Journal of Undergraduate Research
Volume 9, Issue 3 - Spring 2008

Ophthalmic Drug Delivery through Contact Lenses

Michael Abrahamson

ABSTRACT

Current ophthalmic drug delivery systems are insufficient, specifically eye drops, which allow approximately 95% of the drug contained in the drops to be lost due to absorption through the conjunctiva or through the tear drainage.  The use of poly-2-hydroxyethyl methacrylate hydrogels with drug-laden microemulsions has been proposed as a method to deliver drugs to the eye.  The contact lenses restrict the drug from being lost to tear drainage by releasing the drug into two tear layers on either side of the contact lens, where it ultimately diffuses into the eye.  Microemulsions require a unique composition of oil, surfactant, and solution to reach a stable state.  This paper focuses on creating stable microemulsions capable of high drug loading, dispersing these particles throughout a polymer gel, and measuring the release of the drug. 

INTRODUCTION

Currently, approximately 90% of all ophthalmic drugs are delivered using eye drops.  Although convenient and easy to use, eye drops are very inefficient, losing about 95% of the applied drug to absorption by the conjunctiva and through tear drainage.  This significant drug dosage is released into the blood stream, where it can have detrimental side effects elsewhere in the body1-3.  For example, timolol is a nonselective beta blocker that is capable of treating glaucoma by preventing the production of aqueous humor, thus lowering the pressure inside of the eye.  Although useful for this purpose, timolol has several severe side-effects throughout the body, such as cardiac arrhythmias, bronchiospasms, depression, and heart failure. To target the dosage to the eye, it has been proposed to use particle-laden contact lenses for ophthalmic drug delivery.  Ophthalmic drug formulations such as timolol can be encapsulated within microemulsion nanoparticles, then entrapped with a soft poly-2-hydroxyethyl methacrylate (p-HEMA) hydrogel contact lens.  Mircoemulsions are clear, thermodynamically stable liquid mixtures of oil, water, and surfactant.  An oil-in-water microemulsion uses the surfactant to decrease the interfacial tension between droplets of oil and the continuous water phase.  These microemulsions allow highly concentrated drug to diffuse out of the lens allowing for long-term ophthalmic drug delivery.  The drug diffuses from the lens into the tear layers surrounding the lens, ultimately passing into the eye.  This paper focuses on defining the stable compositions of microemulsions used in loading hydrogels and several methods of trapping microemulsions laden with timolol within p-HEMA gels4.

LITERATURE REVIEW

Due to the numerous harmful side-effects of timolol, researchers have previously tried soaking contact lens in drug solution to target delivery to the eye.  When the lens is placed on the eye, the drug diffuses into the pre-lens and post-lens tear layers and eventually into the eye.  The benefits of the lens is the long residence time within the eye and in the limited interaction of these tear layers with the outside tear fluid, thus greatly reducing the undesirable dosage into the bloodstream5,6.  A few clinical studies have shown the soaked lens to give desired therapeutic results7-11.  Researchers have also used microemulsions in ophthalmic drug delivery, although the primary purpose of these studies was to increase the dosage of hydrophobic drugs by entrapping them in the oil phase.  These proved to be ultimately ineffective in decreasing the transfer of drug into the bloodstream since the microemulsions are washed away by tears.  By entrapping the microemulsions in the lens it will prevent them from enter tear circulation.

MATERIALS AND DESIGN

Materials

Hydroxyethyl methacrylate monomer and ethylene glycol dimethacrylate (EGDMA) were purchased from Aldrich Chemicals (St Louis, MO); ethylbutyrate and benzoyl peroxide (BP) (97%) were purchased from Aldrich Chemicals (Milwaukee, WI); Timolol Maleate, Pluronic F127, Dulbecco’s phosphate buffered saline (PBS), sodium caprylate, and sodium hydroxide pellets (99.998%) were purchased from Sigma Chemicals (St Louis, MO); Darocur TPO was kindly provided by Ciba (Tarrytown, NY).

