Anti-PSCA minibody for immunoPET imaging of prostate cancer in preclinical models

Prostate stem cell antigen (PSCA) is a cell surface glycoprotein that has low levels of expression in normal prostate, bladder, and stomach, but is expressed in 83-100%, and overexpressed in 40-100%, of prostate cancers. PSCA expression has been shown to correlate with the Gleason score, seminal vesicle and capsular invasion, advanced clinical stage, androgen independence, MYC gene-amplification, and a poor prognosis. PSCA is also highly overexpressed in the majority of prostate cancer bone metastases (87- 100%) and many metastases to other sites (67% liver, 67-95% lymph node). ImmunoPET is a highly flexible imaging technique that can be utilized to image virtually any cell surface protein using radiolabeled antibodies or antibody fragments. This dissertation describes the use of a radiolabeled anti-PSCA A11 minibody (scFv-CH3 dimer, 80 kDa) for immunoPET imaging of PSCA expression in mouse models of prostate cancer and demonstrates the potential of the A11 minibody for highly specific imaging of local prostate cancer and metastases in the clinic.In this work, 124I and 89Zr-DFO radiolabeling of the anti-PSCA minibody are compared. Imaging with both radionuclides showed specific tumor targeting in 22rv1�PSCA and LAPC-9 tumors and achieved high contrast imaging in both tumor models. However, 124I- labeled A11 minibody achieved superior tumor:soft tissue contrast and was determined to have higher clinical potential. A quantitative method of μPET imaging of both 124I and 89Zr radionuclides using recovery coefficient based partial volume correction was also established and validated.The PSCA promoter contains an androgen response element and expression of PSCA has been shown to be regulated by androgens in the mouse prostate and in human prostate cancer biopsy tissue. In the present work, treatment with the second generation anti-androgen MDV-3100 was demonstrated to cause downregulation of PSCA in LAPC-9 xenografts. In addition, 124I-labeled A11 minibody showed decreased uptake in LAPC-9 xenografts treated with MDV-3100 compared to vehicle controls suggesting a potential use for the A11 minibody in imaging response to androgen deprivation therapy.The highest percentage of prostate cancer metastases are osteoblastic bone metastases and most clinical staging of prostate cancer metastases utilizes 99mTc-MDP or 18F- Fluoride bone scans to image increases in bone turnover in response to these metastases. In this work we demonstrate that 124I-labeled A11 minibody has higher sensitivity and specificity for detecting LAPC-9 intratibial xenografts than 18F-Fluoride bone scans.The A11 minibody binds only to human PSCA (hPSCA) and does not cross-react with the murine homologue mPSCA. In order to address the background uptake caused by endoge- nous hPSCA expression, the creation of an hPSCA knock-in mouse and imaging using the A11 minibody in this model is described. We found a pattern of hPSCA expression in these mice similar to the pattern of normal expression in humans. Uptake of 124I-labeled A11 minibody correlated strongly with the expression of hPSCA, but demonstrated that the overall increase in background uptake of the A11 minibody due to endogenous expression of human PSCA is minimal.The results presented in this work demonstrate robust targeting of PSCA expression by the A11 minibody and clinical investigation of the A11 minibody for imaging local prostate cancer and prostate cancer metastases is warranted