Pharmacokinetic Considerations in the Treatment of Bacterial Keratitis

Abstract

The eye is relatively impermeable to micro-organisms and other environmental elements. However, if corneal integrity is breached by trauma, a sight-threatening bacterial infection can result. Staphylococcus aureus, Pseudomonas aeruginosa, and Streptococcus pneumoniae are the most common bacterial pathogens associated with infection of compromised corneas. Bacterial enzymes and toxins, as well as factors associated with the host immune response, can lead to tissue destruction during corneal infection. For successful therapy, an antibacterial agent must be active against the pathogen and must be able to overcome the permeability barrier of the cornea. Topical application of antibacterial agents adequately delivers drugs to the cornea and aqueous humour. However, drug concentrations at the site of infection are not always sufficient to rapidly kill infective organisms. Infections with antibiotic-resistant strains present an even greater therapeutic challenge. In addition, sterilisation of the cornea by antibacterial agents does not eliminate inflammation and corneal scarring that accompany infection. Steroidal and non-steroidal antiinflammatory agents limit corneal scarring during experimentally induced bacterial keratitis. However, although promising, concomitant use of these drugs with antibacterial agents remains controversial. Two ocular drug delivery systems that provide high and sustained concentrations of drug to ocular tissues are corneal collagen shields and transcorneal iontophoresis. The collagen shield, originally designed as a bandage lens, prolongs drug contact with the cornea. Chemotherapeutic studies of experimental bacterial keratitis demonstrate that shields hydrated with antibacterial agents reduce bacteria in the cornea as well as or better than frequent applications of fortified antibacterial drops. Transcorneal iontophoresis uses an electric current to drive charged drugs into the cornea. In experimentally induced bacterial keratitis, transcorneal iontophoresis of antibacterial agents is superior to topically administered ocular drops for reducing the numbers of bacteria in the cornea. Although both drug delivery systems appear to be well tolerated and nontoxic in animal models, clinical trials in patients are required to determine the usefulness of these drug delivery systems in clinical trials. Based on present experimental results, future therapy of bacterial keratitis will involve efficient drug delivery devices, the use of new antibacterial agents or combinations of presently available antibacterial agents, and careful use of adjuvant anti-inflammatory agents.

Development of a <i>Streptococcus pneumoniae </i>Keratitis Model in Mice

“…S. pneumoniae is one of the most common bacterial pathogens associated with infection of compromised corneas [12,13] . The corneal damage observed in pneumococcal keratitis has been attributed mainly to pneumococcal virulence factors, such as the toxin pneumolysin, that initiate a robust immune response [14][15][16] .…”

A Mucoadhesive Polymer Extracted from Tamarind Seed Improves the Intraocular Penetration and Efficacy of Rufloxacin in Topical Treatment of Experimental Bacterial Keratitis

“…Pseudomonas aeruginosa and Staphylococcus aureus frequently cause severe keratitis that may lead to progressive destruction of the corneal epithelium and stroma (2,3). Infectious keratitis due to these organisms often causes corneal scarring, corneal perforation, and blindness if aggressive and appropriate therapy is not promptly initiated (7,18).…”

“…Since the cornea is not vascularized, it is not readily permeated by systemically administered drugs, which are therefore generally not used for the treatment of keratitis (7). On the other hand, topical treatment may fail to achieve therapeutically active drug levels in the cornea, as continuous tear flow reduces the bioavailability of topically applied antibiotics and the corneal epithelium acts as a barrier against drug penetration.…”

“…On the other hand, topical treatment may fail to achieve therapeutically active drug levels in the cornea, as continuous tear flow reduces the bioavailability of topically applied antibiotics and the corneal epithelium acts as a barrier against drug penetration. For this reason, standard treatment of severe bacterial keratitis requires administration at frequent intervals (every 15 to 60 min for 48 to 72 h) of eyedrops often containing fortified (more concentrated than commercially available solutions) solutions of fluoroquinolones or multiple antibiotics, usually a cephalosporin and an aminoglycoside (7,13,28,29). However, this regimen not only is disruptive to the patient and usually necessitates hospitalization, but it has also been associated with in vitro toxicity to the corneal epithelium (9,22).…”

Management of bacterial keratitis: beyond exorcism towards consideration of organism and host factors

“…96 Transcorneal iontophoresis of antibiotics has also been employed to increase attainable drug concentrations and increase efficacy of antibacterial therapy. 97,98 Antibiotics administered via the parenteral route are relatively poorly absorbed into the noninflamed eye. Bactericidal corneal tissue levels can only be achieved at the risk of systemic toxicity.…”