Methodology and Procedures

Decrease Ethyl Butyrate Solubility. To create a stable microemulsion and minimize the amount of drug outside of the microemulsions, the solubility of the ethyl butyrate in solution must be very low preventing the oil from solubilizing.   Sodium chloride and NaOH were used to increase the ion concentration and pH of the water phase, which will decrease the solubility of the ethyl butyrate.  The solubility limit was determined by small incremental additions of ethyl butyrate until the solution became opaque.  Once the solubility of ethyl butyrate in 60% wt HEMA and 40% wt water was below 1%, the solution could then be used to create stable microemulsions.

Loading timolol into the oil phase. To entrap the timolol within the oil phase, the timolol maleate was fist converted to timolol base.  Approximately 80 mg of timolol maleate was mixed in a test tube with 6 ml of water and 3 g of NaOH.  At this low pH, the timolol maleate will convert to a very hydrophobic timolol base and phase separate in the bottom of the tube.  Five milliliters of the aqueous solution was then carefully removed by pipette ensuring not to remove any of the timolol base, and 400 μl of hyl butyrate was added to extract the timolol base.  The resulting upper phase of ethyl butyrate and timolol base (referred to as TM/EB) was used in creating the microemulsions.

Synthesis of microemulsions. Every microemulsion was made using the same aqueous solution as described above, containing 25 g of 60% wt HEMA and 40% wt water, 0.25 g NaCl, and 3 g of 2N NaOH along with the addition of 0.0425 g of sodium caprylate.  This mixture was kept constant throughout the experiments and well be referred to as the single “pseudo-component” HEMA/water solution.

Since microemulsions will only form in solutions containing specific ratios of HEMA/water solution : Pluronic Surfacant : TM/EB, a ternary phase diagram was constructed to determine the feasible range of compositions.  At each test point chosen on the diagram, the corresponding amount of Pluronic F127 surfactant and TM/EB was added to give the desired composition.  The solution was thoroughly mixed and allowed to stabilize overnight. Those systems that remained opaque were marked as non-microemulsion.  The systems that became clear were tested further using polarized sheets. Due to the shape of microemulsions, they rotate light very poorly.  Each sample was place between perpendicular polarized sheets.  If the sample rotated the light, allowing it to pass through the sheets, it was ruled also to be a non-microemulsion system.  The samples were then further tested by vigorously mixing the system using a vortex mixer.  Any trace amount of the TM/EB which may have phase separated and was not visible through the first eye examination was forced back into the solution.  This caused unstable or minimally phase-separated samples to become opaque.  Any samples which remained clear were ruled to be microemulsions. Each point on the diagram was labeled to indicate if it was a successful microemulsion using an “O” for a success, and an “X” for a failure.  Visible trends could then be used to create phase boundaries.

An example of one stable composition contains the 25 g HEMA/water solution along with 3.6 g of Pluronic F127, 0.61 g of TM/EB.  Fifty-eight other compositions where tested in order to accurately model the phase diagram.

Entrapment of microemulsions in HEMA gels. The microemulsion particles were entrapped within p-HEMA gels using UV-polymerization with Darocur TPO initiator.  One milliliter of the microemulsion solution was added to 1.35 mL of HEMA, 5 ml of ethylene glycol dimethacrylate (EGDMA).  Dissolved oxygen must be removed from solution before using free radical polymerization, since oxygen is capable of acting as an inhibitor. To remove the dissolved oxygen in the solution, the sample was degassed by bubbling nitrogen for 15 minutes.  Three milligrams of Darocur TPO was added o the mixture and allowed to stir for 10 minutes outside of direct light exposure.  Next, the solution was then poured into a mold made of two 3.5” x 3.5” glass plates clapped over a 200 μm plastic spacer.  The mold was set on top of a UVB-light illuminator for 35 minutes.

Polymerization of EGDMA microemulsion. It is possible that the microemulsions could have destabilized during the polymerization process, and it is very difficult to prove they are intact within the gel.  To ensure this does not happen, the microemulsions themselves were first polymerized in solution.  Approximately, 1 ml of EGDMA was degassed for 15 minutes by bubbling nitrogen.  The EGDMA was then loaded with timolol by substituting it for ethyl butyrate in the same process.  The timolol-loaded EGDMA will be referred to as TM/EGDMA.  Eight hundred mL of the TM/EGDMA was combined with 0.0124 g of Darocur TPO and stirred for ten minutes.  A separate 10 g solution of water, NaCl, and 2N NaOH was prepared in the ratio 100 : 1.8 : 10, respectively.  Next, 1.310 g of Pluronic F127 was mixed into the solution until fully dissolved.  A 0.0239 mL aliquot of TM/EGDMA with initiator was added to this solution and allowed to stabilize overnight covered from any light exposure.  The solution was then placed in a Petri dish on a UVB-light illuminator for 35 minutes.  The solution formed some large aggregates; and these large particles were not used in further experiments.

