Advanced Targeted Liposomal Drug Delivery to the Skin
Nina Østergaard Knudsen
In plaque psoriasis, the skin is characterized by the presence of dry, red, raised and scaly plaques, resulting in various changes in the skin barrier function, which challenges the reproducible delivery of anti-psoriatic drugs into the skin. Calcipotriol is a vitamin D3 analogue commonly used for the treatment of psoriasis, since it inhibits proliferation and normalises differentiation of keratinocytes in the lower epidermis during psoriasis. Liposomes can be used for topical application to increase the penetration of drug into the skin and to increase the accumulation of drug in the skin, but the exact mechanism by which liposomes interacts with the skin is not fully understood. However in general, the drug delivery properties of liposomes are largely affected by the physicochemical characteristic of the liposomes as well as the interaction between the vesicles and the skin. Introduction of receptor-specific ligands on the surface of liposomes has been proposed to enhance the therapeutic effect of the drug-loaded liposomes due to specific targeting to diseased cells and increased cellular internalisation.
The aim of the present study was to improve the delivery of calcipotriol into keratinocytes in the viable epidermis of the skin in a way, and thereby increase the therapeutic index of calcipotriol for treatment of psoriasis. The working hypothesis was that topical application of receptor-targeted liposomes can enhance the delivery of calcipotriol to keratinocytes in the lower epidermis of the skin: The function of liposomes would in that respect be to increase the calcipotriol delivery into the skin in areas where the skin is thickened and the barrier function is increased, and the introduction of the receptor-specific ligand would provide an active targeting to keratinocytes in the lower epidermis, when the barrier function of the skin is impaired as it is in a sore, where there is almost free passage to the systemic circulation. Targeting is therefore suggested to reduce exposure of non-target tissue. The engineering of ligand-conjugated liposomes targeting the integrin α2β1 receptor expressed on keratinocytes in the lower epidermis was divided into different steps evaluating the liposomal interaction with the skin and the receptor targeting of the liposomes.
First, ex vivo skin penetrations studies were used to evaluate the interaction between liposomes and the skin, elucidating how the interaction was affected by the skin barrier function and the physicochemical properties of the liposomes, including the integrity of the skin barrier function, the fluidity of the liposomal lipid bilayer and grafting of polymer on the liposomal surface. The penetration of liposome into barrier-impaired skin was significantly increased compared to the penetration into intact skin. In addition, the penetration of lipid into the skin depended on the thermotropic phase behavior of the liposomal lipid bilayer, since lipid penetration was increased from liquid state liposomes as compared to gel state liposomes. The delivery of liposome-intercalated calcipotriol into the skin was also affected by the thermotropic phase behavior of the liposomal lipid bilayer, because an increased co-penetration of calcipotriol and lipid in the upper layers of the skin was observed, when calcipotriol was intercalated into gel state liposomes, whereas an increased penetration of drug and lipid into the lower layers of the skin was observed, when calcipotriol was intercalated into liquid state liposomes. The introduction of the lipopolymer poly(ethylene glycol) (PEG) on the liposomal surface did not affect the penetration of lipid into intact skin. In addition, the delivery of liposome-intercalated calcipotriol into the skin was increased, when the liposomes contained 1 mol% PEG, which introduced an increased deposition of calcipotriol in the SC, whereas no difference in the delivery of calcipotriol into the skin was observed, when the liposomes contained 0 mol%, 0.5 mol% or 5 mol% PEG. However, calcipotriol penetrated the intact skin better than the lipid component, suggesting that at least a fraction of the drug is released from the liposomes during skin migration.
A cyclic liposomal targeting peptide was designed, containing the specific integrin α2β1 receptor binding domain RKKH derived from the jarahagin protein, and were conjugated to the distal ends of PEG-chains on PEGylated liposomes. The introduction of the ligand resulted in an increased interaction with epithelial cells expressing the integrin α2β1 receptor. In keratinocyte cell cultures, the ligand-conjugated liposomes loaded with calcipotriol induced transcription of the gene encoding the antimicrobial peptide cathelicidin, which is activated through the vitamin D3 receptor. This suggests that the liposomes are internalised and that calcipotriol is delivered intracellularly and released in an active form.The RKKH-conjugated liposomes were tested in vivo in a psoriasis-like transgenic mouse model expressing the human integrin α2β1 receptor in the suprabasal layers of the epidermis. However, the phenotype of this transgenic mouse model was poorly characterised, and it was therefore not possible to obtain a good evaluation of the effect of the RKKH-conjugated liposomes in vivo.
In conclusion, the present studies illustrate that the barrier properties of the skin are important for liposomal drug delivery into the skin. This encourages the further development of advanced and targeted drug delivery systems for treatment of psoriasis. The RKKH-conjugated liposomes increased the delivery to epithelial cells expressing the integrin α2β1 receptor cells, and calcipotriol-loaded RKKH-conjugated liposomes were able to deliver biologically active calcipotriol to keratinocytes in vitro, suggesting that the calcipotriol-loaded RKKH-conjugated liposomes can improve the delivery of calcipotriol to the keratinocytes during psoriasis. However, a more throughout investigation of the drug delivery system in vivo is required to evaluate the full potential of the ligand-conjugated liposomes for delivery to the skin.
