International Ophthalmology 16: 23-26, 1992. 9 1992 KluwerAcademic Publishers. Printed in the Netherlands.
Relationship between precorneal retention of viscous eye drops and tear fluid composition A. Ludwig, ~ N.J. van Haeringen, 2 V.M.W. Bodelier 2 & M. Van Ooteghem 1 1 University of Antwerp (UIA), Belgium; 2 The Netherlands Ophthalmic Research Institute, Amsterdam, The Netherlands Accepted 9 July 1991
Key words: precorneal retention, tear fluid composition, viscous eye drops Abstract
The influence of viscolysers on the precorneal residence of a fluorescent tracer was determined, using slit lamp fluorophotometry. The solution acceptability was evaluated by the volunteers by answering a standard questionnaire. The relationship between precorneal retention of viscous eye drops, discomfort and tear fluid composition after instillation of various cellulosic solutions was examined. Irritating hydroxypropylcellulose solution increases the total protein concentration of tears, without change in the ratio of lysozyme to total protein.
Abbreviations: HEC - hydroxyethylcellulose; HPC - hydroxypropylcellulose; HPMC - hydroxypropylmethylcellulose
Introduction
To optimize ocular drug bioavailability from eye drops it will be necessary to minimize the negative influences exerted by the primary protective mechanisms of the eye, namely lacrimation, tear turn over and drainage, on precorneal drug retention. A possible approach to extend drug residence time in the eye, thereby prolonging drug absorption, is to incorporate viscolysers into the vehicle. Solution viscosity would be increased and hence solution drainage would be reduced [1-3]. The aim of the present work is to examine the efficacy of several cellulosic viscolysers on ophthalmic formulations. The objective evaluation of viscous eye drops containing a fluorescent tracer, sodium fluorescein, is done by measuring the fluorescence decay in the precorneal tear film with a slit
lamp fluorophotometer. The subjective assessment of the acceptability of the viscous solutions occurs by answering a standard questionnaire by each volunteer. Moreover the influence of the viscolysers on the tear fluid composition is determined.
Materials and methods
Viscous eye drops The three non-ionic cellulosic viscolysers selected are: hydroxyethylcellulose (HEC) (Natraso1250G, tool. wt. 250000), hydroxypropylcellulose (HPC) (Klucel LF, mol. wt. 95000) both obtained from Hercules Chemicals (Brussels, B) and hydroxypropylmethylcellulose (HPMC) (Celacol 5000, mol. wt. 150000) from Socomer (Brussels, B).
24
A. Ludwig et al.
Physical characterization of the viscous solutions
profile (fluorescence intensity vs time) was recorded using a slit lamp fluorophotometer [4]. The decay profile obeyed fairly well first-order kinetics (correlation coefficient > 0.90). The data were subjected to linear regression analysis by the method of least squares and the elimination rate constant, called tear elimination coefficient, k, were determined [4]. The surface area under the decay curve (AUC) was calculated, using the trapezoidal rule. The tests were repeated on six occasions on consecutive days or a few days interval. The mean k and A U C value of the six experiments on each volunteer with each solution was calculated.
Viscosity determination
Solution acceptability
The viscosity measurements on 3 ml of the sterilized viscous solutions were carried out using a capillary Ostwald viscometer (KPG Viskosimeter Schott Ger~ite, Mainz, FRG) at a constant temperature of 32 + 0.1 ~C. The viscometer was standardized with distilled water. The efflux times determinations were made after a time of 15 minutes needed for equalization of temperature. The effiux times measurements were performed in triplicate and the viscosity of the samples was calculated.
After each experiment the volunteers were asked to give their own evaluation of the viscous solution by answering a standard questionnaire. Questions included were: does your eye hurt, sting, feel watery? Does the drop feel smooth, thick, sticky, gritty or sandy? Does the drop cause blurring of vision? A grading from 0 to 5 was used.
