Adenosine appears to play an important role in tumor growth and metastasis. Synthesized (11)C-adenosine 5'-monophosphate (AMP) has recently been reported as a potential tumor-imaging radiotracer.
A variety of human tumor cell lines (SKOV-3, SCC-15, U251, U87, Raji, and Daudi) were incubated with 3.7 kBq (0.1 microCi) of [2-(3)H]AMP ((3)H-AMP), [5,6-(3)H]FDG ((3)H-FDG), or [2-(3)H]adenosine ((3)H-adenosine) in low-physiologic-glucose serum-free medium. Selected cells were exposed to caffeine, dipyridamole, adenosine 5'-(alpha,beta-methylene)diphosphate (APCP), or unlabeled adenosine before exposure to the radiotracer. R-phycoerythrin-conjugated mouse antihuman monoclonal antibody to human CD73 was used for immunophenotyping. High-performance liquid chromatography was used to characterize the intracellular metabolites of (3)H-AMP after intracellular uptake.
Intracellular uptake of (3)H-AMP was significant-10 to 100 times the uptake of (3)H-FDG, depending on the particular tumor cell line. Preexposure of SKOV-3 cells to caffeine, a competitive inhibitor of adenosine receptors, did not affect cellular uptake of (3)H-AMP. However, preexposure of SKOV-3 cells to dipyridamole, an equilibrative nucleoside transporter inhibitor; APCP, a CD73 (ecto-5'-nucleotidase) inhibitor; or cold adenosine significantly inhibited cellular uptake of (3)H-AMP. SKOV-3 uptake of (3)H-adenosine was inhibited by dipyridamole but not APCP. U251 uptake of (3)H-AMP was significantly inhibited by dipyridamole and APCP. U87 uptake of (3)H-AMP was only partially inhibited by dipyridamole and APCP. However, Raji and Daudi cells had significantly lower uptake of (3)H-AMP than of (3)H-FDG but had significantly increased uptake of (3)H-adenosine, which was inhibited by dipyridamole. Raji and Daudi cells were negative, but the SKOV-3 cells positive, for CD73 cell-surface expression. (3)H-Adenosine metabolites were persistently retained after influx into the cell, predominantly as (3)H-adenosine triphosphate and (3)H-adenosine diphosphate.
Cancer cell lines evaluated in vitro had significantly elevated uptake of radiolabeled AMP, on the order of 10- to 100-fold, in comparison to radiolabeled FDG. The mechanism of intracellular uptake depends predominantly on equilibrative nucleoside transporters after conversion of AMP to adenosine by CD73 in SKOV-3, SCC-15, and U251 cells. Intracellular retention is due to phosphorylation to adenosine triphosphate and adenosine diphosphate. Raji and Daudi cells have low uptake of radiolabeled AMP because of a lack of CD73 expression. This in vitro evaluation using (3)H-AMP with tumor cell lines supports the potential of (11)C-AMP for use in targeting the nucleoside transport pathway in PET imaging of tumors.
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