FAP and neoplasm: The in vivo pharmacokinetic studies by small-animal PET imaging on FAP-transfected HT-1080 xenografts of the most promising 68Ga-labeled candidates of this set (FAPI-21, FAPI-35, FAPI-36, FAPI-46, and FAPI-55), demonstrated rapid tumor accumulation, low background activity, and predominantly renal elimination.