Rheological evaluation and ocular contact time of some carbomer gels for ophthalmic use

Mucoadhesion across scales: Towards the design of protein-based adhesives

Mucoadhesion is a special case of bioadhesion in which a material adheres to soft mucosal tissues. This review elucidates our current understanding of mucoadhesion across length, time, and energy scales by focusing on relevant structural features of mucus. We highlight the importance of both covalent and non-covalent interactions that can be tailored to maximize mucoadhesive interactions, particularly concerning proteinaceous mucoadhesives, which have been explored only to a limited extent so far in the literature. In particular, we highlight the importance of thiol groups, hydrophobic moieties, and charged species inherent to proteins as key levers to fine tune mucoadhesive performance. Some aspects of protein surface modification by grafting specific functional groups or coupling with polysaccharides to influence mucoadhesive performance are examined. Insights from this review offer a physicochemical roadmap to inform the development of biocompatible, protein-based mucoadhesive systems that can fulfil dual roles for both adhesion and delivery of actives, enabling the fabrication of advanced biomedical, nutritional and allied soft material technologies.

Microemulsions as Lipid Nanosystems Loaded into Thermoresponsive In Situ Microgels for Local Ocular Delivery of Prednisolone

This study aimed to develop and evaluate thermoresponsive in situ microgels for the local ocular delivery of prednisolone (PRD) (PRD microgels) to improve drug bioavailability and prolong ocular drug residence time. Lipid nanosystems of PRD microemulsions (PRD-MEs) were prepared and evaluated at a drug concentration of 0.25-0.75%. PRD microgels were prepared by incorporating PRD-MEs into 10 and 12% Pluronic® F127 (F127) or combinations of 12% F127 and 1-10% Kolliphor®P188 (F68). PRD microgels were characterized for physicochemical, rheological, and mucoadhesive properties, eye irritation, and stability. Results showed that PRD-MEs were clear, miscible, thermodynamically stable, and spherical with droplet size (16.4 ± 2.2 nm), polydispersity index (0.24 ± 0.01), and zeta potential (-21.03 ± 1.24 mV). The PRD microgels were clear with pH (5.37-5.81), surface tension (30.96-38.90 mN/m), size, and zeta potential of mixed polymeric micelles (20.1-23.9 nm and -1.34 to -10.25 mV, respectively), phase transition temperature (25.3-36 °C), and gelation time (1.44-2.47 min). The FTIR spectra revealed chemical compatibility between PRD and microgel components. PRD microgels showed pseudoplastic flow, viscoelastic and mucoadhesive properties, absence of eye irritation, and drug content (99.3 to 106.3%) with a sustained drug release for 16-24 h. Microgels were physicochemically and rheologically stable for three to six months. Therefore, PRD microgels possess potential vehicles for local ocular delivery.

Novel in situ gelling ophthalmic drug delivery system based on gellan gum and hydroxyethylcellulose: Innovative rheological characterization, in vitro and in vivo evidence of a sustained precorneal retention time

Achieving drug delivery at the ocular level encounters many challenges and obstacles. In situ gelling delivery systems are now widely used for topical ocular administration and recognized as a promising strategy to improve the treatment of a wide range of ocular diseases. The present work describes the formulation and evaluation of a mucoadhesive and ion-activated in situ gelling delivery system based on gellan gum and hydroxyethylcellulose for the delivery of phenylephrine and tropicamide. First, physico-chemical characteristics were assessed to ensure suitable properties regarding ocular administration. Then, rheological properties such as viscosity and gelation capacity were determined. Gelation capacity of the formulations and the effect of hydroxyethylcellulose on viscosity were demonstrated. A new rheological method was developed to assess the gel resistance under simulated eye blinking. Afterward, mucoadhesion was evaluated using tensile strength test and rheological synergism method in both rotational and oscillatory mode allowing mucoadhesive properties of hydroxyethylcellulose to be point out. Finally, residence time on the ocular surface was investigated in vivo, using cyanine 5.5 dye as a fluorescent marker entrapped in the in situ gelling delivery systems. Residence performance was studied by non-invasive optical imaging on vigilant rabbits, allowing eye blinking and nasolacrimal drainage to occur physiologically. Fluorescence intensity profiles pointed out a prolonged residence time on the ocular surface region for the developed formulations compared to conventional eye drops, suggesting in vitro / in vivo correlations between rheological properties and in vivo residence performances.

Ocular Biodistribution Studies using Molecular Imaging

Classical methodologies used in ocular pharmacokinetics studies have difficulties to obtain information about topical and intraocular distribution and clearance of drugs and formulations. This is associated with multiple factors related to ophthalmic physiology, as well as the complexity and invasiveness intrinsic to the sampling. Molecular imaging is a new diagnostic discipline for in vivo imaging, which is emerging and spreading rapidly. Recent developments in molecular imaging techniques, such as positron emission tomography (PET), single-photon emission computed tomography (SPECT) and magnetic resonance imaging (MRI), allow obtaining reliable pharmacokinetic data, which can be translated into improving the permanence of the ophthalmic drugs in its action site, leading to dosage optimisation. They can be used to study either topical or intraocular administration. With these techniques it is possible to obtain real-time visualisation, localisation, characterisation and quantification of the compounds after their administration, all in a reliable, safe and non-invasive way. None of these novel techniques presents simultaneously high sensitivity and specificity, but it is possible to study biological procedures with the information provided when the techniques are combined. With the results obtained, it is possible to assume that molecular imaging techniques are postulated as a resource with great potential for the research and development of new drugs and ophthalmic delivery systems.

