Convergent synthesis and evaluation of ¹⁸F-labeled azulenic COX₂ probes for cancer imaging

The overall objectives of this research are to (i) develop azulene-based positron emission tomography (PET) probes and (ii) image COX₂ as a potential biomarker of breast cancer. Several lines of research have demonstrated that COX₂ is overexpressed in breast cancer and that its presence correlates with poor prognoses. While other studies have reported that COX₂ inhibition can be modulated and used beneficially as a chemopreventive strategy in cancer, no viable mechanism for achieving that approach has yet been developed. This shortfall could be circumvented through in vivo imaging of COX₂ activity, particularly using sensitive imaging techniques such as PET. Toward that goal, our laboratory focuses on the development of novel ¹⁸F-labled COX₂ probes. We began the synthesis of the probes by transforming tropolone into a lactone, which was subjected to an [8 + 2] cycloaddition reaction to yield 2-methylazulene as the core ring of the probe. After exploring numerous synthetic routes, the final target molecule and precursor PET compounds were prepared successfully using convergent synthesis. Conventional ¹⁸F labeling methods caused precursor decomposition, which prompted us to hypothesize that the acidic protons of the methylene moiety between the azulene and thiazole rings were readily abstracted by a strong base such as potassium carbonate. Ultimately, this caused the precursors to disintegrate. This observation was supported after successfully using an ¹⁸F labeling strategy that employed a much milder phosphate buffer. The ¹⁸F-labeled COX₂ probe was tested in a breast cancer xenograft mouse model. The data obtained via successive whole-body PET/CT scans indicated probe accumulation and retention in the tumor. Overall, the probe was stable in vivo and no defluorination was observed. A biodistribution study and Western blot analysis corroborate with the imaging data. In conclusion, this novel COX₂ PET probe was shown to be a promising agent for cancer imaging and deserves further investigation.