Sodium fluorescein was purchased from Fluka (Buchs, CH). All other chemicals are of analytical grade. All viscous solutions were prepared by incorporating the required amount of viscolysers into an iso-osmotic phosphate buffer solution pH 7.4 (Ph. Helv. V). Sodium fluorescein was added to the vehicles to a final concentration of 0.05% w/w. The solutions were filtered and sterilized by autoclaving. The composition of the various viscous solutions tested is indicated in Table 1.
Osmolality measurement The osmolality of each sterilized viscous solution was determined with a vapor pressure osmometer (model 5500 Wescor, Logan (UT) USA).
Tear fluid analysis Samples (10-20/zl) of tear fluid were collected in glass capillaries with minimal irritation. Total protein was determined with the Bradford method [5], using human serum albumin as standard. Lysozyme activity was measured with a spectrophotometric method, using Micrococcus lysodeikticus as substrate for the bacteriolytic action and human lysozyme as standard [6].
Surface tension measurement The surface tension was measured at 22~ by the Wilhelmy plate method, using a Cahn electrobalance. The measurement was performed 20 sec after pouring th.e solution in the petri dish. This time period was chosen in relation to the blinking dynamics and the mean interblink period of the volunteers.
Slit lamp fluorophotometry Five adult volunteers participated in this study, after informed consent was obtained. The phosphate buffer solution or the viscous solution (10/xl) was instilled in the lower conjunctival sac of the left eye. The fluorescence decay
Table 1. Characteristics of the viscous eye drops. Composition
Viscosity mPa.s
Osmolality Surf. Tension mosm/kg mN/m
phosph, buffer sol. + 0.36% HPMC + 0.64% HPMC + 1.80% HPC + 3.40% HPC + 1.10% HEC + 1.70% HEC
0.9 6.7 23.7 7.4 27.7 9.0 26.5
280 272 282 279 286 278 286
71.9 44.5 49.9 41.8 39.9 61.8 56.9
Precorneal retention and tear fluid composition Results and discussion
Physical characterization of the viscous eye drops The viscosity data reported in Table 1 were obtained with a capillary viscometer, because of the solutions tested exhibit approximate newtonian flow properties. The concentration of the viscolysers used has little influence on the osmolality of the vehicles. The addition of HPC and HPMC reduces the surface tension of the vehicle. The surface tensions obtained are in the same range as the surface tension of lacrimal fluid [7]. HEC exhibits less influence on surface activity.
25
the clearance of the viscolyser from the eye does not necessarily follow the elimination of the tracer [8]. The highest AUC values were observed after instillation of the 3.4% HPC solution. The AUC value is determined by the initial fluorescence of the tear film just after instillation of the viscous solution and by the rate of disappearance of the tracer. Compared to the phosphate buffer solution, firstly a higher initial fluorescence was measured indicating an effect of the viscolyser on the precorneal tear film thickness and secondly the drainage was decreased. The extent of each effect varies both between iso-viscous eye drops and between volunteers. Those high AUC values observed after instillation of a 3.4% HPC solution are partly due to the high waterdragging capacity of HPC.
Tear elimination coefficient The mean value (_+ SEM) of the tear elimination coefficient k and the AUC for the group of five individuals are reported in Table 2. The data show that in general clearance of sodium fluorescein from the precorneal area is found to decrease markedly through addition of viscolysers to the vehicle. But there is a large variation between the volunteers for any viscolyser solution. HEC seems to be the most effective to decrease the elimination rate of sodium fluorescein. In some cases higher elimination rates with respect to phosphate buffer solution are observed after instillation of HPC and HPMC solutions. These solutions cause discomfort and the volunteer reacts by forceful blinking, thus accelerating the drainage of the eye drop instilled. Because no binding between sodium fluorescein and any viscolyser was observed,
Solution acceptability The questionnaire responses are summarized in Table 3. Mean values (+ SEM) of the evaluation over 5 individuals are given. HEC is rated as the most comfortable. HPC and HPMC give rise to complaints of irritation and blurred vision, resulting in lacrimation and a higher blink frequency. Consequently, after instillation of HPMC and HPC eye drops higher k values were observed compared to HEC solutions. A non parametric analysis using the Spearman rank correlation coefficient reveals, however, no significant correlation between tear elimination coefficient k and solution acceptability at 0.05 level. The present study indicates that HEC is the best Table 3. Solution acceptability.
