Synthesis and evaluation of a ^99mTc-MAMA-propyl-thymidine complex as a potential probe for in vivo visualization of tumor cell proliferation with SPECT

Abstract 

Introduction

Cytosolic thymidine kinase (TK1) catalyzes phosphorylation of thymidine to its monophosphate. TK1 activity is closely related with DNA synthesis, and thymidine analogs derivatized with bulky carboranylalkyl groups at the N-3 position were reported to be good substrates for TK1. Accordingly, we have synthesized ^99mTc-MAMA-propyl-thymidine and evaluated it as a potential tumor tracer.

Methods

The bis(S-trityl)-protected MAMA-propyl-thymidine precursor (3-N-[S-trityl-2-mercaptoethyl]-N-[N′-(S-trityl-2-mercaptoethyl)amidoacetyl]-aminopropyl-thymidine) was prepared in three steps, and its structure was confirmed with ¹H NMR and mass spectrometry. Deprotection of the thiols and labeling with ^99mTc were done in a two-step, one-pot procedure, yielding ^99mTc-MAMA-propyl-thymidine, which was analyzed with high-performance liquid chromatography, radio-LC-MS analysis (ESI+) and electrophoresis, and its log P was determined. The biodistribution in normal mice was evaluated, and its biodistribution in a radiation-induced fibrosarcoma (RIF) tumor mouse was compared with that of 3′-deoxy-3′-[¹⁸F] fluorothymidine [¹⁸F]FLT.

Results

^99mTc-MAMA-propyl-thymidine was obtained with a radiochemical yield of 70%. Electrophoresis indicated that the complex is uncharged, and its log P was 1.0. The molecular ion mass of the Tc complex was 589 Da, which is compatible with the hypothesized N₂S₂-oxotechnetium structure. Tissue distribution showed fast clearance from plasma primarily by the hepatobiliary pathway. Whole-body planar imaging after injection of ^99mTc-MAMA-propyl-thymidine in an RIF tumor-bearing mouse showed high uptake in the liver and the intestines. No uptake was observed in the tumor, in contrast to the clear uptake observed for [¹⁸F] FLT visualized with μPET.

Conclusions

Although it has been reported that TK1 accepts large substituents at the N-3 position of the thymine ring, the results of this study show that ^99mTc-MAMA-propyl-thymidine cannot be used as a single photon emission computed tomography tumor tracer, probably because the ^99mTc-MAMA ligand is too bulky to be tolerated by TK1.

Keywords: Thymidine, Technetium-99m, TK1, RIF tumor, Radio-LC-MS