1040-5488/16/9304-0395/0 VOL. 93, NO. 4, PP. 395Y403 OPTOMETRY AND VISION SCIENCE Copyright * 2015 American Academy of Optometry
ORIGINAL ARTICLE
Photoprotection and Extended Drug Delivery by UV Blocking Contact Lenses Kuan-Hui Hsu* and Anuj Chauhan†
ABSTRACT Purpose. Extended release of photo-unstable drugs from ophthalmic inserts is not useful unless the loaded drug is protected from degradation. Because of the recent interest in extended drug delivery from contact lenses, it is critical to assess whether photo-unstable drugs can be stabilized by loading in lenses. Here, we focus on dexamethasone, which is prone to degradation and has been explored as a candidate for extended release from contact lenses for periods ranging from 10 hours to several days. Methods. Degradation rates of dexamethasone were measured in phosphate-buffered saline and after loading in contact lenses. The degradation rates were measured in a humidified, constant-temperature (32-C) chamber with controlled UV exposure. Contact lenses with various degrees of UV blocking were used to explore the relationship between degradation rates and UV exposure. It is known that vitamin E absorbs UV radiation; thus, it was loaded into the lenses to explore the feasibility of reducing the degradation rates. Results. About 85% of dexamethasone degraded in 20 hours in nonYUV blocking lenses, whereas less than 1% degraded in class 1 UV blocking lenses. Incorporation of vitamin E into the nonYUV blocking lenses reduced the fractional degradation to 30%. Degradation rates in phosphate-buffered saline were significantly higher than even in nonYUV blocking contact lenses. Conclusions. The degradation of dexamethasone can be minimized by using a UV blocking contact lens or incorporating vitamin E into a nonYUV blocking lens. Vitamin E incorporation has the dual benefits of improving drug stability and release profiles. (Optom Vis Sci 2016;93:395Y403) Key Words: dexamethasone, degradation, photo-oxidation, contact lens, UV blocker, vitamin E
D
examethasone, a corticosteroid, is a commonly prescribed ophthalmic drug for treatment of inflammatory symptoms such as conjunctivitis and keratitis and also eye injuries.1,2 The dosing of dexamethasone could be as frequent as four to six times a day and potentially last for a few weeks to months,2,3 which could lead to reduced compliance.4,5 Additionally, long-term administration could induce severe adverse effects including glaucoma, visual acuity impairment, and posterior subcapsular cataract formation.6,7 There is also the possibility of systemic adverse effects including diabetes, hemorrhagic ulcers, and osteoporosis.8 Several researchers have proposed that replacing eye drops by drug-eluting contact lenses could reduce systemic side effects because of the significant increase in corneal bioavailability.9Y11 Additional benefits such as improved compliance may be achievable by replacing a high-frequency eye drop instillation schedule with a single insertion of an extended-release contact
*PhD candidate † PhD Department of Chemical Engineering, University of Florida, Gainesville, Florida (both authors).
lens.12Y14 Several researchers have shown that contact lenses can be designed for extended release of dexamethasone lasting a day to as long as a month.15,16 Dexamethasone is photo-unstable with almost complete degradation within 6 hours of sunlight exposure.17 In fact, commercial eye drops containing dexamethasone are stored in white, opaque polyethylene containers to avoid photodegradation.18,19 A contact lensYbased extended-release system is only useful if the degradation duration is much longer than the release duration. The degradation is undesirable because of the reduction in the dose of the active drug, and additionally, the products of the degradation could possibly be toxic.13,19,20 We showed in an earlier study that the oxidation rates of a hydrophilic drug, cysteamine, are significantly reduced by incorporation in vitamin EYcontaining contact lenses.13 This study focuses on determining whether similar improvements in stability against degradation can be achieved for a hydrophobic photounstable drug, dexamethasone. The photodegradation of drugs is dependent on UV exposure, and thus, we specifically compare a class 1 UV blocking contact lens narafilcon B (1-DAY ACUVUE
Optometry and Vision Science, Vol. 93, No. 4, April 2016
396 Extended Drug Delivery by UV Blocking Contact LensesVHsu and Chauhan
TruEye, Vistakon, FL) with lotrafilcon B (O2 OPTIX, CIBA VISION Corp, GA), which blocks a smaller fraction of UV light. We also explore the effect of vitamin E incorporation on drug degradation because vitamin E diffusion barriers increase the drug release durations and also block UV-B and partly UV-A light.21
METHODS Materials Two commercial silicone hydrogel contact lenses, narafilcon B and lotrafilcon B, were purchased from 1SaveOnLens (Blaine, WA). Dexamethasone (Q98%, purity) and (T)->-tocopherol (Q96%, purity) were purchased from Sigma-Aldrich Chemicals (St. Louis, MO). Ethanol (200 proof ) was purchased from Decon Laboratories Inc (King of Prussia, PA). Dulbecco’s phosphate-buffered saline (PBS) was purchased from Mediatech, Inc (Manassas, VA). Highperformance liquid chromatography (HPLC)-grade acetonitrile and water were purchased from Fisher Scientific (Fair Lawn, NJ). All chemicals were used as received without further treatment.
Preparation of Dexamethasone-Loaded Contact Lenses
FIGURE 1. Schematic of the laboratory-built humidified chamber equipped with UV-B transilluminator. The UV intensity from the transilluminator was 16.50 T 0.01 mW/cm2 and the intensity received by the sample was 3.02 T 0.01 mW/cm2.
The lenses were soaked in 3 mL of 0.066 mg/mL dexamethasone in PBS solution for 7 days to load the drug. In this and other experiments, the samples were covered with aluminum foil to minimize dexamethasone degradation during the loading. The drug concentration in the solution was measured after the 7-day loading by UV-vis spectrophotometry (Thermospectronic Genesys 10 UV, Rochester, NY) to determine the total amount of dexamethasone loaded in the lens.
The UV light intensity received by the sample was measured to be 3.02 T 0.01 mW/cm2 by using UV light meter, which is within the very broad range of natural exposure. After 20 hours of exposure, the lens was removed and subjected to multiple extraction steps to elute the remaining drug and the degradation products. In each step, the lens was soaked in 2 mL of PBS for 24 hours and the concentration of dexamethasone in the extracted solution was determined. The experiment was repeated three times each for both narafilcon B and lotrafilcon B.
Measuring Degradation of Dexamethasone Loaded in the Lenses
Identification and Spectra of the Dominant Degradation Products
The UV intensity of sunlight is dependent on the angle of the sun, ozone concentration, season, location, time, and weather conditions.22 As a preliminary study, we restrict this study to a controlled environment of a custom-built humidified chamber using a UVB-10 (Ultra-Lum, Inc, Carson, CA) transilluminator with an intensity of 16.50 mW/cm2 that sharply peaked at 310 nm (Fig. 1). By comparison, the intensity of sunlight in the range 280 to 400 nm was measured to be 13 to 14 mW/cm2 (Sper Scientific light meter, Scottsdale, AZ) at noon in direct sun on a clear day in Florida in October. The intensity decreased significantly to about 1 mW/cm2 under shade or cloudy conditions. These measured values are within the range reported in literature.23,24 To avoid the potential for diffusion of the drug from the lens, the dexamethasone-loaded contact lens was taken out from the drug solution, gently blotted to remove residual solution on the surface, and then placed at the bottom of an 8-mL, 17- by 60-mm clear glass vial (Fisher Scientific Co, Suwanee, GA). The vial was then placed inside a humidified chamber as shown in Fig. 1. The chamber was humidified and maintained at a temperature of roughly 32-C to keep the sample lens hydrated. The transilluminator was placed at the bottom of the chamber, whereas the sample vial was suspended from the top to minimize the exposure of the sample to the heat generated by the transilluminator.
The major photodegradation product of dexamethasone is 17oxodexamethasone with minor presence of 6A-hydroxydexamethasone.25 To verify this and determine the spectra of each component, we exposed 1.5 mL of 0.046 mg/mL dexamethasone solution to UV irradiation for 25 minutes in the humidified chamber and separated the solution into individual components by HPLC. Acquity UPLC system (Waters Corporation, Milford, MA) equipped with a multiwavelength photodiode array detector and Nova-Pak C18 (4 Km particle size, 3.9- by 150-mm column) (Waters Corporation, Dublin, Ireland) was used for the analysis. The mobile phase consisted of 50% HPLC-grade water and 50% acetonitrile at a constant flow rate of 1.0 mL/min. The peak area was calculated at a wavelength of 240 nm.
