Design, Synthesis, and Preclinical Evaluation of Inhibitor-Based Radiotracers for Potential Application in Monitoring CDK4/6 Expression

Cyclin-dependent kinases 4 and 6 (CDK4/6) are essential drivers of cell cycle progression and have been validated as important therapeutic targets in oncology. In this study, we report the design, synthesis, and preclinical validation of a series of novel radiotracers ([68Ga]Ga-PY01–[68Ga]Ga-PY08) based on the pharmacophore of CDK4/6 inhibitor ribociclib. Flexible linkers and functionalized amino acids were incorporated to optimize pharmacokinetic and targeting properties. Among eight radiotracers, [68Ga]Ga-PY03 showed superior pharmacological and pharmacokinetic properties, including high stability, strong CDK4/6 binding affinity and low nonspecific uptake. Micro-PET/CT imaging demonstrated its capability to detect CDK4/6-overexpressed tumors and dynamically monitor CDK4/6 expression post-therapeutic. Importantly, [68Ga]Ga-PY03 also enabled quantitative assessment of CDK4/6 target occupancy, providing a potential tool for therapy response evaluation. In summary, these findings demonstrate the potential of [68Ga]Ga-PY03 as a PET radiotracer to monitor the CDK4/6 expression in tumors.