Preparation, optimization, and characterization of chitosan-coated solid lipid nanoparticles for ocular drug delivery

Effects of nanoparticle size, shape, and zeta potential on drug delivery

Nanotechnology has brought about a significant revolution in drug delivery, and research in this domain is increasingly focusing on understanding the role of nanoparticle (NP) characteristics in drug delivery efficiency. First and foremost, we center our attention on the size of nanoparticles. Studies have indicated that NP size significantly influences factors such as circulation time, targeting capabilities, and cellular uptake. Secondly, we examine the significance of nanoparticle shape. Various studies suggest that NPs of different shapes affect cellular uptake mechanisms and offer potential advantages in directing drug delivery. For instance, cylindrical or needle-like NPs may facilitate better cellular uptake compared to spherical NPs. Lastly, we address the importance of nanoparticle charge. Zeta potential can impact the targeting and cellular uptake of NPs. Positively charged NPs may be better absorbed by negatively charged cells, whereas negatively charged NPs might perform more effectively in positively charged cells. This review provides essential insights into understanding the role of nanoparticles in drug delivery. The properties of nanoparticles, including size, shape, and charge, should be taken into consideration in the rational design of drug delivery systems, as optimizing these characteristics can contribute to more efficient targeting of drugs to the desired tissues. Thus, research into nanoparticle properties will continue to play a crucial role in the future of drug delivery.

Augmented ocular uptake and anti-inflammatory efficacy of decorated Genistein-loaded NLCs incorporated in in situ gel

Genistein (Gen); a naturally occurring isoflavone, acts as a tyrosine kinase inhibitor and efficiently downregulates inflammatory cytokines, which are pivotal in eye inflammation. Also, Gen suffers from sparse ocular bioavailability due to poor solubility. In this work, nanostructured lipid carriers (NLCs) were successfully fabricated by using solid (stearic acid and compritol) and liquid (oleic acid) lipids. The optimized Gen-loaded NLCs showed a nanosize range of 140-246 nm, ≥ 98 % entrapment efficiency, and controlled release over 48 h. The ζ-potential of NLCs was increased from -27.3 mV to 25-27.4 mV due to surface modification with chitosan (CS) or eudragit RS100 (ERS 100). All NLCs showed prominent biocompatibility with enhanced cellular uptake on corneal stromal fibroblasts. Moreover, the different NLCs were incorporated into a mucoadhesive in situ gel. The optimized in situ gel (G9), containing 20 % poloxamers and 0.5 % hydroxyethyl cellulose, exhibited excellent gelling ability within 10.5 s, gelling temperature at 33.1 ± 0.6 ℃, spreadability diameter of 4.73 ± 0.12 cm, shear-thinning behavior, and 20 min ex vivo mucoadhesion time with drug release for 120 h. The in vivo results showed distinguished permeation and distribution potential for ocular delivery. In vivo anti-inflammatory effects after 3 days of treatment with CS-Gen-NLCs/G9 and ERS-Gen-NLCs/G9 revealed a downregulation of interleukin-6 levels in the cornea and retina compared to the untreated group. Our research highlights the promising anti-inflammatory potential of ERS-Gen-NLCs/G9 as an efficient, non-irritant Gen nanodelivery system for managing anterior and posterior ocular inflammation.

Fostering the unleashing potential of nanocarriers-mediated delivery of ocular therapeutics

Ocular delivery is the most challenging aspect in the field of pharmaceutical research. The major hurdle for the controlled delivery of drugs to the eye includes the physiological static barriers such as the complex layers of the cornea, sclera and retina which restrict the drug from permeating into the anterior and posterior segments of the eye. Recent years have witnessed inventions in the field of conventional and nanocarrier drug delivery which have shown considerable enhancement in delivering small to large molecules across the eye. The dynamic challenges associated with conventional systems include limited drug contact time and inadequate ocular bioavailability resulting from solution drainage, tear turnover, and dilution or lacrimation. To this end, various bioactive-based nanosized carriers including liposomes, ethosomes, niosomes, dendrimer, nanogel, nanofibers, contact lenses, nanoprobes, selenium nanobells, nanosponge, polymeric micelles, silver nanoparticles, and gold nanoparticles among others have been developed to circumvent the limitations associated with the conventional dosage forms. These nanocarriers have been shown to achieve enhanced drug permeation or retention and prolong drug release in the ocular tissue due to their better tissue adherence. The surface charge and the size of nanocarriers (10-1000 nm) are the important key factors to overcome ocular barriers. Various nanocarriers have been shown to deliver active therapeutic molecules including timolol maleate, ampicillin, natamycin, voriconazole, cyclosporine A, dexamethasone, moxifloxacin, and fluconazole among others for the treatment of anterior and posterior eye diseases. Taken together, in a nutshell, this extensive review provides a comprehensive perspective on the numerous facets of ocular drug delivery with a special focus on bioactive nanocarrier-based approaches, including the difficulties and constraints involved in the fabrication of nanocarriers. This also provides the detailed invention, applications, biodistribution and safety-toxicity of nanocarriers-based therapeutcis for the ophthalmic delivery.

