Topical drug delivery to the posterior segment of the eye: Thermodynamic considerations

A look to the future: cyclodextrins and cyclodextrin-based drug delivery to the retina

INTRODUCTION

Retinal diseases are a leading cause of vision loss, affecting millions of people worldwide. Current treatment options are based on invasive methods such as intravitreal injections. Therefore, there is a need for alternative therapeutic strategies that are both effective and more patient-friendly.

AREAS COVERED

Topical drug delivery has gained attention as a preferred noninvasive approach, although it is hindered by several ocular barriers. Cyclodextrin (CD)-based nanoparticles have emerged as a promising strategy to overcome these limitations by enhancing drug permeability in the posterior segment of the eye. This review discusses the potential of CDs as enabling pharmaceutical excipients, their role in improving ocular drug bioavailability, and provides examples of CD-based eye drop formulations currently under development or undergoing clinical trials. Also, the role of CDs as active pharmaceutical agents in ophthalmology is discussed.

EXPERT OPINION

CD-based nanoparticle eye drops present a promising solution and have shown clinical success. CDs are approved pharmaceutical excipients for eye drop formulations and can act as active pharmaceutical ingredients for the treatment of inherent retinal diseases. Future innovations in hybrid CD-based delivery systems and integration of novel therapeutic compounds could provide more efficient and targeted treatment options for retinal diseases.

Formulation, characterization, and in vitro release of topical nanoemulsion containing prednisolone-derived corticosteroid

BACKGROUND

Prednisolone-derived corticosteroid (PDC) has anti-inflammatory activity in ocular administration. However, drug administration to the eye is extremely difficult due to the complex structure of the eye. Because of the ability of the eye to retain the drug and its physiology, the bioavailability of drugs applied to the eye is very low.

OBJECTIVE

One of the methods to overcome bioavailability problem is to formulate the drug as a nanoemulsion (NE). NEs are thermodynamically stable, colloidal drug delivery systems. They have small globule size and high surface area. These properties give them the ability to cross the biological membrane and increase the therapeutic efficacy of the drug molecule.

METHODOLOGY

The high energy method was used to create an NE eye drop formulation containing PDC, and the effects of changing homogenization processes on NE formation were investigated. After deciding on the optimum formulation; characterization, assay, and in vitro release studies were performed, and the stability of the formulation was followed for 12 months.

RESULTS

The optimum formulation selected initially had 126.6 ± 40.12 nm and 99.9 ± 1.2% PDC, it had 125.4 ± 41.20 nm and 99.29 ± 1.3% PDC after 12 months in 25 °C 40% RH conditions. Cytotoxicity studies have shown no significant cytotoxic effects in NE-containing PDC.

CONCLUSION

The preparation and optimization of topical NE formulations containing PDC for ocular inflammation treatment were achieved. The developed formulation was stable for 12 months and no toxic effect was found in cell culture studies. This formulation could be useful as an alternative to PDC for ocular applications.

Topical delivery performance of Pickering emulsions stabilized by differently charged spirulina protein isolate/Chitosan composite particles

Pickering emulsions, stabilized by particulate particles, have emerged as a promising vehicle for topical delivery. Herein, Pickering emulsions stabilized by differently charged spirulina protein isolate - chitosan (SC) composite particles were studied for effective topical delivery of α-Bisabolol (ABS). The composite particles were synthesized via electrostatic assembly of spirulina protein isolate (SPI) and chitosan (CS), and their surface charge was assessed using zeta potential measurements. The Pickering emulsions stabilized by SC composite particles with different charges were all stable over 30 days and had a high encapsulation efficiency for ABS. In vitro skin permeation study revealed that positively charged emulsions significantly increased ABS retention within the skin, predominantly in the stratum corneum layer. The underlying delivery mechanism was further explored using attenuated total reflection Fourier transform infrared spectroscopy. Lastly, the influence of particle concentration and oil phase volume fraction on the topical delivery efficiency was conducted to optimize the Pickering formulations. This study provides insight into the role of particle charge in enhancing topical delivery of Pickering emulsions.