Preparation and Evaluation of Topically Applied Azithromycin Based on Sodium Hyaluronate in Treatment of Conjunctivitis

Azithromycin (AZI) eye drops containing sodium hyaluronate (SH) were developed to improve the bioavailability of AZI. Interaction between AZI and SH in the AZI-SH formulation was investigated by differential scanning calorimetry, X-ray diffraction, and ¹H-nuclear magnetic resonance spectroscopy analyses. Moreover, advantages of using SH as an excipient were investigated by comparing physiological properties and pharmacokinetic behaviors of SH-containing AZI eye drops with that of hydroxypropyl methylcellulose (HPMC)-containing formulation. In addition, safety of the developed AZI-SH eye drops was evaluated by in vitro 3-(4,5-dimethyl-2-Thiazyl)-2, 5-diphenyl-2H-tetrazolium bromide assay (MTT assay) and neutral red uptake assay as well as in vivo eye irritation test and acute toxicity test. The results indicated that AZI formed a complex with SH under a slightly acidic condition. The area under the curve (AUC) of AZI in SH-containing formulation was 1.58-fold higher (P < 0.01) than that in HPMC-containing formulation due to the interaction between the amine group of AZI and the carboxyl group of SH, despite of the higher viscosity of HPMC-containing formulation. Safety evaluation showed that AZI-SH eye drops caused no obvious eye irritation and acute toxicity. In conclusion, the developed SH-containing AZI formulation possessing advantages of longer retention time and higher drug availability was a promising drug formulation for topical ocular therapy.

Evaluation of risk factors for development of corneal ulcer after nonocular surgery in dogs: 14 cases (2009-2011)

OBJECTIVE

To evaluate the prevalence of and risk factors for development of corneal ulcers after nonocular surgery performed with general anesthesia in dogs.

DESIGN

Retrospective case-control study.

ANIMALS

14 dogs with development of corneal ulcers after nonocular surgery and 718 control dogs.

PROCEDURES

Medical records of dogs evaluated at the Veterinary Medical Teaching Hospital of Seoul National University from January 2009 to June 2011 were reviewed for assessment of risk factors for development of corneal ulcers.

RESULTS

Among the 732 reviewed cases, 14 (1.9%) dogs of 6 breeds developed a corneal ulcer after nonocular surgery. Duration of anesthesia was significantly longer in dogs with ulcers than dogs without ulcers. The number of medications received and procedures performed were also significantly higher in dogs with ulcers than dogs without ulcers. Dogs with a small skull (OR, 8.59; 95% confidence interval [CI], 1.04 to 70.90) and dogs that received neurosurgery (OR, 21.12; 95% CI, 5.77 to 77.25) were more susceptible to development of corneal ulcers. Also, postoperative application of a fentanyl patch was a risk factor for development of corneal ulcers (OR, 4.53; 95% CI, 1.05 to 19.60).

CONCLUSIONS AND CLINICAL RELEVANCE

Several risk factors were identified for development of corneal ulcers after nonocular surgery was performed with general anesthesia in dogs. Perioperative eye protection strategies and postoperative ophthalmic examination are needed to reduce the occurrence of corneal ulcers and their progression, especially for high-risk dogs and procedures.

A retinyl palmitate-loaded solid lipid nanoparticle system: effect of surface modification with dicetyl phosphate on skin permeation in vitro and anti-wrinkle effect in vivo

Surface-modified solid lipid nanoparticles (SLNs) containing retinyl palmitate (Rpal) were prepared by the hot-melt method using Gelucire 50/13(®) and Precirol ATO5(®). Dicetyl phosphate (DCP) was added to negatively charge the surfaces of the SLNs and thereby enhance the skin distribution properties of Rpal. In vitro skin permeation and in vivo anti-aging studies were performed using SLNs dispersed in a hydrogel. The SLNs were under 100 nm in size with an even polydispersity index (PDI), and the high absolute zeta-potential value was sufficient to maintain the colloidal stability of the SLNs. DCP-modified negative SLNs (DCPmod-SLNs) enhanced the skin distribution of Rpal 4.8-fold and delivered Rpal to a greater depth than did neutral SLNs. The in vivo anti-wrinkle effect of the DCPmod-SLN formulation was Rpal dose-dependent. However, the anti-wrinkle effects of the DCPmod-SLN formulations were significantly different from that of the negative control and effectively prevented the reduction of elastin and superoxide dismutase by UV irradiation. In conclusion, the DCPmod-SLN system presented is a good candidate for topical Rpal delivery.

A novel preparation method for organogels: high-speed homogenization and micro-irradiation

The aim of this work was to prepare organogels of Carbopol 974P NF (C974) in PEG 400 by using a novel technique, high-speed homogenization followed by microwave heating. Triclosan (TCS) was used as a model drug. C974, at concentrations ranging between 2% and 4%, was dispersed in 25 ml of PEG 400, and the dispersion was homogenised for 5 min at 24,000 rpm. The dispersion was either heated at 80°C in water bath under mechanic stirring at 200 rpm or exposed to micro-irradiation (1,200 W/1 h) for 2 min. The formulations prepared with both methods performed a well-structured gel matrix characteristic at 3% and 4% of C974 concentrations. As the concentrations of the polymer increased, the elastic properties also increased. The viscosity profiles indicated a shear-thinning system. DSC data revealed that TCS was dissolved in gel. Skin accumulation ability of TCS had been improved by these novel organogels regardless of the preparation method. TCS was still microbiologically effective after the microwave process was applied. It was determined that microwave heating is a suitable method to obtain C974 organogels. This novel production technique developed might be promising especially in industrial scale when the dramatic reduction in the preparation time and energy were considered.

Design and evaluation of ondansetron liquid suppository for the treatment of emesis

The thermosensitive-mucoadhesive ondansetron liquid suppository (tmOLS) was developed to enhance patient compliance and bioavailability in high-risk patients receiving highly emetogenic therapy and having difficulty in swallowing, The thermosensitive-mucoadhesive liquid suppository bases were formulated using poloxamers (P407 and P188) and hydroxypropylmethyl cellulose (HPMC). The physicochemical properties of the liquid suppository bases were characterized by their gelation temperature, mucoadhesive force, rheological properties, and in vitro release. Rectal mucosal damage following rectal administration of tmOLS in rats was assessed using microscopy. Pharmacokinetic analyses were performed to compare tmOLS administered via the rectal route to ondansetron solution administered orally. The liquid suppository base of tmOLS contained P407, P188, and HPMC in the ratio 18:20:0.8, was in the liquid state at room temperature, underwent gelation at body temperature. Area under the curve and half-life (t1/2) of ondansetron were significantly higher in the tmOLS-treated group, indicating that the formulation bypassed the first-pass metabolism and that it was released slowly from the tmOLS because of the formation of mucoadhesive gel state. Furthermore, the t1/2 of tmOLS was two-fold that of the oral solution. Thus, tmOLS could be administered to patients who have difficulty in swallowing; however, adjustments in dosing interval may be needed.

