Synthesis, In Vitro, and In Silico Evaluation of Organometallic Technetium and Rhenium Thymidine Complexes with Retained Substrate Activity toward Human Thymidine Kinase Type 1

Human cytosolic thymidine kinase (hTK1) has proven to be a suitable target for noninvasive imaging of cancer cell proliferation using radiolabeled substrates such as [F-18]fluorothymidine ([F-18]FLT). However, a thymidine tracer useful for single photon emission tomography (SPECT) based off the inexpensive radionuclide technetium-99m would be of significant interest. In this work, a series of thymidine derivatives labeled with the organometallic [M(CO)(3)](+) core (M = Tc-99m, Re) were synthesized. Neutral, cationic, and anionic complexes were readily formed in aqueous media. and all were substrates of recombinant hTK1 when incubated with ATP. The neutral complexes were phosphorylated to a greater extent than the charged complexes. The extent of phosphorylation was further improved by increasing the spacer length separating thymidine and the organometallic core. A molecular dynamics Simulation Study performed with a modified hTK1 structure Supported the experimental findings. In vitro cell internalization experiments performed if) a human neuroblastoma cell line (SKNMC) showed low uptake of the charged complexes but significant uptake for the neutral, lipophilic complexes with a log P value > 1.