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

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-mereaptoethyl)amidoacetyl]-aminopropyl-thymidine) was prepared in three steps, and its structure was confirmed with H-1 NMR and mass spectrometry. Deprotection of the thiols and labeling with Tc-99m were done in a two-step, one-pot procedure, yielding Tc-99m-MAMA-propyl-thyi-nidine, 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'-[F-18]fluorothymidine[[F-18]FLT]. Results: Tc-99m-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 pPET. 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 Tc-99m-MA MA-propyl-thymidine cannot be used as a single photon emission computed tomography turner tracer, probably because the Tc-99m-MAMA ligand is too bulky to be tolerated by TK1. (c) 2007 Elsevier Inc. All rights reserved.