Rheological, mechanical, and bioadhesive behavior of hydrogels to optimize skin delivery systems

BACKGROUND

Hydrogels are widely used for cutaneous formulations; thereby comparing the bioadhesive properties of polymers with a view to prolong the residence time of topical drugs on the skin would be very useful to design novel topical drug delivery systems.

AIM

The objective of this study was to correlate data from rheological studies and texture profile analysis, with bioadhesion on the skin.

METHODS

Polyacrylic acid polymers used were carbomer homopolymer type A (C971) and type B (C974), and polycarbophil (PP) dispersed in water at various concentrations (0.1, 0.5, 1.0, 1.5, 2.0, 3.0, 5.0%, w/v). Rheological, texture, and bioadhesive properties were determined to compare the hydrogels.

RESULTS

Rheological analysis showed that all samples exhibited pseudoplastic behavior with thixotropy. Texture profile analysis showed that compressibility, hardness, and adhesiveness of the hydrogels were dependent on the polymer concentration, and the cohesion values were high. Bioadhesion of C974 and PP at 0.5 and 2% was of the same magnitude, while all samples of C971 had lower values. The bioadhesion of 5% C974 was the highest, while that 5% PP was lower, possibly because PP showed the greatest hardness and this rigidity may decrease the interaction of the polymer with the skin.

CONCLUSION

A comprehensive comparative rheological and textural analyses of several polymers for topical systems were undertaken in terms of their bioadhesion. Therefore, it is possible to conclude that these polymers can be used for optimization of drug delivery systems on the skin.

Transdermal iontophoretic delivery of timolol maleate

Transdermal iontophoresis would be a promising method for the systemic delivery of water soluble and ionic drugs of relatively high molecular size, including peptides. The objective of the present study was to investigate the effect of biological variable such as guinea pig and human cadaver skin and other variables like drug concentration, current density on the transdermal iontophoretic transport of timolol maleate. The permeation profile of drug using solution and gel formulation was studied and compared. For better bioavailability, better patient compliance, and enhanced delivery, an iontophoretic drug delivery system of a timolol maleate matrix gel was formulated using Carbopol 974P. The study was conducted using silver-silver chloride electrodes across the guinea pig and human cadaver skin. Viscosity measurements and flux calculations indicated the suitability of the Carbopol 974P gel for transdermal iontophoretic delivery of timolol maleate. Anodal iontophoresis with silver-silver chloride electrode significantly increased the timolol maleate skin permeation as compared with the passive permeation study. The amount of timolol maleate transported during iontophoresis was significantly different among the different skins. However, iontophoretic gel formulations provided required flux of drug through human cadaver skin.

Safety of repeated topical ocular administration of a polycarbophil-based formulation in several models of ocular surgery in rabbits

PURPOSE

To evaluate the safety of the polycarbophil-based formulation Durasite in rabbits when administered chronically to intact eyes and acutely to eyes compromised by a corneal epithelial defect, penetrating corneal incision, or laser in situ keratomileusis (LASIK) flap. The rheological properties were evaluated to characterize the behavior of the formulation on the ocular surface.

SETTING

Bausch & Lomb, Rochester, New York, USA.

DESIGN

Experimental study.

METHODS

Intact eyes of albino rabbits received polycarbophil (0.6% or 1.3%) 3 times a day for 1 year. The compromised models using polycarbophil 0.9% were an epithelial defect, penetrating corneal incision, or LASIK flap. Eyes with the epithelial defects were dosed 10 times for 24 hours and then 2 times a day for 2 days, and the defect was monitored with fluorescein. The incision or LASIK eyes were dosed 4 times a day for 11 days starting the day before surgery, with 1 drop just before the surgical procedure. The eyes were examined microscopically. The rheological properties were evaluated using a controlled-stress rheometer with a synthetic tear fluid.

RESULTS

No adverse ocular or systemic effects were observed with polycarbophil after chronic administration. In the compromised models, there were no adverse effects of the polycarbophil. There was no evidence of an anterior chamber reaction or qualitative effects on the corneal endothelium. Rheologically, the polycarbophil-based formulation behaved as a sheer-thinning fluid under physical conditions similar to the ocular surface.

CONCLUSION

Results suggest that the polycarbophil-based formulation, like other shear-thinning formulations, is safe to use in topical ophthalmic pharmaceutical products indicated for chronic use and for treatment of conditions with compromise of the ocular surface.

Formulation and evaluation of in situ gelling systems for intranasal administration of gastrodin

Gastrodin is the major bioactive constituent of the traditional Chinese drug "Tianma." It is used in the treatment of some nervous system diseases and can be transported to the brain via intranasal administration. In the current paper, the development of a novel ion-activated in situ gelling system for the nasal delivery of gastrodin is discussed. An in situ perfusion model was used to determine the absorption-rate constant of gastrodin through rat nasal mucosa. The optimal formulation was determined by measuring the critical cation concentration, anti-dilution capacity, gel expansion coefficient, water-holding capacity, and adhesive capacity. The best formulation consisted of 10% gastrodin, 0.5% deacetylated gellan gum as the gelatinizer, and 0.03% ethylparaben as the preservative. The rheological properties of gastrodin nasal in situ gels were also investigated. The viscosity and elasticity sharply increased at temperatures below 25°C. When physiological concentrations of cations were added into the preparation, the mixture gelled into a semi-solid. The results of an accelerated stability test show that gastrodin nasal in situ gels can be stable for more than 2 years. Mucociliary toxicity was evaluated using the in situ toad palate model and the rat nasal mucociliary method; both models demonstrated no measurable ciliotoxicity. Pharmacodynamic studies suggest that similar acesodyne and sedative effects were induced following intranasal administration of 50 mg/kg gastrodin nasal in situ gels or oral administration of 100 mg/kg gastrodin solution. The in situ gel preparation is a safe and effective nasal delivery system for gastrodin.

