Advances in Ophthalmic Drug Delivery Systems - Part II.

Abstract

Treating various ailments of eye utilizes mostly two strategies i.e. delivery of the therapeutic agent by development of a novel delivery system or by enhancing the permeation of therapeutically active agent by the use of penetration enhancers or by the alteration of its physicochemical properties. One of the important considerations in developing delivery system is the release behavior of the active molecule from the delivery system. In-vitro and in-vivo strategies should be employed for the assessment of the release behavior so as to develop an optimized ophthalmic delivery system, which presents all the advantages over conventional systems of delivering drugs to the eye.

Niosomes

In order to circumvent the limitations of liposomes as chemical instability, oxidative degradation of phospholipids, cost and purity of natural phospholipids, niosomes are developed as they are chemically stable as compared to liposomes and can entrap both hydrophilic and hydrophobic drugs. They are non toxic and do not require special handling techniques (1).

Mucoadhesive dosage forms

This approach relies on vehicles containing polymers which will attach, via non- covalent bonds, to conjunctival mucin (a glycoprotein ) thus remaining in contact with the precorneal tissues until mucin turnover cause elimination of the polymer. Mucoadhesive polymers are usually macromolecular hydrocolloids with numerous hydrophilic functional groups, such as carboxyl -, hydroxyl-, amide and sulphate capable of establishing electrostatic interactions.

Hui and Robinson synthesized polymers of acrylic acid crosslinked with divinyl glycol and 2,5 - dimethyl - 1, 5 - hexadiene and examined their utility in ocular drug delivery. The bioadhesive dosage form showed more bioavailability of the drug as compared to conventional dosage forms (2). Thermes et al evaluated the effect of polyacrylic acid as a bioadhesive polymer on the ocular bioavailability of timolol. It was found that polyacrylic acid prolonged the effect of timolol (3). Park and Robinson synthesized polymers of polycarbophil crosslinked with divinyl glycol and examined their utility in ocular progesterone delivery in rabbit's eyes. The bioadhesive dosage form showed an area under the curve 4.2 times greater than the conventional non-bioadhesive suspension ( AUC calculated from concentration vs. time curve ), over the time course of the study (4).

Particulates ( Nanoparticles and Microparticles)

Particulate polymeric drug delivery systems include micro- and nanoparticles. Particles in the micrometer size range > 1mm are called microparticles or microspheres, whereas those in the nanometer size range < 1mm (1000 nm) are called nanoparticles. Microparticles with a capsule wall enclosing a liquid or solid core are called microcapsules. The upper size limit for microparticles for ophthalmic administration is about 5-10 mm. Above this size, a scratching feeling in the eye can result after ocular application. Microspheres and nonoparticles represent promising drug carriers for ophthalmic application ( Kreuter, 1993) (5). The binding of the drug depends on the physicochemical properties of the drugs as well as of the nano or micro particle polymer. After optimal drug binding to these particles, the drug absorption in the eye is enhanced significantly in comparison to eye drop solutions owing to the much slower elimination rates of the particles. Particulates such as nanoparticles, nanocapsules, submicron emulsions, nanosuspensions improved the bioavailability of ocularly applied drugs (6-14).

Ocular penetration enhancer

One of the principal problems in ocular delivery of drugs is relatively low permeability of these drugs across ocular tissues. The proposed mechanism by which penetration enhancers improve corneal drug transport is by two methods. The first method is an expanded paracellular pathway; i.e. 1) compounds that change the cell cytoskeleton, 2) alter tight junctions by promoting the transport of glucose or amino acids with sodium transport, and 3) choosing cationic molecules. The second approach is to enhance transcellular transport through 1) interaction with lipid membrane, and 2) interaction with the protein component of the cell membrane (Liaw and Robinson, 1993) (15).

Certain penetration enhancers eg. cytochalasin B are used to provide better penetration of drugs into different tissues of eye. Other penetration enhancers can be used as an adjuvant in different delivery systems e.g. solutions and gels etc.

