A new gallium-68 labelled imaging agent for prostate cancer

• Main Author: Nawaz, Saima
• Other Authors: Ballinger, James ; Blower, Philip John
• Published: King's College London (University of London) 2014
• Subjects: 616.99
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.676972

The prostate is an exocrine gland of the male reproductive system. There is a wide array of techniques available for imaging prostate cancer (PCa). However, there is a need to develop a new PET radiotracer to image PCa as the most commonly used PET tracer 18F-FDG is not very successful in imaging of PCa. The aim of this project is to evaluate the chelator CP256 and its bifunctional modificaton (YM103), together with a new molecular targeting vehicle J591-scFv which shows specificity for an extracellular epitope of prostate specific membrane antigen (PSMA), an extensively studied antigen for imaging PCa. The radionuclide used is gallium-68 (68Ga), which is a generator based metallic radionuclide with a half-life of 68 min. A comparative study of CP256 with other gallium chelators such as NOTA, DOTA and HBED showed that CP256 gave a higher radiolabelling yield and greater serum stability. It also gave good radiolabelling yields with other radionuclides such as 111In, 90Y, 99mTc and 89Zr. It was demonstrated that CP256 could chelate 68Ga in vivo, allowing a pre-targeting approach in which an antibody conjugated with CP256 could be administered to a patient, followed hours to days later by 68Ga-acetate or citrate. This technique could be quite useful with 68Ga because of its short half-life. CP256 is a unique and promising chelator as compared to the NOTA and HBED, which are unable to bind with 68Ga which was previously bound to serum protein. The antibody fragment J591-scFv incorporating a His-tag for labelling with 99mTc tricarbonyl was produced and its binding evaluated using human PCa cell lines PC3LN3 (PSMA negative) and a variant engineered to express PSMA (PC3LN3-PSMA). It was observed that J591(scFv) can be radiolabelled with 99mTc(CO)3+ conveniently and efficiently. The labelled product was stable in serum. It showed selective binding to PSMA positive cells compared to PSMA negative cells. A cysteine residue was incorporated into the J591c(scFv) sequence along with a His-tag to facilitate radiolabelling with 68Ga through YM103, a bifunctional chelator which contains a maleimide group to attach with cysteine of J591c(scFv). Conjugated J591c(scFv) gave good (95%) radiolabelling yields with 68Ga within 5 minutes. The radiolabelled conjugated J591c(scFv) showed selective binding with PSMA positive cells (Du145-PSMA) compared to PSMA negative cells (Du145) in vitro. For in vivo evaluation of 68Ga labelled J591c(scFv), one group of male SCID beige mice was injected with Du145-PSMA cells and another group with Du145 cells for tumour growth over the period of a few weeks. For imaging and biodistribution studies mice were injected with 7 MBq/10 μg of radiolabelled conjugated J591c(scFv). The PSMA positive tumour showed clear uptake of the radiotracer compared to the PSMA negative tumour with a specific vs nonspecific uptake ratio of 9.1. Conclusion: CP256 is a unique and versatile chelator for 68Ga. Its radiolabelling yield and reaction conditions are better than available gallium chelators such as DOTA. Its bifunctional version (YM103) conjugated with J591c(scFv) showed quantitative radiolabelling (depending upon concentration) within 5 minutes with 68Ga and showed specific binding to a PSMA positive cell line (Du145-PSMA) and high uptake in PSMA positive tumours as opposed to PSMA negative tumours.