Humanized radioiodinated minibody for imaging of prostate stem cell antigen-expressing tumors.

Purpose: Prostate stem cell antigen (PSCA) is a cell surface glycoprotein that is overexpressed in prostate cancer, including hormone refractory disease. Previous preclinical studies showed the intact anti-PSCA antibodies, 1G8 and hu1G8, localized specifically to PSCA-expressing xenografts. Optimal micro positron emission tomography (microPET) imaging using hu1G8, however, required a delay of 168 hours postinjection. In this study, the 2B3 minibody (an 80-kDa engineered antibody fragment) has been produced for rapid targeting and imaging.

Experimental Design: A gene encoding a PSCA-specific minibody, V(L)-linker-V(H)-hinge-huIgG1 C(H)3, was assembled. The minibody was expressed by secretion from mammalian cells and purified by cation exchange chromatography. Relative affinity and specificity were determined by competition ELISA and flow cytometry. Serial microPET imaging using a 124I-labeled minibody was conducted at 4 and 21 hours in mice bearing LAPC-9 AD, LAPC-9 AI, PC-3, and LNCaP-PSCA human prostate cancer xenografts. Tumor and tissue biodistribution was determined, and region of interest analysis of the images was conducted.

Results: Yields of 20 mg/L purified 2B3 minibody were obtained that showed specific binding to LNCaP-PSCA cells. Purified 2B3 minibody showed specific binding to LNCaP-PSCA cells with an apparent affinity of 46 nmol/L. Radioiodinated 2B3 minibody showed rapid nontarget tissue and blood clearance kinetics (t1/2beta = 11.2 hours). MicroPET scanning using the 124I-2B3 minibody showed both androgen-dependent and -independent tumors as early as 4 hours and excellent high contrast images at 21 hours postinjection.

Conclusions: Imaging PSCA-positive prostate cancer is feasible using an intermediate size antibody fragment at 21 hours.