“To Serve and Protect”: Enzyme Inhibitors as Radiopeptide Escorts Promote Tumor Targeting

Abstract

Radiolabeled octreotide analogs are most successfully being applied today in clinical cancer imaging and treatment. Propagation of this paradigm to other radiopeptide families has been greatly hampered by the inherent poor metabolic stability of systemically administered peptide analogs. We hypothesized that the in vivo coadministration of specific enzyme inhibitors would improve peptide bioavailability and hence tumor uptake. Through single coinjection of the neutral endopeptidase inhibitor phosphoramidon (PA), we were able to provoke remarkable rises in the percentages of circulating intact somatostatin, gastrin, and bombesin radiopeptides in mouse models, resulting in a remarkable increase in uptake in tumor xenografts in mice. Methods: The peptide conjugates [DOTA-Ala 1 ]SS14 (DOTA-Ala-Gly-c[Cys-LysAsn-Phe-Phe-Trp-Lys-Thr-Phe-Thr-Ser-Cys]-OH), PanSB1 (DOTA-PEG 2 -DTyr-Gln-Trp-Ala-Val-bAla-His-Phe-Nle-NH 2 ), and DOTA-MG11 (DOTA-DGlu-Ala-Tyr-Gly-Trp-Met-Asp-Phe-NH 2 ) were labeled with 111 In by 20 min of heating at an acidic pH. Metabolic stability was studied with high-performance liquid chromatography analysis of blood samples collected 5 min after the injection of the test radiopeptide alone or with PA into mice. Biodistribution was studied after injection of each 111 In-labeled radiopeptide alone or after coinjection of PA in tumor-bearing severe combined immunodeficient (SCID) mice. Results: The amount of intact [ 111 In-DOTA-Ala 1 ] SS14 detected in the mouse circulation at 5 min after the injection of PA increased impressively-from less than 2% to 86%-whereas the uptake in AR4-2J xenografts rose from less than 1 percentage injected dose per gram of tissue (%ID/g) to 14 %ID/g at 4 h after injection. Likewise, the coadministration of PA resulted in a marked increase in the amount of circulating intact 111 In-PanSB1-from 12% to 80%-at 5 min after injection, and radioligand uptake in human PC-3 xenografts in SCID mice escalated from less than 4 %ID/g to greater than 21 %ID/g at 4 h after injection. In a similar manner, the coadministration of PA resulted in an equally impressive increase in intact [ 111 In-DOTA]MG11 levels in the mouse bloodstream-from less than 5% to 70%-at 5 min after injection, leading to a remarkable increase in radiotracer uptake-from 2 %ID/g to greater than 15 %ID/g-in both AR4-2J tumors and A431(CCKR1) tumors (i.e., tumors induced by A431 cells transfected to stably express the human cholecystokinin subtype 2 receptor) in mice at 4 h after injection. This effect was well visualized by SPECT/CT imaging of AR4-2J tumor-bearing mice at 4 h after injection. Conclusion: The results of this study clearly demonstrate that the coadministration of key enzyme inhibitors can effectively prolong the survival of radiolabeled peptides in the circulation, securing their safe transit to the target. This strategy clearly provoked an unprecedented increase in radiolabel accumulation in tumor xenografts in mice; this increase might translate into higher diagnostic sensitivity or improved therapeutic...

Evaluation of a Fluorescent and Radiolabeled Hybrid Somatostatin Analog In Vitro and in Mice Bearing H69 Neuroendocrine Xenografts

“…Regarding stability in the bloodstream, peptide molecules are known to be vulnerable to degradation by peptidases (30). Nonetheless, radiolabeled DTPA-and DOTA-linked Tyr 3 -octreotide and Tyr 3 -octreotate have exhibited excellent stability in circulation; they are excreted via the urine mostly in the intact form (30)(31)(32).…”

“…Nonetheless, radiolabeled DTPA-and DOTA-linked Tyr 3 -octreotide and Tyr 3 -octreotate have exhibited excellent stability in circulation; they are excreted via the urine mostly in the intact form (30)(31)(32). Therefore, we expect our hybrid Cy5-111 In-DTPA-Tyr 3 -octreotate peptide analog to exhibit good stability in circulation as well.…”

Radiopeptides for Imaging and Therapy: A Radiant Future

“…A remarkable advance in this area is the discovery of the in vivo enzyme inhibition concept, in which radiopeptides are administered together with an enzyme inhibitor as a safeguard against enzymatic degradation. It has recently been shown that coinjection of a neutral endopeptidase inhibitor, such as phosphoramidon, can stabilize radiolabeled bombesin, minigastrin, and somatostatin analogs in vivo, leading to enhanced tumor uptake and improved tumor visualization (5). We believe that this strategy will have a significant impact on patient care, as it may enhance the diagnostic sensitivity and therapeutic efficacy of radiopeptides.…”

Radiopharmaceuticals for somatostatin receptor imaging

“…neutral endopeptidase, NEP) is responsible for the rapid in vivo breakdown of intravenously administered radiopeptides from at least the somatostatin, bombesin, and gastrin peptide families. Most importantly, this phenomenon can be overcome by enhan cing their supply and accumulation at tumor sites through the mere co-injection of a protease inhibitor, such as phosphoramidon [69]. This approach may result in increased diagnostic sensitivity and therapeutic efficacy being the potential strategy for translation into clinical practice [70,71].…”