Subconjunctival biocompatibility of a viscous bioerodable poly(ortho ester)

Safety evaluation of intracameral and subconjunctival injection of a novel mucoadhesive polysaccharide isolated from Bletilla striata in rabbit eye

PURPOSE

To evaluate the safety of intracameral and subconjunctival injection of a novel mucoadhesive polysaccharide isolated from Bletilla striata in rabbit eye.

METHODS

One hundred microliters (100 μL) of Bletilla striata polysaccharide (BsP) at concentrations of 10, 40, and 80 mg/mL was intracamerally or subconjunctivally injected into rabbit eyes. Phosphate-buffered saline and 10 mg/mL hyaluronic acid solution were also injected as controls. BsP safety was evaluated via clinical follow-up and histological analysis. The dead corneal endothelial cells were observed by vital staining with alizarin red and trypan blue at 14 days after intracameral injection. Finally, in the intracameral injection study, scanning electron microscopy was performed for evaluation of the structure of the corneal endothelium and anterior lens capsules.

RESULTS

Subconjunctival injection of 10 mg/mL BsP does not cause pathological changes or an inflammatory response. Concentration greater than 10 mg/mL of BsP (40 or 80 mg/mL) leaded to a slight inflammatory response, but the rabbits recovered well in 3 days. The pathological observation further confirmed the safety of subconjunctival injection of BsP, and subconjunctival injection of 80 mg/mL BsP caused no lesion of the ocular tissues. Intracameral injection of 80 mg/mL of BsP did not cause a significant inflammatory reaction, and an even lower inflammatory response was observed in rabbits intracamerally injected with 40 or 10 mg/mL BsP. All rabbits intracamerally injected with BsP recovered within 7-14 days. BsP had little effect on the blood-aqueous barrier's integrity when the concentration was 10 mg/mL; at 40 or 80 mg/mL, a mild effect was observed, and the rabbits recovered in 1-3 days. Intracameral BsP injected at a concentration of 80 mg/mL had a negative impact on the corneal endothelium and lens, but concentrations of 40 or 10 mg/mL could be injected safely.

CONCLUSIONS

BsP injection into the subconjunctival space and anterior chamber in rabbits at low concentrations (such as 10 mg/mL) did not have adverse effects.

Effect of β-sheet crystalline content on mass transfer in silk films

The material properties of silk are favorable for drug delivery due to the ability to control material structure and morphology under ambient, aqueous processing conditions. Mass transport of compounds with varying physical-chemical characteristics was studied in silk fibroin films with control of β-sheet crystalline content. Two compounds, vitamin B12 and fluorescein isothiocynate (FITC) labeled lysozyme were studied in a diffusion apparatus to determine transport through silk films. The films exhibited size exclusion phenomenon with permeability coefficients with contrasting trends with increases in β-sheet crystallinity. The size exclusion phenomenon observed with the two model compounds was characterized by contrasting trends in permeability coefficients of the films as a function of β-sheet crystallinity. The diffusivity of the compounds was examined in the context of free volume theory. Apart from the β-sheet crystallinity, size of the compound and its interactions with silk influenced mass transfer. Diffusivity of vitamin B12 was modeled to define a power law relationship with β-sheet crystallinity. The results of the study demonstrate that diffusion of therapeutic agents though silk fibroin films can be directed to match a desired rate by modulating secondary structure of the silk proteins.

Macroporous hydrogels based on 2-hydroxyethyl methacrylate. Part 5: hydrolytically degradable materials

Macroporous hydrogels based on 2-hydroxyethyl methacrylate, 2-ethoxyethyl methacrylate and N-(2-hydroxypropyl)methacrylamide, methacrylic acid and [2-(methacryloyloxy)ethyl]trimethylammonium chloride crosslinked with N,O-dimethacryloylhydroxylamine were prepared. Hydrogels were degraded in a buffer of pH 7.4. Completely water-soluble polymers were obtained over time periods ranging from 2 to 40 days. The process of degradation was followed gravimetrically and by optical and electron microscopy. In vivo biological tests with hydrogels based on copolymers of 2-ethoxyethyl methacrylate/N-(2-hydroxypropyl)methacrylamide were performed.

