Noninvasive Imaging of PSMA in Prostate Tumors with ⁸⁹Zr-Labeled huJ591 Engineered Antibody Fragments: The Faster Alternatives

Engineered antibody fragments offer faster delivery with retained tumor specificity and rapid clearance from nontumor tissues. Here, we demonstrate that positron emission tomography (PET) based detection of prostate specific membrane antigen (PSMA) in prostatic tumor models using engineered bivalent antibodies built on single chain fragments (scFv) derived from the intact antibody, huJ591, offers similar tumor delineating properties but with the advantage of rapid targeting and imaging. ⁸⁹Zr-radiolabeled huJ591 scFv (dimeric scFv-C_H3; ⁸⁹Zr-Mb) and cysteine diabodies (dimeric scFv; ⁸⁹Zr-Cys-Db) demonstrated internalization and similar <i>K</i>_ds (∼2 nM) compared to ⁸⁹Zr-huJ591 in PSMA­(+) cells. Tissue distribution assays established the specificities of both ⁸⁹Zr-Mb and ⁸⁹Zr-Cys-Db for PSMA­(+) xenografts (6.2 ± 2.5% ID/g and 10.2 ± 3.4% ID/g at 12 h p.i. respectively), while minimal accumulation in PSMA(−) tumors was observed. From the PET images, ⁸⁹Zr-Mb and ⁸⁹Zr-Cys-Db exhibited faster blood clearance than the parent huJ591 while tumor-to-muscle ratios for all probes show comparable values across all time points. <i>Ex vivo</i> autoradiography and histology assessed the distribution of the probes within the tumor. Imaging PSMA-expressing prostate tumors with smaller antibody fragments offers rapid tumor accumulation and accelerated clearance; hence, shortened wait periods between tracer administration and high-contrast tumor imaging and lower dose-related toxicity are potentially realized