severe skin disease – Lamellar Ichthyosis. A P2A2-modifed liposomal formulation of the enzyme overcame the problems of insufficient cutaneous delivery and membrane penetration and provided excellent availability and activity in primary keratinocytes as well as restoration of skin integrity (3).
Based on these results we tested the hypothesis that selective cellular uptake of peptide-modified particulate systems into cells is determined by the interplay of particle size and surface charge. Nanocarriers formed with poly-arginine or poly-lysine lipopeptides have been investigated. Whereas the poly-arginine-modified particles were selectively internalised into brain capillary endothelial cells independent on particle size or surface charge density (4), the poly-lysine bearing carriers were preferentially accumulated at the cellular membrane. The results lead to the suggestion that in addition to the sequence of the targeting and uptake-mediating peptides the particle size and peptide density on the surface are decisive for cell recognition and uptake and thus of particular importance for the potential field of application.