Use of Hyaluronic acid

The sodium salt of hyaluronic acid ( SH ) is a high molecular weight biological polymer, made of repeating disaccharide units of glucuronic acid and N-acetyl-b-glucosamine. In the eye, SH is present in the vitreous body and, in lower concentrations, the aqueous humor. SH has several uses in ophthalmic therapy, such as protecting corneal endothelial cells during intraocular surgery, replacing vitreous humor, acting as a tear substitute in the treatment of dry eye, and increasing the precorneal residence time of various drugs.

Snibson et al used gamma scintigraphy to quantify the residence time of 0.2 and 0.3 % sodium salt of hyaluronic acid (SH) in a group of healthy humans and patients with keratoconjunctivitis sicca (KCS). It was found that in KCS patients, the residence time was significantly increased in case of both concentrations. However, residence time was more with 0.3% of SH as compared to 0.2% (16).

Durrani et al studied the influence of molecular weight and formulation pH on the precorneal clearance rate of hyaluronic acid in the rabbit's eye. Hyaluronic acid is a natural polymer which, due to its water retaining capability, binds to cell membranes and can therefore be considered as a putative vehicle for controlled ocular delivery. It was found that bioadhesion was stronger for Healon^Ò at pH 5 than at pH 7.4. It was concluded that high molecular weight hyaluronic acid resides on the ocular surface for a long duration as compared to low molecular weight hyaluronic acid (17).

Durrani et al studied the influence of a mucoadhesive polymer (Carbopol 1342) on the in-vitro release and in-vivo ocular bioavailability of pilocarpine nitrate entrapped in liposomes. The m-vitro release phase of pilocarpine was extended by the presence of the polymer coating The adsorbed film was therefore shown to provide a substantial barrier to drug release. Bioavailability was evaluated in albino rabbits by measuring the intensity and duration of the miotic response. Carbopol 1342 coated REVs showed a larger area under the miotic intensity curve (AUC) and a longer duration of action compared to uncoated REVs. No significant difference of area under the miotic intensity curve and duration of action was found between coated REVs and phosphate-buffered saline (PBS) solution containing the same concentration (0 5% w/v) of pilocarpine nitrate (18).

Use of hydroxy propyl beta cyclodextrins

Cyclodextrins are cyclic oligosaccharides commonly composed of 6-8 a-D-glucose units (a, b, and g, respectively) that have a shape like a truncated cone. The complex has a hydrophobic interior that is capable of encapsulating poorly soluble drugs. The hydrophilic exterior allows for solubilization, thus making these complexes useful for formulation of hydrophobic drugs. The ability of cyclodextrins to solubilize hydrophobic drugs and provide a hydrophilic exterior makes it useful for ocular applications.

The sensitive nature of corneal epithelium precludes the use of certain cyclodextrins due to their toxicity. Jansen et al found that dimethyl-b-cyclodextrin is toxic to the cornea and thus should not be used for corneal ophthalmic formulations (19). Hence, extensive corneal sensitivity studies should be conducted while developing new formulations of cyclodextrins.

Nijhawan et al developed Ophthalmic formulations of ciprofloxacin using inclusion complexes of ciprofloxacin HCl (CFLX HCl) and hydroxypropyl-beta-cyclodextrin (HP-b-CD). The complexes were prepared by the method of freeze drying an evaluated by phase solubility studies, differential scanning calorimetry, powder X-ray diffractiometry, fourier transform infrared spectroscopy and scanning electron microscopy. The ophthalmic solutions formulated using the complexed drug exhibited better stability, biological activity and ocular tolerance in comparison to another ophthalmic solution formulated using uncomplexed drug and also in comparison to a marketed preparation (20). Aktas et al studied the effect of hydroxypropyl beta-cyclodextrin (HP-b-CD) on the corneal permeation of pilocarpine nitrate using isolated rabbit cornea. Pupillary-response pattern to pilocarpine nitrate with and without HP-b-CD was examined in rabbit eye. Corneal permeation of pilocarpine nitrate was found to be four times higher after adding HP-b-CD into the formulation. The reduction of pupil diameter (miosis) by pilocarpine nitrate was significantly increased as a result of HP-b-CD addition into the simple aqueous solution of the active substance. The highest miotic response was obtained with the formulation prepared in a vehicle of Carbopol 940. Results suggested that ocular bioavailability of pilocarpine nitrate could be improved by the addition of HP-b-CD (21).

