Receptor-targeting has been considered a method of choice for drug delivery in oncology because many types of cancer have elevated expression of a specific receptor. Recently, our group has demonstrated that quantification of interacting receptors, particularly homo-dimers, can be achieved in vivo using lifetime-based Förster Resonance Energy Transfer (FRET). However, quantification of FRET is typically performed in microscopy by either intensity- or lifetime-based measurement. Herein, we report on cross-validation between intensity and lifetime-based FRET for in vivo applications. In particular, we demonstrated dynamic in vivo FRET quantification using both intensity and lifetime measurements for assessing transferrin receptor engagement in live intact animals. Using hybridized oligonucleotides as FRET standard, we obtained the same FRET quantification via both intensity and lifetime-based measurements. Overall, both measurement methods provided the same FRET quantification trends of receptor engagement over 120 minutes of imaging. However, intensity FRET approach required 18 measurements (17 of which were used for calibration), whereas lifetime FRET required only one measurement. Hence, macroscopic fluorescence lifetime FRET presents superior, more rapid and simple method for the assessment of targeted drug delivery in longitudinal preclinical studies.
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