Theranostics Targeting Fibroblast Activation Protein in the Tumor Stroma: ⁶⁴Cu- and ²²⁵Ac-Labeled FAPI-04 in Pancreatic Cancer Xenograft Mouse Models

Abstract

Fibroblast activation protein (FAP), which promotes tumor growth and progression, is overexpressed in cancer-associated fibroblasts of many human epithelial cancers. Because of its low expression in normal organs, FAP is an excellent target for theranostics. In this study, we used radionuclides with relatively long half-lives, 64 Cu (half-life, 12.7 h) and 225 Ac (half-life, 10 d), to label FAP inhibitors (FAPIs) in mice with human pancreatic cancer xenografts. Methods: Male nude mice (body weight, 22.5 ± 1.2 g) were subcutaneously injected with human pancreatic cancer cells (PANC-1, n 5 12; MIA PaCa-2, n 5 8). Tumor xenograft mice were investigated after the intravenous injection of 64 Cu-FAPI-04 (7.21 ± 0.46 MBq) by dynamic and delayed PET scans (2.5 h after injection). Static scans 1 h after the injection of 68 Ga-FAPI-04 (3.6 ± 1.4 MBq) were also acquired for comparisons using the same cohort of mice (n 5 8). Immunohistochemical staining was performed to confirm FAP expression in tumor xenografts using an FAP-α-antibody. For radioligand therapy, 225 Ac-FAPI-04 (34 kBq) was injected into PANC-1 xenograft mice (n 5 6). Tumor size was monitored and compared with that of control mice (n 5 6). Results: Dynamic imaging of 64 Cu-FAPI-04 showed rapid clearance through the kidneys and slow washout from tumors. Delayed PET imaging of 64 Cu-FAPI-04 showed mild uptake in tumors and relatively high uptake in the liver and intestine. Accumulation levels in the tumor or normal organs were significantly higher for 64 Cu-FAPI-04 than for 68 Ga-FAPI-04, except in the heart, and excretion in the urine was higher for 68 Ga-FAPI-04 than for 64 Cu-FAPI-04. Immunohistochemical staining revealed abundant FAP expression in the stroma of xenografts. 225 Ac-FAPI-04 injection showed significant tumor growth suppression in the PANC-1 xenograft mice, compared with the control mice, without a significant change in body weight. Conclusion: This proof-of-concept study showed that 64 Cu-FAPI-04 and 225 Ac-FAPI-04 could be used in theranostics for the treatment of FAP-expressing pancreatic cancer. α-therapy targeting FAP in the cancer stroma is effective and will contribute to the development of a new treatment strategy.

Targeting fibroblast activation protein (FAP): next generation PET radiotracers using squaramide coupled bifunctional DOTA and DATA5m chelators

“…xenograft tumor mice (Watabe et al 2020). Table 2 shows a summary of the different models with the corresponding gallium-68 tracer complex and the SUV mean values of PET measurements at 60 min p.i.. Table 2 shows that comparable SUV mean values can be found with [ 68 Ga]Ga-DOTA.-SA.FAPi and [ 68 Ga]Ga-FAPI-04 in the HT-1080 FAP model.…”

“…with the corresponding tumor models. The values of the FAPI-04 component are obtained from the reference literatures(Lindner et al 2018;Watabe et al 2020)…”

FAP: The Next Billion Dollar Nuclear Theranostics Target?

“…The radiation type (a vs. b) also requires optimization. In a preclinical study, a single administration of 225 Ac-FAPI-04 resulted in tumor growth retardation in mice bearing PANC-1 xenografts (30).…”

Current status of radioligand therapy and positron-emission tomography with prostate-specific membrane antigen

“…Thus, there are no reports of the use of PSMA RLT with alpha-ray-emitting radioisotopes thus far. Although delayed in some countries, manufacturing alpha-ray-emitting radioisotopes by cyclotrons is possible, with several results emerging despite its being still in the research stage [10,11].…”