The ethyl butyrate microemulsion solution was replaced by 1 mL of the polymerized EGDMA microemulsion solution and entrapped within a HEMA gel using the same technique above.

Measuring timolol release from polymerized EGDMA microparticle laden hydrogels. After polymerization, the gels were removed from the glass mold and cut in to 1.5 cm x 1.5 cm squares.  The gels were dried overnight to allow any water remaining in the gels to evaporate.  The next day, the exact gel dimensions and gel weight were recorded.  Each gel was then placed in vial with 3 mL phosphate buffer solution, which better mimics a tear environment than DI water.  Periodically, a 1.5 mL aliquot of the solution was temporarily removed and analyzed using UV-visible spectroscopy.  Timolol present in solution will yield an absorbance peak at 295 nm proportional to the fraction of drug released from the gel.  These periodic measurements were repeated until changes in timolol concentration became insignificant.

RESULTS AND DISCUSSION

Solubility of Ethyl Butyrate

Table 1 displays the effect of increasing NaCl concentration on the solubility of ethyl butyrate in the HEMA/water solution.   Increasing the NaCl concentration to one percent weight decreased the solubility from over ten percent to 3.3% wt.

Table 1.
The effect of NaCl on solubility of Ethyl Butyrate in HEMA Solution.
Weight % of NaCl in 60/40 HEMA/Water Solution Weight % of Soluble Ethyl Butyrate in Solution @ 25 deg. C
0 10.1
0.5 6.0
1.0 3.3

 

Table 2 shows a similar effect on ethyl butyrate solubility by adding 2N NaOH to the HEMA/water solution.  The solubility can be decreased to 1.4 percent by adding 4 mL of NaOH to 25 g of HEMA/water.  To more easily achieve the desired 1.0% ethyl butyrate solubility, the effects of both ion concentration and pH were combined by adding 0.25g of NaCl and 3ml of 2N NaOH to meet the desired specification.  This ratio was then used throughout the rest of the experiments.
Table 2. The effect of NaOH on the solubility of Ethyl Butyrate in HEMA Solution.

Table 2.
The effect of NaOH on the solubility of Ethyl Butyrate in HEMA Solution
Milliliters of 2N NaOH in 60/40 HEMA/Water Solution (25g Basis) Weight % of Soluble Ethyl Butyrate in Solution @ 25 deg. C
0 10.1
0.5 8.8
1.0 8.0
2.5 6.5
3.25 2.8
4 1.4

 

Microemulsion pseudo phase diagram

Determination of feasible microemulsion compositions. A total of 59 compositions ranging from 82-97% HEMA/water solution were used to determine the feasible range of microemulsion compositions.  Figure 1 shows a reasonable estimation of the microemulsion phase boundary at a room temperature of 22.8 degrees Celsius.


Figure 1.  Pseudo three component plot of microemulsion system at 22.8 degrees C.
Figure 1.  Pseudo three component plot of microemulsion system at 22.8 degrees C.

Temperature dependence of microemulsion stability. To increase the drug loading capacity of the contact lens and minimize the amount of surfactant required, the microemulsions must have a large oil fraction.  As the temperature of the solution is lowered, it becomes possible to stabilize microemulsions with a larger oil fraction than before (Figures 1-4).  By decreasing the temperature of the microemulsion solution to 5 degrees Celsius, it becomes possible to stabilize microemulsions with an oil fraction of 8.4% wt and an oil-to-surfactant ratio near 1:1.  This greatly increases the drug capacity of the contact lenses.

Figure 2.  Pseudo three component plot of microemulsion system at 20.0 degrees C.
Figure 2.  Pseudo three component plot of microemulsion system at 20.0 degrees C.

