title: 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: BACKGROUND
  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: Laboratory for Radiopharmacy, K.U. Leuven, B-3000 Leuven, Belgium.
authors: Sofie
authors: S
authors: Celen
authors: Tjibbe
authors: T
authors: de Groot
authors: Jan
authors: J
authors: Balzarini
authors: Kathleen
authors: K
authors: Vunckx
authors: Christelle
authors: C
authors: Terwinghe
authors: Peter
authors: P
authors: Vermaelen
authors: Lizette
authors: L
authors: Van Berckelaer
authors: Hubert
authors: H
authors: Vanbilloen
authors: Johan
authors: J
authors: Nuyts
authors: Luc
authors: L
authors: Mortelmans
authors: Alfons
authors: A
authors: Verbruggen
authors: Guy
authors: G
authors: Bormans
mesh_terms: D000818:Animals; D049109:Cell Proliferation; D004353:Drug Evaluation, Preclinical / Q000379:methods; D005240:Feasibility Studies; D005354:Fibrosarcoma / Q000000981:diagnostic imaging* / Q000378:metabolism*; D008657:Metabolic Clearance Rate; D051379:Mice; D008819:Mice, Nude; D015336:Molecular Probe Techniques; D009367:Neoplasm Staging; D009928:Organ Specificity; D015609:Organotechnetium Compounds / Q000737:chemistry / Q000493:pharmacokinetics*; D019275:Radiopharmaceuticals / Q000138:chemical synthesis / Q000493:pharmacokinetics; D014018:Tissue Distribution; D015899:Tomography, Emission-Computed, Single-Photon / Q000379:methods*
chemical_list: C521346:3-N-(2-mercaptoethyl)-N-(N'-(2-mercaptoethyl)amidoacetyl)aminopropylthymidine; D015609:Organotechnetium Compounds; D019275:Radiopharmaceuticals