Synthesis and labeling of CHX-A''-DTPA-DUPA-Pep with gallium-68, zirconium-89 and lutetium-177 for diagnosis and therapy of prostate cancer

Objectives In the last years, the application of small urea based peptides for the targeting of the prostate specific membrane antigen (PSMA) for molecular imaging of prostate carcinoma with PET was of high interest. Nevertheless, therapeutic approaches by PSMA targeting are shown rarely. Previous work with Ga-68 labeled PSMA ligand DUPA-Pep-Desferal showed high affinity to PSMA (KD 42nm) but the used chelator (Desferal) is not suited for chelation of a therapy nuclide as Lu-177. Our goal was the synthesis of DTPA-CHX-A" conjugated DUPA-Pep and the development of suited synthesis strategies for the subsequent radiolabeling with Ga-68 and Zr-89 for diagnostic application and especially with Lu-177 for therapeutic approaches.

Methods DUPA-Pep-DTPA-CHX-A" was synthesized in a one step reaction starting from custom made DUPA-Pep and p-Isothiocyanatobenzyl-DTPA-CHX-A". The dependencies of the RCY of the radiolabeling reaction of DUPA-Pep-DTPA-CHX-A" with Ga-68, Zr-89 and Lu-177 was investigated with respect to peptide amount and reaction time. Radiosyntheses were performed at room temperature in 500 µl HEPES buffer (pH 7.7) with peptide amounts between 10-100 µg. Cell experiments on PSMA positive LNCaP cells were conducted for determination of KD values.

Results DUPA-Pep-DTPA-CHX-A’’ was synthesized in high purity of >98%. Best results (RCY >97%) were obtained with 50 µg of peptide within a reaction time of 10 min in case of radiolabeling with Ga-68 and Lu-177. In case of Zr-89 a reaction time of 60 min was required for practically quantitative labeling. Cell experiments on PSMA positive LNCaP cells resulted in KD value of 12 nM.

Conclusions Optimized reaction conditions for the practically quantitative radiolabeling of DUPA-Pep-DTPA-CHX-A’’ with Ga-68. Zr-89 and Lu-177 were developed. The peptide showed high affinity to PSMA positive LNCaP cells with a KD value of 12 nM.