Recent advances and strategies for nanocarrier-mediated topical therapy and theranostic for posterior eye disease

Posterior eye disorders, such as age-related macular degeneration, diabetic retinopathy, and glaucoma, have a significant impact on human quality of life and are the primary cause of age-related retinal diseases among adults. There is a pressing need for innovative topical approaches to treat posterior eye disorders, as current methods often rely on invasive procedures with inherent risks. Limited success was attained in the realm of topical ophthalmic delivery through non-invasive means. Additionally, there exists a dearth of literature that delves into the potential of this approach for drug delivery and theranostic purposes, or that offers comprehensive design strategies for nanocarrier developers to surmount the significant physiological ocular barriers. This review offers a thorough and up-to-date state-of-the-art overview of 40 studies on therapeutic loaded nanocarriers and theranostic devices that, to the best of our knowledge, represent all successful works that reached posterior eye segments through a topical non-invasive administration. Most importantly, based on the successful literature studies, this review provides a comprehensive summary of the potential design strategies that can be implemented during nanocarrier development to overcome each ocular barrier.

Drug-like properties of tyrosine kinase inhibitors in ophthalmology: Formulation and topical availability

Tyrosine kinase inhibitors (TKIs) can inhibit edema and neovascularization, such as in age-related macular degeneration and diabetic retinopathy. However, their topical administration in ophthalmology is limited by their toxicity and poor aqueous solubility. There are multiple types of TKIs, and each TKI has an affinity to more than one type of receptor. Studies have shown that ocular toxicity can be addressed by selecting TKIs that have a high affinity for specific vascular endothelial growth factor receptors (VEGFRs) but a low affinity for epidermal growth factor receptors (EGFRs). Drugs permeate from the aqueous tear fluid into the eye via passive diffusion. Thus, a sustained high concentration of the dissolved drug in the aqueous tear fluid is essential for a successful delivery to posterior tissues such as the retina. Unfortunately, the aqueous solubility of the TKIs that have the most favorable VEGFR/EGFR affinity ratio, that is, axitinib and cabozantinib, is well below 1 µg/mL, making their topical delivery very challenging. This is a review of the drug-like properties of TKIs that are currently being evaluated or have been evaluated as ophthalmic drugs. These properties include their solubilization, cyclodextrin complexation, and ability to permeate from the aqueous tear fluid to the posterior eye segment.

Polymeric microneedles for the eye: An overview of advances and ocular applications for minimally invasive drug delivery

Ocular drug delivery is challenging due to the presence of tissue barriers and clearance mechanisms. Most widely used topical formulations need frequent application because of poor permeability, short retention, and low bioavailability. Invasive intraocular injections and implants that deliver drugs at the target site are associated with infections, inflammation, and even vision loss post-use. These gaps can be addressed by a delivery platform that can efficiently deliver drug with minimal tissue damage. Microneedles were introduced as a delivery platform for overcoming dermal barriers with minimal tissue damage. After the successful clinical transition of microneedles in the transdermal drug delivery, they are now being extensively studied for ocular applications. The attributes of minimally invasive application and the capability to deliver a wide range of therapeutics make microneedles an attractive candidate for ocular drug delivery. The current manuscript provides a detailed overview of the recent advancements in the field of microneedles for ocular use. This paper reviews research focusing on polymeric microneedles and their pharmaceutical and biopharmaceutical properties. A brief discussion about their clinical translation and regulatory concerns is also covered. The multitude of research articles supports the fact that microneedles are a potential, minimally invasive drug delivery platform for ophthalmic use.

