hHEPATO-Cy5, a Bimodal Tracer for Image-Guided Hepatobiliary Surgery

Liver cancer is a leading cause of cancer deaths worldwide. Surgical resection of superficial hepatic lesions is increasingly guided by the disrupted bile excretion of the fluorescent dye indocyanine green (ICG). To extend this approach to deeper lesions, a dedicated bimodal tracer that facilitates both fluorescence guidance and radioguidance was developed. Methods: A tracer comprising a methylated cyanine-5 (Cy5) fluorescent dye and a mercaptoacetyltriserine chelate (hHEPATO-Cy5) was synthesized and characterized. Cellular uptake and excretion were evaluated in hepatocyte cultures (2-dimensional culture and in vitro lesion model), using a fluorescent bile salt, MitoTracker dye, and methylated Cy5 as a control. After radiolabeling, the pharmacokinetics of ^99mTc-hHEPATO-Cy5 were assessed in mice over 24 h (percentage injected dose and percentage injected dose per gram of tissue, SPECT/CT imaging and fluorescence imaging). The ability to provide real-time fluorescence guidance during robot-assisted hepatobiliary surgery was evaluated in a porcine model using ICG as a reference. Results: The unique molecular signature of hHEPATO-Cy5 promotes hepatobiliary excretion. In vitro studies on hepatocytes showed that where methylated Cy5 remained internalized, hHEPATO-Cy5 showed fast clearance (10 min) similar to that of fluorescent bile salt. In vivo use of ^99mTc-hHEPATO-Cy5 in mice revealed liver accumulation and rapid biliary clearance. The effectiveness of bile clearance was best exemplified by the 2-orders-of-magnitude reduction in count rate for the gallbladder (P = 0.008) over time. During hepatobiliary surgery in a porcine model, hHEPATO-Cy5 enabled fluorescence-based lesion identification comparable to that of ICG. Conclusion: The bimodal ^99mTc-hHEPATO-Cy5 provides an effective means to identify liver lesions. Uniquely, it helps overcome the shortcomings of fluorescence-only approaches by allowing for an extension to in-depth radioguidance.