Current Insights into Targeting Strategies for the Effective Therapy of Diseases of the Posterior Eye Segment

The eye is one a unique sophisticated human sense organ with a complex anatomical structure. It is encased by variety of protective barriers as responsible for vision. There has been a paradigm shift in the prevalence of several major vision threatening ocular conditions with enhanced reliance on computer-based technologies in our workaday life and work-from-home modalities although aging, pollution, injury, harmful chemicals, lifestyle changes will always remain the root cause. Treating posterior eye diseases is a challenge faced by clinicians worldwide. The clinical use of conventional drug delivery systems for posterior eye targeting is restricted by the ocular barriers. Indeed, for overcoming various ocular barriers for efficient delivery of the therapeutic moiety and prolonged therapeutic effect requires prudent and target-specific approaches. Therefore, for efficient drug delivery to the posterior ocular segment, advancements in the development of sustained release and nanotechnology-based ocular drug delivery systems have gained immense importance. Therapeutic efficacy and patient compliance are of paramount importance in clinical translation of these investigative drug delivery systems. This review provides an insight into the various strategies employed for improving the treatment efficacies of the posterior eye diseases. Various drug delivery systems such as systemic and intraocular injections, implants have demonstrated promising outcomes, along with that they have also exhibited side-effects, limitations and strategies employed to overcome them are discussed in this review. The application of artificial intelligence-based technologies along with an appreciation of disease, delivery systems, and patient-specific outcomes will likely enable more effective therapy for targeting the posterior eye segment.

Bedaquiline-Loaded Solid Lipid Nanoparticles Drug Delivery in the Management of Non-Small-Cell Lung Cancer (NSCLC)

Non-small-cell lung cancer (NSCLC) mortality and new case rates are both on the rise. Most patients have fewer treatment options accessible due to side effects from drugs and the emergence of drug resistance. Bedaquiline (BQ), a drug licensed by the FDA to treat tuberculosis (TB), has demonstrated highly effective anti-cancer properties in the past. However, it is difficult to transport the biological barriers because of their limited solubility in water. Our study developed a UPLC method whose calibration curves showed linearity in the range of 5 ng/mL to 500 ng/mL. The UPLC method was developed with a retention time of 1.42 and high accuracy and precision. Its LOQ and LOD were observed to be 10 ng/mL and 5 ng/mL, respectively, whereas in the formulation, capmul MCM C10, Poloxamer 188, and PL90G were selected as solid lipids, surfactants, and co-surfactants, respectively, in the development of SLN. To combat NSCLC, we developed solid lipid nanoparticles (SLNs) loaded with BQ, whereas BQ suspension is prepared by the trituration method using acacia powder, hydroxypropyl methylcellulose, polyvinyl acrylic acid, and BQ. The developed and optimized BQ-SLN3 has a particle size of 144 nm and a zeta potential of (-) 16.3 mV. whereas BQ-loaded SLN3 has observed entrapment efficiency (EE) and loading capacity (LC) of 92.05% and 13.33%, respectively. Further, BQ-loaded suspension revealed a particle size of 1180 nm, a PDI of 0.25, and a zeta potential of -0.0668. whereas the EE and LC of BQ-loaded suspension were revealed to be 88.89% and 11.43%, respectively. The BQ-SLN3 exhibited insignificant variation in particle size, homogeneous dispersion, zeta potential, EE, and LC and remained stable over 90 days of storage at 25 °C/60% RH, whereas at 40 °C/75% RH, BQ-SLN3 observed significant variation in the above-mentioned parameters and remained unstable over 90 days of storage. Meanwhile, the BQ suspension at both 25 °C (60% RH) and 40 °C (75% RH) was found to be stable up to 90 days. The optimized BQ-SLN3 and BQ-suspension were in vitro gastrointestinally stable at pH 1.2 and 6.8, respectively. The in vitro drug release of BQ-SLN3 showed 98.19% up to 12 h at pH 7.2 whereas BQ suspensions observed only 40% drug release up to 4 h at pH 7.2 and maximum drug release of >99% within 4 h at pH 4.0. The mathematical modeling of BQ-SLN3 followed first-order release kinetics followed by a non-Fickian diffusion mechanism. After 24 to 72 h, the IC50 value of BQ-SLN3 was 3.46-fold lower than that of the BQ suspension, whereas the blank SLN observed cell viability of 98.01% and an IC50 of 120 g/mL at the end of 72 h. The bioavailability and higher biodistribution of BQ-SLN3 in the lung tumor were also shown to be greater than those of the BQ suspension. The effects of BQ-SLN3 on antioxidant enzymes, including MDA, SOD, CAT, GSH, and GR, in the treated group were significantly improved and reached the level nearest to that of the control group of rats over the cancer group of rats and the BQ suspension-treated group of rats. Moreover, the pharmacodynamic activity resulted in greater tumor volume and tumor weight reduction by BQ-SLN3 over the BQ suspension-treated group. As far as we are aware, this is the first research to look at the potential of SLN as a repurposed oral drug delivery, and the results suggest that BQ-loaded SLN3 is a better approach for NSCLC due to its better action potential.

