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

Most of the radiolabeled somatostatin analogues (SSAs) are specific for subtype somatostatin receptor 2 (SSTR2). Lack of ligands targeting other subtypes of SSTRs, especially SSTR1, SSTR3, and SSTR5, limited their applications in tumors of low SSTR2 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. Ga-68 was chelated to DOTA-PA1 to obtain Ga-68-DOTA-PA1 radiotracer. The stability, lipophilicity, binding affinity, and binding specificity of Ga-68-DOTA-PA1 and FITC-PA1 were evaluated by various in vitro experiments. Micro-PET imaging of Ga-68-DOTA-PA1 was performed in nude mice bearing A549 lung adenocarcinoma, as compared with Ga-68-DOTA-(Tyr3)-octreotate (Ga-68-DOTA-TATE). Histological analysis of SSTR expression in A549 tumor tissues and human tumor tissues was conducted using immunofluorescence staining and immunohistochemical assay. Ga-68-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 Ga-68-DOTA-PA1 was 1.31-, 1.33-, and 1.90-fold that of Ga-68-DOTA-TATE, which has high binding affinity only for SSTR2, after 2 h incubation in H520, PG, and A549 lung cancer cell lines, respectively. Micro-PET images of 68Ga-DOTA-PA1 showed that the PET imaging signal correlated with the total expression of SSTRs, instead of SSTR2 only, which was measured by Western blotting and immunofluorescence analysis in mice bearing A549 tumors. In summary, a novel PET radiotracer, Ga-68-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.