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Loading Doses and Extended Dosing Intervals in Topical Gentamicin Therapy
Loading Doses and Extended Dosing Intervals in Topical Gentamicin Therapy David B. Glasser, M.D., Susanne Gardner, Pharm.D., John G. Ellis, B.S., and Thomas H. Pettit, M.D.
Loading doses and extended dosing intervals were studied in a rabbit model using topically applied gentamicin in a concentration of 13.6 mglml. Loading doses consisting of one drop every minute for five minutes produced significantly higher gentamicin concentrations in the cornea during the early hours of treatment than regimens using one drop every hour and one drop every 30 minutes. Extended dosing intervals of two and four hours, when used in conjunction with an initial loading dose, produced peak gentamicin levels above 125 ....glg of cornea. However, trough levels with these extended dosing intervals were significantly lower than troughs during hourly gentamicin administration. An extended dosing interval regimen consisting of three drops every two hours maintained peak and trough levels equal to those produced by one drop an hour. INTENSIVE TOPICAL THERAPY with fortified aminoglycoside antibiotics is a mainstay in the treatment of bacterial keratitis, and has resulted in a marked improvement in our ability to sterilize these ulcers and prevent visual loss from corneal scarring and perforation. Factors affecting bacterial killing and antibiotic penetration into the cornea have been demonstrated in several animal models of bacterial keratitis.l" The effectiveness of topical aminoglycoside therapy is enhanced by prompt initiation of treatment, short dosing intervals, the use of concentrated antibiotic solutions, and the presence of an epithelial defect. Clinical and experimental observations have led to gentamicin being used to treat bacterial keratitis in concentrations of 9 to 20 mg/ml, administered hourly
Accepted for publication Jan. 4, 1985. From the Jules Stein Eye Institute and the UCLA Center for the Health Sciences, Los Angeles, California. Reprint requests to Thomas H. Pettit, M.D., Jules Stein Eye Institute, 800 Westwood Plaza, Los Angeles, CA 90024.
©AMERICAN JOURNAL OF OPHTHALMOLOGY
99:329-332,
or, more frequently, around the clock (24 to 96 times/day). Although this is often effective, it has several drawbacks. Epithelial toxicity and mechanical trauma associated with frequent instillation of fortified gentamicin may retard epithelial healing. 7,8 This regimen is also labor intensive and stressful to the patient. Alternatives to hourly and more frequent dosing schedules have not been fully investigated. Loading doses, commonly used for systemic aminoglycoside therapy, produce a rapid increase in serum antibiotic concentrations. Topical antibiotic therapy using loading doses has not been investigated. We compared peak and trough antibiotic levels achieved in the cornea by hourly and half-hourly administration of gentamicin with those achieved by loading doses. A loading dose consists of one drop given every minute for five minutes. We also studied the effect of longer dosing intervals on corneal gentamicin levels.'
Material and Methods We used disks of filter paper soaked in N-heptanol to produce 4-mm central epithelial defects in New Zealand albino rabbits free of external ocular disease. Preliminary studies with fluorescein indicated that round, regular epithelial defects of uniform size were reliably reproduced with this method. The corneas and conjunctivas were thoroughly rinsed with balanced salt solution after application of the heptanolsoaked disks. Gentamicin was prepared in a concentration of 13.6 mg/ml by fortifying commercially available ophthalmic gentamicin solution with injectable drug. Single drops of this fortified gentamicin solution were applied topically to the corneas according to the dosage schedules in Table 1. The size of the drops was approximately 50 ....1. This volume exceeds the capacity of the rabbit (and human) conjunctival sac, but approximates the volume delivered by most commercially available dropper bottles. We made no
MARCH,
1985
329
330
TABLE 1 GENTAMICIN DOSAGE SCHEDULES
Results
GROUP
SCHEDULE
1 2 3 4 5
1 drop every hour 1 drop every 30 minutes Loading dose' every hour Loading dose'; then 1 drop every hour Loading dose'; then 1 drop every 2 hours Loading dose'; then 1 drop every 4 hours 1 drop every minute for 3 minutes, given every 2 hours
6 7
March, 1985
AMERICAN JOURNAL OF OPHTHALMOLOGY
'Loading dose, 1 drop every minute for 5 minutes.