Determination of Dexamethasone Concentration in Aqueous Solution The spectra for the drug and the degradation products obtained from the HPLC were subsequently used to determine the concentration of dexamethasone in aqueous solutions by measuring the absorbance spectra. Because the solution contained multiple components, the spectra were the linear combination of the spectra for each of the components in the solution. A least
Optometry and Vision Science, Vol. 93, No. 4, April 2016
Extended Drug Delivery by UV Blocking Contact LensesVHsu and Chauhan
squares fit method as described in the study of Kim and Chauhan26 was used to distinguish the absorbance from dexamethasone and 17-oxodexamethasone to calculate the concentration of dexamethasone in the mixture solution. The validity of this approach was confirmed by showing that the linear combination of dexamethasone and 17-oxodexamethasone fitted the measured spectra with less than 1% mean square error.
Effect of Vitamin E Incorporation on Degradation Rates Contact lenses were rinsed with deionized water to remove the package solution and then dried in air. Vitamin E-ethanol solutions of desired concentrations were prepared by adding vitamin E to ethanol followed by 3 hours of magnetic stirring. The dried narafilcon B and lotrafilcon B were soaked in 3 mL of vitamin E-ethanol solution with concentrations of 33 and 50 mg/mL, respectively, for 24 hours. After the soaking, the contact lenses were taken out, gently blotted, and dried in air again. The dried weights of contact lenses, both before and after vitamin E loading, were measured to determine the vitamin E loading. In this study, 15% vitamin EYloaded narafilcon B and 20% vitamin EYloaded lotrafilcon B were prepared. These vitamin E loading concentrations were chosen to increase the drug release duration to about 24 hours. The vitamin EYloaded lenses were then soaked in 3 mL of drug solution for 14 days. The drug and vitamin EYloaded lenses were tested for drug degradation by following the same procedure as described above. The experiment was repeated three times each for both narafilcon B and lotrafilcon B lenses.
Measuring Degradation of Dexamethasone Loaded in the Lens Hydrated in PBS The lens was loaded with dexamethasone by soaking in 3 mL of 0.07 mg/mL of dexamethasone solution. After reaching equilibrium, the lens was taken out of the drug solution and gently blotted and then placed in an 8-mL, 17- by 60-mm clear glass vial containing 2 mL of PBS solution. The glass vial was sealed to avoid evaporation and exposed to UV light in the humidified chamber at 32-C for variable periods ranging from 20 to 120 minutes. During the UV exposure, to avoid the degradation product reentry into the lens, the PBS solution was refreshed every 20 minutes in the first hour and then every 30 minutes. After the desired duration of UV exposure, the lenses were taken out of the vial and were subjected to the extraction process described above. The experiment was repeated three times. The mass of unreacted dexamethasone in the lens decreases with time not only because of reaction but also because of diffusion into the PBS. To compare these effects, we estimate the mass of dexamethasone released into the PBS by modeling the diffusion and then comparing that with the mass of drug extracted from the lens.16 The model parameters diffusivity D and partition coefficient K were obtained by measuring release of dexamethasone and fitting the data to a diffusion-controlled transport model.16 Briefly, narafilcon B lens was soaked in 3 mL of 0.07 mg/mL dexamethasone-PBS solution for 7 days, which was long enough to reach equilibrium. The drug-loaded lens was soaked in 2 mL of PBS, and the concentration of dexamethasone released to the PBS
397
was measured periodically by UV-vis spectrophotometry. The experiment was repeated three times.
Measuring Degradation of Dexamethasone Dissolved in Solution The same setup as described above was also used to measure the degradation rates of the drug dissolved in PBS, except that the contact lens was replaced by 1.5 mL of solution with 0.046 mg/mL of dissolved dexamethasone. The sealed vial was exposed to UV light from the transilluminator for a certain duration and the concentration of dexamethasone was then measured in the solution. The experiment was repeated 10 times with exposure times of 5, 10, 30, 40, 50, 60, 70, 90, 105, and 120 minutes and one sample for each time point.
Statistical Analysis The statistical comparison of dexamethasone degraded amount in each type of lens was analyzed by analysis of variance F test and Tukey multiple comparisons of means under a completely randomized design (> = 0.05). The analysis was performed in statistical software R (version R-2.15.2 for Windows, R Development Core Team).
RESULTS Determination of Dexamethasone Concentration in Aqueous Solution The UV spectra of the degradation products were obtained by exposing dexamethasone solution of 0.0465 mg/mL to UV irradiation in the humidified chamber (Fig. 1) for 25 minutes and then analyzing the solution by HPLC. The HPLC chromatogram
FIGURE 2. The individual spectra of dexamethasone and 17-oxodexamethasone. The spectrum of 17-oxodexamethasone was magnified 10 times.
Optometry and Vision Science, Vol. 93, No. 4, April 2016
398 Extended Drug Delivery by UV Blocking Contact LensesVHsu and Chauhan
(figure not shown) showed a dominant peak at the elution time for dexamethasone, a smaller peak with an absorbance of about 10% compared with dexamethasone, and finally a minor peak with an absorbance of about 1% compared with the dominant peak. These two peaks were interpreted as corresponding to the major degradation product (17-oxodexamethasone) and the minor product (6A-hydroxydexamethasone).25 The spectra of dexamethasone and its major degradation product (Fig. 2) were used as the reference spectra in the least squares fit method. As shown in Fig. 2, the spectrum of dexamethasone has a peak at 240 nm whereas 17-oxodexamethasone has a linear and much smaller absorbance
response in the wavelength range of 220 to 270 nm. As a further test of the accuracy of the method based on the least squares fit, we compared the degradation of dexamethasone after exposure for 25 minutes. For this sample, the HPLC data and the least squares fit yielded 49.67 and 48.43% degradation, which supports the validity of our approach.
Dexamethasone Degradation in Contact Lenses The unreacted drug remaining in the contact lens after the 20-hour exposure was determined by multiple cycles of extraction
FIGURE 3. The transient absorbance spectra in the PBS solution after soaking (A) lotrafilcon B, (B) 20% vitamin EYloaded lotrafilcon B, (C) narafilcon B, and (D) 15% vitamin EYloaded narafilcon B that were exposed to 20 hours of UV radiation. The legend indicates the time the spectra were measured after soaking in the PBS. The experiment was conducted in triplicate with multiple extraction cycles for each. The data shown are for the first extraction cycle for just one of the experiments. The solid lines are the fits to the data.
Optometry and Vision Science, Vol. 93, No. 4, April 2016
Extended Drug Delivery by UV Blocking Contact LensesVHsu and Chauhan
in PBS. Determining the drug loaded required just the equilibrium spectra, but the transient spectra were measured to validate the assumption of a single degradation product. Fig. 3A shows the transient absorbance spectra in the PBS solution after soaking the lotrafilcon B lens that was exposed to 20 hours of UV radiation. Data are shown only for the first extraction cycle, although six cycles were used to extract the drug. The solid lines are fits to the data assuming that the spectra are a linear combination of that from the drug dexamethasone and the main degradation product 17-oxodexamethasone. Fig. 3B to D show similar data for the lotrafilcon B lens with 20% vitamin E and narafilcon B without and with 15% vitamin E, respectively. In each case, the solid lines are good fits to the data, strongly suggesting that there is only one dominant degradation product in these experiments. The values of fractional degradation of dexamethasone loaded in contact lenses after 20 hours of UV exposure is listed in Table 1 for narafilcon B and lotrafilcon B lenses, both for control and for vitamin EYloaded lenses. The result showed that roughly 80% of dexamethasone was degraded in lotrafilcon B. With 20% vitamin E loaded, the percentage of degraded dexamethasone was reduced to about 30%, which was significantly different from that of pure lotrafilcon B (p G 10j6). The degradation was negligible in narafilcon B lenses, and vitamin E incorporation did not have a significant impact on degradation (p = 0.491). The degradation in narafilcon B is significantly different from that in lotrafilcon B with or without vitamin E (p G 10j6).
