Peptide receptor scintigraphy and peptide receptor radionuclide therapy using radiolabeled somatostatin receptor (SSTR) agonists are successfully used in the clinic for imaging and treatment of neuroendocrine tumors. Contrary to the paradigm that internalization and the resulting accumulation of radiotracers in cells is necessary for efficient tumor targeting, recent studies have demonstrated the superiority of radiolabeled SSTR antagonists for imaging purposes, despite little to no internalization in cells. However, studies comparing the therapeutic antitumor effects of radiolabeled SSTR agonists versus antagonists are lacking. The aim of this study was to directly compare the therapeutic effect of Lu-177-DOTA-octreotate, an SSTR agonist, and Lu-177-DOTA-JR11, an SSTR antagonist. Methods: We analyzed radiotracer uptake (both membrane-bound and internalized fractions) and the produced DNA double-strand breaks, by determining the number of p53 binding protein 1 foci, after incubating SSTR2-positive cells with Lu-177-diethylene triamine pentaacetic acid, Lu-177-DOTAToctreotate, or Lu-177-DOTA-JR11. Also, biodistribution studies were performed in tumor-xenografted mice to determine the optimal dose for therapy experiments. Afterward, in vivo therapy experiments comparing the effect of Lu-177-DOTA-octreotate and Lu-177-DOTA-JR11 were performed in this same animal model. Results: We found a 5-times-higher uptake of Lu-177-DOTA-JR11 than of Lu-177-DOTA-octreotate. The major part (88% +/- 1%) of the antagonist uptake was membrane bound, whereas 74% +/- 3% of the total receptor agonist uptake was internalized. Cells treated with Lu-177-DOTA-JR11 showed 2 times more p53-binding protein 1 foci than cells treated with Lu-177-DOTA-octreotate. Biodistribution studies with Lu-177-DOTA-JR11 (0.5 mu g/30 MBq) resulted in the highest tumor radiation dose of 1.8 0.7 Gy/MBq, 4.4 times higher than the highest tumor radiation dose found for Lu-177-DOTA-octreotate. In vivo therapy studies with Lu-177-DOTA-octreotate and Lu-177-DOTA-JR11 resulted in a tumor growth delay time of 18 +/- 5 and 26 +/- 7 d, respectively. Median survival rates were 43.5, 61, and 71 d for the control group, Lu-177-DOTA-octreotate group, and the Lu-177-DOTA-JR11-treated group, respectively. Conclusion: On the basis of these results, we concluded that the use of radiolabeled SSTR antagonists such as JR11 might enhance peptide receptor scintigraphy and peptide receptor radionuclide therapy of neuroendocrine tumors and provide successful imaging and therapeutic strategies for cancer types with relatively low SSTR expression.