Administration routes for SSTR- / PSMA- and FAP-directed theranostic radioligands in mice

Introduction: The NETTER-1, VISION, and TheraP trials prove efficacy of repeat intravenous (i.v.) application of small radioligands. Application by subcutaneous (s.c.), intraperitoneal (i.p.), or oral (p.o.) access are important alternatives and may yield comparable or favorable organ and tumor radioligand uptake. Here, we assess organ and tumor biodistribution for various radioligand application routes in healthy mice and models of somatostatin receptor (SSTR)-, prostate-specific membrane antigen (PSMA)-, and fibroblast activation protein (FAP)- expressing cancer. Methods: Healthy and tumor-bearing male C57BL/6 or NOD SCID Gamma mice, respectively, were applied with a mean of 6.0±0.5 MBq ⁶⁸Ga DOTATOC (RM1-SSTR allograft), 5.3±0.3 MBq ⁶⁸Ga-PSMA11 (RM1-PSMA allograft) or 4.8±0.2 MBq ⁶⁸Ga-FAPI46 (HT1080 FAP xenograft) i.v., i.p., s.c. or p.o.. In vivo positron emission tomography and ex vivo biodistribution in tumor, organs, and at the injection site were assessed up to 5h post injection (p.i.). Healthy mice were monitored for up to 7 days after the last scan for signs of stress or adverse reactions. Results: After i.v., i.p. and s.c. radioligand administration, average residual activity at the injection site was <17%IA/g (1h p.i.), <10%IA/g (2h p.i.) and ≤4%IA/g (4h p.i.) for all radioligands. Following oral administration ≥50%IA/g remained within the intestines until 4h p.i.. Biodistribution in organs of healthy mice was nearly equivalent following i.v., i.p., and s.c. application at 1h p.i. and all subsequent timepoints (≤1%IA/g for liver, blood and bone marrow; 11.2±1.4%IA/g for kidneys). In models for SSTR-, PSMA- and FAP-expressing cancer, tumor uptake was higher or equivalent for i.p./s.c. versus i.v. injection at 5h p.i. (ex vivo): SSTR: 7.2±1.0%IA/g (P = 0.0197) / 6.5±1.3%IA/g (P = 0.0827) versus 2.9±0.3%IA/g; PSMA: 3.4±0.8%IA/g (P = 0.9954) / 3.9±0.8%IA/g (P = 0.8343) versus 3.3±0.7%IA/g and FAP: 1.1±0.1%IA/g (P = 0.9805) / 1.1±0.1%IA/g (P = 0.7446) versus 1.0±0.2%IA/g. Conclusion: In healthy mice, biodistribution of small theranostic ligands following i.p. or s.c. application is nearly equivalent compared to i.v. injection. S.c. administration resulted in highest absolute SSTR tumor and tumor-to-organ uptake as compared to the i.v. route, warranting further clinical assessment.