**1. Introduction**

Liposomes are a well-accepted drug delivery system and still the most common nanomedicines in clinical trials [1]. Most liposomal carriers are coated with polyethylene glycol (PEG) to prolong their residence times in blood circulation [2,3], but there are still problems related to reduced uptake into the target cells [3,4], immunogenicity [5,6], and undesired protein interactions [7]. Therefore, alternatives for PEG were proposed, including polyvinylpyrrolidone (PVP) [8], polycarbonates [9], pullulan [10], polyoxazolines [11], and hyaluronic acid (HA) or hyaluronan [12].

HA is a negatively charged natural polysaccharide of d-glucuronic acid and *N*-acetyl-dglucosamine disaccharide units that are connected via alternating β-1,4 and β-1,3 glycosidic bonds. HA contains carboxyl, hydroxyl, and amino groups that can be derivatized. HA is biocompatible, biodegradable, non-toxic, and less immunogenic than PEG [12,13]. In addition, HA binds the CD44 receptors that are overexpressed in melanomas, lymphomas, breast cancer, lung, and rectal tumor cells [12] and present in the retinal pigment epithelium [14]. Thus, the HA coating combines steric stabilization ("stealth") and active targeting functions. Furthermore, HA improves liposomal stability in freeze-drying due to its high water-binding capacity [15].

Passive drug release from liposomes is often erratic and inadequate [16], but this can be improved by external triggers such as light [17,18]. Light activation is feasible for drug release in superficial organs (eye and skin), as well as in deeper tissues with light guides, and light parameters can be optimized for treatment (e.g., wavelength, light intensity, exposure time, and beam size) [19]. We developed light-activated PEG-coated liposomes using indocyanine green (ICG) as the light-sensitizing compound [20–22]. Light-activated release of small and large compounds was achieved with near-infrared (NIR) light with only five-second light exposures [22–24]. Amphiphilic ICG can be integrated into the liposomal bilayers or clustered with the hydrophilic coating on the liposome surface (e.g., PEG). ICG is a fluorescent dye approved by the European Medicines Agency (EMA) [23] and the United States (US) Food and Drug Administration (FDA) [24] for angiographic and lymphatic system imaging in the clinics [25].

The acquired protein corona determines the "biological identity" and fate of liposomes in a biological environment [26,27]. We demonstrated that ICG clustered with PEG on the liposome surface influenced the structure and composition of the protein corona, including the beneficial enrichment of "dysopsonin" molecules such as clusterin and apolipoprotein E [28]. Even though the liposomal protein coronas were characterized in plasma, their formation in the vitreous humor of the eye was studied only in our recent publication [29]. The structure and contents of the vitreal liposome corona were not dependent on the composition of 50 nm anionic light-activated liposomes [28]. This is in line with earlier observations with silica and gold nanoparticles in dilute dog vitreous samples [30]. The slight increase (10–12%) in liposomal diameter due to corona formation did not affect the mobility of the liposomes in the vitreous humor [29].

HA is an endogenous component in the vitreous humor and, in that respect, an interesting coating material for ocular liposomes. This report describes the development of light-activated liposomes with HA coating. We produced HA–lipid conjugates in-house and characterized the HA-coated liposomes for drug release, stability, protein corona formation, and mobility in the vitreous humor.
