Towards early detection of age-related macular degeneration with tetracyclines and FLIM

Recently, we discovered microscopic spherules of hydroxyapatite (HAP) in aged human sub-retinal pigment epithelial (sub-RPE) deposits in the retinas of aged humans (PMID: 25605911), and developed evidence that the spherules may act to nucleate the growth of sub-RPE deposits such as drusen. Drusen are clinical hallmarks of age-related macular degeneration (AMD). We found that tetracycline-family antibiotics, long known to stain HAP in teeth and bones, also stained the HAP spherules, but in general the HAP-bound fluorescence excitation and emission spectra overlapped with the well-known autofluorescence of the RPE overlying drusen, making them difficult to resolve. However, we also found that certain tetracyclines exhibited substantial increases in fluorescence lifetime upon binding to HAP, and moreover these lifetimes were substantially greater than those previously observed (Dysli, et al., 2014) for autofluorescence in the human retina in vivo. Thus we were able to image the HAP spherules by fluorescence lifetime imaging microscopy (FLIM) in cadaveric retinas of aged humans. These findings suggest that FLIM imaging of tetracycline binding to HAP could become a diagnostic tool for the development and progression of AMD.