Evaluation of ocular drug delivery systems

Ocular drug delivery systems are evaluated by various methods. The ocular in-vitro studies include design of specialised apparatus. The ocular in-vivo studies were done in rabbits and include tear pH measurements, pharmacodynamic studies and scintigraphy to assess precorneal residence of formulations.

(1) In-vitro evaluation methods :


A number of approaches are used by different workers to conduct in-vitro evaluation of controlled ocular drug delivery systems. These include bottle method, modified rotating basket/ paddle method and flow through apparatus.etc.

(a) Bottle method

In this method, dosage forms are placed in the culture bottles containing phosphate buffer at pH 7.4. The culture bottles are shaken in a thermostatic water bath at 37°C. A sample of medium is taken out at appropriate intervals and analyzed for drug contents.

(b) Diffusion method

An appropriate simulator apparatus is used in this method. Drug solution is placed in the donor compartment and buffer medium is placed in the receptor compartment. An artificial membrane or goat cornea is placed in between donor and receptor compartment. Drug diffused in receptor compartment is measured at various time intervals.

(c) Modified rotating basket method

In this method, dosage form is placed in a basket assembly connected to a stirrer. The assembly is lowered into a jacketed beaker containing buffer medium. The temperature of system is maintained at 37°C. A sample of medium is taken out at appropriate time intervals and analyzed for drug content.

(d) Modified rotating paddle apparatus

In this method, diffusion cells (those that are used for analysis of semi-solid formulations) are placed in the flask of rotating paddle apparatus. The buffer medium is placed in the flask and paddle is rotated at 50 rpm. The entire unit is maintained at 37+0.5° C. Aliquots of samples are removed at appropriate time intervals and analyzed for drug content.

(e) Flow through devices

There are obvious and insurmountable limitations to the official dissolution testing apparatus concerning maintenance of sink condition for drugs that saturate rapidly in large volumes of medium. The in-homogenicity of the solution in the rotating basket and poor reproducibility led to enhanced use of flow through devices. A constant fluid circulation apparatus is used as a flow through device. The apparatus consist of a glass dissolution cell, a continuous duty oscillating pump, a water bath and a reservoir. The dosage form is placed in the reservoir of the dissolution medium. The whole assembly is maintained at the temperature of 37°C. The dissolution medium is circulated through the apparatus. Sampling of medium is done at various time intervals and analyzed for drug content.

Ali and Sharma had fabricated flow through cell for the determination of in- vitro release of drug from ocular inserts (22). Sultana et al modified the same apparatus by introducing jacketed flask and eye (23).

(2) In – vivo evaluation methods

The controlled ocular drug delivery systems can be evaluated for its pharmacokinetic and pharmacodynamic profiles. The main objective of the pharmacokinetic studies is to determine the drug release from the dosage form to the eye. Rabbit is used as an experimental animal because of a number of anatomical and physiological ocular similarities and also due to larger size of the eye. Pharmacokinetic studies are performed by measuring drug concentration in various eye tissues eg. lens, cornea, iris, ciliary body, retina sclera, aqueous and vitreous humour in rabbits. The intraocular pressure of the eye is measured with a tonometer. Ocular pharmacokinetic studies can also be carried out by tear fluid sampling, which is a non-invasive technique. Usually, disposible glass capillaries of 1ml capacity are used for sampling. The samples are collected from the marginal tear strip of the rabbits. The capillary force fills the tube rapidly and the small volume collected does not interfere with the ocular pharmacokinetics. Extreme care must be taken to avoid any corneal contact and possible induced lacrimation. To withdraw aqueous humour, rabbits are anaesthetized with ketamine and aqueous humour about 200ml is withdrawn from the anterior chamber using 1ml syringe with 26 guage needle. Vitreous samples are also obtained with 20 gauge needle. The entire cornea, lens, and iris-ciliary body are also removed and analyzed for the drug content (24).