attempt to increase the amount of medication retained in the conjunctival sac by using a pouch technique of delivery or by preventing the animals from blinking or wiping their eyes. We used a standardized scale to gauge the degree of conjunctival hyperemia and ocular discharge associated with delivery of the medication. 9 Two animals (four corneas per determination) were killed at various intervals during dosing to determine peak and trough levels of gentamicin. Tissue samples were taken five minutes after the last drop for the peak studies. Trough levels were measured in Groups 1, 5, 6, and 7 by taking tissue samples immediately before the scheduled doses at four and eight hours. The corneas were rinsed, excised, and individually weighed and homogenized in a microblender. Masked samples were frozen and gentamicin concentrations were determined by inhibition of
Staphylococcus epidermidis. A two-sample t-test was performed on the data with P < .05 considered statistically significant. TABLE
InOammatory response-Conjunctival hyperemia and ocular discharge increased steadily during gentamicin delivery, and decreased after the drug was discontinued. Eyes receiving one drop each hour (Group 1) showed little inflammation during the first 18 hours of drug administration. Conjunctival injection and discharge in these eyes increased between 18 and 24 hours. Eyes given one drop every 30 minutes (Group 2) developed a greater degree of external ocular inflammation than the eyes in Group 1. The greatest inflammatory response occurred in eyes receiving sequential (hourly) loading doses (Group 3). Eyes given a single loading dose followed by one drop everyone, two, or four hours (Groups 4, 5, and 6, respectively) developed minimal inflammatory responses, as did eyes given three drops every two hours (Group 7). Gentamicin penetration-Table 2 shows the gentamicin peak levels measured in the cornea. In Group 1, peak levels increased gradually over eight hours, then reached a plateau with the administration of one drop of gentamicin every hour. One drop every 30 minutes (Group 2) produced a more rapid increase in gentamicin levels in the cornea. These levels were significantly higher at all times than those achieved in Group 1. Sequential loading doses produced remarkably high gentamicin levels within one hour. These levels were significantly higher than those achieved by one drop an hour (Group 1) at all times studied, and higher than those achieved by one drop every 30 minutes during the first hour of treatment (Group 2). During the first four hours of gentamicin administration, a single loading dose followed by one drop each hour (Group 4) produced significantly higher peaks
2
PEAK LEVELS OF GENTAMICIN MEAN (:!: S.E.M.) PEAK LEVELS OF GENTAMICIN (ILQ/G OF CORNEA) GROUP
HOUR 1
1
41.9 ± 6 128.0 ± 121 489.0 ± 8111
2 3 4
5 6 7 'Peak level after 17 hours of treatment. 'P <.05 compared to Group 1. Ip <.05 compared to Group 2.
HOUR 2
62.9 ± 262.0 ± 367.0 ± 110.0 ± 139.0 ±
14 281 611 1411 2111
59.7 ± 101
HOUR 4
95.6 ± 323.0 ± 495.0 ± 155.0 ± 146.0 ± 162.0 ± 119.0 ±
9 61' 112' 21' 26 18t! 171
HOURS
HOUR 12
125 ± 20 252 ± 20 1 468±1131 167 ± 41 178 ± 24 128 ± 221 135 ± 171
140 ± 19' 254 ± 171 448 ± 961
Vol. 99, No.3
c Q) I:
8 E e C'
::L
210 180 150 Figure (Glasser and associates). Mean
120
I:
'0
°ec
± S.E.M. gentamicin peak levels in the
cornea in Groups 1, 4, 5, 6, and 7. Shaded areas indicate P < .05 compared to Group 1. NO, no data. There were four corneas per determination.
90
60 1= Q)
C>
30
4
2
Hours of Trea1ment
than those produced in Group 1 (Figure) but one drop every 30 minutes (Group 2) produced higher peaks than a single loading dose followed by one drop each hour. These differences were significant after two hours, but not after four or eight hours of treatment. A single loading dose followed by extended dosing intervals (Groups 5 and 6) and three drops every two hours (Group 7) produced peak gentamicin levels higher than or similar to those attained by one drop every hour (Group 1) (Figure). Gentamicin trough levels measured in the cornea are shown in Table 3. There were no significant differences between trough levels with three drops every two hours (Group 7) and one drop an hour (Group 1). Both of these dosing regimens maintained troughs significantly higher than one drop every two or four hours after a loading dose (Groups 5 and 6).