Chitosan-based thermosensitive hydrogel as a promising ocular drug delivery system: Preparation, characterization, and in vivo evaluation

The purpose of this study was to evaluate the feasibility of in situ thermosensitive hydrogel based on chitosan in combination with disodium α-d-Glucose 1-phosphate (DGP) for ocular drug delivery system. Aqueous solution of chitosan/DGP underwent sol–gel transition as temperature increased which was flowing sol at room temperature and then turned into non-flowing hydrogel at physiological temperature. The properties of gels were characterized regarding gelation time, gelation temperature, and morphology. The sol-to-gel phase transition behaviors were affected by the concentrations of chitosan, DGP and the model drug levocetirizine dihydrochloride (LD). The developed hydrogel presented a characteristic of a rapid release at the initial period followed by a sustained release and remarkably enhanced the cornea penetration of LD. The results of ocular irritation demonstrated the excellent ocular tolerance of the hydrogel. The ocular residence time for the hydrogel was significantly prolonged compared with eye drops. The drug-loaded hydrogel produced more effective anti-allergic conjunctivitis effects compared with LD aqueous solution. These results showed that the chitosan/DGP thermosensitive hydrogel could be used as an ideal ocular drug delivery system in terms of the suitable sol–gel transition temperature, mild pH environment in the hydrogel as well as the organic solvent free.

In situ gelling gelrite/alginate formulations as vehicles for ophthalmic drug delivery

The objective of this study was to develop an ion-activated in situ gelling vehicle for ophthalmic delivery of matrine. The rheological properties of polymer solutions, including Gelrite, alginate, and Gelrite/alginate solution, were evaluated. In addition, the effect of formulation characteristics on in vitro release and in vivo precorneal drug kinetic of matrine was investigated. It was found that the optimum concentration of Gelrite solution for the in situ gel-forming delivery systems was 0.3% (w/w) and that for alginate solution was 1.4% (w/w). The mixture of 0.2% Gelrite and 0.6% alginate solutions showed a significant enhancement in gel strength at physiological condition. On the basis of the in vitro results, the Gelrite formulations of matrine-containing alginate released the drug most slowly. For each tested polymer solution, the concentration of matrine in the precorneal area was higher than that of matrine-containing simulated tear fluid (STF) almost at each time point (p < 0.05). The area under the curve of formulation 16 (0.2%Gelrite/0.6%alginate) was 4.65 times greater than that of containing matrine STF. Both the in vitro release and in vivo pharmacological studies indicated that the Gelrite/alginate solution had the better ability to retain drug than the Gelrite or alginate solutions alone. The tested formulation was found to be almost non-irritant in the ocular irritancy test. The overall results of this study revealed that the Gelrite/alginate mixture can be used as an in situ gelling vehicle to enhance ocular retention.

A poloxamer/chitosan in situ forming gel with prolonged retention time for ocular delivery

The aim of the present work was to obtain an ophthalmic delivery system with improved mechanical and mucoadhesive properties that could provide prolonged retention time for the treatment of ocular diseases. For this, an in situ forming gel comprised of the combination of a thermosetting polymer, poly (ethylene oxide)-poly (propylene oxide)-poly (ethylene oxide) (PEO-PPO-PEO, poloxamer), with a mucoadhesive agent (chitosan) was developed. Different polymer ratios were evaluated by oscillatory rheology, texture and mucoadhesive profiles. Scintigraphy studies in humans were conduced to verify the retention time of the formulations developed. The results showed that chitosan improves the mechanical strength and texture properties of poloxamer formulations and also confers mucoadhesive properties in a concentration-dependent manner. After a 10-min instillation of the poloxamer/chitosan 16:1 formulation in human eyes, 50-60% of the gel was still in contact with the cornea surface, which represents a fourfold increased retention in comparison with a conventional solution. Therefore, the developed formulation presented adequate mechanical and sensorial properties and remained in contact with the eye surface for a prolonged time. In conclusion, the in situ forming gel comprised of poloxamer/chitosan is a promising tool for the topical treatment of ocular diseases.

Ocular ciprofloxacin hydrochloride mucoadhesive chitosan-coated liposomes

The aim of this work is to improve the ocular bioavailability of ciprofloxacin hydrochloride (CPX) through the preparation of ocular mucoadhesive chitosan (CS)-coated liposomes. Liposomes were prepared by the thin film hydration technique, using different molar ratios of L-α-phosphatidylcholine (PC), cholesterol (CH), stearylamine (SA) and dicetyl phosphate (DP). CS was used to coat the optimal liposomal formulae. The prepared formulae were characterized regarding encapsulation efficiency (%EE), particle size, physical morphology and in vitro drug release. The in vivo characterization of the prepared formulae was performed through evaluating the level of CPX in the external eye tissue of nine albino rabbits. Results showed an alteration in release rate and %EE of CPX from liposomal formulae upon varying the molar ratios of the lipid bilayer composition. The optimal liposomal formulae F1 (10:0, PC:CH), F12 (10:0:0.5, PC:CH:SA) and F15 (10:0:1, PC:CH:DP), showed % EE of 38.5 ± 2.10, 39.65 ± 1.85 and 30.05 ± 0.75 and % in vitro release after 8 hours (Q(8h)) of 78.15 ± 2.4, 54.07 ± 2.3 and 62.14 ± 2.9, respectively. In vitro drug release and in vivo results confirmed that CS-coated liposomal formulae have exhibited a higher retention of CPX. Consequently, CS-coated liposomes could be a promising approach to increase the ocular bioavailability of CPX.

