Assessing antibiotic efficacy using pharmacodynamic measures

ByTerrence P. O'Brien, MD

Terrence P. O'Brien

Achieving and sustaining sufficiently therapeutic antibiotic concentrations at the infection site are necessary for rapid eradication of pathogens in ocular surface infections or external diseases. Ineffective or insufficient amounts of antibiotic can exacerbate chronic bacterial colonization of eyelid margins, selecting for resistant strains to predominate, which then contribute to spillover of infectious products into the tear film. Moreover, the selection pressure can expand populations of resistant pathogens in the conjunctiva. To effectively treat ocular surface infections or external diseases, clinicians must use a biocompatible agent that achieves high concentration in the conjunctiva, tear film, and cornea.

Ophthalmic clinicians have the advantage of administering antibiotics directly to the site of infection on the ocular surface. High ocular concentrations of antibiotic can be achieved using topical medications because tissue dynamics allow the agent to concentrate in the lid, conjunctiva, tears, and cornea. The principal indications for topical antibiotic agents include therapy of conjunctivitis, blepharitis, blepharoconjunctivitis, and bacterial keratitis. Topical antibiotics are also used to prevent or reduce the development of infection after ophthalmic surgery such as cataract surgery, glaucoma surgery, corneal surgery, and laser refractive procedures such as LASIK.

A medication’s dosing regimen is important because fewer doses lead to improved patient compliance.^1-3 Patients will likely comply with a once-a-day dosing schedule better than they would with a dosing schedule of four times a day.^4-7 This, in turn, will reduce the risk for bacterial resistance. A medication that is equally efficacious with fewer doses reduces exposure and selection pressure for the development of resistance, provided the agent is effective in eradicating the bacteria at the concentration and the frequency with which it is administered. To treat severe, sight-threatening infections in the central part of the cornea, clinicians must apply topical antibiotics every 15 to 30 minutes, an exhausting process for both the patient and the physician. An ideal topical medication is biocompatible with the eye and achieves equal efficacy with less frequent dosing (Figure 1). Azithromycin solution in DuraSite requires the administration of at least 50% fewer drops than other ocular antibiotics such as Vigamox (moxifloxacin 0.5% ophthalmic solution, Alcon Laboratories, Inc.), Zymar (gatifloxacin, 0.3% ophthalmic solution, Allergan, Inc.) and tobramycin, 0.3% ophthalmic suspension.

FDA-approved dosing schedule of drugs indicated for bacterial conjunctivitis
Figure 1. FDA-approved dosing schedule of drugs indicated for bacterial conjunctivitis

Figure 1. FDA-approved dosing schedule of drugs indicated for bacterial conjunctivitis

Figure 1. Dosing regimen is important because fewer doses leads to improved patient compliance. A medication that is equally effective with fewer doses reduces exposure and selection pressure for the development of resistance.

Source: Respective full prescribing information for each ocular antibiotic.

Sustaining high tissue concentration

Potency as assessed by in vitro susceptibility testing is a frequently conducted pharmacodynamic measure of an antibiotic. Microbiologists identify the minimum inhibitory concentration (MIC), the minimum bactericidal concentration (MBC), and dynamically integrate these values with the antibiotic concentrations in tissue, specifically the C_max, to assess antibiotic efficacy at a particular site. The C_max,is the maximum concentration of antibiotic achieved in the tissue after a single dose and can be measured in the conjunctiva, tear film, eyelid tissue, or cornea. The AUC is the area under the concentration-time curve at steady state over 24 hours. It is correlated with how long drug concentrations persist in tissue. The higher the number, the longer the drug is present.

An eye drop can be developed to achieve high tissue concentration by increasing the formula concentration, increasing the frequency of application, or dosing via a drug delivery vehicle that allows the drop to remain in contact with the ocular surface, which subsequently prevents washout by tears or rapid blinking. Increasing the frequency of application is the standard approach for the treatment of severe infections such as keratitis, for which hourly dosing is sometimes used in the early stages of treatment to achieve high corneal tissue concentrations. Increasing ocular antibiotic concentrations in the formulation of eye drops presents a challenge to developers because antibiotics have limits of solubility. In addition, concentrations that are too high can become toxic to the delicate ocular surface cells as well as keratocytes, corneal endothelium, and intraocular cells. Azithromycin, 1% ophthalmic solution has a delivery vehicle, DuraSite (InSite Vision, Inc.), that contains polycarbophil and electrolytes similar to constituents of the tear film. DuraSite stabilizes aqueous azithromycin and increases its residence time in the conjunctiva, as shown in animal studies.^8,9The formulation permits sustained release of the drug from the vehicle and extended content with the ocular surface. The result is a higher concentration of drug in the tear film, conjunctiva, superficial epithelial cells, and cornea (Figure 2).