Dexamethasone Degradation in Contact Lenses Hydrated in PBS
FIGURE 4. Dexamethasone release profile from narafilcon B lenses. The experiment data are presented as mean T SD with n = 3. The average amount of dexamethasone initially loaded in the lens was 60.41 Kg and the average amount released to the PBS at equilibrium was 35.67 Kg. The solid line is the fitting result based on one-dimensional diffusion equation.
fraction of unreacted dexamethasone in the lens was calculated as the ratio of the measured mass of dexamethasone and the initial drug loading are included in the table. The two fractions are comparable, which shows that the drug did not degrade significantly even after 120 minutes of UV exposure with complete hydration.
The low degradation rates reported above could potentially be attributed to incomplete hydration of the lens and thus degradation rates were also measured in lenses soaked in PBS. Phosphatebuffered saline was replaced periodically to minimize reentry of any component into the lens, and the mass of each component released was estimated using a mathematical model. The model was first validated by fitting the release of the drug from narafilcon B lenses soaked in PBS (Fig. 4). The solid line in the figure is the model fit based on the one-dimensional diffusion equation with best-fit values of 1.176 10j4 mm2/h and 66.83 for diffusivity D and partition coefficient K, respectively. The fitting is good with rootmean-square error equal to 1.51%. The model was used to calculate the fraction of drug remaining in the lens after 20, 40, 60, 90, and 120 minutes, and these values are listed in Table 2. Also, the
Dexamethasone Degradation in Solution
TABLE 1.
DISCUSSION
Percentages of dexamethasone not degraded in contact lenses during the 20 hours of UV-B illumination in the customized chamber Dexamethasone not degraded, %
Contact lens
21.17 T 1.93 70.56 T 1.71 98.11 T 1.52 101.72 T 5.16
Pure lotrafilcon B 20% Vitamin EYloaded lotrafilcon B Pure narafilcon B 15% Vitamin EYloaded narafilcon B Data are presented as mean T SD (n = 3).
399
The fractional degradation of dexamethasone in solution is plotted as a function of time in Fig. 5 and the spectra of the solution after exposure of varying durations are shown in Fig. 6. The solid lines in Fig. 6 are good fits to the data, strongly suggesting that there is only one dominant degradation product in these experiments. The solid line in Fig. 5 is a model fit based on a firstorder reaction; that is,
% of degraded dexamethasone ¼ 1j exp ðjktÞ
ð1Þ
The data fit the model with a rate constant k of 0.029/min. The data show that dexamethasone was almost degraded in the PBS solution after 2 hours of UV light illumination.
Degradation Mechanism Dexamethasone can degrade into at least five known products and several more that have not been identified.25 Dexamethasone can degrade from various mechanisms, but the most relevant path for ophthalmic drug delivery is photodegradation. The UV degradation (366 nm) leads to the formation of six or more distinct products, but the major product is 17-oxodexamethasone,25 which is also produced by oxidation of dexamethasone, suggesting that the major pathway of UV-induced degradation is photo-oxidation. The molecular structures of dexamethasone and 17-oxodexamethasone
Optometry and Vision Science, Vol. 93, No. 4, April 2016
400 Extended Drug Delivery by UV Blocking Contact LensesVHsu and Chauhan TABLE 2.
Model-predicted percentage of dexamethasone that did not release to PBS during the UV exposure in the humidified chamber and the actual percentage of dexamethasone recovered from multiple extractions after UV exposure UV exposure time
20 min
40 min
60 min
90 min
120 min
% of dexamethasone that did not release 85.81 73.64 63.19 52.11 42.98 % of dexamethasone recovered from multiple extractions 90.56 T 7.90* 77.37 T 3.40* 65.12 T 4.29* 52.05 T 4.50* 41.14 T 4.13* *Data are presented as mean T SD (n = 3).
are shown in Fig. 7. We considered 17-oxodexamethasone as the only degradation product, and the concentrations of the parent molecule dexamethasone and the degradation product 17-oxodexamethasone were obtained by fitting the UV-vis spectra of the mixture to a linear combination of the spectra of the two components. The good agreement (G1% root-mean-square error) between the measured spectra and the fitted linear combination spectra strongly suggests the validity of our approach.
Degradation of Drug Loaded in Contact Lenses The results in Table 1 list the degradation fraction of drug loaded in the contact lenses after 20 hours of UV exposure. About 80% of drug degraded in the lotrafilcon B lens, whereas the fraction degraded was negligible in narafilcon B lens. Incorporation of 20% vitamin E into the lotrafilcon lenses significantly reduced the fractional degradation to about 30%, but vitamin E did not impact degradation dynamics in the narafilcon lenses. Narafilcon B lenses are class 1 UV blockers, that is, at least 99% UV-B and 90% UV-A radiation is blocked by the lens, whereas lotrafilcon B lenses block only about 25% of UV radiation ranging from 240 to 300 nm and less than 10% from 300 to 400 nm. The significant difference in the UV blocking profile is the likely reason for the improved drug stability in the narafilcon B lenses. Further support for the importance
of UV blocking in drug stability was provided by the decrease in degradation rates on incorporation of vitamin E into the lotrafilcon B lenses. Incorporation of 20% vitamin E into lotrafilcon B significantly increases the UV-B absorption by the lens, with almost 100% absorption below 300 nm and an average absorption of about 30% from 300 to 400 nm.21 Thus, a significant fraction of the radiation provided by the UV-B transilluminator is absorbed by the lens, thereby reducing the exposure of the drug to radiation. Because the drug degradation is through the photo-oxidation pathway, the rates could also be reduced by reduction in oxygen available for the reaction. However, because of the high oxygen permeability of the silicone hydrogel lenses and the minor effect of vitamin E incorporation on the oxygen permeability, it is likely that oxygen concentrations are not rate limiting in these cases.21 The degradation in the pure narafilcon B lenses was already negligible after 20 hours; hence, it was not possible to discern whether vitamin E incorporation has any impact on the degradation rates.
Degradation of Drug Loaded in Contact Lenses Hydrated in PBS To verify that the lenses were fully hydrated in the custom-built humidified chamber, similar tests were repeated with narafilcon B lenses soaked in PBS. As shown in Fig. 4, the release duration of
FIGURE 5. Percentage of degraded dexamethasone in PBS solution induced by UV light exposure in the customized chamber. The experiment was repeated 10 times with one experiment for each time point. The solid line is the fitting result based on a first-order reaction model (equation 1). Optometry and Vision Science, Vol. 93, No. 4, April 2016
Extended Drug Delivery by UV Blocking Contact LensesVHsu and Chauhan
401
FIGURE 6. The absorbance spectra of dexamethasone-PBS solutions after exposure to various periods of UV light as indicated in the legend. A total of 10 separate experiments were conducted with each experiment lasting a different time duration. The solid lines are fits to the data.
dexamethasone from narafilcon B is less than 10 hours; thus, it is impossible to measure the degradation percentage after 20 hours of UV exposure as the drug would diffuse out from the lens. To gauge the degradation of the drug in the lens during loading in contact lenses, we compared the fraction of drug in the lens if there was no degradation with the actual fraction of undegraded dexamethasone recovered from the lens. The fractions of drug remaining in the lens without considering the reaction were obtained from the model and are listed in Table 2 for various times. The fractions of drug extracted from the lens after various durations of soaking with UV exposure are also included in the table. Statistical analysis showed that the two fractions are not significantly different for all time points with p values of 0.6541, 0.4419, 0.7343, 0.9916, and 0.7368 for exposure of 20, 40, 60, 90, and 120 minutes, respectively. This shows that the extent of reaction is negligible even after 120 minutes, which is consistent with the data obtained from the tests in which the lens was not soaked in solution but kept hydrated by controlling the humidity in the chamber.
solution and the lotrafilcon lens was comparable; thus, the reduction in the degradation rates is likely attributed to the significant difference in the environment of the drug loaded in the lens
Degradation of Drug Dissolved in PBS The fractional degradation of the drug dissolved in PBS shown in Fig. 5 fits a first-order reaction model with a rate constant of 0.029/min. The first-order model again assumes that the drug degrades into only one product and the rate is not limited by the oxygen concentration. The rate constant obtained by the fitting will depend on the intensity of the UV radiation as the reaction mechanism is photo-oxidation. The good fit between the model and the data further supports the assumption of a single key degradation product. The data also show that the drug degrades almost entirely in about 2 hours, which is much more rapid compared with even the lotrafilcon B lens, which had about 80% degradation after 20 hours. The UV exposure in both PBS
FIGURE 7. Structure of (A) dexamethasone and (B) the main degradation product 17-oxodexamethasone from UV-induced or oxidative degradation.