Table 2. Mean value (_+ SEM) of k (10 -3 s 1) and A U C of five individuals.
Solution
k
AUC
phosph, buffersol. 0.36% HPMC 0.64% HPMC 1.80% HPC 3.40% HPC 1.10% HEC 1.70% HEC
12.10_+ 4.33 10.80 _+ 6.11 7.24_+ 4.61 11.14 -+ 7.99 8.59_+ 5.93 5.70_+ 3.23 3.88_+ 1.38
16.20_+ 4.13 21.52 _+ 9.22 33.64 + 14.60 24.63 _+ 9.86 42.81 -+ 19.42 23.35_+ 8.71 27.67_+ 15.77
Questions
A
B
C
Solution 0.36% HPMC 0.64% HPMC 1.80% HPC 3.40% HPC 1.10% HEC 1.70% HEC
2.0_+ 0.7 2.8 _+ 0.8 2.4 _+ 0.5 3.6_+ 0.9 0.4_+ 0.9 1.2+ 1.1
1.8_+ 1.1 2.4 _+ 1.5 2.0_+ 1.0 2.6 + 1.5 1.4_+ 0.9 1.6_+ 0.9
0 . 8 + 0.8 2.4_+ 0.5 1.0_+ 0.7 2.6 _+ 0.5 0.2+ 0.4 1.2-+ 1.1
A: no irritation (0), mild (1), hurting (2), stinging (3-5); B: eye water (0-2), lacrimation (3-4), overflow (5); C: blurred vision (0-5).
26
A. Ludwig et al.
tolerated solution with all volunteers. At any viscosity it produces a homogeneous film on the eye, observed with the slit-lamp during recording of the decay profile. At the contrary the HPC and HPMC solutions do not mix easily with lacrimal fluid and irregular films were observed. Moreover their reduced surface tension could be the reason for the irritation elicited, as noted in a previous study after instillation of non-ionic surfactant solutions [9].
Protein in tear fluid The viscous solutions, containing 3.4% HPC and 1.7% HEC, were chosen to investigate their influence on tear protein concentration. If leaking of plasma proteins from conjunctival vessels or stimulation of tear flow should occur after instillation of the viscous eye drop, the protein composition of the tear fluid could be changed. Protein was determined in tears collected before and 30 min after instillation of 10/zl of the viscous eye drop and as a control also after instillation of the vehicle, consisting of phosphate buffer solution. The 3.4% HPC caused a significant increase in total protein of the tears, but not 1.7% HEC or the vehicle (Table 4). The lysozyme concentration after 3.4% HPC also was increased in the same ratio as the total protein.
Conclusions
The addition of cellulose polymers to ophthalmic solutions in order to increase the viscosity of eye drops enhances the ocular retention of sodium fluorescein. HEC is much better tolerated than the more surface active HPMC and HPC. Irritative 3.4% HPC solution increases signifTable 4. Mean values (+ SEM) of tear proteins of five individuals.
before instill. after phosph sol after 1.70% HEC after 3.40% HPC
Total protein (g/l)
Lysozyme (g/l)
7.2 + 1.2 6.0 _+ 2.3 7.2 _+ 2.2 12.4 + 1.7"
2.16 + 0.44
3.22 + 0.29*
* Significance of difference vs before instillation: P < 0.05 (Wilcoxon's signed rank test).
icantly the total protein concentration of tears. Because the lysozyme concentration increased in parallel with total protein a systematic and not a local origin, such as leakage of plasma proteins into the tears, may be the underlying condition of the protein increase. This increase may be due to a removal of water from the tear film during the prolonged corneal contact time of the viscous eye drop, either by increased evaporation from the disturbed tear film or by osmotic attraction of water into the underlying epithelium. In both explanations the lower surface tension (39.9 mN/m) of the 3.4% HPC solution, as compared to the higher value (56.9 mN/m) of the 1.7% HEC solution, may be held responsible for the observed increase in protein in the tears.