Development of an ex vivo method for evaluation of precorneal residence of topical ophthalmic formulations

This paper describes the development of an ex vivo perfusion method for the evaluation of topical ophthalmic formulations. The perfusion system developed consisted of a perfusion chamber, two precision pumps to control the in/out flow rates to simulate the tear flow rate, and a fluorescence microscope imager. Freshly excised rat cornea was used as a biomembrane. Fluorescein (FITC) was used as a marker. Residence time was determined by measuring fluorescence intensity over time after application of the formulation to the cornea. In addition, viscoelastic properties of the formulations were measured and correlated to the retention times. The perfusion method easily differentiated formulations based on the retention time on the cornea: For example, a 0.3% hydroxypropyl methylcellulose formulation had a short retention time of <10 min. Addition of 0.25% carboxymethylcellulose increased the retention time from less than 10 min to over 16 min, and addition of 0.1% Carbopol further increased retention time to over 40 min. For alginate formulations, the retention time was significantly longer in the presence of 0.06% calcium chloride than that of 0.006% calcium chloride. The longer residence time at a higher Ca++ concentration can be attributed to the greater elastic modulus associated with the gel. Interestingly, however, a hyaluronate formulation displayed a very long retention time but has low viscoelastic moduli. This suggests that the mucoadhesive properties may not always be discernable by the rheological properties. The ex vivo perfusion method may in fact provide more meaningful information with regard to retention times of formulations.

Light Scattering as Spectroscopic Tool for the Study of Disperse Systems Useful in Pharmaceutical Sciences

The use of colloidal systems in pharmaceutical formulations, for addressing the issue of selective and controlled drug delivery or for improving drug availability, requires an accurate previous characterization of their chemical and physical properties. Light scattering is a useful and non-invasive method to study the structure and conformation of colloids in a wide space-scale, encompassing nanometric- to micrometric-sized particles, as well as their size distribution, surface electrostatic potential and aggregation phenomena occurring under proper conditions. In this review the physical bases of the light scattering approach are described and many examples are reported to discuss the examination of various multiphase systems useful in pharmaceutical fields.

Pluronic F127-g-poly(acrylic acid) copolymers as in situ gelling vehicle for ophthalmic drug delivery system

To prolong the precorneal resident time and improve ocular bioavailability of the drug, Pluronic F127-g-poly(acrylic acid) copolymers were studied as in situ gelling vehicle for ophthalmic drug delivery system. The rheological properties and in vitro drug release of Pluronic-g-PAA copolymer gels were investigated. The rheogram and in vitro drug release studies indicated that the drug release rates decreased as acrylic acid/Pluronic molar ratio and copolymer solution concentration increased. But the drug concentration had no obvious effect on drug release. The release rates of the drug from such copolymer gels were mainly dependent on the gel dissolution. In vivo resident experiments showed the drug resident time and the total resident amount in rabbit's conjunctiveal sac increased by 5.0 and 2.6 folds for in situ gel, compared with eye drops. The decreased loss angle at body temperature and prolonged precorneal resident time also indicated that the copolymer gels had bioadhesive properties. These in vivo experimental results, along with the rheological properties and in vitro drug release studies, demonstrated that in situ gels containing Pluronic-g-PAA copolymer may significantly prolong the drug resident time and thus improve bioavailability. Pluronic-g-PAA copolymer can be a promising in situ gelling vehicle for ophthalmic drug delivery system.

Viscoelastic properties of Carbopol 940 gels and their relationships to piroxicam diffusion coefficients in gel bases

PURPOSE

This study was conducted to determine the effect of formula compositions on viscoelastic properties of piroxicam gels using Carbopol 940 as a gelling agent and to determine the relationships between viscoelastic properties of Carbopol 940 gel bases and diffusion coefficients of piroxicam in gel bases.

METHODS

Piroxicam gels (1.0% w/w) were prepared by using Carbopol 940 as a gelling agent and varying Carbopol 940 concentrations, glycerin, and sodium chloride contents. The in vitro release of piroxicam from gel bases to the receiving media, isotonic phosphate buffer solution (pH 7.4), were carried out using Franz-modified cell. The piroxicam diffusion coefficients were obtained by Higuchi's equation. Rheological property measurements of gel samples were performed via a cone and plate fluid rheometer. Relationships between viscoelastic properties of gel samples and piroxicam diffusion in gel bases were analyzed by Pearson's test at a p value of less than 0.05.

RESULTS

All piroxicam gels exhibited predominantly elastic solid behavior whose magnitude depended on Carbopol 940 concentration. Preparations containing good solvent exhibited more elastic solid characters. In contrast, the piroxicam gels containing higher sodium chloride contents possessed more viscous fluid behavior. Analyzed by Pearson's test at a p value of less than 0.05, piroxicam diffusion coefficients were directly proportional to loss tangent, but were inversely proportional to storage modulus, loss modulus, complex modulus, and viscosity.

CONCLUSIONS

There is a potential for predicting drug diffusion coefficients from their correlations to rheological parameters. This could be beneficial to the formulation design of transdermal drug delivery systems including mucoadhesive drug delivery systems.

The influence of the use of viscosifying agents as dispersion media on the drug release properties from PLGA nanoparticles

Poly(lactide-co-glycolide) nanoparticles incorporating ciprofloxacin HCl were prepared by means of a W/O/W emulsification solvent evaporation method. The physicochemical properties of these particles were evaluated by measuring particle size, zeta potential and drug loading efficiency. Gamma-sterilised nanoparticles were dispersed in different isoviscous polymer solutions, commonly used as vehicles in eye drops. The influence of gamma-irradiation of the viscosifying agents on the drug release properties of the dispersed nanoparticles was evaluated with respect to release in mannitol solution. The viscosity of the polymer solutions prepared was measured by flow rheometry and thereby the influence of temperature and sterilisation by autoclaving on viscosity was examined. Before and after freeze-drying and subsequent sterilisation by gamma-irradiation, the polymer solutions were also characterised by dynamic stress sweep and dynamic frequency sweep oscillation measurements to deduce possible structural changes. A possible relationship between the differences in ciprofloxacin release from the nanoparticles suspended in the various media and the network structure or rheological behaviour of the polymers was investigated.