Intraocular implants for extended drug delivery: therapeutic applications

An overview of ocular implants with therapeutic application potentials is provided. Various types of implants can be used as slow release devices delivering locally the needed drug for an extended period of time. Thus, multiple periocular or intraocular injections of the drug can be circumvented and secondary complications minimized. The various compositions of polymers fulfilling specific delivery goals are described. Several of these implants are undergoing clinical trials while a few are already commercialized. Despite the paramount progress in design, safety and efficacy, the place of these implants in our clinical therapeutic arsenal remains limited. Miniaturization of the implants allowing for their direct injection without the need for a complicated surgery is a necessary development avenue. Particulate systems which can be engineered to target specifically certain cells or tissues are another promising alternative. For ocular diseases affecting the choroid and outer retina, transscleral or intrasscleral implants are gaining momentum.

Poly(ortho esters)From Concept to Reality†

The development of poly(ortho esters) dates back to the early 1970s, and during that time, four distinct families were developed. These polymers can be prepared by a transesterification reaction or by the addition of polyols to diketene acetals, and it is the latter method that has proven to be preferred one. The latest polymer, now under intense development, incorporates a latent acid segment in the polymer backbone that takes advantage of the acid-labile nature of the ortho ester linkages and allows control over erosion rates. By use of diols having selected chain flexibility, polymers that range from hard, brittle materials to materials that have a gel-like consistency at room temperature can be obtained. Drug release from solid materials will be illustrated with 5-fluorouacil and bovine serum albumin, and drug release from gel-like materials will be illustrated with mepivacaine, now in Phase II clinical trials as a delivery system to treat post-operative pain. A brief summary of preclinical toxicology studies is also presented.

Ocular biocompatibility of a poly(ortho ester) characterized by autocatalyzed degradation

The biocompatibility of autocatalyzed poly(ortho ester) (POE(70)LA(30)), a viscous, hydrophobic, bioerodible polymer, was investigated. POE(70)LA(30) was synthesized, sterilized by gamma irradiation, and injected in rabbit eyes at adequate volumes through subconjunctival, intracameral, intravitreal, and suprachoroidal routes. Clinical examinations were performed postoperatively at regular time points for 6 mo, and histopathologic analysis was carried out to confirm tissular biocompatibility. After subconjunctival injection, the polymer was well tolerated and persisted in the subconjunctival space for about 5 weeks. In the case of intracameral injections, polymer biocompatibility was good; the POE(70)LA(30) bubble was still present in the anterior chamber for up to 6 mo after injection. No major histopathologic anomalies were detected, with the exception of a localized Descemet membrane thickening. After intravitreal administration, POE(70)LA(30) biocompatibility was excellent, and no inflammatory reaction could be detected during the observation period. The polymer was degraded in approximately 3 mo. Suprachoroidal injections of POE(70)LA(30) were reproducible and well tolerated. POE(70)LA(30) triggered a slight elevation of the retina and choroid upon clinical observation. The polymer was detectable in the suprachoroidal space for about 6 mo. No inflammatory reaction and no major retinal anomalies could be detected by histology. In conclusion, POE(70)LA(30) appears to be a promising biomaterial for intraocular application, potentially providing sustained drug delivery over an extended period of time, with a good tolerance.

Biocompatibility of implantable synthetic polymeric drug carriers: focus on brain biocompatibility

Numerous polymeric biomaterials are implanted each year in human bodies. Among them, drug delivery devices are potent novel powerful therapeutics for diseases which lack efficient treatments. Controlled release systems are in direct and sustained contact with the tissues, and some of them degrade in situ. Thus, both the material itself and its degradation products must be devoid of toxicity. The knowledge and understanding of the criteria and mechanisms determining the biocompatibility of biomaterials are therefore of great importance. The classical tissue response to a foreign material leads to the encapsulation of the implant, which may impair the drug diffusion in the surrounding tissue and/or cause implant failure. This tissue response depends on different factors, especially on the implantation site. Indeed, several organs possess a particular immunological status, which may reduce the inflammatory and immune reactions. Among them, the central nervous system is of particular interest, since many pathologies still need curative treatments. This review describes the classical foreign body reaction and exposes the particularities of the central nervous system response. The recent in vivo biocompatibility studies of implanted synthetic polymeric drug carriers are summarized in order to illustrate the behavior of different classes of polymers and the methodologies used to evaluate their tolerance.