TABLE 3 TRO\JGH LEVELS OF GENTAMICIN MEAN (:t S.E.M.) TROUGH LEVELS OF GENTAMICIN (ILII!G OF CORNEA) GROUP
1 5 6 7
331
Topical Gentamicin Therapy
HOUR 4
50.2 25.7 18.4 55.3
± 4.6 ± 4.4· ± 2.4·
± 5.2
"P <.005 compared to Groups 1 and 7.
HOURS
62.9 ± 4.0 28.6 ± 3.8·
16.4 ± 0.7" 48.1 ± 4.1
a
Discussion Several factors other than dosing techniques may influence drug levels produced in the cornea by a given concentration of topically applied antibioticS.I O•11 Patient factors in antibiotic penetration include tear dynamics, blinking rate, integrity of the corneal epithelium, and external vascularity and inflammation. Drug factors in antibiotic penetration include lipid and aqueous solubility, concentration, vehicle properties (which may influence contact time), and preservative properties. Breakdown of the intercellular junctions or the presence of a frank defect in the corneal epithelium allows for greater stromal penetration of a hydrophilic drug such as gentamicin. 5 •10 •11 Central epithelial defects present in this animal model simulate conditions in ulcerated eyes. Benzalkonium chloride is a preservative in the gentamicin ophthalmic solution used to prepare the fortified drops for this study. Benzalkonium chloride also possesses surfactant properties and is known to interfere with epithelial integrity;" thereby increasing permeability. These factors probably contributed to the remarkably high gentamicin levels achieved by all of the regimens. The degree of tearing and external ocular inflammation may also affect the corneal penetration of topically applied drugs. Excessive tearing may dilute the administered drug. Increased inflammation and vascularity are associated with more rapid removal of the drug from the tear film." These factors are variable during the course of bacterial keratitis in humans.
332
March, 1985
AMERICAN JOURNAL OF OPHTHALMOLOGY
Nevertheless, the data suggest that loading doses, particularly sequential loading doses, produce higher gentamicin concentrations in the cornea than single doses during the early hours of treatment. Therapy for bacterial keratitis may be initiated with one drop every minute for five minutes. This loading dose may be repeated in 30 to 60 minutes, followed by a single drop every 30 to 60 minutes. The potential for corneal endothelial toxicity from sustained high levels of gentamicin attainable with continued sequential hourly loading doses is unknown. The use of antibiotics containing benzalkonium chloride may increase epithelial toxicity, but may also increase drug penetration. The extended dosing interval data were interesting. Peaks above 100 ""gIg of cornea were attained with three drops every two hours (Group 7) and with dosing intervals of up to four hours when used in conjunction with an initial loading dose (Groups 4,5, and 6). However, the decline of antibiotic levels in the cornea between doses remains a concern. The only extended dosing interval regimen tested that maintained troughs equal to those seen with hourly dosing was the administration of three drops every two hours (Group 7). Although all of the regimens tested maintained troughs many times above the mean bactericidal concentrations for most pathogens, including Pseudomonas aeruginosa, this does not necessarily assure effective bacterial killing. The concentration of gentamicin required to eradicate 99% of viable organisms in a rat model of bacterial keratitis was 25 to 260 times higher than the concentration required to produce the same effect in vitro (T. Roussel, P. Badenoch, and D. Coster, unpublished data). Therefore, further investigation into the effectiveness of extended dosing interval techniques is required before their clinical application can be considered. ACKNOWLEDGMENT
The Schering Corporation provided the gentamicin used in this study.
