Synthesis, in vitro and in silico assessment of organometallic Rhenium(I) and Technetium(I) thymidine complexes

Thymidine kinases have been identified as suitable targets for non-invasive imaging of gene therapy and cancer. Thus, there is a high interest in new, reliable and inexpensive radiolabeled thymidine analogues for these applications. In this study we present the synthesis and in vitro evaluation of M(CO)₃-complexes of thymidine (M = ^99mTc, Re) for potential use in SPECT tumor imaging. 5′-amino-5′-deoxythymidine was derivatized at position C5′ with spacers of various lengths (∼0-30 Å) carrying tridentate metal chelating entities such as iminodiacetic acid and picolylamine-N-monoacetic acid. The nucleoside derivatives were reacted with the precursors [ReBr₃(CO)₃]²⁻ and [^99mTc(OH₂)₃(CO)₃]⁺, respectively. The organometallic thymidine complexes have been fully characterized by means of IR, NMR and mass spectrometry. Enzyme kinetic studies revealed mixed inhibition of the human cytosolic thymidine kinase with K_i values ranging from 4.4 to 334 μM for all thymidine complexes. Competitive inhibition of herpes simplex virus type 1 thymidine kinase was only achieved when thymidine and the metal core were separated by a spacer of approximately 30 Å length. These findings were supported by in silico molecular docking and molecular dynamic experiments.