References 1. Blaug SM, Canada AT. Relationship of viscosity, contact time and prolongation of action of methylcellulose containing ophthalmic solutions. Am J Hosp Pharm 1965; 22: 662-6. 2. Hardberger R, Hanna C, Boyd CM. Effects of drug vehicles on ocular contact time. Arch Ophthalmol 1975; 93: 42-5. 3. Trueblood JH, Rossomondo RM, Carlton WH, Wilson LA. Corneal contact times of ophthalmic vehicles. Arch Ophthalmol 1975; 93: 127-30. 4. Ludwig A, Van Ooteghem M. The study of the precorneal dynamics of ophthalmic solutions by fluorophotometry. Pharm Acta Helv 1986; 85: 59-65. 5. Bradford MM. Rapid and sensitive method for the quantitation of microgram quantities of protein utilizing the principle of protein-dye binding. Anal Biochem 1976; 72: 248-53. 6. Ensink FTE, van Haeringen NJ. Pitfalls in the assay of lysozyme in human tear fluid. Ophthalmic Res 1977; 9: 366-73. 7. Tiffany JM, Winter N, Bliss G. Tear film stability and tear surface tension. Curt Eye Res 1989; 8: 507-15. 8. Ludwig A, Van Ooteghem M. The evaluation of viscous ophthalmic vehicles by slit lamp fluorophotometry in humans. Int J Pharmaceutics 1989; 54: 95-102. 9. Ludwig A, Van Ooteghem M. Influence of the surface tension of eye drops on the retention of a tracer in the precomeal area of human eyes. J Pharm Belg 1988; 43: 157-62.
Address for offprints: A. Ludwig, University of Antwerp, Dept. Pharmaceutical Scs., Universiteitsplein 1, 2610 Wilrijk, Belgium
Relationship between precorneal retention of viscous eye drops and tear fluid composition A. Ludwig, ~ N.J. van Haeringen, 2 V.M.W. Bodelier 2 & M. Van Ooteghem 1 1 University of Antwerp (UIA), Belgium; 2 The Netherlands Ophthalmic Research Institute, Amsterdam, The Netherlands Accepted 9 July 1991
Key words: precorneal retention, tear fluid composition, viscous eye drops Abstract
The influence of viscolysers on the precorneal residence of a fluorescent tracer was determined, using slit lamp fluorophotometry. The solution acceptability was evaluated by the volunteers by answering a standard questionnaire. The relationship between precorneal retention of viscous eye drops, discomfort and tear fluid composition after instillation of various cellulosic solutions was examined. Irritating hydroxypropylcellulose solution increases the total protein concentration of tears, without change in the ratio of lysozyme to total protein.
Abbreviations: HEC - hydroxyethylcellulose; HPC - hydroxypropylcellulose; HPMC - hydroxypropylmethylcellulose
Introduction
To optimize ocular drug bioavailability from eye drops it will be necessary to minimize the negative influences exerted by the primary protective mechanisms of the eye, namely lacrimation, tear turn over and drainage, on precorneal drug retention. A possible approach to extend drug residence time in the eye, thereby prolonging drug absorption, is to incorporate viscolysers into the vehicle. Solution viscosity would be increased and hence solution drainage would be reduced [1-3]. The aim of the present work is to examine the efficacy of several cellulosic viscolysers on ophthalmic formulations. The objective evaluation of viscous eye drops containing a fluorescent tracer, sodium fluorescein, is done by measuring the fluorescence decay in the precorneal tear film with a slit
lamp fluorophotometer. The subjective assessment of the acceptability of the viscous solutions occurs by answering a standard questionnaire by each volunteer. Moreover the influence of the viscolysers on the tear fluid composition is determined.