Discovery and Potential Utility of a Novel Non-Invasive Ocular Delivery Platform

To this day, the use of oily eye drops and non-invasive retinal delivery remain a major challenge. Oily eye drops usually cause ocular irritation and interfere with the normal functioning of the eye, while ocular injections for retinal drug delivery cause significant adverse effects and a high burden on the healthcare system. Here, the authors report a novel topical non-invasive ocular delivery platform (NIODP) through the periorbital skin for high-efficiency anterior and posterior ocular delivery in a non-human primate model (NHP). A single dose of about 7 mg JV-MD2 (omega 3 DHA) was delivered via the NIODP and reached the retina at a Cmax of 111 µg/g and the cornea at a Cmax of 66 µg/g. The NIODP also delivered JV-DE1, an anti-inflammatory agent in development for dry eye diseases, as efficiently as eye drops did to the anterior segments of the NHP. The topical NIODP seems to transport drug candidates through the corneal pathway to the anterior and via the conjunctiva/sclera pathway to the posterior segments of the eye. The novel NIODP method has the potential to reshape the landscape of ocular drug delivery. This is especially the case for oily eye drops and retinal delivery, where the success of the treatment lies in the ocular tolerability and bioavailability of drugs in the target tissue.

Impact of Apparatus and Adapter on In vitro Drug Release of Ophthalmic Semisolid Drug Products

PURPOSE

In vitro release testing (IVRT) is a widely used tool for evaluating the quality and performance of drug products. However, standardized sample adaptors or drug release apparatus setups for IVRT studies are still lacking for ophthalmic ointments. The aim of this study was to provide a better understanding of the impact of apparatus and sample adaptor setups on IVRT of ophthalmic ointments.

METHODS

Dexamethasone (DEX), a steroidal ingredient commonly used in ophthalmic drug products, was selected as a model drug. Ointments were prepared by mixing DEX in white petrolatum using a high shear mixer. A novel two-sided adapter was developed to increase the drug release surface area. DEX ointment was placed in one-sided or two-sided release adaptors coupled with 1.2 μm polyethersulfone membrane, and the drug release was studied in different USP apparatuses (I, II, and IV).

RESULTS

The sample adaptor setups had a minimal impact on cumulative drug release amount per area or release rate while USP IV apparatus with agitated flow enhanced drug release rates. The USP apparatus I with a two-sided semisolid adapter, which uses membranes on both sides, showed dramatically higher cumulative drug release and discriminative release profiles when evaluating ophthalmic formulations.

CONCLUSIONS

USP apparatuses and sample adaptors are critical considerations for IVRT. Two-sided semisolid adapter provides higher cumulative release, facilitating the discrimination between low drug content ophthalmic ointment formulations with good sensitivity and repeatability without affecting the drug release rate.

Controlled Drug Delivery Device for Cornea Treatment and Novel Method for Its Testing

A new solution for local anesthetic and antibiotic delivery after eye surgery is presented. A contact lens-shaped collagen drug carrier was created and loaded by Levofloxacin and Tetracaine with a riboflavin crosslinked surface layer, thus impeding diffusion. The crosslinking was confirmed by Raman spectroscopy, whereas the drug release was investigated using UV-Vis spectrometry. Due to the surface barrier, the drug gradually releases into the corneal tissue. To test the function of the carrier, a 3D printed device and a new test method for a controlled drug release, which mimics the geometry and physiological lacrimation rate of the human eye, were developed. The experimental setup with simple geometry revealed that the prepared drug delivery device can provide the prolonged release profile of the pseudo-first-order for up to 72 h. The efficiency of the drug delivery was further demonstrated using a dead porcine cornea as a drug recipient, without the need to use live animals for testing. Our drug delivery system significantly surpasses the efficiency of antibiotic and anesthetic eyedrops that would have to be applied approximately 30 times per hour to achieve the same dose as that delivered continuously by our device.

Implantable sustained-release drug delivery systems: a revolution for ocular therapeutics

PURPOSE

Due to the inimitable anatomical structure of the eyeball and various physiological barriers, conventional ocular local administration is often complicated by apparent shortcomings, such as limited bioavailability and short drug retention. Thus, developing methods for sustainable, safe and efficient drug delivery to ocular target sites has long been an urgent need. This study briefly summarizes the barriers to ocular drug administration and various ocular drug delivery routes and highlights recent progress in ocular implantable sustained-release drug delivery systems (DDSs) to provide literature evidence for developing novel ocular implants for sustained drug delivery.