Optometry and Vision Science, Vol. 93, No. 4, April 2016
402 Extended Drug Delivery by UV Blocking Contact LensesVHsu and Chauhan
compared with that in the solution. Most importantly, the availability of oxygen in the lens could be lower compared with that in the solution. The silicone hydrogel contact lenses are designed to be highly permeable to oxygen, which mainly partitions and diffuses through the silicone phase of the bicontinuous matrix. The drug loaded in the contact lens would partition in both the silicone and the hydrogel phases, with a higher partition coefficient expected in the silicone phase owing to the hydrophobicity of the drug. Whereas the silicone phase certainly has a high oxygen solubility, the dissolved oxygen is likely closely associated, potentially surface adsorbed, to the silicone polymer, and thus may not be available to the drug molecules partitioned in the silicone phase.
Clinical Relevance Drug-eluting contact lenses are excellent candidates for treatment of anterior chamber diseases. Many researchers are designing contact lenses for extended release of several drugs including dexamethasone as a potential replacement for eye drops.12,26,27 The extended release is an important feature of the lenses but a long release duration may not be useful for unstable drugs. In fact, the release duration from an optimally designed drug-eluting lens should be less than the degradation time to ensure that the drug released into the tear film is the active molecule and not the degradation products, which lack the therapeutic activity of the drug and, in some cases, could also be toxic.13,19,20 Here, we show that vitamin EYloaded contact lenses protect dexamethasone from degradation; hence, the fractional degradation during the release duration is small. Thus, vitamin EYloaded contact lenses are promising for extended delivery of dexamethasone and potentially other unstable drugs as well.
CONCLUSIONS There is a growing interest in drug-eluting devices for extended delivery of ophthalmic drugs. Several ophthalmic drugs such as dexamethasone are unstable and thus extended release may not be useful unless the encapsulation method also provides a stabilizing effect. This study shows that the degradation rates of drug loaded in contact lenses are much lower than the rates in solution. Furthermore, the degree of UV blocking significantly impacts the degradation rates, with negligible degradation in a class 1 UV blocking lens. Even in a nonYUV blocking lens, the degradation rates are significantly lower than those in solution, and further reduction in the degradation rates can be achieved by incorporating vitamin E into the lenses. Vitamin E incorporation is very useful because of its effect on increasing the release duration, but equally importantly, it also blocks the UV light, which can have a protective effect on the eye, and also reduces the degradation rates of the drugs like dexamethasone. Although the results of this study are encouraging, the drug degradation under in vivo conditions could be significantly different owing to the differences in sun exposure compared with the simulated environment. Thus, future in vivo studies are necessary to gauge the benefits of vitamin E incorporation in contact lenses to reduce the drug degradation under typical wear conditions. Received August 4, 2014; accepted February 14, 2015.
REFERENCES 1. Wagoner MD. Chemical injuries of the eye: current concepts in pathophysiology and therapy. Surv Ophthalmol 1997;41:275Y313. 2. Gaudio P. A review of evidence guiding the use of corticosteroids in the treatment of intraocular inflammation. Ocul Immunol Inflamm 2004;12:169Y92. 3. Dexamethasone Dosing Information. Drugs.com. Available at: http://www.drugs.com/mtm/dexamethasone-ophthalmic.html. Accessed October 26, 2014. 4. Schwartz GF, Quigley HA. Adherence and persistence with glaucoma therapy. Surv Ophthalmol 2008;53:S57Y68. 5. Schwartz GF. Compliance and persistency in glaucoma follow-up treatment. Curr Opin Ophthalmol 2005;16:114Y21. 6. Schwartz B. The response of ocular pressure to corticosteroids. Int Ophthalmol Clin 1966;6:929Y89. 7. Urban RC, Jr., Cotlier E. Corticosteroid-induced cataracts. Surv Ophthalmol 1986;31:102Y10. 8. Clark AF, Yorio T. Ophthalmic drug discovery. Nat Rev Drug Discov 2003;2:448Y59. 9. Li CC, Chauhan A. Modeling ophthalmic drug delivery by soaked contact lenses. Ind Eng Chem Res 2006;45:3718Y34. 10. Xinming L, Yingde C, Lloyd AW, Mikhalovsky SV, Sandeman SR, Howel CA, Liewen L. Polymeric hydrogels for novel contact lensbased ophthalmic drug delivery systems: a review. Cont Lens Anterior Eye 2008;31:57Y64. 11. Alvarez-Lorenzo C, Hiratani H, Concheiro A. Contact lenses for drug delivery: achieving sustained release with novel systems. Am J Drug Del 2006;4:131Y51. 12. Bengani LC, Hsu KH, Gause S, Chauhan A. Contact lenses as a platform for ocular drug delivery. Expert Opin Drug Del 2013;10: 1483Y96. 13. Hsu KH, Fentzke RC, Chauhan A. Feasibility of corneal drug delivery of cysteamine using vitamin E modified silicone-hydrogel contact lenses. Eur J Pharm Biopharm 2013;85:531Y40. 14. Nordstrom BL, Friedman DS, Mozaffari E, Quigley HA, Walker AM. Persistence and adherence with topical glaucoma therapy. Am J Ophthalmol 2005;140:598Y606. 15. Kim J, Conway A, Chauhan A. Extended delivery of ophthalmic drugs by silicone hydrogel contact lenses. Biomaterials 2008;29: 2259Y69. 16. Kim J, Peng CC, Chauhan A. Extended release of dexamethasone from silicone-hydrogel contact lenses containing vitamin E. J Control Release 2010;148:110Y6. 17. Kawabata K, Sugihara K, Sanoh S, Kitamura S, Ohta S. Photodegradation of pharmaceuticals in the aquatic environment by sunlight and UV-A, -B and -C irradiation. J Toxicol Sci 2013;38: 215Y23. 18. Piechocki JT, Thoma K. Photodegradation and photoprotection in clinical practice. In: Piechocki JT, Thoma K, eds. Pharmaceutical Photostability and Photostabilization Technology. London, UK: Informa Healthcare; 2010;397Y423. 19. Klick S, Muijselaar PG, Waterval J, Eichinger T, Korn C, Gerding TK, Debets AJ, Sanger-Van de Griend C, Van den Beld C, Somsen GW, De Jong GJ. Toward a generic approach for stress testing of drug substances and drug products. Pharm Technol 2005;29:48Y57. 20. Samuel NT, Kenkare N, Lackey PD, Chen J. Contact lenses containing carotenoid and method for making same. US patent 8784867 B2. July 22, 2014. Available at: https://www.google.com/ patents/US8784867. Accessed March 26, 2015.
Optometry and Vision Science, Vol. 93, No. 4, April 2016
Extended Drug Delivery by UV Blocking Contact LensesVHsu and Chauhan 21. Peng CC, Kim J, Chauhan A. Extended delivery of hydrophilic drugs from silicone-hydrogel contact lenses containing vitamin E diffusion barriers. Biomaterials 2010;31:4032Y47. 22. Bauer DR. Interpreting weathering acceleration factors for automotive coatings using exposure models. Polym Degrad Stabil 2000;69:307Y16. 23. Getter KL, Rowe DB, Cregg BM. Solar radiation intensity influences extensive green roof plant communities. Urban Forest Urban Green 2009;8:269Y81. 24. Lee DW, Downum KR. The spectral distribution of biologically active solar radiation at Miami, Florida, USA. Int J Biometeorol 1991;35:48Y54. 25. Chen Q, Zielinski D, Chen J, Koski A, Werst D, Nowak S. A validated, stability-indicating HPLC method for the determination of
403
dexamethasone related substances on dexamethasone-coated drugeluting stents. J Pharm Biomed Anal 2008;48:732Y8. 26. Kim J, Chauhan A. Dexamethasone transport and ocular delivery from poly(hydroxyethyl methacrylate) gels. Int J Pharm 2008;353: 205Y22. 27. Hsu KH, Gause S, Chauhan A. Review of ophthalmic drug delivery by contact lenses. J Drug Deliv Sci Tec 2014;24:123Y35.