Findings from a study conducted on New Zealand white rabbits showed that a single drop of AzaSite (azithromycin, 1% ophthalmic solution in DuraSite, Inspire Pharmaceuticals, Inc.) achieves high concentrations in the tear film and bulbar conjunctiva. Researchers administered one drop of either 0.5% or 1% AzaSite solution to each eye bilaterally and collected conjunctiva and tear film samples at 0.5, 1, 2, 4, 8, 12, and 24 hours postadministration. Using assays that employed high pressure liquid chromatography and mass spectral analysis, researchers identified the concentration of antibiotic in tissues and reported mean levels of concentration of 300 µg/g in the tear film and 82 µg/g in the bulbar conjunctiva 30 minutes after initial application. ¹⁰ These values ranked above the MIC of a majority of pathogens causing conjunctival or ocular surface disease. Researchers also found significant concentrations in tissue at 24 hours postadministration of a single drop. The mean conjunctival concentration was reported to be 16 µg/g, which is higher than the MIC ₅₀ of most common pathogens causing ocular surface or external ocular infection. ¹⁰ Current human studies reveal similar trends in tissue concentration. These characteristics minimize the number of drops needed during a therapeutic course.

Ocular tissue concentrations of azithromycin in rabbit conjunctiva
Figure 2. Ocular tissue concentrations of azithromycin in rabbit conjunctiva

Figure 2. Ocular tissue concentrations of azithromycin in rabbit conjunctiva

Figure 2. The C_maxis the maximum concentration achieved in superior conjunctiva after a single dose. The area under the curve (AUC) is a measure of how long concentrations persist in the tissue. The C_max and AUC of AzaSite is 18- and 12-fold higher, respectively, than those of a topical solution of 1% azithromycin without DuraSite. This C_max is higher than the MIC₅₀of all known pathogens causing conjunctival or ocular surface disease.

Source: Inspire Pharmaceuticals, Inc.

In vitro testing

A stronger correlation exists between in vitro data and systemic infection than between in vitro data and ocular surface infection. In vitro data more directly correlates to achievable concentrations in the serum and blood can be predictive of effective antibiotic treatment for systemic infections such as endocarditis, meningitis, or sinusitis. Breakpoints have not been established or standardized for antibiotics in ocular tissue. In vitro methods of assessing antibiotic efficacy eliminate host factors that may affect the presentation or potency of a drug and concentrate solely on the pathogen and its intrinsic susceptibility to an agent considered for treatment. Physiologic factors that affect susceptibility include immunomodulatory effects unique to both the antibiotic and individual patients. In vivo measures have a number of variables, some of which are uncontrolled, that render one patient more susceptible to infection than another. Alternatively, these variables may also predict the individual response to infection. For example, a young, healthy patient who contracts an upper respiratory infection may recover quickly, whereas an infant or elderly patient may need more time to recover and may have greater challenges in fighting off the infection. The same is true of eye infections.

My colleagues Darlene Miller, DHSc, Eduardo Alfonso, MD, and I at the Ocular Microbiology Laboratory at Bascom Palmer Eye Institute conducted a comparative in vitro analysis of ocular isolates that caused conjunctivitis in humans. This study compared the in vitro potency of 1% azithromycin in DuraSite with that of fluoroquinolones, specifically moxifloxacin and gatifloxacin. The study results showed similar susceptibility for azithromycin, moxifloxacin, and gatifloxacin, especially to more common pathogens (Figure 3).¹¹ For Staphylococcus epidermidis, 31 isolates were established as the cause of conjunctivitis, and no statistically significant difference could be identified in the percentage susceptible for azithromycin vs fluoroquinolones. Similarly, for Haemophilus influenzae, 27 isolates were established and again, identical correlation was found. For Staphylococcus aureus and Streptococcus pneumoniae, fluoroquinolones were shown to be slightly more effective than azithromycin, but this finding is of uncertain statistical significance given the lack of standardized breakpoints and the vagaries of comparing in vitro testing between different classes of antibiotic agents. In vitro study results do not necessarily predict in vivo efficacy because concentration and exposure time may aid in eradication, even when in vitro potency suggests that an agent is less effective.