METHODS

We conducted a comprehensive search of studies on ocular implantable sustained-release DDSs in PubMed and Web of Science using the following keywords: ocular, implantable and drug delivery system. More than 400 papers were extracted. Publications focused on sustained and controlled drug release were primarily considered. Experimental articles involving DDSs that cannot be implanted into the eye through surgeries and cannot be inserted into ocular tissues in solid form were excluded. Approximately 143 publications were reviewed to summarize the most current information on the subject.

RESULTS

In recent years, numerous ocular sustained-release DDSs using lipids, nanoparticles and hydrogels as carriers have emerged. With unique properties and systematic design, ocular implantable sustained-release DDSs are able to continuously maintain drug release, effectively sustain the therapeutic concentration in target tissues, and substantially enhance the therapeutic efficacy. Nevertheless, few ocular implantable sustained-release DDSs have been available in clinical use.

CONCLUSIONS

Ocular implantable sustained-release DDSs have become a new focus in the field of ocular drug development through unique designs and improvements in the materials of drug carriers, administration methods and dosage forms. With more ocular implantable sustained-release DDSs being commercialized, ocular therapeutics may be revolutionized.

Cellulose Acetate Phthalate-Based pH-Responsive Cyclosporine A-Loaded Contact Lens for the Treatment of Dry Eye

Cyclosporine A (CsA) as an eye drop is an effective treatment for dry eye. However, it has potential side effects and a short ocular residence time. To overcome these obstacles, we developed a cellulose acetate phthalate-based pH-responsive contact lens (CL) loaded with CsA (CsA-CL). The CsA was continuously released from the CsA-CL at physiological conditions (37 °C, pH 7.4) without an initial burst. CsA was well-contained in the selected storage condition (4 °C, pH 5.4) for as long as 90 days. In safety assays, cytotoxicity, ocular irritation, visible light transmittance, and oxygen permeability were in a normal range. CsA concentrations in the conjunctiva, cornea, and lens increased over time until 12 h. When comparing the therapeutic efficacy between the normal control, experimental dry eye (EDE), and treatment groups (CsA eye drop, naïve CL, and CsA-CL groups), the tear volume, TBUT, corneal fluorescein staining at 7 and 14 days, conjunctival goblet cell density, and corneal apoptotic cell counts at 14 days improved in all treatment groups compared to EDE, with a significantly better result in the CsA-CL group compared with other groups (all p < 0.05). The CsA-CL could be an effective, stable, and safe option for inflammatory dry eye.

Aqueous Prostaglandin Eye Drop Formulations

Glaucoma is one of the leading causes of irreversible blindness worldwide. It is characterized by progressive optic neuropathy in association with damage to the optic nerve head and, subsequently, visual loss if it is left untreated. Among the drug classes used for the long-term treatment of open-angle glaucoma, prostaglandin analogues (PGAs) are the first-line treatment and are available as marketed eye drop formulations for intraocular pressure (IOP) reduction by increasing the trabecular and uveoscleral outflow. PGAs have low aqueous solubility and are very unstable (i.e., hydrolysis) in aqueous solutions, which may hamper their ocular bioavailability and decrease their chemical stability. Additionally, treatment with PGA in conventional eye drops is associated with adverse effects, such as conjunctival hyperemia and trichiasis. It has been a very challenging for formulation scientists to develop stable aqueous eye drop formulations that increase the PGAs' solubility and enhance their therapeutic efficacy while simultaneously lowering their ocular side effects. Here the physiochemical properties and chemical stabilities of the commercially available PGAs are reviewed, and the compositions of their eye drop formulations are discussed. Furthermore, the novel PGA formulations for glaucoma treatment are reviewed.

