Noninvasive Bioluminescence Imaging of Serum Albumins in Living Mice

Bioluminescence (BL) is an emerging optical readout that has been extensively used in various bioassays and molecular imaging systems. In this study, we present the bioanalytical application of marine luciferins as an excellent optical indicator for noninvasive imaging of serum albumins. We synthesized 30 kinds of regioisomeric coelenterazine (CTZ) analogs and investigated their specificities for major serum proteins from various species. The results found that some of the CTZ analogs exhibited surprisingly specific optical signals upon binding with the serum albumins. These CTZ analogs showed diverse emission spectra ranging from 495 to 558 nm according to the albumin species used acting as pseudoluciferases. The selective albumin indicators, TS1 and TS2, exhibited long and linear dose-response curves and were sensitive enough to determine clinically normal and abnormal (microalbuminuria) ranges of albumins in saliva and urine. The sensitivity of this assay is superior to that of the conventional Bromocresol purple (BCP) method. We further demonstrated the advantages of the albumin indicators through noninvasive imaging of liver-albumin in vivo in living mice. The in vivo and ex vivo imaging results confirmed that the CTZ analog TS2 can sensitively image the liver-albumin in vivo with high signal-to-background ratio. This study paves a new way to make use of CTZ analogs for noninvasive albumin imaging and conceptualizes the pseudoluciferase-based imaging. The distinct in vivo imaging of serum albumins can potentially aid clinicians in providing insight into patients' liver function and other vital factors needed for whole-body homeostasis.