Controlled delivery of metoclopramide using an injectable semi-solid poly(ortho ester) for veterinary application

In animal health care, current therapeutic regimens for gastrointestinal disorders require repeated oral or parenteral dosage forms of anti-emetic agents. However, fluctuations of plasma concentrations produce severe side effects. The aim of this work is to develop a subcutaneous and biodegradable controlled release system containing metoclopramide (MTC). Semi-solid poly(ortho ester)s (POE) prepared by a transesterification reaction between trimethyl orthoacetate and 1,2,6,-hexanetriol were investigated as injectable bioerodible polymers for the controlled release of MTC. MTC is present in the polymeric matrix as a solubilised form and it is released rapidly from the POE by erosion and diffusion because of its acidic character and its high hydrosolubility. If a manual injection is desired, only low molecular weight can be used. However, low molecular weight POEs release the drug rapidly. In order to extend polymer lifetime and decrease drug release rate, a sparingly water-soluble base Mg(OH)(2) was incorporated to the formulation. It was possible to produce low molecular weight POE that can be manually injected and releasing MTC over a period of several days.

Therapeutic applications of viscous and injectable poly(ortho esters)

Poly(ortho esters) (POE) are hydrophobic and bioerodible polymers that have been investigated for pharmaceutical use since the early 1970s. Among the four described generations of POE, the third (POE III) and fourth (POE IV) are promising viscous and injectable materials which have been investigated in numerous biomedical applications. POE III has been extensively studied for ophthalmic drug delivery, it presents an excellent biocompatibility and is currently being investigated as a vehicle for sustained drug delivery to treat diseases of the posterior segment of the eye. POE IV is distinguishable by a highly reproducible and controlled synthesis, a higher hydrophobicity, and an excellent biocompatibility. It is currently under development for a variety of applications, such as ocular delivery, periodontal disease treatment and applications in veterinary medicine. This review will also focus on new perspectives for this promising family of polymers, such as guided tissue regeneration, treatment of osteoarthritis, as well as peptide and protein delivery.

Slow release of the antimetabolite 5-fluorouracil (5-FU) from modified Baerveldt glaucoma drains to prolong drain function

Drainage devices are routinely placed in the eyes of patients with glaucoma to reduce intraocular pressure (IOP) by providing controlled outflow of fluid (aqueous humor) via a filtering bleb. However, the natural wound healing response often interferes with fluid outflow by thickening the walls of the bleb over time, so that these devices rarely remain functional for more than 5 years. We investigated the use of controlled release of an antimetabolite, 5-fluorouracil (5-FU), within glaucoma drains to determine if the wound healing response could be reduced and the useful life span of the device increased. Collagen plugs containing 1.125 mg of 5-FU were placed in the silicone tubes of modified Baerveldt glaucoma drains. Eight drains with 5-FU and eight drains without 5-FU were implanted in one eye each of 16 New Zealand white rabbits: the contralateral eyes served as unoperated controls. Results were evaluated in terms of IOP, fibrous capsule thickness, macrophage density. and presence of type III collagen surrounding the drain plate, 3 and 6 months after implantation. In general, eyes implanted with antimetabolite-containing drains demonstrated significantly lower values for all evaluated parameters at 3 months and lower or equal values at 6 months, compared with the eyes not receiving 5-FU and the unoperated controls, indicating improved IOP-lowering function, reduced bleb wall thickness, and earlier achievement of a steady-state wound healing response. All eyes remained healthy throughout the 6-month duration of the study with no cytotoxicity complications in any of the eyes. Thus, biodegradable plugs placed within the silicone tubes of glaucoma drains can safely deliver 5-FU to filtering blebs over time, which could prolong the functional life of the bleb by decreasing the thickness of the anterior fibrous capsule and permitting sufficient fluid outflow to reduce IOP to physiological levels.