Recent Progress of Solid Lipid Nanoparticles and Nanostructured Lipid Carriers as Ocular Drug Delivery Platforms

Sufficient ocular bioavailability is often considered a challenge by the researchers, due to the complex structure of the eye and its protective physiological mechanisms. In addition, the low viscosity of the eye drops and the resulting short ocular residence time further contribute to the observed low drug concentration at the target site. Therefore, various drug delivery platforms are being developed to enhance ocular bioavailability, provide controlled and sustained drug release, reduce the number of applications, and maximize therapy outcomes. Solid lipid nanoparticles (SLNs) and nanostructured lipid carriers (NLCs) exhibit all these benefits, in addition to being biocompatible, biodegradable, and susceptible to sterilization and scale-up. Furthermore, their successive surface modification contributes to prolonged ocular residence time (by adding cationic compounds), enhanced penetration, and improved performance. The review highlights the salient characteristics of SLNs and NLCs concerning ocular drug delivery, and updates the research progress in this area.

Recent Progress in Chitosan-Based Nanomedicine for Its Ocular Application in Glaucoma

Glaucoma is a degenerative, chronic ocular disease that causes irreversible vision loss. The major symptom of glaucoma is high intraocular pressure, which happens when the flow of aqueous humor between the front and back of the eye is blocked. Glaucoma therapy is challenging because of the low bioavailability of drugs from conventional ocular drug delivery systems such as eye drops, ointments, and gels. The low bioavailability of antiglaucoma agents could be due to the precorneal and corneal barriers as well as the low biopharmaceutical attributes of the drugs. These limitations can be overcome by employing nanoparticulate drug delivery systems. Over the last decade, there has been a lot of interest in chitosan-based nanoparticulate systems to overcome the limitations (such as poor residence time, low corneal permeability, etc.) associated with conventional ocular pharmaceutical products. Therefore, the main aim of the present manuscript is to review the recent research work involving the chitosan-based nanoparticulate system to treat glaucoma. It discusses the significance of the chitosan-based nanoparticulate system, which provides mucoadhesion to improve the residence time of drugs and their ocular bioavailability. Furthermore, different types of chitosan-based nanoparticulate systems are also discussed, namely nanoparticles of chitosan core only, nanoparticles coated with chitosan, and hybrid nanoparticles of chitosan. The manuscript also provides a critical analysis of contemporary research related to the impact of this chitosan-based nanomedicine on the corneal permeability, ocular bioavailability, and therapeutic performance of loaded antiglaucoma agents.

