Topical delivery of siRNA to psoriatic skin model using high molecular weight chitosan derivatives: In vitro and in vivo studies.

Psoriasis is a chronic inflammatory skin disease that, like other immune-mediated conditions, may benefit from small interfering RNA (siRNA)-based therapies, which are emerging as a promising alternative by addressing several limitations of current treatments. In this study, topical formulations of chitosan-based vectors were developed to deliver siRNA targeting tumor necrosis factor alpha (TNFα) to inflamed skin. Grafting diisopropylethylamine (DIPEA) and polyethylene glycol (PEG) onto the chitosan backbone enhanced siRNA delivery efficiency under physiological conditions, forming robust polymeric vectors with high structural and colloidal stability. These vectors provided siRNA protection against RNAse degradation and oxidative damage. Additionally, the chitosan derivatives displayed lysozyme-mediated biodegradability comparable to native chitosan, while PEG was released in response to reductive environments, supporting controlled vector disassembly. The PEGylated DIPEA-chitosan/siRNA polyplexes demonstrated positive zeta potentials (up to + 11 mV), particle sizes of 100-200 nm, and very low cytotoxicity in keratinocyte and fibroblast cell lines. In vitro, the polyplexes achieved TNFα knockdown levels (65%) in RAW macrophages, comparable to those obtained with Lipofectamine™. Topical formulations showed enhanced interaction of vectors with skin models (Strat-M^® and porcine ear skin) compared to naked siRNA. Furthermore, in vivo studies indicated that hair follicles were a key route for polyplexes to penetrate deeper skin layers. A rodent model of psoriasis induced by imiquimod was treated topically with these vectors, resulting in approximately a 50% reduction in TNFα levels at inflammation sites, decreased immune cell infiltration, and preservation of epidermal structure. These findings collectively underscore the potential of DIPEA-chitosan-based vectors for topical siRNA-based therapies.