Materials and methods
Viscous eye drops The three non-ionic cellulosic viscolysers selected are: hydroxyethylcellulose (HEC) (Natraso1250G, tool. wt. 250000), hydroxypropylcellulose (HPC) (Klucel LF, mol. wt. 95000) both obtained from Hercules Chemicals (Brussels, B) and hydroxypropylmethylcellulose (HPMC) (Celacol 5000, mol. wt. 150000) from Socomer (Brussels, B).
24
A. Ludwig et al.
Physical characterization of the viscous solutions
profile (fluorescence intensity vs time) was recorded using a slit lamp fluorophotometer [4]. The decay profile obeyed fairly well first-order kinetics (correlation coefficient > 0.90). The data were subjected to linear regression analysis by the method of least squares and the elimination rate constant, called tear elimination coefficient, k, were determined [4]. The surface area under the decay curve (AUC) was calculated, using the trapezoidal rule. The tests were repeated on six occasions on consecutive days or a few days interval. The mean k and A U C value of the six experiments on each volunteer with each solution was calculated.
Viscosity determination
Solution acceptability
The viscosity measurements on 3 ml of the sterilized viscous solutions were carried out using a capillary Ostwald viscometer (KPG Viskosimeter Schott Ger~ite, Mainz, FRG) at a constant temperature of 32 + 0.1 ~C. The viscometer was standardized with distilled water. The efflux times determinations were made after a time of 15 minutes needed for equalization of temperature. The effiux times measurements were performed in triplicate and the viscosity of the samples was calculated.
After each experiment the volunteers were asked to give their own evaluation of the viscous solution by answering a standard questionnaire. Questions included were: does your eye hurt, sting, feel watery? Does the drop feel smooth, thick, sticky, gritty or sandy? Does the drop cause blurring of vision? A grading from 0 to 5 was used.
Sodium fluorescein was purchased from Fluka (Buchs, CH). All other chemicals are of analytical grade. All viscous solutions were prepared by incorporating the required amount of viscolysers into an iso-osmotic phosphate buffer solution pH 7.4 (Ph. Helv. V). Sodium fluorescein was added to the vehicles to a final concentration of 0.05% w/w. The solutions were filtered and sterilized by autoclaving. The composition of the various viscous solutions tested is indicated in Table 1.
Osmolality measurement The osmolality of each sterilized viscous solution was determined with a vapor pressure osmometer (model 5500 Wescor, Logan (UT) USA).
Tear fluid analysis Samples (10-20/zl) of tear fluid were collected in glass capillaries with minimal irritation. Total protein was determined with the Bradford method [5], using human serum albumin as standard. Lysozyme activity was measured with a spectrophotometric method, using Micrococcus lysodeikticus as substrate for the bacteriolytic action and human lysozyme as standard [6].
Surface tension measurement The surface tension was measured at 22~ by the Wilhelmy plate method, using a Cahn electrobalance. The measurement was performed 20 sec after pouring th.e solution in the petri dish. This time period was chosen in relation to the blinking dynamics and the mean interblink period of the volunteers.
Slit lamp fluorophotometry Five adult volunteers participated in this study, after informed consent was obtained. The phosphate buffer solution or the viscous solution (10/xl) was instilled in the lower conjunctival sac of the left eye. The fluorescence decay
Table 1. Characteristics of the viscous eye drops. Composition
Viscosity mPa.s
Osmolality Surf. Tension mosm/kg mN/m
phosph, buffer sol. + 0.36% HPMC + 0.64% HPMC + 1.80% HPC + 3.40% HPC + 1.10% HEC + 1.70% HEC
0.9 6.7 23.7 7.4 27.7 9.0 26.5
280 272 282 279 286 278 286
71.9 44.5 49.9 41.8 39.9 61.8 56.9
Precorneal retention and tear fluid composition Results and discussion
Physical characterization of the viscous eye drops The viscosity data reported in Table 1 were obtained with a capillary viscometer, because of the solutions tested exhibit approximate newtonian flow properties. The concentration of the viscolysers used has little influence on the osmolality of the vehicles. The addition of HPC and HPMC reduces the surface tension of the vehicle. The surface tensions obtained are in the same range as the surface tension of lacrimal fluid [7]. HEC exhibits less influence on surface activity.
