Intravitreal amikacin in the treatment of bacterial endophthalmitis

Pharmacokinetics of intravitreal antibiotics in endophthalmitis

Intravitreal antibiotics are the mainstay of treatment in the management of infectious endophthalmitis. Basic knowledge of the commonly used intravitreal antibiotics, which includes their pharmacokinetics, half-life, duration of action and clearance, is essential for elimination of intraocular infection without any iatrogenic adverse effect to the ocular tissue. Various drugs have been studied over the past century to achieve this goal. We performed a comprehensive review of the antibiotics which have been used for intravitreal route and the pharmacokinetic factors influencing the drug delivery and safety profile of these antibiotics. Using online resources like PubMed and Google Scholar, articles were reviewed. The articles were confined to the English language only. We present a broad overview of pharmacokinetic concepts fundamental for use of intravitreal antibiotics in endophthalmitis along with a tabulated compendium of the intravitreal antibiotics using available literature. Recent advances for increasing bioavailability of antibiotics to the posterior segment with the development of controlled drug delivery devices are also described.

Incidence of endophthalmitis related to intravitreal injection of bevacizumab and ranibizumab

PURPOSE

To report the overall incidence of endophthalmitis associated with office based intravitreal injections of bevacizumab and ranibizumab.

METHODS

This is a retrospective, consecutive, multicenter case series involving four large clinical sites. Included were all patients receiving at least one injection of intravitreal bevacizumab or intravitreal ranibizumab. Follow-up after each injection was at least 4 weeks.

RESULTS

A total of 12,585 injections of intravitreal bevacizumab and 14,320 injections of intravitreal ranibizumab were given during the study period. Infectious endophthalmitis developed in three patients after administration of bevacizumab and in three patients after administration of ranibizumab. Four of these patients were culture positive. Rates of endophthalmitis were 0.02% and 0.02%, respectively, with an overall rate of 0.02%.

CONCLUSION

The rate of endophthalmitis associated with intravitreal bevacizumab and ranibizumab is low, with an incidence of approximately 1 in 4,500 injections.

In vivo transscleral iontophoresis of amikacin to rabbit eyes

The objectives of these studies were to determine the amount and distribution of the aminoglycoside antibiotic amikacin delivered to rabbit eyes following transscleral iontophoresis and to determine the inter-study reproducibility of delivery over three identical studies. New Zealand White rabbits (N = 6 per dose group) were treated with a 200-mg/mL amikacin solution at 0, 2, 3 or 4 mA of (+) DC current for 20 minutes. Amikacin concentrations in eye tissues were highest with the 4-mA treatment. Concentrations for all three studies at this current were approximately 5.4, 40, 41, 343, and 92 mcg/g in the vitreous humor, anterior segment, non-treated hemisphere of the sclera, treated hemisphere of the sclera, and retina/choroid, respectively. These values were approximately 27, 50, 40, 10, and 13 fold greater than in the 0-mA control group and are well above the in vitro minimum inhibitory concentrations (MICs) for this drug. Inter-study reproducibility (measured as %CV) depended on the tissue type and treatment group and ranged from 8% for the retina/choroid to 51% for the anterior segment in the 4-mA group. Pretreatment with topical proparacaine hydrochloride local anesthetic did not affect amikacin delivery and total drug delivered was not affected by delivery time for the same total charge administered. Therapeutically relevant amounts of amikacin were delivered into eye tissues in a reproducible and controllable manner.

Controversies in the management of open-globe injuries involving the posterior segment

