Modulation sensing of fluorophores in tissue: A new approach to drug compliance monitoring

We describe a method to detect the presence of fluorophores in scattering m edia, including intralipid suspensions and chicken muscle covered with skin . The fluorophores were rhodamine 800 (Rh800) and indocyanine green (IcG), both of which can be excited at long wavelengths where there is minimal abs orption by tissues. These fluorophores were dissolved in intralipid or in c hicken muscle under skin. A method to approximate the fluorophore concentra tion in such samples was developed using a long lifetime reference fluoroph ore in a polymer film placed immediately on the illuminated surface of the sample. Because of the long lifetime of the reference film, the modulation of its emission at low frequencies near 2 MHz is near zero. Since the lifet ime of Rh800 and IcG are below 2 ns the modulation of the combined emission is a measure of the intensity of the fluorophore (Rh800 or IcG) relative t o the long lifetime reference. Using this method we were able to measure th e concentration-dependent intensities of Rh800 and IcG in an intralipid sus pension. Additionally, micromolar concentrations of these probes could be d etected in chicken muscles, even when the muscle was covered with a layer o f chicken skin. The presence of an India ink absorber in the intralipid had only a moderate effect on the modulation values. We suggest the use of thi s transdermal detection of long-wavelength fluorophores as a noninvasive me thod to monitor patient compliance when taking medicines used for treatment of chronic diseases such as AIDS or tuberculosis. (C) 1999 Society of Phot o-Optical Instrumentation Engineers. [S1083-3668(99)00504-3].