Quercetin Loaded Cationic Solid Lipid Nanoparticles in a Mucoadhesive In Situ Gel-A Novel Intravesical Therapy Tackling Bladder Cancer

The study aim was to develop an intravesical delivery system of quercetin for bladder cancer management in order to improve drug efficacy, attain a controlled release profile and extend the residence time inside the bladder. Either uncoated or chitosan coated quercetin-loaded solid lipid nanoparticles (SLNs) were prepared and evaluated in terms of colloidal, morphological and thermal characteristics. Drug encapsulation efficiency and its release behaviour were assessed. Furthermore, cytotoxicity of SLNs on T-24 cells was evaluated. Ex vivo studies were carried out using bovine bladder mucosa. Spherical SLNs (≈250 nm) ensured good entrapment efficiencies (EE > 97%) and sustained drug release up to 142 h. Cytotoxicity profile revealed concentration-dependent toxicity recording an IC50 in the range of 1.6−8.9 μg/mL quercetin. SLNs were further dispersed in in situ hydrogels comprising poloxamer 407 (20%) with mucoadhesive polymers. In situ gels exhibited acceptable gelation temperatures (around 25 °C) and long erosion time (24−27 h). SLNs loaded gels displayed remarkably enhanced retention on bladder tissues relative to SLNs dispersions. Coated SLNs exhibited better penetration abilities compared to uncoated ones, while coated SLNs dispersed in gel (G10C-St-QCT-SLNs-2) showed the highest penetration up to 350 μm. Hence, G10C-St-QCT-SLNs-2 could be considered as a platform for intravesical quercetin delivery.

Lipid Nanoparticles for the Posterior Eye Segment

This review highlights the application of lipid nanoparticles (Solid Lipid Nanoparticles, Nanostructured Lipid Carriers, or Lipid Drug Conjugates) as effective drug carriers for pathologies affecting the posterior ocular segment. Eye anatomy and the most relevant diseases affecting the posterior segment will be summarized. Moreover, preparation methods and different types and subtypes of lipid nanoparticles will also be reviewed. Lipid nanoparticles used as carriers to deliver drugs to the posterior eye segment as well as their administration routes, pharmaceutical forms and ocular distribution will be discussed emphasizing the different targeting strategies most recently employed for ocular drug delivery.

Toxoplasma gondii Proteasome Subunit Alpha Type 1 with Chitosan: A Promising Alternative to Traditional Adjuvant

As an important zoonotic protozoan, Toxoplasma gondii (T. gondii) has spread around the world, leading to infections in one-third of the population. There is still no effective vaccine or medicine against T. gondii, and recombinant antigens entrapped within nanospheres have benefits over traditional vaccines. In the present study, we first expressed and purified T. gondii proteasome subunit alpha type 1 (TgPSA1), then encapsulated the recombinant TgPSA1 (rTgPSA1) in chitosan nanospheres (CS nanospheres, rTgPSA1/CS nanospheres) and incomplete Freund’s adjuvant (IFA, rTgPSA1/IFA emulsion). Antigens entrapped in CS nanospheres reached an encapsulation efficiency of 67.39%, and rTgPSA1/CS nanospheres showed a more stable release profile compared to rTgPSA1/IFA emulsion in vitro. In vivo, Th1-biased cellular and humoral immune responses were induced in mice and chickens immunized with rTgPSA1/CS nanospheres and rTgPSA1/IFA emulsion, accompanied by promoted production of antibodies, IFN-γ, IL-4, and IL-17, and modulated production of IL-10. Immunization with rTgPSA1/CS nanospheres and rTgPSA1/IFA emulsion conferred significant protection, with prolonged survival time in mice and significantly decreased parasite burden in chickens. Furthermore, our results also indicate that rTgPSA1/CS nanospheres could be used as a substitute for rTgPSA1/IFA emulsion, with the optimal administration route being intramuscular in mass vaccination. Collectively, the results of this study indicate that rTgPSA1/CS nanospheres represent a promising vaccine to protect animals against acute toxoplasmosis.

Comparative Study of Specific Loss Power and Transverse Relaxivity of Spinel Ferrite Nanoensembles Coated With Chitosan and Polyethylene Glycol

