Prostate Stem Cell Antigen (PSCA) is a cell surface protein expressed by a majority of prostate cancers. Over the past 4 years, we have (1) validated PSCA as a target for prostate cancer therapy (2) shown that PSCA may be a marker of PTEN null prostate cancers (3) demonstrated that a monoclonal antibody targeting PSCA inhibits prostate cancer progression and metastasis in vivo (4) shown that antibody against PSCA induces cell death by an Fc-independent mechanism and (5) developed humanized PSCA antibodies that retain activity and may be used in the clinic. In parallel, a fully human anti-PSCA antibody was developed in collaboration with industry and successfully translated into Phase I trials in men with metastatic prostate cancer. In the current application, we propose to develop and test a radiolabeled PSCA antibody fragment to molecularly image prostate cancer with SPECT and PET scans. A PSCA-targeted molecular imaging probe has the potential to improve the ability to detect advanced and metastatic prostate cancer. It may be used to classify tumors at the molecular level based on the level and extent of PSCA expression, which may identify tumors with PTEN mutation or that respond to PSCA antibody therapy. Finally, a PSCA- targeted molecular imaging probe might be used to monitor response to therapy (eg. PSCA antibody therapy or drugs that target the PIS kinase-PTEN-Akt pathway). A humanized PSCA minibody has already been constructed and yields striking images of PSCA-positive xenografts with excellent pharmacokinetics (i.e high tumor uptake and rapid clearance from normal tissues). The Aims of this study are (1) to develop and test engineered antibody fragments in contolled biological systems (2) to assess PSCA antibody fragments to image transgenic models of prostate cancer, and (3) to perform a pilot imaging trial in men with metastatic prostate cancer. These studies have the potential to advance prostate cancer imaging in the age of targeted therapies and to link more closely the diagnosis and treatment of this disease. Layman statement: This application proposes to develop a new test to identify metastatic prostate cancer and to classify tumors at the molecular level by a noninvasive means. We will develop an antibody that is radiolabeled and binds to the protein PSCA on the surface of prostate cancer cells. Mice and then men administered this antibody will be imaged with SPECT scans in order to detect localized or metastatic prostate cancer.