Screening and Preclinical Evaluation of Novel Radiolabeled Anti-Fibroblast Activation Protein-α Recombinant Antibodies.

Fibroblast activation protein-α (FAPα) is selectively overexpressed in tumor-associated fibroblasts in more than 90% of epithelial tumors, and may be a good target for anticancer treatment, for example, using an anti-FAPα recombinant antibody (rAb) labeled with radionuclides. In the present report, the radiolabeling and preclinical evaluation of novel anti-FAPα rAbs were investigated. Two novel anti-FAPα VHHs (AMS002-1 and AMS002-2) with high binding affinity to FAPα were selected from an antibody phage library. The anti-FAPα VHHs were then fused with the Fc fragment of human IgG4 to create two VHH-Fc rAbs. The VHH-Fc rAbs were radiolabeled with Zr and Lu. The radiolabeled products were evaluated by radioligand-binding assays using FAPα-expressing cells. The biodistribution and tumor-targeting properties were investigated by small-animal PET/CT. AMS002-1-Fc, which showed promising tumor-targeting properties in Zr-microPET imaging, was radiolabeled with Lu for efficacy study on HT1080 tumor-bearing mice and monitored with SPECT/CT imaging. The two VHH-Fc rAbs with good affinity with K values in low nanomolar range were identified. Both PET/CT imaging with Zr-AMS002-1-Fc rAb and SPECT/CT imaging with Lu-AMS002-1-Fc rAb demonstrated highest tumor uptakes at 72 h p.i. and long tumor retention in the preclinical models. Furthermore, biodistribution analysis revealed high tumor uptake of Zr-AMS002-1-Fc at 48 h p.i. with the value of 6.91% ± 2.08% ID/g. Finally, radioimmunotherapy with Lu-AMS002-1-Fc rAb delayed the tumor growth without significant weight loss in mice with HT1080 xenografts. The tumor size of untreated control group was 2.59 times larger compared with the treatment group with Lu-AMS002-1-Fc at day 29. Zr/Lu-AMS002-1-Fc represent a pair of promising radiopharmaceuticals for theranostics on FAPα-expressing tumors.