Optometry and Vision Science, Vol. 93, No. 4, April 2016
Anuj Chauhan Department of Chemical Engineering University of Florida Gainesville, FL 32611-6005 e-mail: [email protected]
ORIGINAL ARTICLE
Photoprotection and Extended Drug Delivery by UV Blocking Contact Lenses Kuan-Hui Hsu* and Anuj Chauhan†
ABSTRACT Purpose. Extended release of photo-unstable drugs from ophthalmic inserts is not useful unless the loaded drug is protected from degradation. Because of the recent interest in extended drug delivery from contact lenses, it is critical to assess whether photo-unstable drugs can be stabilized by loading in lenses. Here, we focus on dexamethasone, which is prone to degradation and has been explored as a candidate for extended release from contact lenses for periods ranging from 10 hours to several days. Methods. Degradation rates of dexamethasone were measured in phosphate-buffered saline and after loading in contact lenses. The degradation rates were measured in a humidified, constant-temperature (32-C) chamber with controlled UV exposure. Contact lenses with various degrees of UV blocking were used to explore the relationship between degradation rates and UV exposure. It is known that vitamin E absorbs UV radiation; thus, it was loaded into the lenses to explore the feasibility of reducing the degradation rates. Results. About 85% of dexamethasone degraded in 20 hours in nonYUV blocking lenses, whereas less than 1% degraded in class 1 UV blocking lenses. Incorporation of vitamin E into the nonYUV blocking lenses reduced the fractional degradation to 30%. Degradation rates in phosphate-buffered saline were significantly higher than even in nonYUV blocking contact lenses. Conclusions. The degradation of dexamethasone can be minimized by using a UV blocking contact lens or incorporating vitamin E into a nonYUV blocking lens. Vitamin E incorporation has the dual benefits of improving drug stability and release profiles. (Optom Vis Sci 2016;93:395Y403) Key Words: dexamethasone, degradation, photo-oxidation, contact lens, UV blocker, vitamin E
D
examethasone, a corticosteroid, is a commonly prescribed ophthalmic drug for treatment of inflammatory symptoms such as conjunctivitis and keratitis and also eye injuries.1,2 The dosing of dexamethasone could be as frequent as four to six times a day and potentially last for a few weeks to months,2,3 which could lead to reduced compliance.4,5 Additionally, long-term administration could induce severe adverse effects including glaucoma, visual acuity impairment, and posterior subcapsular cataract formation.6,7 There is also the possibility of systemic adverse effects including diabetes, hemorrhagic ulcers, and osteoporosis.8 Several researchers have proposed that replacing eye drops by drug-eluting contact lenses could reduce systemic side effects because of the significant increase in corneal bioavailability.9Y11 Additional benefits such as improved compliance may be achievable by replacing a high-frequency eye drop instillation schedule with a single insertion of an extended-release contact
*PhD candidate † PhD Department of Chemical Engineering, University of Florida, Gainesville, Florida (both authors).
lens.12Y14 Several researchers have shown that contact lenses can be designed for extended release of dexamethasone lasting a day to as long as a month.15,16 Dexamethasone is photo-unstable with almost complete degradation within 6 hours of sunlight exposure.17 In fact, commercial eye drops containing dexamethasone are stored in white, opaque polyethylene containers to avoid photodegradation.18,19 A contact lensYbased extended-release system is only useful if the degradation duration is much longer than the release duration. The degradation is undesirable because of the reduction in the dose of the active drug, and additionally, the products of the degradation could possibly be toxic.13,19,20 We showed in an earlier study that the oxidation rates of a hydrophilic drug, cysteamine, are significantly reduced by incorporation in vitamin EYcontaining contact lenses.13 This study focuses on determining whether similar improvements in stability against degradation can be achieved for a hydrophobic photounstable drug, dexamethasone. The photodegradation of drugs is dependent on UV exposure, and thus, we specifically compare a class 1 UV blocking contact lens narafilcon B (1-DAY ACUVUE
Optometry and Vision Science, Vol. 93, No. 4, April 2016
396 Extended Drug Delivery by UV Blocking Contact LensesVHsu and Chauhan
TruEye, Vistakon, FL) with lotrafilcon B (O2 OPTIX, CIBA VISION Corp, GA), which blocks a smaller fraction of UV light. We also explore the effect of vitamin E incorporation on drug degradation because vitamin E diffusion barriers increase the drug release durations and also block UV-B and partly UV-A light.21
METHODS Materials Two commercial silicone hydrogel contact lenses, narafilcon B and lotrafilcon B, were purchased from 1SaveOnLens (Blaine, WA). Dexamethasone (Q98%, purity) and (T)->-tocopherol (Q96%, purity) were purchased from Sigma-Aldrich Chemicals (St. Louis, MO). Ethanol (200 proof ) was purchased from Decon Laboratories Inc (King of Prussia, PA). Dulbecco’s phosphate-buffered saline (PBS) was purchased from Mediatech, Inc (Manassas, VA). Highperformance liquid chromatography (HPLC)-grade acetonitrile and water were purchased from Fisher Scientific (Fair Lawn, NJ). All chemicals were used as received without further treatment.
Preparation of Dexamethasone-Loaded Contact Lenses
FIGURE 1. Schematic of the laboratory-built humidified chamber equipped with UV-B transilluminator. The UV intensity from the transilluminator was 16.50 T 0.01 mW/cm2 and the intensity received by the sample was 3.02 T 0.01 mW/cm2.
The lenses were soaked in 3 mL of 0.066 mg/mL dexamethasone in PBS solution for 7 days to load the drug. In this and other experiments, the samples were covered with aluminum foil to minimize dexamethasone degradation during the loading. The drug concentration in the solution was measured after the 7-day loading by UV-vis spectrophotometry (Thermospectronic Genesys 10 UV, Rochester, NY) to determine the total amount of dexamethasone loaded in the lens.
The UV light intensity received by the sample was measured to be 3.02 T 0.01 mW/cm2 by using UV light meter, which is within the very broad range of natural exposure. After 20 hours of exposure, the lens was removed and subjected to multiple extraction steps to elute the remaining drug and the degradation products. In each step, the lens was soaked in 2 mL of PBS for 24 hours and the concentration of dexamethasone in the extracted solution was determined. The experiment was repeated three times each for both narafilcon B and lotrafilcon B.
Measuring Degradation of Dexamethasone Loaded in the Lenses
Identification and Spectra of the Dominant Degradation Products
The UV intensity of sunlight is dependent on the angle of the sun, ozone concentration, season, location, time, and weather conditions.22 As a preliminary study, we restrict this study to a controlled environment of a custom-built humidified chamber using a UVB-10 (Ultra-Lum, Inc, Carson, CA) transilluminator with an intensity of 16.50 mW/cm2 that sharply peaked at 310 nm (Fig. 1). By comparison, the intensity of sunlight in the range 280 to 400 nm was measured to be 13 to 14 mW/cm2 (Sper Scientific light meter, Scottsdale, AZ) at noon in direct sun on a clear day in Florida in October. The intensity decreased significantly to about 1 mW/cm2 under shade or cloudy conditions. These measured values are within the range reported in literature.23,24 To avoid the potential for diffusion of the drug from the lens, the dexamethasone-loaded contact lens was taken out from the drug solution, gently blotted to remove residual solution on the surface, and then placed at the bottom of an 8-mL, 17- by 60-mm clear glass vial (Fisher Scientific Co, Suwanee, GA). The vial was then placed inside a humidified chamber as shown in Fig. 1. The chamber was humidified and maintained at a temperature of roughly 32-C to keep the sample lens hydrated. The transilluminator was placed at the bottom of the chamber, whereas the sample vial was suspended from the top to minimize the exposure of the sample to the heat generated by the transilluminator.
The major photodegradation product of dexamethasone is 17oxodexamethasone with minor presence of 6A-hydroxydexamethasone.25 To verify this and determine the spectra of each component, we exposed 1.5 mL of 0.046 mg/mL dexamethasone solution to UV irradiation for 25 minutes in the humidified chamber and separated the solution into individual components by HPLC. Acquity UPLC system (Waters Corporation, Milford, MA) equipped with a multiwavelength photodiode array detector and Nova-Pak C18 (4 Km particle size, 3.9- by 150-mm column) (Waters Corporation, Dublin, Ireland) was used for the analysis. The mobile phase consisted of 50% HPLC-grade water and 50% acetonitrile at a constant flow rate of 1.0 mL/min. The peak area was calculated at a wavelength of 240 nm.