(3) In vitro – in vivo correlation (25)

In establishing a good in-vitro-in-vivo correlation, one has to consider not only the pharmaceutics aspects of the controlled release drug delivery systems but also the biopharmaceutics and pharmacokinetics of the therapeutic agent in the body after its delivery from a drug delivery system, as well as the pharmacodynamics of the therapeutic agent at the site of drug actions

For in-vivo-in-vitro correlation for the cumulative percentage drug release, scatter diagrams are plotted between cumulative percent drug release in- vivo on the X-axis. Versus cumulative percent drug release in– vitro on Y–axis. Several workers have established the correlation of in-vitro and in-vivo data by performing regression analysis. The value of correlation coefficient indicated the suitability of the in-vitro method and adaptibility of the delivery system to the biological system (26).

A high value of correlation coefficient showed good in-vitro and in-vivo correlation. The design of the evaluation methods should take into account the anatomical, physiological and metabolic considerations for the eye.

Ticho et al evaluated the release of pilocarpine from ophthalmic emulsion using the dialysis bag technique. The dialysis bag was filled with the preparation and it was suspended in normal saline at 37°C. The saline samples were withdrawn at various time intervals and analyzed for drug content (27).

Mitra and Mikkelson obtained, miosis-time profile using rabbits following the instillation of 25.0 ml of 1% pilocarpine nitrate solution. It was found that the area under the miosis-time profiles and the maximum observed pupillary diameter changes, decreased as the citrate buffer concentration was increased (28).

Schoenwald and Huang determined the permeability characteristics of a group of b-blocking agent across excised rabbit corneas. Various correlation were determined for the log permeability coefficient as a sum of log functions of the partition (or distribution) coefficient, molecular weight and/or degree of ionization (29).

Ahmad and Patton utilized an in-vivo technique to study the effect of pH and buffer capacity on the precorneal disposition and ocular penetration of pilocarpine in the rabbit eyes. Test solutions were prepared in 0.0667 M phosphate buffer at various pH (4.5, 6.0 and 7.2) and at various phosphate buffer concentrations (0M, 0.00667 M, 0.0667 M, 0.1M) at pH 4.5. It appeared that following the instillation of pilocarpine nitrate solutions buffered below the physiological pH of the lacrimal fluid, the extent of depression of the tear film pH and the tear pH re-equilibration time depend not only on the pH of the solution but also on the precorneal fluid dynamics, and the buffer capacities of the instilled solution and that of the tears. In order to ensure optimum ocular penetration of pilocarpine, the system should not depress the tear film pH appreciably, and should allow rapid tear pH re-equilibration (30).

Saettone et. al tested mydriatic activity of 0.2% tropicamide in humans and rabbits using polymers carboxymethyl cellulose, low molecular weight hydroxypropyl cellulose, medium moleculer weight hydroxy propyl cellulose, polyvinyl pyrrolidone and polyvinyl alcohol. All vehicles increased the ocular bioavailability of drug in both humans and rabbits when compared with a non-viscous solution (31).

Saettone et al found that in rabbits, administration of pilocarpine in the hydrogel vehicles doubled the drug bioavailability (as expressed by the area under the miotic response vs time curve, AUC) with respect to aqueous solution of the drug, however, no statistical differences were apparent in AUC values of the hydrogels (32).

Ahmad et al examined the diffusion of different drugs like propanolol, timolol, penbutol and nadolol across the isolated corneal and scleral membrane of the rabbit using a two chamber glass diffusion cells. The scleral permeability was found higher than the respective corneal permeability (33).