Comparative susceptibility of ocular isolates to AzaSite and fourth-generation fluoroquinolones
Figure 3. Comparative susceptibility of ocular isolates to AzaSite and fourth-generation fluoroquinolones

Figure 3. Comparative susceptibility of ocular isolates to AzaSite and fourth-generation fluoroquinolones

Figure 3. Study results for ocular bacterial isolates showed similar susceptibility for Haemophilus influenzae and Staphylococcus epidermidis species to 1% azithromycin in DuraSite (AzaSite) and fourth-generation fluoroquinolones. Differences in susceptibility for Staphylococcus aureus and Streptococcus pneumoniae were not statistically significant. n= number of isolates.

Source: Bascom Palmer Eye Institute

Conclusion

Although in vitro data provide some indication of which antibiotics will eradicate specific bacterial isolates, one cannot absolutely predict a clinically significant difference in the ability of different ocular antibiotics to eradicate conjunctival or ocular surface pathogens with topical administration based on these results. Clinicians also cannot rely solely on experimental in vivo clinical results because successful eradication may cause significant damage to the delicate cells and tissues in the eye. The clinician must consider both in vitro data as well as monitor the in vivo response to antibiotics. No single piece of data can conclusively predict antibiotic efficacy for treating an infection for a given patient at a given site, especially the eye, although in vitro potency data can reveal trends or likely responses.

Clinical trial results are required to conclusively determine optimal antibiotics for specific infections. Phase 3 trials evaluating 1% azithromycin in DuraSite found clinical outcomes to be similar to those of aminoglycosides.^12,13Later comparisons of the data revealed that the cure rates were similar to that reported for 4th generation fluoroquinolones.¹³Macrolides, such as AzaSite, are shown to be efficacious because they exhibit good in vitro efficacy, achieve high tissue concentrations and sustain effective concentrations throughout the treatment period.

References

Kardas P. Comparison of patient compliance with once-daily and twice-daily antibiotic regimens in respiratory tract infections: results of a randomized trial. J Antimicrob Chemother. 2007;59:531-536.

Kass MA, Gordon M, Morley RE Jr, Meltzer DW, Goldberg JJ. Compliance with topical timolol treatment. Am J Ophthalmol. 1987;103:188-193.

Winfield AJ, Jessiman D, Williams A, Esakowitz L. A study of the causes of non-compliance by patients prescribed eyedrops. Br J Ophthalmol. 1990;74:477-480.

Perez-Gorricho B, Ripoll M, PACE Study Group. Does short-course antibiotic therapy better meet patient expectations? Int J Antimicrob Agents. 2003; 21:222-228.

Kardas P. Patient compliance with antibiotic treatment for respiratory tract infections. J Antimicrob Chemother. 2002;49:897-903.

Zimmerman TJ, Zalta AH. Facilitating patient compliance in glaucoma therapy. Surv Ophthalmol. 1983;28(suppl):252-258.

Richter A, Anton SE, Kock P, Dennett SL. The impact of reducing dose frequency on health outcomes. Clin Ther. 2003;25:8:2307-2335.

Harper DG, Chen CE, Friedlaender MH. Controlled comparison of two fluorometholone formulations in the antigen challenge model of allergic conjunctivitis. CLAO J. 1995;21:256-260.

Keller N, Tsao SW, Chadrasekaran K, Hatfield JM. Ocular kinetics of extended duration fluorometholone (0.1%) formulations vs ointment and aqueous suspension. Invest Ophthalmol Vis Sci. Abstract 1461. Presented at: The Association for Research in Vision and Ophthalmology Annual Meeting. May 2-7, 1993; Sarasota, Fla.

Si EC, Bowman LM, Roy SD. Ocular bioavailability and systemic levels of an ophthalmic formulation of azithromycin, ISV-401. Invest Ophthalmol Vis Sci. 2003. E-Abstract 1461. Presented at: The Association for Research in Vision and Ophthalmology Annual Meeting. May 4-9, 2003; Fort Lauderdale, Fla.

Miller D, Alfonso E, O’Brien TP. Newer Horizons in Ocular Infective Therapy: Progress in the management of external and ocular surface infections. Presented at Hawaiian Eye Annual Meeting. January 20-25, 2008; Waikoloa, Hi.

Protzko E, Bowman L, Abelson M, Shapiro A; AzaSite Clinical Study Group. Phase 3 safety comparisons for 1.0% azithromycin in polymeric mucoadhesive eye drops versus 0.3% tobramycin eye drops for bacterial conjunctivitis. Invest Ophthalmol Vis Sci. 2007;48:3425-3429.

Abelson MB, Protzko E, Shapiro A, Graces-Soldana A, Bowman L; AzaSite Study Group. A randomized trial assessing the clinical efficacy and microbial eradication of 1% azithromycin ophthalmic solution vs. tobramycin in adult and pediatric subjects with bacterial conjunctivitis. Dove Medical Press. Clin Ophthalmol. 2007(1);177-182.