Synthesis and evaluation of a (99m)Tc-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 (99m)Tc-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 (1)H NMR and mass spectrometry. Deprotection of the thiols and labeling with (99m)Tc were done in a two-step, one-pot procedure, yielding (99m)Tc-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'-[(18)F] fluorothymidine [(18)F]FLT.

RESULTS:

(99m)Tc-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 N(2)S(2)-oxotechnetium structure. Tissue distribution showed fast clearance from plasma primarily by the hepatobiliary pathway. Whole-body planar imaging after injection of (99m)Tc-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 [(18)F] FLT visualized with muPET.

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 (99m)Tc-MAMA-propyl-thymidine cannot be used as a single photon emission computed tomography tumor tracer, probably because the (99m)Tc-MAMA ligand is too bulky to be tolerated by TK1.

• Authors: Sofie Celen, Tjibbe de Groot, Jan Balzarini, Kathleen Vunckx, Christelle Terwinghe, Peter Vermaelen, Lizette Van Berckelaer, Hubert Vanbilloen, Johan Nuyts, Luc Mortelmans, Alfons Verbruggen, Guy Bormans
• Journal: Nuclear medicine and biology (Nucl Med Biol) Vol. 34 Issue 3 Pg. 283-91 (Apr 2007) ISSN: 0969-8051 [Print] United States
• PMID: 17383578 (Publication Type: Evaluation Study, Journal Article, Research Support, Non-U.S. Gov't)

Chemical References

• 3-N-(2-mercaptoethyl)-N-(N'-(2-mercaptoethyl)amidoacetyl)aminopropylthymidine
• Organotechnetium Compounds
• Radiopharmaceuticals

Topics

• Animals
• Cell Proliferation
• Drug Evaluation, Preclinical (methods)
• Feasibility Studies
• Fibrosarcoma (diagnostic imaging, metabolism)
• Metabolic Clearance Rate
• Mice
• Mice, Nude
• Molecular Probe Techniques
• Neoplasm Staging
• Organ Specificity
• Organotechnetium Compounds (chemistry, pharmacokinetics)
• Radiopharmaceuticals (chemical synthesis, pharmacokinetics)
• Tissue Distribution
• Tomography, Emission-Computed, Single-Photon (methods)