There are numerous unresolved issues and controversies regarding the management of open-globe injuries involving the posterior segment. These areas include, but are not limited to, the following issues. Although vitrectomy has been shown to improve visual outcomes and allow retention of the eye in many cases, the extent of visual improvement is often limited because of the nature of the injury. Timing of vitrectomy surgery has been and will continue to be debated by proponents of early versus delayed intervention. The multiple features of acute ocular injury make it very difficult to interpret retrospective data regarding the most appropriate timing for surgical intervention. The use of prophylactic cryotherapy, in the setting of a scleral laceration with possible retinal damage, is not as controversial at present, as there is now sufficient data indicating that cryotherapy may actually exacerbate intraocular proliferation and worsen the situation. The role and benefit of a prophylactic scleral buckle is very widely contested, and it is not known if it truly decreases the risk of subsequent retinal detachment. Another area of debate centers on the use of antibiotics. When there is a known clinical infection, intravitreal antibiotics are the mainstays of therapy. However, in the absence of clinical infection, the use of prophylactic antibiotics and their routes of administration are quite controversial. Although there are significant data regarding the use of antibiotics in the postoperative setting, this information cannot be extrapolated into the setting of open-globe injuries, as organisms and virulence factors differ. Similarly, the use of vitrectomy versus vitreous tap in the setting of traumatic endophthalmitis is not fully resolved, although vitrectomy is used in most cases to repair concurrent damage from the injury itself. Finally, the placement of intraocular lenses in the acute trauma setting is controversial, as the risk of complications is quite high. Prospective, controlled clinical studies have not been done. This article reviews pertinent data regarding these management issues and controversies, and provides recommendations for treatment based on the available published data and the authors' personal experience.

Endophthalmitis

Endophthalmitis is an inflammatory reaction of intraocular fluids or tissues. Infectious endophthalmitis is one of the most serious complications of ophthalmic surgery. Occasionally, infectious endophthalmitis is the presenting feature of an underlying systemic infection. Successful management of infectious endophthalmitis depends on timely diagnosis and institution of appropriate therapy. Recognition of the different clinical settings in which endophthalmitis occurs and awareness of the highly variable presentation it may have facilitate timely diagnosis. Biopsy of intraocular fluid/tissue is the only method that permits reliable diagnosis and treatment. The different presenting clinical settings, a rational approach to diagnosis (i.e., when, what, and how to biopsy), and the treatment of infectious endophthalmitis are reviewed.

Exogenous endophthalmitis initially treated without systemic antibiotics

PURPOSE

In 1987, the authors reported the successful eradication of infection in 16 patients with culture-proven acute exogenous bacterial endophthalmitis using intravitreal but no systemic antibiotics. They retrospectively reviewed additional consecutive cases since then to determine if the initial omission of systemic antibiotics remained reasonable.

METHODS

Twenty patients had culture-proven endophthalmitis. Four patients initially received systemic antibiotics for orbital cellulitis (2 patients), prevention of a possible scleral buckle infection (1 patient), and ascending cholangitis (1 patient). The remaining 16 patients were treated initially with intravitreal antibiotics only.

FINDINGS

Three of these additional 16 patients ultimately required systemic antibiotics for orbital cellulitis (1 patient), infectious scleritis (1 patient), and prevention of central nervous system infection with Neisseria meningitidis (1 patient). Only in one patient who had a neglected endophthalmitis and in whom an orbital cellulitis ultimately developed were we unable to clear the intraocular infection. In the overall series of 32 patients, cultures yielded staphylococcal species in 16 eyes, gram-positive bacilli in 3, streptococcal infection in 5, gram-negative cocci in 1, and gram-negative bacilli in 7. Half of the 14 specimens (1 aqueous and 13 vitreal) collected at the time of 16 reinjections in 13 eyes yielded organisms. Half (16/32) of the eyes attained visual acuity of 20/40 or better; 87.5% (28/32) attained visual acuity of 20/400 or better.

CONCLUSIONS

Therapy with intravitreal antibiotics without systemic antibiotics is reasonable, unless the infection has extended (or is at risk to extend) beyond the globe. Such evidence includes an elevated temperature or leukocyte count, corneal ring abscess, proptosis, loss of extraocular movements, scleral abscesses or infectious scleritis, and, perhaps, the presence of a scleral buckle.

Intravitreal pharmacokinetics of liposome-encapsulated amikacin in a rabbit model

BACKGROUND

Intravitreal injection of antibiotics has become a standard therapy for bacterial endophthalmitis. The duration of effective antimicrobial levels in the vitreous after single injection, however, may not be long enough to get optimal response. The authors prepared liposome-encapsulated amikacin for prolonging the duration of intravitreal therapeutic concentrations and investigated the intravitreal pharmacokinetics of the liposomes and amikacin in phosphate buffer solution (PBS) as control.

