Optimization of the near-infrared fluorescence labeling for in vivo monitoring of a protein drug distribution in animal model

The objective of this study is to optimize the parameters in labeling near-infrared (NIR)fluorescent dye cypate to protein drugs for in vivo optical imaging of drug distributions in animal model. L-ASparaginase (L-ASNase) was used as a protein drug model for the study. To achieve this goal, various labeling conditions, including different catalysts, feed ratios of all components, pH conditions, temperatures, and reacting durations, were investigated. The dye-to-protein (D/P) ratio and enzymatic activity were designated as the metric to evaluate the labeling process. The stability of the cypate-protein conjugate in blood serum and its distribution in small animals were subsequently inspected. Results showed that feed ratio of L-ASNase and the pH value played the most important role in adjusting the labeling efficiency. Reaction duration and temperature had less effect on the dye-to-protein labeling properties. The optimal condition for the labeling of cypate to L-ASNase was 4 h reaction duration at 4 °C and pH 8.5 under catalysis by HOBt/HBTU. The dynamic distribution in animal model displayed that the labeled L-ASNase firstly accumulated in liver and cleared from the enteron system. This study demonstrated that the NIR image system combined with NIR probe has the capability to trace the dynamics of protein drugs in animals for drug development.