GRPR and severe combined immunodeficiency: Interestingly, when directly comparing the GRPR antagonist-comprising [177Lu]Lu-18 and the earlier described GRPR agonist-comprising analog [177Lu]Lu-16 [87], evaluated in PC3-bearing SCID (severe combined immunodeficiency) or immunodeficient nude mice, respectively, [177Lu]Lu-16 showed a significantly less favorable pharmacokinetic profile with much lower tumor-to-non-tumor ratios, especially in pancreas but also in liver, stomach, spleen, kidneys, and intestines.