Efficacité et tolérance d’un gel de carbomère fluide versus un gel de carbomère classique lors du traitement du syndrome sec

AIM

To compare the risk/benefit for C974P (a 0.25% fluid carbomer gel in a vial allowing dropwise instillation) versus a conventional carbomer gel.

MATERIAL AND METHODS

During this multicenter, randomized, investigator-masked trial, patients with dry eye syndrome were treated with C974P or C940. Control visits were planned on day 7, day 28 (efficacy evaluation) and day 56 (tolerance evaluation). The main criterion was dry eye symptoms globally assessed by a visual analog scale (VAS). The non-inferiority limit for the between-group difference of VAS changes was 10 mm.

RESULTS

In the population of 169 patients (87 patients for C974P, 82 for C940), C974P was at least as effective as C940 on symptoms (non-inferiority hypothesis confirmed). The mean VAS value fell by one third in the two groups. The objective signs improved identically in the two groups: corneal staining by fluorescein (p=0.96), rose Bengal score (p=0.73), and lacrimal break-up time (p=0.73). The dosage adaptation was slightly lower than three instillations per day (p=0.16). The adverse events were mild or moderate.

CONCLUSION

C974P galenic changes are able to reach the same level of efficacy on dry eye symptoms and ocular surface damages as the conventional tube carbomer gels.

Characterization and in vivo evaluation of ocular bioadhesive minitablets compressed at different forces

The influence of the compression force on the physical properties, the in vitro release and the in vivo behavior of ocular minitablets is evaluated in the present study. The bioerodible minitablets (Ø 2 mm, 6 mg) were produced at different compression forces. The crushing strength, friability, water uptake, hydration and swelling of the minitablets both in vitro as well as in vivo after application in the cul-de-sac were evaluated. The friability remained below 1% only for the minitablets made at 0.500 and 0.750 kN. The crushing strength measured was 3.53+/-0.98, 12.34+/-1.69 and 18.64+/-2.37 N for minitablets made at 0.250, 0.500 and 0.750 kN, respectively. The full hydration time equalled 20 and 30 min for minitablets compressed at 0.250 kN and 0.500-0.750 kN, respectively. Increasing the compression force resulted in a decreased swelling capacity. The in vivo release was evaluated in healthy volunteers using a non-invasive method to measure the apparent sodium fluorescein concentration in the tearfilm-cornea compartment as a function of time. The longest residence time of the fluorescent tracer at the administration site was obtained by the minitablets compressed at 0.750 kN. The in vitro release was evaluated with three different dissolution methods: the reciprocating cylinder method, vials in an oscillatory shaking bath and a static method with vials. The best correlation with the in vivo behavior of the matrix minitablets was obtained with the shaking bath method.

Preparation of in situ-forming ophthalmic gels of ciprofloxacin hydrochloride for the treatment of bacterial conjunctivitis: in vitro and in vivo studies

Sol-to-gel systems of ciprofloxacin hydrochloride were prepared utilizing the phase transition properties of hydroxy propyl methyl cellulose K 15 M grade (HPMC) and carbopol 934. The sol-to-gel systems were sterilized by gamma radiation and/or filtration. The sol-to-gel systems were evaluated for rheological characteristics, in vitro release behavior, microbial efficacy, in vivo release behavior, and efficacy against induced bacterial conjunctivitis in rabbits' eyes. Concentration in aqueous humor was determined and stability studies were carried out as per the ICH guidelines. The system passed the test for sterility. The sol-to-gel system exhibited a zero-order drug release pattern over 24 h in in vitro release studies. The drug was active against selected microorganisms in microbial efficacy studies. Better improvement in artificially induced bacterial conjunctivitis in rabbits' eyes was observed in animals treated with the sol-to-gel system compared with marketed eye drops. Drug concentration in aqueous humor was greater than the minimum inhibitory concentration (MIC 90) against selected microorganisms. The shelf-life of the product was >2 years.

Thermosetting gels with modulated gelation temperature for ophthalmic use: the rheological and gamma scintigraphic studies

For ophthalmic drug delivery, Pluronic F127 solutions have a phase transition temperature too low for them to be instilled into the eye at room temperature. Refrigerator storage is usually required to make administration easier, whereas the potential irritation of cold to the sensitive ocular tissues may result in poor topical bioavailability. The purpose of this study is to develop a thermosetting gel with a suitable phase transition temperature by combining Pluronic analogs and to examine the influence of incorporating mucoadhesive polysaccharide, sodium hyaluronate (HA-Na), on the ocular retention of the gel. Dynamic rheological method and single photon emission computing tomography (SPECT) technique were used to ex/in vivo evaluate the thermosetting gels, respectively. An optimized formulation containing 21% F127 and 10% F68 increased the phase transition temperature by 9 degrees C as evaluated by elasticity modulus compared to that of individual 21% F127 solution. Rheological behaviors of the Pluronic solutions showed that the combined Pluronic formulation was free flowing liquid below 25 degrees C and converted to a firm gel under the physiological condition. Furthermore, this formulation possessed the highest viscosity both before and after tear dilution at 35 degrees C. Gamma scintigraphic data demonstrated that the clearance of the thermosetting gel labeled with 99mTc-DTPA was significantly delayed with respect to the phosphate buffered solution, and at least a threefold increase of the corneal residence time was achieved. However, no further improvement in the ocular retention was observed when adding HA-Na into the thermosetting gel due to the substantially decreased gel strength.

