Elsevier

Journal of Controlled Release

Volume 211, 10 August 2015, Pages 105-117
Journal of Controlled Release

Novel biodegradable polyesteramide microspheres for controlled drug delivery in Ophthalmology

https://doi.org/10.1016/j.jconrel.2015.05.279Get rights and content

Abstract

Most of the posterior segment diseases are chronic and multifactorial and require long-term intraocular medication. Conventional treatments of these pathologies consist of successive intraocular injections, which are associated with adverse effects. Successful therapy requires the development of new drug delivery systems able to release the active substance for a long term with a single administration. The present work involves the description of a new generation of microspheres based on poly(ester amide)s (PEA), which are novel polymers with improved biodegradability, processability and good thermal and mechanical properties. We report on the preparation of the PEA polymer, PEA microspheres (PEA Ms) and their characterization. PEA Ms (~ 15 μm) were loaded with a lipophilic drug (dexamethasone) (181.0 ± 2.4 μg DX/mg Ms). The in vitro release profile of the drug showed a constant delivery for at least 90 days. Based on the data from a performed in vitro release study, a kinetic ocular model to predict in vivo drug concentrations in a rabbit vitreous was built. According to the pharmacokinetic simulations, intravitreal injection of dexamethasone loaded PEA microspheres would provide release of the drug in rabbit eyes up to 3 months. Cytotoxicity studies in macrophages and retinal pigment epithelial cells revealed a good in vitro tolerance of the microsystems. After sterilization, PEA Ms were administered in vivo by subtenon and intravitreal injections in male Sprague–Dawley rats and the location of the microspheres in rat eyes was monitored. We conclude that PEA Ms provide an alternative delivery system for controlling the delivery of drugs to the eye, allowing a novel generation of microsphere design.

Introduction

Most of the diseases affecting the posterior segment of the eye are related with visual impairment and blindness. The effective treatment of these pathologies is one of the major challenges in drug delivery as most of them are chronic and multifactorial. Among them, aged-related macular degeneration, diabetic retinopathies and glaucoma produce irreversible visual damage and blindness [1]. These diseases are becoming more and more prevalent in the aging populations, and nowadays tens of millions of patients are affected worldwide. Depending on the disease, the medications should be delivered to the retinal cells, retinal pigment epithelium or choroid. Furthermore, therapeutic concentrations of the active substance in the intraocular target site have to be maintained during a long period of time.

Due to the ocular barriers, it is difficult to deliver effective drug concentrations to the posterior tissues of the eye using non-invasive routes such as topical or systemic administration [2]. It is well known that after topical administration only very low drug concentrations are reached in the retina and choroid [3]. This is due to the obstacles of drug penetration that include the short residence time of formulations on the ocular surface, the presence of tissue barriers (cornea, lens, conjunctiva, sclera), and flow mediated drug loss factors (conjunctival blood flow, aqueous humor flow) that limit the drug access to the retina and choroid. Although systemic administration is used to deliver some drugs to the eye (e.g., corticosteroids), this route is restricted by the systemic toxicity of the drugs and reduced access to the target site, mainly due to the blood-aqueous and blood-retinal barriers [2].

The most effective method of drug delivery to the back of the eye is through intraocular administrations, mainly intravitreal injections. However, intravitreal administration is an invasive mode of drug delivery and it is sometimes associated with adverse effects (endophthalmitis, hemorrhages, damage of lens or retinal detachment) and it requires frequent visits of the patients to the clinics. Besides, most low molecular weight drugs have short intravitreal half-lives (2–10 h), so they have to be administered frequently to be clinically feasible. Controlled drug delivery systems, such as nano- and microcarriers, as well as implants, able to release and maintain effective active substance levels over long periods of time, would prolong the dosing interval to months [4], [5]. Biodegradable micro- and nanoparticulate systems are emerging therapeutic tools as they can be administered as a conventional injection by periocular (subconjunctival, subtenon, juxtascleral) and intraocular routes and they are cleared from the site of administration over time.

Ophthalmic drug delivery systems can be made with a variety of biodegradable materials such as polyesters (lactide and glycolide copolymers, polycaprolactones, poly(β-hydroxybutyrates)), polyamides (including natural polymers such as collagen, gelatin and albumin), heteropolysaccharides (chitosan) or lactic and glycolic acid polymers and copolymers, among others. Poly-lactic-co-glycolic acid (PLGA) has been widely used for the development of a number of drug delivery systems, such as the intraocular commercialized implant loaded with dexamethasone (Ozurdex®). The advantages of biodegradable implants over the non-erodible devices in the clinical practice have promoted the interest in novel polymers adequate for intraocular drug delivery purposes.

