The results of this study are consistent with other tear film studies conducted in humans, horses and rabbits. We found that the concentration of drug in the tear film rapidly declined exponentially in the first 50 minutes, linear on logarithmic axes, and then plateaued and became variable as the limit of detection for the drug assay was approached. After one hour, the low tear drug levels were variable. Other tear film pharmacokinetic studies in horses, dogs, and rabbits, showed a large amount of variation of tear antimicrobial concentrations
]. The authors suggested that this variation may be explained by inter-animal variation and low sample numbers
]. Similar to these studies, we also had one sample in each the 5 min, 2 hr, 4 hr and 6 hr groups that were below the limit of quantification. The previous studies hypothesized that this occurred from not fully extracting the drug from the strips, from errors in the amount of drug in the syringe because of air, and from loss from blinking after the drop was given
]. In the previous studies the animal’s other eye was normal, but in our study the 4 hr and 6 hr samples were from the same horse
]. This is suggestive of sensitivity to the drug resulting in increased tearing.
We showed that the mean tobramycin in the tear film reached the MIC90
of 16 mg/L after 68.5 minutes, indicating that therapeutic concentrations of the drug were maintained for over an hour, compared to the 10 minutes reported in the rabbit tears
]. Our result of an initial concentration of 905 mg/L of tobramycin at time 0 is in agreement with the estimates from a previous study on tear volume in horses
]. In addition, when our results are compared to the same study, tear production is likely the main factor involved in tobramycin clearance from the surface of the eye
]. The horses used in the study were normal, however it would be expected that tobramycin levels in the tear film of a horse with a corneal ulcer would decrease more quickly than a normal horse, as corneal ulceration causes increased tear production.
Tear production is also impacted by drugs, including sedatives and general anesthetics. A study in dogs found that sedation with xylazine alone did not impact tear production, butorphanol caused a mild decrease in tear production, while the combination of the two drugs significantly decreased tear production
]. Another study showed that in horses intravenous xylazine did not affect tear production, while general anesthesia with halothane caused a decrease in tear production
]. This suggests that the xylazine used for sedation in the study did not impact the tear production. It is possible that the combination of xylazine and butorphanol used in the fractious horse caused decreased tear production, and subsequent elevated concentrations of tobramycin in the tear film. When the individual measurements from this horse were examined, the concentrations were in the middle of the range compared to the other horses in the group. This suggests that it is unlikely that the sedation had a significant effect.
for gram positive and gram negative ocular isolates from humans has previously been reported as 16 mg/L for tobramycin
], but there is limited published data regarding the aminoglycoside susceptibilities for bacteria isolated from horses
]. For equine keratitis isolates tested by use of minimum inhibitory concentration methods for tobramycin, susceptibility criteria were <4 mg/L for susceptible, 8 mg/L for moderately susceptible, and >16 mg/L for resistant isolates
Treatment of bacterial infections requires therapeutic levels of antimicrobials within the tissues. Maintaining therapeutic concentrations of drug could be achieved by improving drug retention in the tears, by using higher concentrations of the drug, and by more frequent drug administration
]. In this sense, a study in rabbits found higher concentrations of tobramycin in the tears and within the ocular tissues when the drug was combined with xanthan gum in a product called TobraDex ST® (Alcon Laboratories Inc., Fort Worth, TX, USA) versus tobramycin alone
]. Exposure to the tear pH causes the ionic bond between the xanthan gum and drug molecules to break, increasing the viscosity of the drop when it is within the tear film, thus prolonging the retention of the drug in the tears
]. This study found that tear tobramycin concentration was 8 and 12.5 times higher in the formulation using xanthan gum 10 and 60 minutes after topical application respectively
]. Interestingly, increasing retention in the tears did not affect the aqueous humour concentrations of the drugs
Using higher drug concentrations is another way to achieve therapeutic levels within infected tissues. A study in rabbits demonstrated that tear and corneal tobramycin levels increased proportionally with the concentration of drug administered
]. Tobramycin was not detected in the cornea or aqueous 15 minutes following application of a 0.3% solution and concentrations greater than 1.1% were required to achieve penetration into the aqueous. However, tobramycin concentrations higher than 0.3% caused a significant decrease in the rate of corneal healing following hourly drug application
]. Further, the use of 1.1% and 4.0% tobramycin solutions administered hourly compared to 0.3% administered every 15 minutes did not statistically reduce the number of colony forming units (CFU) in Pseudomonas aeruginosa
keratitis in rats
]. Interestingly, an in vitro study showed that tobramycin affected the migration of canine corneal epithelial cells less than all the other antimicrobial agents tested, including ciprofloxacin
]. This suggests that tobramycin has the least impact on corneal wound healing, making it potentially more advantageous in the treatment of superficial corneal ulcerations
A recent study in horses examined the ocular penetration of ciprofloxacin and moxifloxacin through intact corneal epithelium following topical application
]. Therapeutic levels of both drugs were found in the tear film and cornea, but similar to the tobramycin studies, aqueous humor concentrations of moxifloxacin and ciprofloxacin were low and undetectable, respectively
]. Further research is needed to determine the penetration of tobramycin into the ocular tissues.
In addition to pharmacokinetics considerations, the development of antimicrobial resistance is a concern in both human and veterinary ophthalmology. While a study from Florida found that Pseudomonas aeruginosa
showed a statistically significant increase in resistance to tobramycin over several years, other studies found that 100% of their cultures were susceptible to the drug
]. In order to minimize the development of further antimicrobial resistance, tobramycin should be used judiciously or in combination with other antibiotics based on pharmacokinetics studies in each species