We synthesized spinel ferrite nanoensembles (MnFe2O4, CoFe2O4, and Fe3O4) using the chemical co-precipitation method and characterized their physical, chemical, and magnetic properties by X-ray diffraction (XRD), transmission electron microscopy (TEM), physical properties measurement system (PPMS), Mössbauer spectroscopy, Fourier transform infrared spectroscopy (FTIR), dynamic light scattering (DLS) and Raman spectroscopy. Their relaxation properties and potential for hyperthermia therapy were determined using nuclear magnetic resonance (NMR) and cell viability assay, respectively. XRD and TEM data confirmed that the particle core sizes were 6–9 nm before coating while their sizes increased to 10–14 nm and 14–20 nm after coating with chitosan and polyethylene glycol (PEG), respectively. Mössbauer spectroscopy showed superparamagnetic behavior for MnFe2O4 nanoparticles and ferrimagnetic behavior for the CoFe2O4 and Fe3O4 nanoparticles. A detailed studies of MH loops of all three ferrites before and after coating showed surface functionalization by a large reduction of coercivity and anisotropy. The successful coating was further confirmed by the peak shifts in the FTIR spectra of the particles whereas Raman spectra of coated ferrites also displayed the characteristic absorption patterns and suppression of the ferrite peaks suggesting successful coating. The induced heating profile of the nanoparticles in stable suspension was tested with a radio frequency magnetic field of 76 mT and a frequency of 400 kHz. High mortality (>98%) of 9 L gliosarcoma cancer cells by hyperthermia suggested that these nanoparticles could be used for cancer therapy. Transverse relaxivities (r2) determined by NMR for chitosan-coated MnFe2O4, CoFe2O4, and Fe3O4 nanoparticles were 297 (±22), 353 (±26), and 345 (±13), mM−1S−1, while for PEG-coated nanoparticles are 165 (±22), 146 (±14), and 159 (±07) mM−1S−1, respectively. Overall these spinel ferrite nanoensembles show great promise for cancer theranostics research applications.

Lipid-Based Nanocarriers for Ophthalmic Administration: Towards Experimental Design Implementation

Nanotherapeutics based on biocompatible lipid matrices allow for enhanced solubility of poorly soluble compounds in the treatment of ophthalmic diseases, overcoming the anatomical and physiological barriers present in the eye, which, despite the ease of access, remains strongly protected. Micro-/nanoemulsions, solid lipid nanoparticles (SLN) or nanostructured lipid carriers (NLC) combine liquid and/or solid lipids with surfactants, improving drug stability and ocular bioavailability. Current research and development approaches based on try-and-error methodologies are unable to easily fine-tune nanoparticle populations in order to overcome the numerous constraints of ocular administration routes, which is believed to hamper easy approval from regulatory agencies for these systems. The predictable quality and specifications of the product can be achieved through quality-by-design (QbD) implementation in both research and industrial environments, in contrast to the current quality-by-testing (QbT) framework. Mathematical modelling of the expected final nanoparticle characteristics by variation of operator-controllable variables of the process can be achieved through adequate statistical design-of-experiments (DoE) application. This multivariate approach allows for optimisation of drug delivery platforms, reducing research costs and time, while maximising the understanding of the production process. This review aims to highlight the latest efforts in implementing the design of experiments to produce optimised lipid-based nanocarriers intended for ophthalmic administration. A useful background and an overview of the different possible approaches are presented, serving as a starting point to introduce the design of experiments in current nanoparticle research.

Unconventional Constituents and Shared Molecular Architecture of the Melanized Cell Wall of C. neoformans and Spore Wall of S. cerevisiae

The fungal cell wall serves as the interface between the cell and the environment. Fungal cell walls are composed largely of polysaccharides, primarily glucans and chitin, though in many fungi stress-resistant cell types elaborate additional cell wall structures. Here, we use solid-state nuclear magnetic resonance spectroscopy to compare the architecture of cell wall fractions isolated from Saccharomyces cerevisiae spores and Cryptococcus neoformans melanized cells. The specialized cell walls of these two divergent fungi are highly similar in composition. Both use chitosan, the deacetylated derivative of chitin, as a scaffold on which a polyaromatic polymer, dityrosine and melanin, respectively, is assembled. Additionally, we demonstrate that a previously identified but uncharacterized component of the S. cerevisiae spore wall is composed of triglycerides, which are also present in the C. neoformans melanized cell wall. Moreover, we identify a tyrosine-derived constituent in the C. neoformans wall that, although it is not dityrosine, is a non-pigment constituent of the cell wall. The similar composition of the walls of these two phylogenetically distant species suggests that triglycerides, polyaromatics, and chitosan are basic building blocks used to assemble highly stress-resistant cell walls and the use of these constituents may be broadly conserved in other fungal species.