We have studied a new generation of microspheres based on poly(ester amide)s (PEA). The PEAs are amino acid containing biodegradable polymers combining ester and amide groups in the polymer chain. This chemical structure contributes to improved biodegradability, processability and mechanical properties of the materials (via intra- or inter-chain hydrogen bonding interactions though its amide groups). Furthermore, an important polymer feature is that the current composition of PEA predominantly degrades through a surface erosion mechanism [6]. These materials have already demonstrated good biocompatibility showing little or no inflammation both in vitro and in vivo [7] including in an ophthalmic setting [8]. Extruded PEA fibrils have been implanted in both periocular (subconjunctival) and intravitreal routes in a rabbit experimental model. Readouts after 1, 3, 5 days and 2, 4, 8 weeks have shown excellent material tolerance and tissue biocompatibility. Further work [9] investigated blood and cellular in vitro responses of PEA. The findings of the study revealed that monocytes adherent to PEA secreted reduced levels of the pro-inflammatory interleukins (IL)-6 and IL-1β into the culture supernatant relative to those on comparative polymers but secreted significantly higher amounts of the anti-inflammatory mediator, IL-1 receptor antagonist. A PEA coating on cardiovascular stent has been reported in a phase III, 2-armed clinical study [10]. The 24-months follow up of this study reports absence of Major Adverse Cardiac Events (MACE) and suggests that the tested PEA-coated stent is safe [11].

The present work is focused on the study of the ability of biodegradable polyesteramide to form microspheres for ophthalmic drug delivery purposes. Microspheres present several advantages among other ophthalmic drug delivery forms for different reasons: (a) drugs encapsulated in microspheres are protected from degradation and physiological clearance, (b) the release kinetics of the drug can be adjusted by varying the technological parameters of these systems and (c) microspheres can be injected as a suspension using conventional needles (27-34G) without surgery [12].

This study shows the synthesis of polyesteramide (PEA) polymers and the preparation, sterilization, “in vitro tolerance” and delivery characteristics of microspheres (Ms) made from PEA and loaded with a lipophilic drug model (dexamethasone). After studying the impact of gamma sterilization on the properties of these systems, we have built a kinetic ocular model with in vitro release data to predict in vivo drug concentrations in a rabbit vitreous model. Then, we studied the impact of the different reagents, solutions and processes required to perform histological procedures, on the properties of Ms, in order to establish the most appropriate inclusion techniques for in vivo studies. Finally, we have analyzed the behavior of PEA Ms after injection in the subtenon space and in the vitreous humor of rats with the aim of determining whether or not a satisfactory amount of PEA Ms were placed in the desired locations.

Section snippets

Material

Polyvinyl alcohol 67 kDa (PVA) was provided by Merck (Darmstadt, Germany). Acetonitrile (ACN), dichloromethane (DCM) and methanol (MET) were purchased from Sigma-Aldrich (Schenelldorf, Germany). Tetrahydrofuran (THF) was supplied by Teknokroma (Barcelona, Spain). Super gradient acetonitrile (ACN) was purchased from Lab-Scan (Madrid, Spain). Dexamethasone (DX), dimethyl sulphoxide (DMSO) and 3-(4-5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) were obtained from Sigma-Aldrich

Polymer characterization

The obtained PEA product was of number average molecular weight (Mn) of 49 kDa and narrow dispersity index (ĐM) as revealed by GPC analysis (Table 1).

The 1H NMR spectrum was in full agreement with the anticipated chemical structure of the polymer. The 1H NMR spectrum of the purified polymer is in agreement with the chemical structure as shown in Fig. 3.

The aromatic signals 14 at 7.35–7.25 ppm correspond well to aliphatic multiplet at 3.20–3.10 ppm indicating no side ester groups have been lost in

Discussion

Current developments in the therapy of ophthalmic chronic diseases are directed towards the use of new drug delivery systems that allow sustained therapeutic levels of therapeutic molecules at ocular target tissues. Due to the unique characteristics of ocular barriers, controlled drug delivery has become a challenge for scientists, especially when the target site is located in the posterior segment of the eye. Among the intraocular drug delivery systems, implants and microspheres have been

Conclusions

Amino acid based polyesteramides were successfully formulated into microspheres (~ 15 μm), which showed good in vitro tolerance with macrophages and retinal pigment epithelial cells. The microspheres can be readily sterilized by gamma irradiation and administered using 27-32G needles within 30 min of suspension preparation. Loading and release studies with dexamethasone revealed high drug encapsulation efficiency (~ 85%) with a controlled delivery for 90 days. Pharmacokinetic simulations indicate

Acknowledgments

This work was performed within the PANOPTES project, which is supported by funding from the NMP Theme of the Cooperation Programme, under the 7th Research Framework Programme of the European Union (project number 246180).

References (30)

  • S. Resnikoff et al.

    Global data on visual impairment in the year 2002

    Bull. World Health Organ.

    (2004)
  • R. Gaudana et al.

    Ocular drug delivery

    AAPS J.

    (2010)
  • R. Herrero-Vanrell et al.

    Nano and microtechnologies for ophthalmic administration, an overview

    J. Drug Deliv. Sci. Technol.

    (2013)
  • H. Sun et al.

    alpha-Amino acid containing degradable polymers as functional biomaterials: rational design, synthetic pathway, and biomedical applications

    Biomacromolecules

    (2011)
  • M. Kropp et al.

    Biocompatibility of poly(ester amide) (PEA) microfibrils in ocular tissues

    Polymers

    (2014)
  • Cited by (0)

    View full text