Q: How might the U.S. Food and Drug Administration (FDA) go about choosing a particular species of Acanthamoeba for efficacy testing? What should the testing protocol include?
At the FDA workshop on Microbiological Testing for Contact Lens Care Products in January, a panel of experts discussed several factors that figure into a practical measurement of solution efficacy against Acanthamoeba. Topics of the meeting included strain selection, test methodology and the controlled culturing of both trophozoites and cysts.
Expert participants from optometry, ophthalmology and the microbiology community selected a strain of A. castellanii as the proposed testing standard.1,2
This strain grows well in the laboratory and has given consistent results in disinfection assays over many years, says Simon Kilvington, Ph.D., director of microbiology for Advanced Medical Optics (AMO).
A. castellanii also shares many similar features of other strains, so its results would be pertinent and therefore would make it an effective standard. We have studied other keratitis strains of A. polyphaga and A. hatchetti, and we have not found significant differences in the trophozoite susceptibility to common contact lens disinfectant solutions, adds Dr. Kilvington.
But, some attendees did suggest future testing of additional strains and emphasized the importance of an ongoing collection of isolates.1,2
The testing scenario selection took into consideration ease of trophozoite and cyst growth in the medium and repeatability.1 Trophozoites should be cultured in a standardized bacteria-free broth medium, says Dr. Kilvington. A challenge test assay in which the rate of Acanthamoeba kill can be assessed is the preferred method.
Though culturing trophozoites in an axenic medium isnt precisely real world, its controlled and repeatable.1
Another focus of this testing, says Dr. Kilvington, is the tendency for Acanthamoeba trophozoites, upon exposure to some contact lens solutions, to encyst. In this stage, it is much more difficult to eradicate. The efficacy against cysts must also be addressed, he says. Numerous methods have been developed to induce this resistant form of the organism; however, for a given disinfectant, differing efficacy results are obtained depending on the method used to produce the cysts. The FDA workshop participants agreed upon using Neffs constant pH broth, which produces a large number of cysts in seven days.1
The group further recommended a medium challenge (3 or 4 log reduction), allowing for some microbes to survive and thereby creating a formula to predict the kill rate.1 Survivors may be counted in a non-nutrient agar broth spiked with either E. coli or Enterobacter. While this challenge size may not accurately reflect real-world situations, attendees pointed out that the infective dose in cases of Acanthamoeba keratitis is not yet actually known.
Also, testing procedures will now include a contact lens; direct inoculation with a known amount of Acanthamoeba is the proposed method.1,2
So, what is the next step? Given the high degree of concern in the clinical and science communities, a working draft of a protocol for test methodology will be created based on results of the workshop, and information should be published in the Federal Register by summer 2009. The Ophthalmic Devices Advisory Panel, which hosted this hearing, may be asked to review the new standards by fall 2009. Final standards are expected to be implemented by January 2011.1
Thanks to H. Dwight Cavanagh, M.D., Ph.D., of the University of Texas Southwestern Medical Center, for his assistance with this column.
1. Closing statements, FDA Microbiology Workshop. January 23, 2009. Available at: www.jcahpo.org/clmw/pdf/fda_postmeeting2.pdf (Accessed February 24, 2009).2. Sindt CW. Workshop on Acanthamoeba testing. Rev Cornea Contact Lenses 2009 Mar;146(2):10.