PEGylated Lipid Polymeric Nanoparticle-Encapsulated Acyclovir for In Vitro Controlled Release and Ex Vivo Gut Sac Permeation

Currently, pharmaceutical research is directed wide range for developing new drugs for oral administration to target disease. Acyclovir formulation is having common issues of short half-life and poor permeability, causing messy treatment which results in patient incompliance. The present study formulates a lipid polymeric hybrid nanoparticles for antiviral acyclovir (ACV) agent with Phospholipon® 90G (lecithin), chitosan, and polyethylene glycol (PEG) to improve controlled release of the drugs. The study focused on the encapsulation of the ACV in lipid polymeric particle and their sustained delivery. The formulation developed for the self-assembly of chitosan and lecithin to form a shell encapsulating acyclovir, followed by PEGylation. Optimisation was performed via Box-Behnken Design (BBD), forming nanoparticles with size of 187.7 ± 3.75 nm, 83.81 ± 1.93% drug-entrapped efficiency (EE), and + 37.7 ± 1.16 mV zeta potential. Scanning electron microscopy and transmission electron microscopy images displayed spherical nanoparticles formation. Encapsulation of ACV and complexity with other physical parameters are confirmed through analysis using Fourier transform infrared spectroscopy, differential scanning calorimetry, and X-ray diffraction. Nanoparticle produced was capable of achieving 24-h sustained release in vitro on gastric and intestinal environments. Ex vivo study proved the improvement of acyclovir's apparent permeability from 2 × 10-6 to 6.46 × 10-6 cm s-1. Acyclovir new formulation was achieved to be stable up to 60 days for controlled release of the drugs. Graphical abstract.

Chitosan and its Derivatives for Ocular Delivery Formulations: Recent Advances and Developments

Chitosan (CS) is a hemi-synthetic cationic linear polysaccharide produced by the deacetylation of chitin. CS is non-toxic, highly biocompatible, and biodegradable, and it has a low immunogenicity. Additionally, CS has inherent antibacterial properties and a mucoadhesive character and can disrupt epithelial tight junctions, thus acting as a permeability enhancer. As such, CS and its derivatives are well-suited for the challenging field of ocular drug delivery. In the present review article, we will discuss the properties of CS that contribute to its successful application in ocular delivery before reviewing the latest advances in the use of CS for the development of novel ophthalmic delivery systems. Colloidal nanocarriers (nanoparticles, micelles, liposomes) will be presented, followed by CS gels and lenses and ocular inserts. Finally, instances of CS coatings, aiming at conferring mucoadhesiveness to other matrixes, will be presented.

Biofabrication of Chitosan-Based Nanomedicines and Its Potential Use for Translational Ophthalmic Applications

Drug delivery to the anterior and posterior segment of eye remains a challenge. Nanoparticle-mediated drug delivery has indicated some promise. The presented review aims to summarize recent advancements in chitosan-based nanotherapies for ocular drug delivery and the challenges encountered during the process. Significant research using chitosan, a cationic linear polymer, is being conducted for ocular drug delivery. A vast number of publications exploit the mucoadhesive properties of the polymer, which arise due to interactions between the amino acids of chitosan and the sialic acid residues in mucous. The high degree of crosslinking in chitosan nanoparticles facilitates a dramatic increase in ocular drug retention of the desired drug, which subsequently helps in ocular penetration and improving the bioavailability of the drugs. A noted decrease in the initial burst of the drug is the basis for developing sustained drug release formulation using biodegradable and biocompatible chitosan polymer. In vitro as well as in vivo studies have indicated enhancement in the uptake, accumulation, and removal of chitosan nanoparticles from the site of delivery. In summary, chitosan- or modified-chitosan-based nanoparticles are being widely tested as drug carriers for treatment of bacterial and viral infections, glaucoma, age-related macular degeneration, and diabetic retinopathy.

Advances and limitations of drug delivery systems formulated as eye drops

Topical instillation of eye drops remains the most common and easiest route of ocular drug administration, representing the treatment of choice for many ocular diseases. Nevertheless, low ocular bioavailability of topically applied drug molecules can considerably limit their efficacy. Over the last several decades, numerous drug delivery systems (DDS) have been developed in order to improve drug bioavailability on the ocular surfaces. This review systematically covers the most recent advances of DDS applicable by topical instillation, that have shown better performance in in vivo models compared to standard eye drop formulations. These delivery systems are based on in situ forming gels, nanoparticles and combinations of both. Most of the DDS have been developed using natural or synthetic polymers. Polymers offer many advantageous properties for designing advanced DDS including biocompatibility, gelation properties and/or mucoadhesiveness. However, despite the high number of studies published over the last decade, there are several limitations for clinical translation of DDS. This review article focuses on the recent advances for the development of ocular drug delivery systems. In addtion, the potential challenges for commercialization of new DDS are presented.