References 1. Hyndiuk, R.: Experimental Pseudomonas keratitis. Trans. Am. Ophthalmol. Soc. 79:541, 1981. 2. Baum, J.: Treatment of bacterial ulcers of the cornea in the rabbit. A comparison of administration by eye drops and subconjunctival injections. Trans. Am. Ophthalmol. Soc. 80:369, 1982. 3. Davis, S. D., Sarff, L. D., and Hyndiuk, R. A.: Antibiotic therapy of experimental Pseudomonas keratitis in guinea pigs. Arch. Ophthalmol. 95:1638, 1977. 4. - - : Comparison of therapeutic routes in experimental Pseudomonas keratitis. Am. J. Ophthalmol. 87:710, 1979. 5. - - : Topical tobramycin therapy of experimental Pseudomonas keratitis. An evaluation of some factors that potentially enhance efficacy. Arch. Ophthalmol. 96:123, 1978. 6. Kupferman, A., and Leibowitz, H.: Topical antibiotic therapy of Pseudomonas aeruginosa keratitis. Arch. Ophthalmol. 97:1699, 1979. 7. Stern, G., Schemmer, G., Farber, R., and Gorovoy, M.: Effect of topical antibiotic solutions on corneal epithelial wound healing. Arch. Ophthalmol. 101:644, 1983. 8. Petroutsos, G., Guimaraes, R., Giraud, J., and Pouliquen, Y.: Antibiotics and corneal epithelial wound healing. Arch. Ophthalmol. 101:1775, 1983. 9. McDonald, T., and Shadduck, J.: Eye irritation. In Marzulli, F. A., and Howard, I. M. (eds.): Advances in Modern Toxicology, Dermatotoxicology, and Pharmacology. New York, John Wiley and Sons, 1977, vol. 4, p. 162. 10. Shell, J.: Pharmacokinetics of topically applied ophthalmic drugs. Surv. Ophthalmol. 26:207, 1982. 11. Maurice, D., and Mishima, S.: Ocular pharmacokinetics. In Sears, M. L. (ed.): Pharmacology of the Eye. New York, Springer-Verlag, 1984, p. 19. 12. Pfister, R., and Burstein, N.: The effect of ophthalmic drugs, vehicles, and preservatives on corneal epithelium. A scanning electron microscope study. Invest. Ophthalmol. 15:246, 1976. 13. Matoba, A. Y., Wilhelmus, K. R., Robinson, N. M., and Jones, D. B.: Concentration of tobramycin in preocular tear film following topical application. ARVO Abstracts. Supplement to Invest. Ophthalmol. Vis. Sci. St. Louis, C. V. Mosby, 1982, p. 215.
Loading doses and extended dosing intervals were studied in a rabbit model using topically applied gentamicin in a concentration of 13.6 mglml. Loading doses consisting of one drop every minute for five minutes produced significantly higher gentamicin concentrations in the cornea during the early hours of treatment than regimens using one drop every hour and one drop every 30 minutes. Extended dosing intervals of two and four hours, when used in conjunction with an initial loading dose, produced peak gentamicin levels above 125 ....glg of cornea. However, trough levels with these extended dosing intervals were significantly lower than troughs during hourly gentamicin administration. An extended dosing interval regimen consisting of three drops every two hours maintained peak and trough levels equal to those produced by one drop an hour. INTENSIVE TOPICAL THERAPY with fortified aminoglycoside antibiotics is a mainstay in the treatment of bacterial keratitis, and has resulted in a marked improvement in our ability to sterilize these ulcers and prevent visual loss from corneal scarring and perforation. Factors affecting bacterial killing and antibiotic penetration into the cornea have been demonstrated in several animal models of bacterial keratitis.l" The effectiveness of topical aminoglycoside therapy is enhanced by prompt initiation of treatment, short dosing intervals, the use of concentrated antibiotic solutions, and the presence of an epithelial defect. Clinical and experimental observations have led to gentamicin being used to treat bacterial keratitis in concentrations of 9 to 20 mg/ml, administered hourly
Accepted for publication Jan. 4, 1985. From the Jules Stein Eye Institute and the UCLA Center for the Health Sciences, Los Angeles, California. Reprint requests to Thomas H. Pettit, M.D., Jules Stein Eye Institute, 800 Westwood Plaza, Los Angeles, CA 90024.