Drug Carriers: A Review on the Most Used Mathematical Models for Drug Release

Carriers are protective transporters of drugs to target cells, facilitating therapy under each points of view, such as fast healing, reducing infective phenomena, and curing illnesses while avoiding side effects. Over the last 60 years, several scientists have studied drug carrier properties, trying to adapt them to the release environment. Drug/Carrier interaction phenomena have been deeply studied, and the release kinetics have been modeled according to the occurring phenomena involved in the system. It is not easy to define models’ advantages and disadvantages, since each of them may fit in a specific situation, considering material interactions, diffusion and erosion phenomena, and, no less important, the behavior of receiving medium. This work represents a critical review on main mathematical models concerning their dependency on physical, chemical, empirical, or semi-empirical variables. A quantitative representation of release profiles has been shown for the most representative models. A final critical comment on the applicability of these models has been presented at the end. A mathematical approach to this topic may help students and researchers approach the wide panorama of models that exist in literature and have been optimized over time. This models list could be of practical inspiration for the development of researchers’ own new models or for the application of proper modifications, with the introduction of new variable dependency.

Aqueous eye drops containing drug/cyclodextrin nanoparticles deliver therapeutic drug concentrations to both anterior and posterior segment

Using topical application to deliver therapeutic concentrations of drugs to the posterior segment of the eye remains very challenging. As a result, posterior segment diseases are usually treated by intravitreal injection or implant. While topical treatments are commonly used for anterior segment conditions, they sometimes require frequent applications. Eye drop formulations based on γ-cyclodextrin (γCD)-based nanoparticle aggregates were developed, which in animal models and clinical studies deliver therapeutic concentrations of drugs (dorzolamide and dexamethasone) to both anterior and posterior segments of the eye. An early study in humans showed dorzolamide/γCD eye drops could achieve comparable intraocular pressure decreases to commercial dorzolamide eye drops, but with less frequent application. Pilot studies with dexamethasone/γCD eye drops suggested that they could be effective in a range of conditions, including diabetic macular oedema, cystoid macular oedema and vitritis secondary to uveitis, postcataract surgery inflammation and postoperative treatment in trabeculectomy. Phase II studies with similar dexamethasone/γCD nanoparticle eye drops in diabetic macular oedema and postcataract surgery inflammation have recently been completed. This technology has the potential to be used with other classes of drug molecules and to replace or complement invasive treatments, providing safer, non-invasive therapies, particularly for posterior segment conditions, that can be self-administered as eye drops by patients.

Topical drug delivery to the retina: obstacles and routes to success

INTRODUCTION

Retinal diseases are one of the main reasons for vision loss where all available drug treatments are based on invasive drug administration such as intravitreal injections. Despite huge efforts and some promising results in animal models, almost all delivery technologies tested have failed in human trials. There are however examples of clinically effective topical delivery systems such as fast dissolving aqueous eye drop suspensions.

AREAS COVERED

Six obstacles to topical drug delivery to the eye have been identified and discussed in some details. These obstacles consist of static membrane barriers to drug permeation into the eye, dynamic barriers such as the lacrimal drainage and physiochemical barriers such as low thermodynamic activity. It is explained how and why these obstacles hamper drug permeation and how different technologies, both those that are applied in marketed drug products and those that are under investigation, have addressed these obstacles.

EXPERT OPINION

The reason that most topical drug delivery systems have failed to deliver therapeutic drug concentrations to the retina is that they do not address physiochemical barriers such as the thermodynamic activity of the permeating drug molecules. Topical drug delivery to the retina has only been successful when the static, dynamic, and physiochemical barriers are addressed simultaneously.