Optimisation of carbomer viscous eye drops: an in vitro experimental design approach using rheological techniques

The optimisation of the in vitro interaction between several poly(acrylic acid) derivatives (Carbopol 1342P NF, Carbopol 974P and Carbopol 980 NF) and mucin was performed by an analysis technique combining oscillatory shear rheology and experimental design in order to improve the formulation of carbomer viscous eye drops.First, standard oscillation procedures were used to characterise the polyacrylic acid and mucin dispersions, and to investigate the influence of several polymer-related factors (concentration, preparation, type of polymer used) on the rheological properties. Second, an experimental plan design was developed to investigate the effect of polymer-related factors on the mucoadhesive indexes (MAI(G') and MAI(G")) which were calculated using the viscoelastic data obtained from polymer/mucin, polymer/tearfluid and mucin/tearfluid mixtures. Optimal mucoadhesive interactions were determined based on the experimental design results. Finally, the optima were fully characterised rheologically to further verify the mucoadhesive capacity. The main conclusion is that the factor influencing most explicitly the mucoadhesive interaction of the viscous eye drop is the mucin concentration and neither the type of polyacrylic acid, nor its concentration.

Ocular preparations: the formulation approach

The main aim of pharmacotherapeutics is the attainment of an effective drug concentration at the intended site of action for a sufficient period of time to elicit the response. A major problem being faced in ocular therapeutics is the attainment of an optimal concentration at the site of action. Poor bioavailability of drugs from ocular dosage forms is mainly due to the tear production, non-productive absorption, transient residence time, and impermeability of corneal epithelium. This article reviews: (1) the barriers that decrease the bioavailability of an ophthalmic drug; (2) the objectives to be considered in producing optimal formulations; and (3) the approaches being used to improve the corneal penetration of a drug molecule and delay its elimination from the eye. The focus of this review is on the recent developments in topical ocular drug delivery systems, the rationale for their use, their drug release mechanism, and the characteristic advantages and limitations of each system. In addition, the review attempts to give various analytical procedures including the animal models and other models required for bioavailability and pharmacokinetic studies. The latter can aid in the design and predictive evaluation of newer delivery systems. The dosage forms are divided into the ones which affect the precorneal parameters, and those that provide a controlled and continuous delivery to the pre- and intraocular tissues. The systems discussed include: (a) the commonly used dosage forms such as gels, viscosity imparting agents, ointments, and aqueous suspensions; (b) the newer concept of penetration enhancers, phase transition systems, use of cyclodextrins to increase solubility of various drugs, vesicular systems, and chemical delivery systems such as the prodrugs; (c) the developed and under-development controlled/continuous drug delivery systems including ocular inserts, collagen shields, ocular films, disposable contact lenses, and other new ophthalmic drug delivery systems; and (d) the newer trends directed towards a combination of drug delivery technologies for improving the therapeutic response of a non-efficacious drug. The fruitful resolution of the above-mentioned technological suggestions can result in a superior dosage form for both topical and intraocular ophthalmic application.

Alginic acid effect on carteolol ocular pharmacokinetics in the pigmented rabbit

The effect of alginic acid addition to 1% or 2% carteolol solutions on the ocular penetration of the drug has been evaluated in the pigmented rabbit. During single dose studies, an increase in bioavailability ranging from 40% to 60% was observed in the aqueous humor and in the iris-ciliary body. During repeated dose studies, this increased ocular bioavailability of carteolol in the presence of alginic acid led to an equivalent concentration in the target tissue, although the dosage was only once a day compared with twice a day for the usual carteolol eyedrops. 14C-carteolol distribution studies demonstrated the binding of carteolol in pigmented ocular tissues. Thus, the presence of alginic acid as a new excipient supports a possible decrease in dosage regimen, while retaining sufficient ocular bioavailability to lower intraocular pressure.

Incorporation of small quantities of surfactants as a way to improve the rheological and diffusional behavior of carbopol gels

This paper analyzes the effects of Tween 80, Pluronic F-127, sodium dodecylsulfate (SDS), and benzalkonium chloride on the macro and microviscosity of Carbopol 934NF (0.25-0.50 g/dl) pharmaceutical gels. Carbopol/surfactant interactions, which were reflected in changes in the intrinsic viscosity of the polymer and in shifts of IR spectra bands of films, considerably modified the rheological properties of the gel (flow and oscillatory rheometry) and the diffusion coefficients of polystyrene particles (dynamic light scattering, DLS). At pH 4, any surfactant at a concentration of 0.01 g/dl promoted interpolymer connections producing an open three-dimensional network with maximum viscous and elastic moduli, which does not disturb the diffusive movement of polystyrene particles. An increase in non-ionic surfactant (0.05-0.50 g/dl) gradually decreased viscosity and elasticity since there were more surfactant molecules to surround each carbopol particle, forming intrapolymeric micelles and breaking the interpolymer connections. This macroscopic effect is, however, not reflected in a decrease but in an increase in microviscosity (estimated by DLS) owing to the formation of larger carbopol/surfactant aggregates and free micelles that contribute significantly to the obstruction of the diffusional path. Both ionic surfactants decreased macroviscosity owing to ionic aggregation (benzalkonium chloride) or increase in ionic strength (mainly SDS), while the repercussion on the diffusion of polystyrene particles was dramatically different, and was hindered (due to the carbopol/surfactant aggregates) or enhanced (due to the shrinking of carbopol microgels), respectively. At pH 7.4, the ionization of the carboxylic groups produced an expansion of the polymer chains accompanied by a huge increase in viscosity and elasticity and a decrease in diffusion coefficients in comparison with those obtained at pH 4. The effects of the surfactants were similar to those observed at pH 4 but less intense. Chloramphenicol release studies (Franz-Chien cells) revealed that 0.01 g/dl surfactant did not affect the diffusion while a change in pH dramatically altered the process. The results show that by choosing the appropriate proportion of the most suitable surfactant, it is possible to modulate the flow behavior, elastic properties, and diffusional microenvironment of carbopol gels, without losing the pH-dependent gelling ability, which could improve the suitability of carbopol gels for drug delivery through different routes.