Determination of Dexamethasone Concentration in Aqueous Solution The spectra for the drug and the degradation products obtained from the HPLC were subsequently used to determine the concentration of dexamethasone in aqueous solutions by measuring the absorbance spectra. Because the solution contained multiple components, the spectra were the linear combination of the spectra for each of the components in the solution. A least
Optometry and Vision Science, Vol. 93, No. 4, April 2016
Extended Drug Delivery by UV Blocking Contact LensesVHsu and Chauhan
squares fit method as described in the study of Kim and Chauhan26 was used to distinguish the absorbance from dexamethasone and 17-oxodexamethasone to calculate the concentration of dexamethasone in the mixture solution. The validity of this approach was confirmed by showing that the linear combination of dexamethasone and 17-oxodexamethasone fitted the measured spectra with less than 1% mean square error.
Effect of Vitamin E Incorporation on Degradation Rates Contact lenses were rinsed with deionized water to remove the package solution and then dried in air. Vitamin E-ethanol solutions of desired concentrations were prepared by adding vitamin E to ethanol followed by 3 hours of magnetic stirring. The dried narafilcon B and lotrafilcon B were soaked in 3 mL of vitamin E-ethanol solution with concentrations of 33 and 50 mg/mL, respectively, for 24 hours. After the soaking, the contact lenses were taken out, gently blotted, and dried in air again. The dried weights of contact lenses, both before and after vitamin E loading, were measured to determine the vitamin E loading. In this study, 15% vitamin EYloaded narafilcon B and 20% vitamin EYloaded lotrafilcon B were prepared. These vitamin E loading concentrations were chosen to increase the drug release duration to about 24 hours. The vitamin EYloaded lenses were then soaked in 3 mL of drug solution for 14 days. The drug and vitamin EYloaded lenses were tested for drug degradation by following the same procedure as described above. The experiment was repeated three times each for both narafilcon B and lotrafilcon B lenses.
Measuring Degradation of Dexamethasone Loaded in the Lens Hydrated in PBS The lens was loaded with dexamethasone by soaking in 3 mL of 0.07 mg/mL of dexamethasone solution. After reaching equilibrium, the lens was taken out of the drug solution and gently blotted and then placed in an 8-mL, 17- by 60-mm clear glass vial containing 2 mL of PBS solution. The glass vial was sealed to avoid evaporation and exposed to UV light in the humidified chamber at 32-C for variable periods ranging from 20 to 120 minutes. During the UV exposure, to avoid the degradation product reentry into the lens, the PBS solution was refreshed every 20 minutes in the first hour and then every 30 minutes. After the desired duration of UV exposure, the lenses were taken out of the vial and were subjected to the extraction process described above. The experiment was repeated three times. The mass of unreacted dexamethasone in the lens decreases with time not only because of reaction but also because of diffusion into the PBS. To compare these effects, we estimate the mass of dexamethasone released into the PBS by modeling the diffusion and then comparing that with the mass of drug extracted from the lens.16 The model parameters diffusivity D and partition coefficient K were obtained by measuring release of dexamethasone and fitting the data to a diffusion-controlled transport model.16 Briefly, narafilcon B lens was soaked in 3 mL of 0.07 mg/mL dexamethasone-PBS solution for 7 days, which was long enough to reach equilibrium. The drug-loaded lens was soaked in 2 mL of PBS, and the concentration of dexamethasone released to the PBS
397
was measured periodically by UV-vis spectrophotometry. The experiment was repeated three times.
Measuring Degradation of Dexamethasone Dissolved in Solution The same setup as described above was also used to measure the degradation rates of the drug dissolved in PBS, except that the contact lens was replaced by 1.5 mL of solution with 0.046 mg/mL of dissolved dexamethasone. The sealed vial was exposed to UV light from the transilluminator for a certain duration and the concentration of dexamethasone was then measured in the solution. The experiment was repeated 10 times with exposure times of 5, 10, 30, 40, 50, 60, 70, 90, 105, and 120 minutes and one sample for each time point.
Statistical Analysis The statistical comparison of dexamethasone degraded amount in each type of lens was analyzed by analysis of variance F test and Tukey multiple comparisons of means under a completely randomized design (> = 0.05). The analysis was performed in statistical software R (version R-2.15.2 for Windows, R Development Core Team).
RESULTS Determination of Dexamethasone Concentration in Aqueous Solution The UV spectra of the degradation products were obtained by exposing dexamethasone solution of 0.0465 mg/mL to UV irradiation in the humidified chamber (Fig. 1) for 25 minutes and then analyzing the solution by HPLC. The HPLC chromatogram
FIGURE 2. The individual spectra of dexamethasone and 17-oxodexamethasone. The spectrum of 17-oxodexamethasone was magnified 10 times.
Optometry and Vision Science, Vol. 93, No. 4, April 2016
398 Extended Drug Delivery by UV Blocking Contact LensesVHsu and Chauhan
(figure not shown) showed a dominant peak at the elution time for dexamethasone, a smaller peak with an absorbance of about 10% compared with dexamethasone, and finally a minor peak with an absorbance of about 1% compared with the dominant peak. These two peaks were interpreted as corresponding to the major degradation product (17-oxodexamethasone) and the minor product (6A-hydroxydexamethasone).25 The spectra of dexamethasone and its major degradation product (Fig. 2) were used as the reference spectra in the least squares fit method. As shown in Fig. 2, the spectrum of dexamethasone has a peak at 240 nm whereas 17-oxodexamethasone has a linear and much smaller absorbance
response in the wavelength range of 220 to 270 nm. As a further test of the accuracy of the method based on the least squares fit, we compared the degradation of dexamethasone after exposure for 25 minutes. For this sample, the HPLC data and the least squares fit yielded 49.67 and 48.43% degradation, which supports the validity of our approach.
Dexamethasone Degradation in Contact Lenses The unreacted drug remaining in the contact lens after the 20-hour exposure was determined by multiple cycles of extraction
FIGURE 3. The transient absorbance spectra in the PBS solution after soaking (A) lotrafilcon B, (B) 20% vitamin EYloaded lotrafilcon B, (C) narafilcon B, and (D) 15% vitamin EYloaded narafilcon B that were exposed to 20 hours of UV radiation. The legend indicates the time the spectra were measured after soaking in the PBS. The experiment was conducted in triplicate with multiple extraction cycles for each. The data shown are for the first extraction cycle for just one of the experiments. The solid lines are the fits to the data.
Optometry and Vision Science, Vol. 93, No. 4, April 2016
Extended Drug Delivery by UV Blocking Contact LensesVHsu and Chauhan
in PBS. Determining the drug loaded required just the equilibrium spectra, but the transient spectra were measured to validate the assumption of a single degradation product. Fig. 3A shows the transient absorbance spectra in the PBS solution after soaking the lotrafilcon B lens that was exposed to 20 hours of UV radiation. Data are shown only for the first extraction cycle, although six cycles were used to extract the drug. The solid lines are fits to the data assuming that the spectra are a linear combination of that from the drug dexamethasone and the main degradation product 17-oxodexamethasone. Fig. 3B to D show similar data for the lotrafilcon B lens with 20% vitamin E and narafilcon B without and with 15% vitamin E, respectively. In each case, the solid lines are good fits to the data, strongly suggesting that there is only one dominant degradation product in these experiments. The values of fractional degradation of dexamethasone loaded in contact lenses after 20 hours of UV exposure is listed in Table 1 for narafilcon B and lotrafilcon B lenses, both for control and for vitamin EYloaded lenses. The result showed that roughly 80% of dexamethasone was degraded in lotrafilcon B. With 20% vitamin E loaded, the percentage of degraded dexamethasone was reduced to about 30%, which was significantly different from that of pure lotrafilcon B (p G 10j6). The degradation was negligible in narafilcon B lenses, and vitamin E incorporation did not have a significant impact on degradation (p = 0.491). The degradation in narafilcon B is significantly different from that in lotrafilcon B with or without vitamin E (p G 10j6).
