Design, Synthesis, and Biological Evaluation of 68 Ga-DOTA–PA1 for Lung Cancer: A Novel PET Tracer for Multiple Somatostatin Receptor Imaging

摘要

Most of the radiolabeled somatostatin analogues (SSAs) are specific for subtype somatostatin receptor 2 (SSTR_(2)). Lack of ligands targeting other subtypes of SSTRs, especially SSTR_(1,) SSTR_(3), and SSTR_(5), limited their applications in tumors of low SSTR_(2) expression, including lung tumor. In this study, we aimed to design and synthesize a positron emission tomography (PET) radiotracer targeting multi-subtypes of SSTRs for PET imaging. PA1 peptide and its conjugate with 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) chelator or fluorescein isothiocyanate (FITC) at the N-terminal of the lysine position were synthesized. ~(68)Ga was chelated to DOTA–PA1 to obtain ~(68)Ga-DOTA–PA1 radiotracer. The stability, lipophilicity, binding affinity, and binding specificity of ~(68)Ga-DOTA–PA1 and FITC–PA1 were evaluated by various in vitro experiments. Micro-PET imaging of ~(68)Ga-DOTA–PA1 was performed in nude mice bearing A549 lung adenocarcinoma, as compared with ~(68)Ga-DOTA-(Tyr3)-octreotate (~(68)Ga-DOTA-TATE). Histological analysis of SSTR expression in A549 tumor tissues and human tumor tissues was conducted using immunofluorescence staining and immunohistochemical assay. ~(68)Ga-DOTA–PA1 had high radiochemical yield and radiochemical purity of over 95% and 99%, respectively. The radiotracer was stable in vitro in different buffers over a 2 h incubation period. Cell uptake of ~(68)Ga-DOTA–PA1 was 1.31-, 1.33-, and 1.90-fold that of ~(68)Ga-DOTA-TATE, which has high binding affinity only for SSTR_(2), after 2 h incubation in H520, PG, and A549 lung cancer cell lines, respectively. Micro-PET images of ~(68)Ga-DOTA–PA1 showed that the PET imaging signal correlated with the total expression of SSTRs, instead of SSTR_(2) only, which was measured by Western blotting and immunofluorescence analysis in mice bearing A549 tumors. In summary, a novel PET radiotracer, ~(68)Ga-DOTA–PA1, targeting multi-subtypes of SSTRs, was successfully synthesized and was confirmed to be useful for PET imaging. It may have potential as a noninvasive PET radiotracer for imaging SSTR-positive tumors.