An antibody-radionuclide conjugate targets fibroblast activation protein for cancer therapy

Abstract

Purpose: Fibroblast activation protein is one of the most attractive targets for tumor diagnosis and therapy. There have been many successful clinical translations with small molecules and peptides, yet only a few anti-FAP antibody diagnostic or therapeutic agents have been reported. Antibodies often feature good tumor selectivity and long tumor retention, which may be a better-match with therapeutic radionuclides (e.g.,¹⁷⁷Lu, ²²⁵Ac) for cancer therapy. Here we report a ¹⁷⁷Lu-labeled anti-FAP antibody, PKU525, as a therapeutic radiopharmaceutical for FAP-targeted radiotherapy.

Methods: The anti-FAP antibody is produced as a derivative of sibrotuzumab. The pharmacokinetics and blocking study are performed with ⁸⁹Zr-labeled antibody by PET imaging. The conjugation strategies have been screened and tested with SPECT imaging through ¹⁷⁷Lu-labeling. The biodistribution and radiotherapy studies are performed on ¹⁷⁷Lu-labeled anti-FAP antibody in NU/NU mice-bearing HT-1080-FAP tumors.

Results: A multiple time-point PET imaging study shows that the tumor accumulation of [⁸⁹Zr]Zr-DFO-PKU525 is intense, selective, and relatively rapid. The time activity curve indicates that the tumor uptake continually increases until reaches the highest uptake (SUVmax = 18.4 ± 2.3, n = 4) at 192 h, then gradually declines. Radioactivity rapidly cleared from the blood, liver, and other major organs, resulting in high tumor-to-background ratios. An in vivo blocking experiment suggests that [⁸⁹Zr]Zr-DFO-PKU525 is FAP-specific and the uptake in FAP-negative tumors is almost negligible. Ex vivo biodistribution study shows that the tumor uptake of [¹⁷⁷Lu]Lu-DOTA-NCS-PKU525 is 23.04 ± 5.11% ID/g, 33.2 ± 6.36% ID/g, 19.87 ± 6.84% ID/g and 19.02 ± 5.90% ID/g at 24 h, 96 h, 168 h, and 240 h after injection (n = 5), which is corroborated with the PET imaging. In therapeutic assays, multiple doses of [¹⁷⁷Lu]Lu-DOTA-NCS-PKU525 have been tested in tumor-bearing mice, and the data suggests that 3.7 MBq may be sufficient to completely suppress the tumor growth in mice without showing observable side effects.

Conclusion: A FAP-targeted antibody-radionuclide conjugate was developed and evaluated in vitro and in vivo. Its tumor accumulation is rapid and high with a clean background. It remarkably suppresses the tumors in mice while the side effect is almost negligible, showing that it is promising for further clinical translational studies.

Keywords: Antibody; Fibroblast activation protein; Targeted radionuclide therapy.