Ahmad and Chaudhari prepared pilocarpine solutions with equimolar acetate, phosphate and citrate buffers. Four solutions in single 25 ml dose were given to 6 concious rabbits and in-vivo tear pH was determined with pH sensitive paper at different time intervals and lacrimal fluid pH-time profiles were made. Phosphate buffer was used as a buffer of choice (34).

Grass and Robinson conducted ultrastructural analysis to augment results of classical kinetic studies. Scanning electron microscopy (SEM) allowed visual inspection of cellular junctions on corneal epithelium and endothelium. Results suggested that hydrophilic compounds were preferentially located in intercellular spaces, whereas hydrophobic compounds were associated with the lipid structures of the tissue (35).

Grove et al instilled 1% solution of L- 653.328 in different concentrations of hydroxy ethyl cellulose to rabbits and ocular concentrations were measured in cornea, aqueous humor, iris and ciliary body. It was found that maximum drug concentrations in all three ocular sites increased concomittantly with increase in viscosity of polymers (36).

Thermes et al measured the ocular bioavailability of 0.5% Timoptol (in PVA and bioadhesive polymer-polyacrylic acid) in cornea, aqueous humour,iris and ciliary body of rabbit’s eye. It was found that bioadhesive polymer gave highest timolol concentration in ocular tissues (37).

Sasaki et al used a cylindrical cell to characterize the absorption properties of ocular membranes in-situ. Rabbits were anaesthetized and placed in a position to brought one eye in the upright position. A plastic cylindrical cell was then fitted over the cornea, sclera and conjunctiva of the eye. Tilisolol solutions were filled into the cells and samples of 0.5 ml was collected at 10 minutes and analyzed for drug content. It was found that in-situ method using a cylindrical cell was a useful method for investigation of the ocular absorption of ophthalmic drugs (38).

Desai and Blanchard formulated gel preparations of pilocarpine hydrochloride with pluronic F127 and assessed the in-vivo performance of the formulations using miosis in albino rabbit as a measure of ocular bioavailability. An improvement in bioavailability of pilocarpine was obtained as compared to conventional eyedrops (39).

Conclusion

Many in-vitro release techniques/apparatus were designed and used by several authors. The in-vivo assessment of ocular drug activity was done by measuring concentration of drug in ocular tissues, mucoadhesion, ocular retention, tear pH and intraocular pressure in rabbit’s eye. There is a need to develop a model in-vitro release apparatus which would greatly mimic conditions of the eye and gives a high correlation with in-vivo drug release in animals.

Different in-vitro and in-vivo methods of evaluation of ophthalmic drug delivery system are summarized in Table.no.1 and 2

Table 1: In-Vitro methods used for release studies

Table.2: In-vivo methods for release studies

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Yasmin Sultana, Rahul Jain, Rahul Rathod, Asgar Ali, M. Aqil^*

Department of Pharmaceutics, Faculty of Pharmacy, Hamdard University, New Delhi 110062, INDIA.

* Corresponding author Dr. Mohd. Aqil received his doctorate in Pharmacy from Hamdard University, New Delhi (India). He has authored and/or coauthored over 25 publications. His research interests include Drug Delivery Systems, pharmacovigilance and drug utilization review. He is currently working as Lecturer at Faculty of Pharmacy, Hamdard University, New Delhi. His current job responsibilities include teaching UG/PG classes as well as supervising research. He is a Life Member of Indian Pharmaceutical Association, Society of Pharmacovigilance (India) and Association of Pharmaceutical Teachers of India .

Contact info:

Tel. # +91-11-26059688 ext. 5632

E-mail- mailto:yas2312003@yahoo.com

Prof. Asgar Ali is a Professor of Pharmaceutics at Jamia Hamdard University, New Delhi. He is M. Pharm., Ph.D, CIC, D.Sc. and has more than 75 papers published in National and International Journals. He has presented more than 60 research papers at various conferences and written 2 books. His area of research interest is Novel/ controlled drug delivery systems.