25
the clearance of the viscolyser from the eye does not necessarily follow the elimination of the tracer [8]. The highest AUC values were observed after instillation of the 3.4% HPC solution. The AUC value is determined by the initial fluorescence of the tear film just after instillation of the viscous solution and by the rate of disappearance of the tracer. Compared to the phosphate buffer solution, firstly a higher initial fluorescence was measured indicating an effect of the viscolyser on the precorneal tear film thickness and secondly the drainage was decreased. The extent of each effect varies both between iso-viscous eye drops and between volunteers. Those high AUC values observed after instillation of a 3.4% HPC solution are partly due to the high waterdragging capacity of HPC.
Tear elimination coefficient The mean value (_+ SEM) of the tear elimination coefficient k and the AUC for the group of five individuals are reported in Table 2. The data show that in general clearance of sodium fluorescein from the precorneal area is found to decrease markedly through addition of viscolysers to the vehicle. But there is a large variation between the volunteers for any viscolyser solution. HEC seems to be the most effective to decrease the elimination rate of sodium fluorescein. In some cases higher elimination rates with respect to phosphate buffer solution are observed after instillation of HPC and HPMC solutions. These solutions cause discomfort and the volunteer reacts by forceful blinking, thus accelerating the drainage of the eye drop instilled. Because no binding between sodium fluorescein and any viscolyser was observed,
Solution acceptability The questionnaire responses are summarized in Table 3. Mean values (+ SEM) of the evaluation over 5 individuals are given. HEC is rated as the most comfortable. HPC and HPMC give rise to complaints of irritation and blurred vision, resulting in lacrimation and a higher blink frequency. Consequently, after instillation of HPMC and HPC eye drops higher k values were observed compared to HEC solutions. A non parametric analysis using the Spearman rank correlation coefficient reveals, however, no significant correlation between tear elimination coefficient k and solution acceptability at 0.05 level. The present study indicates that HEC is the best Table 3. Solution acceptability.
Table 2. Mean value (_+ SEM) of k (10 -3 s 1) and A U C of five individuals.
Solution
k
AUC
phosph, buffersol. 0.36% HPMC 0.64% HPMC 1.80% HPC 3.40% HPC 1.10% HEC 1.70% HEC
12.10_+ 4.33 10.80 _+ 6.11 7.24_+ 4.61 11.14 -+ 7.99 8.59_+ 5.93 5.70_+ 3.23 3.88_+ 1.38
16.20_+ 4.13 21.52 _+ 9.22 33.64 + 14.60 24.63 _+ 9.86 42.81 -+ 19.42 23.35_+ 8.71 27.67_+ 15.77
Questions
A
B
C
Solution 0.36% HPMC 0.64% HPMC 1.80% HPC 3.40% HPC 1.10% HEC 1.70% HEC
2.0_+ 0.7 2.8 _+ 0.8 2.4 _+ 0.5 3.6_+ 0.9 0.4_+ 0.9 1.2+ 1.1
1.8_+ 1.1 2.4 _+ 1.5 2.0_+ 1.0 2.6 + 1.5 1.4_+ 0.9 1.6_+ 0.9
0 . 8 + 0.8 2.4_+ 0.5 1.0_+ 0.7 2.6 _+ 0.5 0.2+ 0.4 1.2-+ 1.1
A: no irritation (0), mild (1), hurting (2), stinging (3-5); B: eye water (0-2), lacrimation (3-4), overflow (5); C: blurred vision (0-5).
26
A. Ludwig et al.
tolerated solution with all volunteers. At any viscosity it produces a homogeneous film on the eye, observed with the slit-lamp during recording of the decay profile. At the contrary the HPC and HPMC solutions do not mix easily with lacrimal fluid and irregular films were observed. Moreover their reduced surface tension could be the reason for the irritation elicited, as noted in a previous study after instillation of non-ionic surfactant solutions [9].