METHODS

The liposome-encapsulated amikacin was prepared by reverse-phase evaporation method. The intravitreal pharmacokinetics of the liposomes was compared with amikacin in PBS by fluorescence polarization immunoassay. Albino rabbits were randomly distributed into 12 groups. Rabbits in groups 1 to 6 and in groups I to VI (control groups) received an intravitreal injection of the liposome-encapsulated amikacin and amikacin in PBS, respectively.

RESULTS

The encapsulation rate of amikacin was 91%. The time of 50% spontaneous degradation (half-life) of the liposomes in PBS (38 degrees C, pH 7.4) was 47.6 days, and the time of 50% release (half-life) of the drug from the liposomes in PBS was 84.8 hours. The vitreous amikacin concentrations in groups 1 to 6 were significantly greater (P < 0.05) than those in control groups I to VI in every time interval, except in groups 1 to 3 at 1 hour after injection. The difference was particularly obvious in the endophthalmitis groups. The clearance of encapsulated amikacin in vitreous appeared to be related to the state of blood-ocular barrier and to the structural integrity of vitreous. The distribution, the absorption, and the elimination of encapsulated amikacin in vitreous showed the first-order kinetics.

CONCLUSION

The liposome-encapsulated amikacin prolonged half-life of the drug in vitreous. The results of the pharmacokinetic analysis suggested that in endophthalmitis, especially in severe cases, the liposomes may be preferable to conventional preparation.

Effects of inflammation and surgery on amikacin levels in the vitreous cavity

Intraocular injection of amikacin is increasingly used in the treatment of endophthalmitis. We injected 400 micrograms of amikacin into the vitreous cavity of rabbit eyes to study its pharmacokinetics. Phakic, aphakic, and aphakic vitrectomized eyes were injected, and inflamed eyes were compared to control eyes. Vitreous concentrations were determined at two, eight, 24, and 48 hours, and clearance rates were calculated. Amikacin is cleared considerably more quickly from aphakic (half-life, 14.3 hours) than phakic control eyes (half-life, 25.5 hours) and even more quickly from aphakic vitrectomized eyes (half-life, 7.0 hours). Inflammation substantially increased the rate of clearance in aphakic eyes. In inflamed aphakic and aphakic vitrectomized eyes, vitreous drug levels were equal to or below the minimal inhibitory concentration for most organisms considered sensitive to amikacin at 24 hours. Supplementation of intraocular antibiotics may therefore be required in clinical settings.

Intravitreal antibiotic therapy with vancomycin and aminoglycoside: examination of the retinal toxicity of repetitive injections after vitreous and lens surgery

The combination of vancomycin and an aminoglycoside antibiotic is frequently injected into aphakic/vitrectomized eyes for the treatment of endophthalmitis. At 2 weeks after lens and vitreous removal, rabbit eyes received an injection of a combination of 1 mg vancomycin and either 400 micrograms amikacin or 100 micrograms gentamicin. Furthermore, the effects of repeating the same combination a second and third time at intervals of 48 h were examined by light and transmission electron microscopy at 7 days to 4 months after the last injection. A single injection of either combination produced no toxicity. After 2 or 3 sets of injections of either combination, 13 of 16 eyes displayed retinal toxicity manifested by macrophages in the subretinal space, disorganization of the outer segments and retinal pigment epithelium, and discontinuities in Bruch's membrane. These results indicate that whereas a single injection of these combinations is not toxic to aphakic/vitrectomized eyes, repetitive injections may result in increasing toxicity.

Microbial endophthalmitis resulting from ocular trauma

Twenty-seven cases of culture-positive endophthalmitis that developed after ocular trauma were reviewed. The intraocular culture specimens showed a virulent microbiologic spectrum with Bacillus sp as the most common isolate (8 eyes). The visual prognosis was poor, with only 22% of patients retaining 20/400 or better vision. This level of vision was achieved in 2 of 22 (9%) bacterial cases and in four of five (80%) fungal cases. Retinal detachment (5 cases) or retinal breaks (2 cases) at the time of the initial injury had a uniformly poor visual prognosis. Postoperative retinal detachment not associated with phthisis bulbi occurred in five eyes, three of which had successful retinal reattachment surgery. Delayed onset retinal detachment after successful initial management of traumatic endophthalmitis had a greater frequency of successful retinal reattachment surgery.