A novel class of potential positron emission tomography (PET) radiotracers for imaging aminopeptidase N (also known as APN or CD13) and cannabinoid type 1 (CB1) receptors were designed and synthesised with an efficient chemical strategy. Both targets have remarkable diagnostic and therapeutic potential, in fact the CD13 receptors are over-expressed during tumour angiogenesis and the CB1 receptors are highly expressed in the brain playing important functions in several pathophysiological processes. The target compounds were obtained by means of oxime-bio-conjugation between fluoro-deoxy-carbohydrates, used as prosthetic groups, and hydroxylamine-functionalised cyclic NGR (asparagine-glycine-arginine) motif sequences for CD13 receptor and rimonabant-type pyrazoles for the CB1 receptor. In particular, aminooxy-cyclic NGR peptides were conjugated with the novel prosthetic group 5-FDR (5-fluoro-5-deoxy-D-ribose) and the aminooxy- pyrazole-type cannabinoid molecules were conjugated with both 5-FDR and with FDG (2-fluoro-2-deoxy-D-glucose). 5-FDR proved to be superior to FDG, as the bioconjugation reaction occurred in milder conditions (room temperature vs 100 °C) and at faster rate. Furthermore, we observed that the rate of the oxime bond formation depends on the solubility of the aminooxy-functionalized core used. In fact, the bioconjugation with hydrophilic cyclic aminooxy-NGR peptides was faster than in the case of lipophilic aminooxy-pyrazoles (10 min vs 20-30 min). The receptor affinity is decreased in the case of the CB1 receptors after conjugation with the fluoro-carbohydrates. This is not observed with the conjugated NGR peptides, which maintain similar affinity for the CD13 receptor compared with the unconjugated NGR. In conclusion, we have developed an efficient strategy for the synthesis of a novel class of CD13 ligands, which may be also produced in radiofluorinated form, and explored a novel bioconjugation strategy for CB1 receptor ligands. Both may have important applications in the development of PET tracers.
|Publisher||University of Aberdeen|
|Source Sets||Ethos UK|
|Type||Electronic Thesis or Dissertation|
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