Protein in tear fluid The viscous solutions, containing 3.4% HPC and 1.7% HEC, were chosen to investigate their influence on tear protein concentration. If leaking of plasma proteins from conjunctival vessels or stimulation of tear flow should occur after instillation of the viscous eye drop, the protein composition of the tear fluid could be changed. Protein was determined in tears collected before and 30 min after instillation of 10/zl of the viscous eye drop and as a control also after instillation of the vehicle, consisting of phosphate buffer solution. The 3.4% HPC caused a significant increase in total protein of the tears, but not 1.7% HEC or the vehicle (Table 4). The lysozyme concentration after 3.4% HPC also was increased in the same ratio as the total protein.
Conclusions
The addition of cellulose polymers to ophthalmic solutions in order to increase the viscosity of eye drops enhances the ocular retention of sodium fluorescein. HEC is much better tolerated than the more surface active HPMC and HPC. Irritative 3.4% HPC solution increases signifTable 4. Mean values (+ SEM) of tear proteins of five individuals.
before instill. after phosph sol after 1.70% HEC after 3.40% HPC
Total protein (g/l)
Lysozyme (g/l)
7.2 + 1.2 6.0 _+ 2.3 7.2 _+ 2.2 12.4 + 1.7"
2.16 + 0.44
3.22 + 0.29*
* Significance of difference vs before instillation: P < 0.05 (Wilcoxon's signed rank test).
icantly the total protein concentration of tears. Because the lysozyme concentration increased in parallel with total protein a systematic and not a local origin, such as leakage of plasma proteins into the tears, may be the underlying condition of the protein increase. This increase may be due to a removal of water from the tear film during the prolonged corneal contact time of the viscous eye drop, either by increased evaporation from the disturbed tear film or by osmotic attraction of water into the underlying epithelium. In both explanations the lower surface tension (39.9 mN/m) of the 3.4% HPC solution, as compared to the higher value (56.9 mN/m) of the 1.7% HEC solution, may be held responsible for the observed increase in protein in the tears.
References 1. Blaug SM, Canada AT. Relationship of viscosity, contact time and prolongation of action of methylcellulose containing ophthalmic solutions. Am J Hosp Pharm 1965; 22: 662-6. 2. Hardberger R, Hanna C, Boyd CM. Effects of drug vehicles on ocular contact time. Arch Ophthalmol 1975; 93: 42-5. 3. Trueblood JH, Rossomondo RM, Carlton WH, Wilson LA. Corneal contact times of ophthalmic vehicles. Arch Ophthalmol 1975; 93: 127-30. 4. Ludwig A, Van Ooteghem M. The study of the precorneal dynamics of ophthalmic solutions by fluorophotometry. Pharm Acta Helv 1986; 85: 59-65. 5. Bradford MM. Rapid and sensitive method for the quantitation of microgram quantities of protein utilizing the principle of protein-dye binding. Anal Biochem 1976; 72: 248-53. 6. Ensink FTE, van Haeringen NJ. Pitfalls in the assay of lysozyme in human tear fluid. Ophthalmic Res 1977; 9: 366-73. 7. Tiffany JM, Winter N, Bliss G. Tear film stability and tear surface tension. Curt Eye Res 1989; 8: 507-15. 8. Ludwig A, Van Ooteghem M. The evaluation of viscous ophthalmic vehicles by slit lamp fluorophotometry in humans. Int J Pharmaceutics 1989; 54: 95-102. 9. Ludwig A, Van Ooteghem M. Influence of the surface tension of eye drops on the retention of a tracer in the precomeal area of human eyes. J Pharm Belg 1988; 43: 157-62.
Address for offprints: A. Ludwig, University of Antwerp, Dept. Pharmaceutical Scs., Universiteitsplein 1, 2610 Wilrijk, Belgium