©AMERICAN JOURNAL OF OPHTHALMOLOGY
99:329-332,
or, more frequently, around the clock (24 to 96 times/day). Although this is often effective, it has several drawbacks. Epithelial toxicity and mechanical trauma associated with frequent instillation of fortified gentamicin may retard epithelial healing. 7,8 This regimen is also labor intensive and stressful to the patient. Alternatives to hourly and more frequent dosing schedules have not been fully investigated. Loading doses, commonly used for systemic aminoglycoside therapy, produce a rapid increase in serum antibiotic concentrations. Topical antibiotic therapy using loading doses has not been investigated. We compared peak and trough antibiotic levels achieved in the cornea by hourly and half-hourly administration of gentamicin with those achieved by loading doses. A loading dose consists of one drop given every minute for five minutes. We also studied the effect of longer dosing intervals on corneal gentamicin levels.'
Material and Methods We used disks of filter paper soaked in N-heptanol to produce 4-mm central epithelial defects in New Zealand albino rabbits free of external ocular disease. Preliminary studies with fluorescein indicated that round, regular epithelial defects of uniform size were reliably reproduced with this method. The corneas and conjunctivas were thoroughly rinsed with balanced salt solution after application of the heptanolsoaked disks. Gentamicin was prepared in a concentration of 13.6 mg/ml by fortifying commercially available ophthalmic gentamicin solution with injectable drug. Single drops of this fortified gentamicin solution were applied topically to the corneas according to the dosage schedules in Table 1. The size of the drops was approximately 50 ....1. This volume exceeds the capacity of the rabbit (and human) conjunctival sac, but approximates the volume delivered by most commercially available dropper bottles. We made no
MARCH,
1985
329
330
TABLE 1 GENTAMICIN DOSAGE SCHEDULES
Results
GROUP
SCHEDULE
1 2 3 4 5
1 drop every hour 1 drop every 30 minutes Loading dose' every hour Loading dose'; then 1 drop every hour Loading dose'; then 1 drop every 2 hours Loading dose'; then 1 drop every 4 hours 1 drop every minute for 3 minutes, given every 2 hours
6 7
March, 1985
AMERICAN JOURNAL OF OPHTHALMOLOGY
'Loading dose, 1 drop every minute for 5 minutes.
attempt to increase the amount of medication retained in the conjunctival sac by using a pouch technique of delivery or by preventing the animals from blinking or wiping their eyes. We used a standardized scale to gauge the degree of conjunctival hyperemia and ocular discharge associated with delivery of the medication. 9 Two animals (four corneas per determination) were killed at various intervals during dosing to determine peak and trough levels of gentamicin. Tissue samples were taken five minutes after the last drop for the peak studies. Trough levels were measured in Groups 1, 5, 6, and 7 by taking tissue samples immediately before the scheduled doses at four and eight hours. The corneas were rinsed, excised, and individually weighed and homogenized in a microblender. Masked samples were frozen and gentamicin concentrations were determined by inhibition of
Staphylococcus epidermidis. A two-sample t-test was performed on the data with P < .05 considered statistically significant. TABLE
InOammatory response-Conjunctival hyperemia and ocular discharge increased steadily during gentamicin delivery, and decreased after the drug was discontinued. Eyes receiving one drop each hour (Group 1) showed little inflammation during the first 18 hours of drug administration. Conjunctival injection and discharge in these eyes increased between 18 and 24 hours. Eyes given one drop every 30 minutes (Group 2) developed a greater degree of external ocular inflammation than the eyes in Group 1. The greatest inflammatory response occurred in eyes receiving sequential (hourly) loading doses (Group 3). Eyes given a single loading dose followed by one drop everyone, two, or four hours (Groups 4, 5, and 6, respectively) developed minimal inflammatory responses, as did eyes given three drops every two hours (Group 7). Gentamicin penetration-Table 2 shows the gentamicin peak levels measured in the cornea. In Group 1, peak levels increased gradually over eight hours, then reached a plateau with the administration of one drop of gentamicin every hour. One drop every 30 minutes (Group 2) produced a more rapid increase in gentamicin levels in the cornea. These levels were significantly higher at all times than those achieved in Group 1. Sequential loading doses produced remarkably high gentamicin levels within one hour. These levels were significantly higher than those achieved by one drop an hour (Group 1) at all times studied, and higher than those achieved by one drop every 30 minutes during the first hour of treatment (Group 2). During the first four hours of gentamicin administration, a single loading dose followed by one drop each hour (Group 4) produced significantly higher peaks
2
PEAK LEVELS OF GENTAMICIN MEAN (:!: S.E.M.) PEAK LEVELS OF GENTAMICIN (ILQ/G OF CORNEA) GROUP
HOUR 1
1
41.9 ± 6 128.0 ± 121 489.0 ± 8111
2 3 4
5 6 7 'Peak level after 17 hours of treatment. 'P <.05 compared to Group 1. Ip <.05 compared to Group 2.