Improved biocompatibility of a viscous bioerodible poly(ortho ester) by controlling the environmental pH during degradation

The poly(ortho ester), POE, used in this investigation, is a viscous bioerodible polymer (8 kDa), which rapidly degrades into a triol and an acidic by-product, acetic acid. In order to improve biocompatibility, we have evaluated the addition of various basic excipients, such as sodium acetate, hydroxyapatite, calcium carbonate and magnesium hydroxide, which buffered and neutralized the acidic degradation product and prolonged the polymer lifetime and drug release. This decrease of POE degradation rate results in a decreased rate of formation of the acidic by-product. Similarly, a POE of higher molecular weight (14 kDa) has been tested. Sodium acetate was too hydrophilic to affect the drug release and the biocompatibility of the polymer, whereas the presence of magnesium hydroxide markedly prolonged the drug release and improved the acceptability of the polymer. The increased molecular weight POE did not improve biocompatibility and a similar but delayed, inflammatory reaction was observed.

A viscous bioerodible poly(ortho ester) as a new biomaterial for intraocular application

The biocompatibility of a viscous, hydrophobic, bioerodible poly(ortho ester) (POE) intended for intraocular application was investigated. POE was evaluated as a blank carrier and as containing modulators of degradation. Each formulation was injected intracamerally and intravitreally in rabbit eyes, and clinical and histological examinations were performed postoperatively for 2 weeks. In the case of intracameral injections, polymer biocompatibility appeared to depend on the amount injected in the anterior chamber. When 50 microL was administered, the polymer degraded within 2 weeks, and clinical observations showed good biocompatibility of POE with no toxicity to the ocular tissues or increase in intraocular pressure. The injection of a larger volume, 100 microL, of POE, appeared inappropriate because of direct contact of polymeric material with the corneal endothelium, and triggered reversible edema and inflammation in the anterior chamber of the eye that regressed after a few days. After intravitreal administration, POE was well tolerated and no inflammatory reaction developed during the observation period. The polymer degraded slowly, appearing as a round whitish bubble in the vitreous cavity. The presence of modulators of degradation both improved POE biocompatibility and prolonged polymer lifetime in the eye. POE appears to be a promising biomaterial for clinical intraocular application.

Sterilization, storage stability and in vivo biocompatibility of poly(trimethylene carbonate)/poly(adipic anhydride) blends

Biodegradable blends of poly(trimethylene carbonate) (PTMC) and poly(adipic anhydride) (PAA) have been proven to be strong candidates for controlled drug delivery polymers in vitro. We now report on the stability, sterilizability and in vivo local tissue response of these matrices. Blend matrices were sterilized by beta-radiation or ethylene oxide gas treatment, stored at different times and temperatures, and analyzed for changes in physicochemical properties. Moisture uptake at different relative humidities and storage times was determined. Sterilization procedures induced hydrolysis of the matrices. Ethylene oxide gas sterilization had a significantly more marked effect upon the matrix properties than radiation treatment. The onset of degradation was reflected in a decrease of crystallinity and molecular weight along with a change of blend composition. A similar onset of matrix degradation was observed upon storage in air. The physicochemical properties of the blends were well preserved upon storage under argon atmosphere. Biocompatibility of PTMC/PAA implants was assessed in the anterior chamber of rabbits eyes for 1 month. At selected post-operative time points, aqueous humor was analyzed for white blood cells and the corneal thickness was measured. The results suggest good biocompatability of PTMC-rich matrices, whereas fast eroding PAA-rich matrices caused inflammatory responses, due to a burst release of degradation products.

In vitro drug release from self-catalyzed poly(ortho ester): case study of 5-fluorouracil

Self-catalyzed poly(ortho esters) are a new variation of linear poly(ortho esters) prepared by the addition of diols to the diketene acetal 3,9-diethylidene-2,4,8,10-tetraoxaspiro[5,5]undecane where dimer segments of lactic acid or glycolic acid are built into the polymer backbone. By varying the concentration of these segments, polymer erosion rate can be controlled. The present investigation describes the in vitro drug release characteristics from these new polymers. Because poly(ortho esters) have potential applications for the delivery of antifibroblastic agents for example after glaucoma-filtering surgery, the in vitro release studies were evaluated using 5-fluorouracil as the active compound. It was shown that a mole ratio of 90/10 or 80/20 diol/diol-lactate incorporated into the polymer lead to a release of 5-fluorouracil by an erosion process. Smaller amounts of diol-lactate lead to a concomitant drug release by diffusion and erosion. It was also shown that the release rate depends on the alkyl chain length of the diol in the polymer backbone but it does not depend on the drug loading.