A new long acting ophthalmic formulation of carteolol containing alginic acid

Alginic acid was evaluated as a potential vehicle in ophthalmic solutions for prolonging the therapeutic effect of carteolol. This anionic vehicle was expected to slow down drug elimination by the lacrimal flow, both by undergoing in-situ gel formation and by interacting with the mucus. In vitro studies indicated that carteolol is released slowly from alginic acid formulations, suggesting an ionic interaction. The adhesive behavior of alginic acid solution was better than that of another polymer, hydroxyethylcellulose (HEC). Intraocular pressure (IOP) measurements of rabbit eyes treated with a 1% carteolol formulation with or without alginic acid showed that this polymer significantly extended the duration of the pressure-reducing effect of carteolol to 8 h. The increased ocular bioavailability of 1% carteolol in the presence of alginic acid led to an equivalent concentration in the target tissue although administration was only once a day compared with twice a day for 1% carteolol alone. The overall results of this study indicate that the alginic-acid vehicle is an excellent drug carrier, well tolerated, and could be used for the development of a long-acting ophthalmic formulation of carteolol.

Influence of lecithin on some physical chemical properties of poloxamer gels: rheological, microscopic and in vitro permeation studies

Thermoreversible gels may be used in delivery systems which require a sol-gel transition at body temperature. The influence of the addition of lecithin, a permeation enhancer, on the rheological and in vitro permeation properties of poloxamer 407 gels was investigated. Light microscopy and rheological parameters were used to characterize the microscopic structure of the formulations which showed non Newtonian behaviour, pseudoplastic flow with a yield value. Increased concentrations of lecithin increased the thixotropy, yield value, apparent viscosity, and the gelation temperature of the gels. Light microscopy showed the formation of micellar structures by the addition of lecithin, which may account for changes in rheological properties. In vitro permeation of a model drug, triamcinolone acetonide, was decreased when the lecithin concentration was increased. The presence of lecithin in the poloxamer gel improved the characteristics for topical drug delivery.

Rheological studies of the gelation of deacetylated gellan gum (Gelrite) in physiological conditions

Gels have been successfully used to increase the mucosal contact time and hence the bioavailability of nasal and ophthalmic formulations. The use of in situ gelling polymers requires a rapid sol-gel transition that produces a strong gel for an optimal contact time. In this study, the rheological behaviour of deacetylated gellan gum (Gelrite) was analysed in order to better understand the reasons for the good performance in humans. Thermal scans were used to study gel formation and other changes in the structure of the samples when the macromolecular and ionic contents were altered. The effect the different ions in tear fluid (Na+, K+, Ca2+) had on the gel strength and the consequences of dilution due to the ocular protective mechanisms were examined. Na+ was found to be the most important gel-promoting ion in vivo. It was also found that gels are formed in tear fluid even when the concentration of Gelrite) is only 0.1%. Samples with concentrations of Gelrite of 0.5-1% do not require more ions than 10-25% of those in tear fluid to form gels. These two findings can partly explain the good performance of Gelrite in vivo. Gels with a high elastic modulus can thus be formed even though dilution of instilled drops takes place.

Nasal mucoadhesive delivery systems of the anti-parkinsonian drug, apomorphine: influence of drug-loading on in vitro and in vivo release in rabbits

Lyophilized polyacrylic acid powder formulations loaded with apomorphine HCl were prepared and the influence of drug loading on in vitro release and in vivo absorption studied after intranasal administration in rabbits. These formulations prepared with Carbopol 971P, Carbopol 974P and polycarbophil sustained apomorphine release both in vitro and in vivo. The in vitro release rate and mechanism were both influenced by the drug loading. There was no large influence of drug loading on the time to achieve the peak (Tmax) for a particular polymer, but Tmax differed between different polymers. For a particular drug loading, the Tmax from Carbopol 971P was the slowest compared with that for Carbopol 974P and polycarbophil; however, only the Tmax from Carbopol 971P loaded with 15% w/w of apomorphine was significantly longer than polycarbophil of similar drug loading (P=0.0386). The trend further observed was that increasing drug loading led to increased peak plasma concentration and area under the curve (AUC). In the second part of this study, a mixture containing an immediate release component and sustained release formulation was administered in an attempt to increase the initial plasma level, as this could be therapeutically beneficial. Only one peak plasma concentration was observed and the initial plasma concentrations were no higher than those obtained with solely sustained release formulation. The Tmax, the peak plasma drug concentration (Cmax) and AUC from the lactose-containing formulation were lower than the formulation without lactose but the differences were only marginally statistically significant for Cmax (P=0.0911) and AUC (P=0.0668), but not Tmax (P=0.2788).

Rheological evaluation of Gelrite in situ gels for ophthalmic use

One of the reasons for the relatively low bioavailability of conventional eye drops is their short precorneal contact times. In situ gels are promising ocular drug delivery systems since they are conveniently dropped into the eye as a liquid whereafter they undergo a transition into a gel. Due to their elastic properties hydrogels resist ocular drainage leading to longer contact times. In this paper the rheology of Gelrite in situ gels was studied. A complementary in vivo study for determining precorneal contact times in humans and in rabbits was performed. The elastic moduli of the gels increased with increasing concentration of electrolytes. At physiological concentration of the electrolytes, the elasticity of the gels was independent of Gelrite concentration. The human contact times increased up to 20 h with decreasing osmolality of the formulations. The results indicate that a high rate of the sol/gel transition results in long contact times.

Rheological evaluation of poloxamer as an in situ gel for ophthalmic use

The contact time of a vehicle on the cornea is of utmost importance for ocular drug delivery. In the present study rheological measurements were performed to study the gel and the sol-gel transition of an in situ gel, Poloxamer 407. The rheological measurements and a small in vivo study of ocular residence times in humans were used to evaluate poloxamer as an ocular vehicle. An increasing concentration of poloxamer resulted in a slightly increasing elasticity of the gels and a decreasing sol-gel transition temperature. The contact time increased with increasing concentration of poloxamer which could be explained and correlated with the rheology of poloxamer solutions/gels mixed with simulated tear fluid. The maximum contact time for the preparations studied was about 1 h. The poloxamer system did not seem to be promising as an ophthalmic in situ gel due to the strong concentration dependence of the sol-gel transition temperature combined with the dilution that occurs in the eye.