Prostate cancer patients can benefit from non-invasive and more accurate diagnosis, as well as improved visualization during surgery. ImmunoPET can provide information on location and extent of the disease, while fluorescent image-guided surgery can distinguish cancerous tissue during resection. Prostate Stem Cell Antigen (PSCA) is upregulated in the majority of prostate cancers and metastases and is therefore a promising target for imaging. Engineered antibody fragments, such as the minibody, exhibit ideal imaging characteristics due to fast blood clearance for high target-to-background images at short imaging times post-injection. A dual-labeled minibody can reveal the whole-body PSCA-expressing tumor burden by PET, followed by intraoperative visualization of margins by fluorescence.

The fully humanized anti-PSCA A11 minibody was engineered with a C-terminal cys-tag (A11 cMb) for site-specific labeling by thiol-directed chemistry. In order to radiolabel with 89Zr, a metal chelator desferrioxamine (DFO) was site-specifically conjugated to A11 cMb by maleimide chemistry, while direct iodination was used to label with 124I. Singly labeled 89Zr- and 124I-A11 cMb successfully imaged subcutaneous PSCA (+) and (-) 22Rv1 human prostate adenocarcinoma tumors in nude mice, resulting in positive-to-negative tumor ratios of 2.5:1 and 8:1, respectively. Both 89Zr-A11 cMb and 124I-A11 cMb cleared from blood by 22 hours. As expected for the residualizing radiometal, 89Zr-A11 cMb resulted in retention in the organs of clearance (liver and kidneys).

For dual-modality imaging, A11 cMb was site-specifically conjugated with Cy5.5-maleimide and radiolabeled with 124I. PET imaging with 124I-Cy5.5-A11 cMb in the subcutaneous 22Rv1 tumor model resulted in a positive-to-negative tumor ratio of 13:1. The PSCA (+) tumors were subsequently visualized by fluorescence in situ and ex vivo with high contrast in comparison to PSCA (-) tumors and tissues. In an orthotopic model, PSCA (+) 22Rv1 cells were implanted in the prostate, and therefore 89Zr was used in order reduce interfering signal from clearance to the bladder. A11 cMb was site-specifically conjugated with Cy5.5 and randomly labeled with SCN-DFO, then radiolabeled with 89Zr. 89Zr-Cy5.5-A11 cMb successfully imaged the prostate tumor, resulting in a 3:1 tumor-to-blood ratio. Fluorescence imaging clearly distinguished prostate tumor from adjacent tissues including seminal vesicles. In conclusion, a single injection of the dual-labeled A11 cMb can visualize tumor burden by immunoPET and fluorescence imaging. This humanized probe has the potential for clinical translation for primary and metastatic prostate cancer detection and surgical guidance that can ultimately enhance treatment success.

Citation Format: Wen-Ting K. Tsai, Kirstin A. Zettlitz, Richard Tavaré, Felix B. Salazar, Robert E. Reiter, Anna M. Wu. Dual-modality immunoPET/fluorescence imaging of prostate cancer utilizing 89Zr- or 124I-Cy5.5-anti-PSCA cys-minibody [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1856. doi:10.1158/1538-7445.AM2017-1856