Dexamethasone Degradation in Contact Lenses Hydrated in PBS
FIGURE 4. Dexamethasone release profile from narafilcon B lenses. The experiment data are presented as mean T SD with n = 3. The average amount of dexamethasone initially loaded in the lens was 60.41 Kg and the average amount released to the PBS at equilibrium was 35.67 Kg. The solid line is the fitting result based on one-dimensional diffusion equation.
fraction of unreacted dexamethasone in the lens was calculated as the ratio of the measured mass of dexamethasone and the initial drug loading are included in the table. The two fractions are comparable, which shows that the drug did not degrade significantly even after 120 minutes of UV exposure with complete hydration.
The low degradation rates reported above could potentially be attributed to incomplete hydration of the lens and thus degradation rates were also measured in lenses soaked in PBS. Phosphatebuffered saline was replaced periodically to minimize reentry of any component into the lens, and the mass of each component released was estimated using a mathematical model. The model was first validated by fitting the release of the drug from narafilcon B lenses soaked in PBS (Fig. 4). The solid line in the figure is the model fit based on the one-dimensional diffusion equation with best-fit values of 1.176 10j4 mm2/h and 66.83 for diffusivity D and partition coefficient K, respectively. The fitting is good with rootmean-square error equal to 1.51%. The model was used to calculate the fraction of drug remaining in the lens after 20, 40, 60, 90, and 120 minutes, and these values are listed in Table 2. Also, the
Dexamethasone Degradation in Solution
TABLE 1.
DISCUSSION
Percentages of dexamethasone not degraded in contact lenses during the 20 hours of UV-B illumination in the customized chamber Dexamethasone not degraded, %
Contact lens
21.17 T 1.93 70.56 T 1.71 98.11 T 1.52 101.72 T 5.16
Pure lotrafilcon B 20% Vitamin EYloaded lotrafilcon B Pure narafilcon B 15% Vitamin EYloaded narafilcon B Data are presented as mean T SD (n = 3).
399
The fractional degradation of dexamethasone in solution is plotted as a function of time in Fig. 5 and the spectra of the solution after exposure of varying durations are shown in Fig. 6. The solid lines in Fig. 6 are good fits to the data, strongly suggesting that there is only one dominant degradation product in these experiments. The solid line in Fig. 5 is a model fit based on a firstorder reaction; that is,
% of degraded dexamethasone ¼ 1j exp ðjktÞ
ð1Þ
The data fit the model with a rate constant k of 0.029/min. The data show that dexamethasone was almost degraded in the PBS solution after 2 hours of UV light illumination.
Degradation Mechanism Dexamethasone can degrade into at least five known products and several more that have not been identified.25 Dexamethasone can degrade from various mechanisms, but the most relevant path for ophthalmic drug delivery is photodegradation. The UV degradation (366 nm) leads to the formation of six or more distinct products, but the major product is 17-oxodexamethasone,25 which is also produced by oxidation of dexamethasone, suggesting that the major pathway of UV-induced degradation is photo-oxidation. The molecular structures of dexamethasone and 17-oxodexamethasone
Optometry and Vision Science, Vol. 93, No. 4, April 2016
400 Extended Drug Delivery by UV Blocking Contact LensesVHsu and Chauhan TABLE 2.
Model-predicted percentage of dexamethasone that did not release to PBS during the UV exposure in the humidified chamber and the actual percentage of dexamethasone recovered from multiple extractions after UV exposure UV exposure time
20 min
40 min
60 min
90 min
120 min
% of dexamethasone that did not release 85.81 73.64 63.19 52.11 42.98 % of dexamethasone recovered from multiple extractions 90.56 T 7.90* 77.37 T 3.40* 65.12 T 4.29* 52.05 T 4.50* 41.14 T 4.13* *Data are presented as mean T SD (n = 3).
are shown in Fig. 7. We considered 17-oxodexamethasone as the only degradation product, and the concentrations of the parent molecule dexamethasone and the degradation product 17-oxodexamethasone were obtained by fitting the UV-vis spectra of the mixture to a linear combination of the spectra of the two components. The good agreement (G1% root-mean-square error) between the measured spectra and the fitted linear combination spectra strongly suggests the validity of our approach.
Degradation of Drug Loaded in Contact Lenses The results in Table 1 list the degradation fraction of drug loaded in the contact lenses after 20 hours of UV exposure. About 80% of drug degraded in the lotrafilcon B lens, whereas the fraction degraded was negligible in narafilcon B lens. Incorporation of 20% vitamin E into the lotrafilcon lenses significantly reduced the fractional degradation to about 30%, but vitamin E did not impact degradation dynamics in the narafilcon lenses. Narafilcon B lenses are class 1 UV blockers, that is, at least 99% UV-B and 90% UV-A radiation is blocked by the lens, whereas lotrafilcon B lenses block only about 25% of UV radiation ranging from 240 to 300 nm and less than 10% from 300 to 400 nm. The significant difference in the UV blocking profile is the likely reason for the improved drug stability in the narafilcon B lenses. Further support for the importance
of UV blocking in drug stability was provided by the decrease in degradation rates on incorporation of vitamin E into the lotrafilcon B lenses. Incorporation of 20% vitamin E into lotrafilcon B significantly increases the UV-B absorption by the lens, with almost 100% absorption below 300 nm and an average absorption of about 30% from 300 to 400 nm.21 Thus, a significant fraction of the radiation provided by the UV-B transilluminator is absorbed by the lens, thereby reducing the exposure of the drug to radiation. Because the drug degradation is through the photo-oxidation pathway, the rates could also be reduced by reduction in oxygen available for the reaction. However, because of the high oxygen permeability of the silicone hydrogel lenses and the minor effect of vitamin E incorporation on the oxygen permeability, it is likely that oxygen concentrations are not rate limiting in these cases.21 The degradation in the pure narafilcon B lenses was already negligible after 20 hours; hence, it was not possible to discern whether vitamin E incorporation has any impact on the degradation rates.
Degradation of Drug Loaded in Contact Lenses Hydrated in PBS To verify that the lenses were fully hydrated in the custom-built humidified chamber, similar tests were repeated with narafilcon B lenses soaked in PBS. As shown in Fig. 4, the release duration of
FIGURE 5. Percentage of degraded dexamethasone in PBS solution induced by UV light exposure in the customized chamber. The experiment was repeated 10 times with one experiment for each time point. The solid line is the fitting result based on a first-order reaction model (equation 1). Optometry and Vision Science, Vol. 93, No. 4, April 2016
Extended Drug Delivery by UV Blocking Contact LensesVHsu and Chauhan
401
FIGURE 6. The absorbance spectra of dexamethasone-PBS solutions after exposure to various periods of UV light as indicated in the legend. A total of 10 separate experiments were conducted with each experiment lasting a different time duration. The solid lines are fits to the data.
dexamethasone from narafilcon B is less than 10 hours; thus, it is impossible to measure the degradation percentage after 20 hours of UV exposure as the drug would diffuse out from the lens. To gauge the degradation of the drug in the lens during loading in contact lenses, we compared the fraction of drug in the lens if there was no degradation with the actual fraction of undegraded dexamethasone recovered from the lens. The fractions of drug remaining in the lens without considering the reaction were obtained from the model and are listed in Table 2 for various times. The fractions of drug extracted from the lens after various durations of soaking with UV exposure are also included in the table. Statistical analysis showed that the two fractions are not significantly different for all time points with p values of 0.6541, 0.4419, 0.7343, 0.9916, and 0.7368 for exposure of 20, 40, 60, 90, and 120 minutes, respectively. This shows that the extent of reaction is negligible even after 120 minutes, which is consistent with the data obtained from the tests in which the lens was not soaked in solution but kept hydrated by controlling the humidity in the chamber.
solution and the lotrafilcon lens was comparable; thus, the reduction in the degradation rates is likely attributed to the significant difference in the environment of the drug loaded in the lens
Degradation of Drug Dissolved in PBS The fractional degradation of the drug dissolved in PBS shown in Fig. 5 fits a first-order reaction model with a rate constant of 0.029/min. The first-order model again assumes that the drug degrades into only one product and the rate is not limited by the oxygen concentration. The rate constant obtained by the fitting will depend on the intensity of the UV radiation as the reaction mechanism is photo-oxidation. The good fit between the model and the data further supports the assumption of a single key degradation product. The data also show that the drug degrades almost entirely in about 2 hours, which is much more rapid compared with even the lotrafilcon B lens, which had about 80% degradation after 20 hours. The UV exposure in both PBS
FIGURE 7. Structure of (A) dexamethasone and (B) the main degradation product 17-oxodexamethasone from UV-induced or oxidative degradation.