HOUR 2
62.9 ± 262.0 ± 367.0 ± 110.0 ± 139.0 ±
14 281 611 1411 2111
59.7 ± 101
HOUR 4
95.6 ± 323.0 ± 495.0 ± 155.0 ± 146.0 ± 162.0 ± 119.0 ±
9 61' 112' 21' 26 18t! 171
HOURS
HOUR 12
125 ± 20 252 ± 20 1 468±1131 167 ± 41 178 ± 24 128 ± 221 135 ± 171
140 ± 19' 254 ± 171 448 ± 961
Vol. 99, No.3
c Q) I:
8 E e C'
::L
210 180 150 Figure (Glasser and associates). Mean
120
I:
'0
°ec
± S.E.M. gentamicin peak levels in the
cornea in Groups 1, 4, 5, 6, and 7. Shaded areas indicate P < .05 compared to Group 1. NO, no data. There were four corneas per determination.
90
60 1= Q)
C>
30
4
2
Hours of Trea1ment
than those produced in Group 1 (Figure) but one drop every 30 minutes (Group 2) produced higher peaks than a single loading dose followed by one drop each hour. These differences were significant after two hours, but not after four or eight hours of treatment. A single loading dose followed by extended dosing intervals (Groups 5 and 6) and three drops every two hours (Group 7) produced peak gentamicin levels higher than or similar to those attained by one drop every hour (Group 1) (Figure). Gentamicin trough levels measured in the cornea are shown in Table 3. There were no significant differences between trough levels with three drops every two hours (Group 7) and one drop an hour (Group 1). Both of these dosing regimens maintained troughs significantly higher than one drop every two or four hours after a loading dose (Groups 5 and 6).
TABLE 3 TRO\JGH LEVELS OF GENTAMICIN MEAN (:t S.E.M.) TROUGH LEVELS OF GENTAMICIN (ILII!G OF CORNEA) GROUP
1 5 6 7
331
Topical Gentamicin Therapy
HOUR 4
50.2 25.7 18.4 55.3
± 4.6 ± 4.4· ± 2.4·
± 5.2
"P <.005 compared to Groups 1 and 7.
HOURS
62.9 ± 4.0 28.6 ± 3.8·
16.4 ± 0.7" 48.1 ± 4.1
a
Discussion Several factors other than dosing techniques may influence drug levels produced in the cornea by a given concentration of topically applied antibioticS.I O•11 Patient factors in antibiotic penetration include tear dynamics, blinking rate, integrity of the corneal epithelium, and external vascularity and inflammation. Drug factors in antibiotic penetration include lipid and aqueous solubility, concentration, vehicle properties (which may influence contact time), and preservative properties. Breakdown of the intercellular junctions or the presence of a frank defect in the corneal epithelium allows for greater stromal penetration of a hydrophilic drug such as gentamicin. 5 •10 •11 Central epithelial defects present in this animal model simulate conditions in ulcerated eyes. Benzalkonium chloride is a preservative in the gentamicin ophthalmic solution used to prepare the fortified drops for this study. Benzalkonium chloride also possesses surfactant properties and is known to interfere with epithelial integrity;" thereby increasing permeability. These factors probably contributed to the remarkably high gentamicin levels achieved by all of the regimens. The degree of tearing and external ocular inflammation may also affect the corneal penetration of topically applied drugs. Excessive tearing may dilute the administered drug. Increased inflammation and vascularity are associated with more rapid removal of the drug from the tear film." These factors are variable during the course of bacterial keratitis in humans.
