Small Animal Clinical Pharmacology - CYF MEDICAL DISTRIBUTION

ANTIBACTERIAL DRUG CLASSES
of drug characteristics is required to select an appropriate
antibacterial.
For topically applied drugs to penetrate the cornea,
they must be able to pass through the lipid epithelium
and endothelium as well as the hydrophilic corneal
stroma. Antibacterials that have good penetration
through the cornea include chloramphenicol, erythromycin,
tetracyclines and fluoroquinolones.
The blood–aqueous and blood–retina barriers severely
limit drugs given systemically that are not lipid soluble
or are highly protein bound. However, if the eye is
inflamed the blood–eye barriers break down. Drugs that
have good intraocular penetration in normal eyes
include chloramphenicol, lincomycin, sulfonamides and
the fluoroquinolones. Drugs that are poorly liposoluble,
such as ampicillin and amikacin, must be given by subconjunctival
injection to achieve a therapeutic intraocular
concentration.
ANTIBACTERIAL DRUG CLASSES
Aminoglycosides
Aminoglycosides, especially gentamicin, are commonly
used in ophthalmology because of their efficacy against
Pseudomonas spp. Because of widespread and often
inappropriate use of gentamicin, bacterial resistance is
increasing. For this reason it is judicious to restrict the
use of aminoglycosides to severe, infected corneal ulcers.
Culture and sensitivity is recommended because of gentamicin
resistance and cross-resistance by Pseudomonas
spp to other aminoglycosides.
Gentamicin
Although gentamicin is commonly used to treat corneal
ulcers in veterinary practice, its use should be restricted
to treatment of severe infected corneal ulceration. There
is no benefit in using gentamicin as a prophylactic treatment
for corneal ulceration.
Formulations and dose rates
DOGS AND CATS
For severe vision-threatening infections, such as a melting cornea,
gentamicin should be used frequently, e.g. hourly, and as a fortifi ed
preparation. Commercially, gentamicin is available as a 3 mg/mL
ophthalmic preparation.
When treating severe infected corneal ulcers, gentamicin can be
fortifi ed to a concentration of 10 mg/mL by adding injectable gentamicin
to the ophthalmic preparation. Following systemic administration,
relatively low concentrations of gentamicin are reached in the
tears, cornea and anterior chamber.
Tobramycin
Tobramycin is very similar to gentamicin and can be
fortified for use in serious infections.
Neomycin
Neomycin is usually combined with polymyxin and
bacitracin in ophthalmic preparations (drops or ointments).
It has been reported to cause local allergic
reactions.
Bacitracin
Bacitracin is used in a number of eye preparations in
combination with other antibacterials, usually neomycin
and polymyxin. It has a bactericidal action, chiefly
against Gram-positive organisms. There is limited penetration
into the eye.
Chloramphenicol
Chloramphenicol is commonly used in ophthalmology
as it can penetrate the intact cornea due to the high lipid
solubility of the nonionized form. When administered
systemically, it is able to penetrate the blood–aqueous
and blood–retina barriers.
Chloramphenicol is bacteriostatic, with a broad spectrum
of activity against both Gram-negative and Grampositive
organisms. It is not effective against Pseudomonas
so is often used in combination with polymyxin B. Chloramphenicol
is also effective against Chlamydophila.
Clindamycin
Clindamycin is usually used in ophthalmology for the
treatment of Toxoplasma infections. Toxoplasmosis
usually causes posterior chorioretinitis. Systemic clindamycin
is selectively concentrated into the choroid and
retina. Clindamycin can also be given as a subconjunctival
injection. The usual dose is 35 mg per subconjunctival
injection.
Clinical data have shown that oral clindamycin at a
dose of 25 mg/kg/d for 14–42 d may reduce the ophthalmic
signs of ocular toxoplasmosis in the cat.
Erythromycin
Erythromycin is available as a 0.5% ointment but is not
commonly used. It is effective against Gram-positive
organisms and Chlamydophila. Erythromycin has poor
penetration of the blood–aqueous and blood–retina
barriers.
Fluoroquinolones
Fluoroquinolones are effective against aerobic
Gram-positive and Gram-negative bacteria. They are
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