Synthesis and Biological Evaluation of Novel Tc-Labeled Palbociclib Derivatives Targeting Cyclin-Dependent Kinase 4/6 (CDK4/6) as Potential Cancer Imaging Agents.

Cancer results from cell proliferation that exceeds normal growth control. There are various specific proteins that control and regulate the cell cycle, such as cyclin-dependent kinases (CDKs), cyclins, and retinoblastoma protein (pRb). The aberration of the cyclin D-CDK4/6-INK4-pRb pathway occurs frequently in cancers; thus, CDK4/6 is an attractive target for the development of radiopharmaceuticals for tumor imaging. In this study, we chose palbociclib, which was approved by the FDA for treating ER+/HER2- advanced breast cancer as the target vector and the isonitrile group, which can coordinate strongly with the [Tc(CO)] core as the bifunctional chelator, to develop four novel Tc-labeled radiotracers for tumor imaging. The ligands (, , , and were synthesized by reacting palbociclib with isocyanide-containing active esters and then radiolabeling with a [Tc(CO)] core to produce radiotracers (Tc-, Tc-, Tc-, and Tc- with high radiochemical purity (>95%) and good stability in vitro. The structures of the Tc complexes were identified by preparation and characterization of the corresponding stable rhenium complexes. Partition coefficient results indicated that these complexes were lipophilic. A kinase inhibition assay demonstrated the high affinity of the stable Re complexes for CDK4. A cell study showed that all four complexes had substantial uptake by MCF-7 cells and could be significantly inhibited by palbociclib and nonradiolabeled ligand, indicating a CDK4/6-specific uptake mechanism. Biodistribution studies in nude mice bearing MCF-7 tumors showed that the complexes had obvious accumulation in tumors at 2 h postinjection. Tc- exhibited the highest tumor uptake and tumor/blood ratio, whereas Tc- showed the highest tumor/muscle ratio. The micro-SPECT/CT study showed that complex Tc- had visible uptake at the tumor site, and the accumulation was clearly reduced in the image after pretreatment with palbociclib, further indicating CDK4/6 specificity. All the results showed that the Tc-labeled complexes in this work have the potential for tumor imaging.