Cyclodextrin-based formulation of carbonic anhydrase inhibitors for ocular delivery - A review

Carbonic anhydrase inhibitors (CAIs) are used as systemic and topical agents for lowering intraocular pressure (IOP) in patients with glaucoma. Owing to the wide distribution of CAs and their physiological functions in various tissues, systemic administration of CAIs may lead to unwanted side effects. Thus, exploration of drugs targeting the specific CA isoenzyme in ocular tissues and application of the same as topical eye drops would be desirable. However, the anatomical and physiological barriers of the eyes can limit drug availability at the site. The very low aqueous solubility of CAI agents can further hamper drug bioavailability, consequently resulting in insufficient therapeutic efficacy. Solubilization of drugs using cyclodextrin (CD) complexes can enhance both solubility and permeability of the drugs. The use of CD for such purposes and development and testing of topical CAI eye drops containing CD have been discussed in detail. Further, pharmaceutical nanotechnology platforms were discussed in terms of investigation of their IOP-lowering efficacies. Future prospects in drug discovery and the use of CD nanoparticles and CD-based nanocarriers to develop potential topical CAI formulations have also been described here.

Intellective and stimuli-responsive drug delivery systems in eyes

Stimuli-responsive drug delivery systems have attracted widespread attention in recent years since they can control drug release in a spatiotemporal manner and can achieve tunable drug release according to patient's physiological or pathological condition. In this review, we briefly introduce the drug delivery barriers and drug delivery systems in the anterior and posterior segment of eyes, and collect the recent advances in stimuli-responsive drug delivery systems in eyes for controlled drug release in response to exogenous stimuli (ultrasound, magnetic stimulus, electrical stimulus, and light) or endogenous stimuli (enzyme, active oxygen species, temperature, ions, and pH). In addition, the design and mechanisms of the stimuli-responsive drug delivery systems have been summarized in this review, and the advantages and limitations are also briefly discussed.

Bio-Distribution and Pharmacokinetics of Topically Administered γ-Cyclodextrin Based Eye Drops in Rabbits

The purpose of this study was to evaluate the ocular pharmacokinetics, bio-distribution and local tolerability of γ-cyclodextrin (γCD) based irbesartan 1.5% eye drops and candesartan 0.15% eye drops after single and multiple topical administration in rabbit eyes. In this randomized, controlled study, a total number of 59 New Zealand White albino rabbits were consecutively assigned to two study groups. Group 1 (n = 31) received irbesartan 1.5% and group 2 (n = 28) candesartan 0.15% eye drops. In both groups, single dose and multiple administration pharmacokinetic studies were performed. Rabbits were euthanized at five predefined time points after single-dose administration, whereas multiple-dose animals were dosed for 5 days twice-daily and then euthanized 1 h after the last dose administration. Drug concentration was measured by using liquid chromatography-tandem mass spectrometry (LC-MS/MS) in the retinal tissue, vitreous humor, aqueous humor, corneal tissue and in venous blood samples. Pharmacokinetic parameters including maximal drug concentration (C_max), time of maximal drug concentration (T_max), half-life and AUC were calculated. To assess local tolerability, six additional rabbits received 1.5% irbesartan eye drops twice daily in one eye for 28 days. Tolerability was assessed using a modified Draize test and corneal sensibility by Cochet Bonnet esthesiometry. Both γCD based eye drops were rapidly absorbed and distributed in the anterior and posterior ocular tissues. Within 0.5 h after single administration, the C_max of irbesartan and candesartan in retinal tissue was 251 ± 142 ng/g and 63 ± 39 ng/g, respectively. In the vitreous humor, a C_max of 14 ± 16 ng/g for irbesartan was reached 0.5 h after instillation while C_max was below 2 ng/g for candesartan. For multiple dosing, the observed C_mean in retinal tissue was 338 ± 124 ng/g for irbesartan and 36 ± 10 ng/g for candesartan, whereas mean vitreous humor concentrations were 13 ± 5 ng/g and <2 ng/g, respectively. The highest plasma concentrations of both irbesartan (C_max 5.64 ± 4.08 ng/mL) and candesartan (C_max 4.32 ± 1.04 ng/mL) were reached 0.5 h (T_max) after single administration. Local tolerability was favorable with no remarkable differences between the treated and the control eyes. These results indicate that irbesartan and candesartan in γCD based nanoparticle eye drops can be delivered to the retinal tissue of the rabbit’s eye in pharmacologically relevant concentrations. Moreover, safety and tolerability profiles appear to be favorable in the rabbit animal model.