Optometry and Vision Science, Vol. 93, No. 4, April 2016
402 Extended Drug Delivery by UV Blocking Contact LensesVHsu and Chauhan
compared with that in the solution. Most importantly, the availability of oxygen in the lens could be lower compared with that in the solution. The silicone hydrogel contact lenses are designed to be highly permeable to oxygen, which mainly partitions and diffuses through the silicone phase of the bicontinuous matrix. The drug loaded in the contact lens would partition in both the silicone and the hydrogel phases, with a higher partition coefficient expected in the silicone phase owing to the hydrophobicity of the drug. Whereas the silicone phase certainly has a high oxygen solubility, the dissolved oxygen is likely closely associated, potentially surface adsorbed, to the silicone polymer, and thus may not be available to the drug molecules partitioned in the silicone phase.
Clinical Relevance Drug-eluting contact lenses are excellent candidates for treatment of anterior chamber diseases. Many researchers are designing contact lenses for extended release of several drugs including dexamethasone as a potential replacement for eye drops.12,26,27 The extended release is an important feature of the lenses but a long release duration may not be useful for unstable drugs. In fact, the release duration from an optimally designed drug-eluting lens should be less than the degradation time to ensure that the drug released into the tear film is the active molecule and not the degradation products, which lack the therapeutic activity of the drug and, in some cases, could also be toxic.13,19,20 Here, we show that vitamin EYloaded contact lenses protect dexamethasone from degradation; hence, the fractional degradation during the release duration is small. Thus, vitamin EYloaded contact lenses are promising for extended delivery of dexamethasone and potentially other unstable drugs as well.
CONCLUSIONS There is a growing interest in drug-eluting devices for extended delivery of ophthalmic drugs. Several ophthalmic drugs such as dexamethasone are unstable and thus extended release may not be useful unless the encapsulation method also provides a stabilizing effect. This study shows that the degradation rates of drug loaded in contact lenses are much lower than the rates in solution. Furthermore, the degree of UV blocking significantly impacts the degradation rates, with negligible degradation in a class 1 UV blocking lens. Even in a nonYUV blocking lens, the degradation rates are significantly lower than those in solution, and further reduction in the degradation rates can be achieved by incorporating vitamin E into the lenses. Vitamin E incorporation is very useful because of its effect on increasing the release duration, but equally importantly, it also blocks the UV light, which can have a protective effect on the eye, and also reduces the degradation rates of the drugs like dexamethasone. Although the results of this study are encouraging, the drug degradation under in vivo conditions could be significantly different owing to the differences in sun exposure compared with the simulated environment. Thus, future in vivo studies are necessary to gauge the benefits of vitamin E incorporation in contact lenses to reduce the drug degradation under typical wear conditions. Received August 4, 2014; accepted February 14, 2015.
REFERENCES 1. Wagoner MD. Chemical injuries of the eye: current concepts in pathophysiology and therapy. Surv Ophthalmol 1997;41:275Y313. 2. Gaudio P. A review of evidence guiding the use of corticosteroids in the treatment of intraocular inflammation. Ocul Immunol Inflamm 2004;12:169Y92. 3. Dexamethasone Dosing Information. Drugs.com. Available at: http://www.drugs.com/mtm/dexamethasone-ophthalmic.html. Accessed October 26, 2014. 4. Schwartz GF, Quigley HA. Adherence and persistence with glaucoma therapy. Surv Ophthalmol 2008;53:S57Y68. 5. Schwartz GF. Compliance and persistency in glaucoma follow-up treatment. Curr Opin Ophthalmol 2005;16:114Y21. 6. Schwartz B. The response of ocular pressure to corticosteroids. Int Ophthalmol Clin 1966;6:929Y89. 7. Urban RC, Jr., Cotlier E. Corticosteroid-induced cataracts. Surv Ophthalmol 1986;31:102Y10. 8. Clark AF, Yorio T. Ophthalmic drug discovery. Nat Rev Drug Discov 2003;2:448Y59. 9. Li CC, Chauhan A. Modeling ophthalmic drug delivery by soaked contact lenses. Ind Eng Chem Res 2006;45:3718Y34. 10. Xinming L, Yingde C, Lloyd AW, Mikhalovsky SV, Sandeman SR, Howel CA, Liewen L. Polymeric hydrogels for novel contact lensbased ophthalmic drug delivery systems: a review. Cont Lens Anterior Eye 2008;31:57Y64. 11. Alvarez-Lorenzo C, Hiratani H, Concheiro A. Contact lenses for drug delivery: achieving sustained release with novel systems. Am J Drug Del 2006;4:131Y51. 12. Bengani LC, Hsu KH, Gause S, Chauhan A. Contact lenses as a platform for ocular drug delivery. Expert Opin Drug Del 2013;10: 1483Y96. 13. Hsu KH, Fentzke RC, Chauhan A. Feasibility of corneal drug delivery of cysteamine using vitamin E modified silicone-hydrogel contact lenses. Eur J Pharm Biopharm 2013;85:531Y40. 14. Nordstrom BL, Friedman DS, Mozaffari E, Quigley HA, Walker AM. Persistence and adherence with topical glaucoma therapy. Am J Ophthalmol 2005;140:598Y606. 15. Kim J, Conway A, Chauhan A. Extended delivery of ophthalmic drugs by silicone hydrogel contact lenses. Biomaterials 2008;29: 2259Y69. 16. Kim J, Peng CC, Chauhan A. Extended release of dexamethasone from silicone-hydrogel contact lenses containing vitamin E. J Control Release 2010;148:110Y6. 17. Kawabata K, Sugihara K, Sanoh S, Kitamura S, Ohta S. Photodegradation of pharmaceuticals in the aquatic environment by sunlight and UV-A, -B and -C irradiation. J Toxicol Sci 2013;38: 215Y23. 18. Piechocki JT, Thoma K. Photodegradation and photoprotection in clinical practice. In: Piechocki JT, Thoma K, eds. Pharmaceutical Photostability and Photostabilization Technology. London, UK: Informa Healthcare; 2010;397Y423. 19. Klick S, Muijselaar PG, Waterval J, Eichinger T, Korn C, Gerding TK, Debets AJ, Sanger-Van de Griend C, Van den Beld C, Somsen GW, De Jong GJ. Toward a generic approach for stress testing of drug substances and drug products. Pharm Technol 2005;29:48Y57. 20. Samuel NT, Kenkare N, Lackey PD, Chen J. Contact lenses containing carotenoid and method for making same. US patent 8784867 B2. July 22, 2014. Available at: https://www.google.com/ patents/US8784867. Accessed March 26, 2015.
Optometry and Vision Science, Vol. 93, No. 4, April 2016
Extended Drug Delivery by UV Blocking Contact LensesVHsu and Chauhan 21. Peng CC, Kim J, Chauhan A. Extended delivery of hydrophilic drugs from silicone-hydrogel contact lenses containing vitamin E diffusion barriers. Biomaterials 2010;31:4032Y47. 22. Bauer DR. Interpreting weathering acceleration factors for automotive coatings using exposure models. Polym Degrad Stabil 2000;69:307Y16. 23. Getter KL, Rowe DB, Cregg BM. Solar radiation intensity influences extensive green roof plant communities. Urban Forest Urban Green 2009;8:269Y81. 24. Lee DW, Downum KR. The spectral distribution of biologically active solar radiation at Miami, Florida, USA. Int J Biometeorol 1991;35:48Y54. 25. Chen Q, Zielinski D, Chen J, Koski A, Werst D, Nowak S. A validated, stability-indicating HPLC method for the determination of
403
dexamethasone related substances on dexamethasone-coated drugeluting stents. J Pharm Biomed Anal 2008;48:732Y8. 26. Kim J, Chauhan A. Dexamethasone transport and ocular delivery from poly(hydroxyethyl methacrylate) gels. Int J Pharm 2008;353: 205Y22. 27. Hsu KH, Gause S, Chauhan A. Review of ophthalmic drug delivery by contact lenses. J Drug Deliv Sci Tec 2014;24:123Y35.
Optometry and Vision Science, Vol. 93, No. 4, April 2016
Anuj Chauhan Department of Chemical Engineering University of Florida Gainesville, FL 32611-6005 e-mail: [email protected]
