Automated radiosynthesis and preclinical evaluation of Al[18F]F-NOTA-P-GnRH for PET imaging of GnRH receptor-positive tumors

Introduction: Gonadotropin releasing hormone (GnRH) receptor is overexpressed in many human tumors. Previously we developed a F-18-labelled GnRH peptide. Although the GnRH-targeted PET probe can be clearly visualized by microPET imaging in a PC-3 xenograft model, clinical applications of the probe have been limited by complex labeling procedures, poor radiochemical yield, and unwanted accumulation in GnRH receptor negative tissues. In this study, we have designed a new F-18-labelled GnRH peptide that is more amenable to clinical development. Methods: GnRH peptide analogues NOTA-P-GnRH was synthesized and automated radiolabeled with F-18 using a Al[F-18]F complex on a modified PET-MF-2V-IT-I synthesis module. The GnRH receptor affinities of AIF-NOTA-P-GnRH and NOTA-P-GnRH were determined by in vitro competitive binding assay. For in vitro characterization determination of stability and partition coefficients were carried out, respectively. Dynamic microPET and biodistribution studies of Al[F-18]F-NOTA-P-GnRH were evaluated in xenograft tumor mouse models. Results: The total radiochemical synthesis and purification of Al[F-18]F-NOTA-P-GnRH was completed within 35 min with a decay-corrected yield of 35 +/- 10%. The logP value of Al[F-18]F-NOTA-P-GnRH was -2.74 +/- 0.04 and the tracer was stable in phosphate-buffered saline, and bovine and human serum. The IC50 values of AlF-NOTA-P-GnRH and NOTA-P-GnRH were 116 nM and 56.2 nM, respectively. Dynamic PET imaging together with ex vivo biodistribution analyses revealed that Al[F-18]F-NOTA-P-GnRH was clearly delineated in both PC-3 and MDA-MB-231 xenografted tumors. Conclusion: Al[F-18]F-NOTA-P-GnRH can be efficiently produced on a commercially available automated synthesis module and has potential for use in clinical diagnosis of GnRH receptor-positive tumors. Advances in knowledge: Our studies developed the automated radiosynthesis of a new F-18-labelled GnRH tracer and predinical evaluation for future clinical application. Implications for patient care: Quantitative and noninvasive imaging of GnRH expression would provide information for diagnosis and treatment of cancer patients. (C) 2020 Elsevier Inc. All rights reserved.