332
March, 1985
AMERICAN JOURNAL OF OPHTHALMOLOGY
Nevertheless, the data suggest that loading doses, particularly sequential loading doses, produce higher gentamicin concentrations in the cornea than single doses during the early hours of treatment. Therapy for bacterial keratitis may be initiated with one drop every minute for five minutes. This loading dose may be repeated in 30 to 60 minutes, followed by a single drop every 30 to 60 minutes. The potential for corneal endothelial toxicity from sustained high levels of gentamicin attainable with continued sequential hourly loading doses is unknown. The use of antibiotics containing benzalkonium chloride may increase epithelial toxicity, but may also increase drug penetration. The extended dosing interval data were interesting. Peaks above 100 ""gIg of cornea were attained with three drops every two hours (Group 7) and with dosing intervals of up to four hours when used in conjunction with an initial loading dose (Groups 4,5, and 6). However, the decline of antibiotic levels in the cornea between doses remains a concern. The only extended dosing interval regimen tested that maintained troughs equal to those seen with hourly dosing was the administration of three drops every two hours (Group 7). Although all of the regimens tested maintained troughs many times above the mean bactericidal concentrations for most pathogens, including Pseudomonas aeruginosa, this does not necessarily assure effective bacterial killing. The concentration of gentamicin required to eradicate 99% of viable organisms in a rat model of bacterial keratitis was 25 to 260 times higher than the concentration required to produce the same effect in vitro (T. Roussel, P. Badenoch, and D. Coster, unpublished data). Therefore, further investigation into the effectiveness of extended dosing interval techniques is required before their clinical application can be considered. ACKNOWLEDGMENT
The Schering Corporation provided the gentamicin used in this study.
References 1. Hyndiuk, R.: Experimental Pseudomonas keratitis. Trans. Am. Ophthalmol. Soc. 79:541, 1981. 2. Baum, J.: Treatment of bacterial ulcers of the cornea in the rabbit. A comparison of administration by eye drops and subconjunctival injections. Trans. Am. Ophthalmol. Soc. 80:369, 1982. 3. Davis, S. D., Sarff, L. D., and Hyndiuk, R. A.: Antibiotic therapy of experimental Pseudomonas keratitis in guinea pigs. Arch. Ophthalmol. 95:1638, 1977. 4. - - : Comparison of therapeutic routes in experimental Pseudomonas keratitis. Am. J. Ophthalmol. 87:710, 1979. 5. - - : Topical tobramycin therapy of experimental Pseudomonas keratitis. An evaluation of some factors that potentially enhance efficacy. Arch. Ophthalmol. 96:123, 1978. 6. Kupferman, A., and Leibowitz, H.: Topical antibiotic therapy of Pseudomonas aeruginosa keratitis. Arch. Ophthalmol. 97:1699, 1979. 7. Stern, G., Schemmer, G., Farber, R., and Gorovoy, M.: Effect of topical antibiotic solutions on corneal epithelial wound healing. Arch. Ophthalmol. 101:644, 1983. 8. Petroutsos, G., Guimaraes, R., Giraud, J., and Pouliquen, Y.: Antibiotics and corneal epithelial wound healing. Arch. Ophthalmol. 101:1775, 1983. 9. McDonald, T., and Shadduck, J.: Eye irritation. In Marzulli, F. A., and Howard, I. M. (eds.): Advances in Modern Toxicology, Dermatotoxicology, and Pharmacology. New York, John Wiley and Sons, 1977, vol. 4, p. 162. 10. Shell, J.: Pharmacokinetics of topically applied ophthalmic drugs. Surv. Ophthalmol. 26:207, 1982. 11. Maurice, D., and Mishima, S.: Ocular pharmacokinetics. In Sears, M. L. (ed.): Pharmacology of the Eye. New York, Springer-Verlag, 1984, p. 19. 12. Pfister, R., and Burstein, N.: The effect of ophthalmic drugs, vehicles, and preservatives on corneal epithelium. A scanning electron microscope study. Invest. Ophthalmol. 15:246, 1976. 13. Matoba, A. Y., Wilhelmus, K. R., Robinson, N. M., and Jones, D. B.: Concentration of tobramycin in preocular tear film following topical application. ARVO Abstracts. Supplement to Invest. Ophthalmol. Vis. Sci. St. Louis, C. V. Mosby, 1982, p. 215.