 

Figure 3.  Pseudo three component plot of microemulsion system at 15.0 degrees C.
Figure 3.  Pseudo three component plot of microemulsion system at 15.0 degrees C.

 

Figure 4.  Pseudo three component plot of microemulsion system at 5.0 degrees C.
Figure 4.  Pseudo three component plot of microemulsion system at 5.0 degrees C.

Polymerization of EGDMA microemulsions

Following polymerization of the EGDMA, a sample of the microemulsion was prepared and analyzed using a SEM microscope (Images 1-3).  The resulting images show the microemulsions were successfully polymerized and resulted a uniform dispersion of microparticles approximately 2 µm in diameter.  Microemulsions are typically much smaller in size, therefore these particles are likely aggregates of several microemulsions.

Image 1. SEM image of polymerized EGDMA microemulsion
Image 1. SEM image of polymerized EGDMA microemulsion.


Image 2. SEM image of polymerized EGDMA microemulsion
Image 2. SEM image of polymerized EGDMA microemulsion.


Image 3. SEM image of polymerized EGDMA microemulsion.
Image 3. SEM image of polymerized EGDMA microemulsion.

Drug Release from gels

Drug release from Ethyl Butyrate microemulsion gels. These experiments were performed by Chi Chung-Li to determine the equilibrium release time for the microemulsion-laden gel.  These equilibrium experiments were done with a gel composition of 25 g HEMA/water solution : 3 g Pluronic F127 : 0.5 g TM/EB.  These gels lost 17.5% of the entrapped drug during a 5 hour initial soak in 200 mL of DI water to remove excess HEMA monomer.  The results of drug release experiments (Figure 5) show that about 8% of the entrapped drug diffuses out in a period of about 10 days.  The release profiles are encouraging because the gels continue to release the drug for a period of about 25 days with water replacement at rates comparable to the conditions in the eye, and the equilibrium time is 10 days, which implies that the gels will release drug for 10 days even under perfect sink conditions.  However, it must be noted that these release experiments were performed in DI water, which may not be a good mimic of the tear environment, particularly for ionizable drugs such as timolol.

Figure 5.  Timolol released in DI water from ethyl butyrate microemulsion laden pHEMA gels (n = 4)
Figure 5.  Timolol released in DI water from ethyl butyrate microemulsion laden pHEMA gels (n = 4).

Drug release from polymerized EGDMA microparticle laden gels. These experiments were performed to determine the release rate of timolol from the polymerized EGDMA microparticles when entrapped within a HEMA gel.  The resulting data (Figure 6) indicated that the microparticles released a significant amount of timolol for only the first hour making it unsuitable for contact lens applications.  Although this system did not provide prolonged release of timolol, this method when used with other types of cross-linking within the microemulsions may provide sufficient resistance to be useful.

Figure 6. Timolol released in PBS from polymerized EGDMA microparticle laden pHEMA gels (n = 2)
Figure 6. Timolol released in PBS from polymerized EGDMA microparticle laden pHEMA gels (n = 2).

SUMMARY

The feasible region of compositions for which the HEMA/water solution : Pluronic F127 : TM/EB system will form stable microemulsion was defined for a series of temperatures.  The capability of increasing the oil fraction by lowering temperature allows a very large amount of timolol to be dissolved within the oil phase.  The stable microemulsions were added to HEMA and polymerized into transparent hydrogels.  Although this system released for 25 days, it was performed in DI water which may not be a good mimic of the eyes’ environment.

EGDMA microemulsions were successfully polymerized as evident through the SEM.  The resulting particles were slightly larger than expected and are likely to be aggregates of the original microemulsions.  The timolol release from this system into PBS was rapid and too short for contact lens application, but this method may be useful in slightly different conditions.  By loading these microemulsions with larger drugs or using a more closely packing cross linker, these particles may provide sufficient resistance resulting in long term drug release.

ACKNOWLEDGEMENTS

I would like to thank Dr. Anuj Chauhan for giving me this research opportunity and for the time he has devoted to helping me.  I would also like to thank Chi-Chung Li for teaching me the laboratory techniques used to study them.  Chi Chung-Li also performed the drug release experiments from the ethyl butyrate microemulsion gels.

REFERENCES

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