Amide-to-triazole switch vs. in vivo NEP-inhibition approaches to promote radiopeptide targeting of GRPR-positive tumors

IntroductionRadiolabeled bombesin (BBN)-analogs have been proposed for diagnosis and therapy of gastrin-releasing peptide receptor (GRPR)-expressing tumors, such as prostate, breast and lung cancer. Metabolic stability represents a crucial factor for the success of this approach by ensuring sufficient delivery of circulating radioligand to tumor sites. The amide-to-triazole switch on the backbone of DOTA-PEG4-[Nle14]BBN(7-14) (1) was reported to improve the in vitro stability of resulting 177Lu-radioligands. On the other hand, in-situ inhibition of neutral endopeptidase (NEP) by coinjection of phosphoramidon (PA) was shown to significantly improve the in vivo stability and tumor uptake of biodegradable radiopeptides. We herein compare the impact of the two methods on the bioavailability and localization of 177Lu-DOTA-PEG4-[Nle14]BBN(7-14) analogs in GRPR-positive tumors in mice.MethodsThe 1,4-disubstituted [1-3]-triazole was used to replace one (2: Gly11-His12; 3: Ala9-Val10) or two (4: Ala9-Val10 and Gly11-His12) peptide bonds in 1 (reference) and all compounds were labeled with 177Lu. Each of [177Lu]1-[177Lu]4 was injected without (control) or with PA in healthy mice. Blood samples collected 5 min post-injection (pi) were analyzed by HPLC. Biodistribution of [177Lu]1-[177Lu]4 was conducted in SCID mice bearing human prostate adenocarcinoma PC-3 xenografts at 4 h pi. Groups of 4 animals were injected with radioligand, alone (controls), or with coinjection of PA, or of a mixture of PA and excess and [Tyr4]BBN to determine GRPR-specificity of uptake (Block).ResultsThe in vivo stability of the radioligands was: [177Lu]1 (25% intact), [177Lu]2 (45% intact), [177Lu]3 (30% intact) and [177Lu]4 (40% intact). By PA-coinjection these values notably increased to 90%-93%. Moreover, treatment with PA induced an impressive and GRPR-specific uptake of all radioligands in the PC-3 xenografts at 4 h pi: [177Lu]1: 4.7 ± 0.4 to 24.8 ± 4.9%ID/g; [177Lu]2: 8.3 ± 1.2 to 26.0 ± 1.1%ID/g; [177Lu]3: 6.6 ± 0.4 to 21.3 ± 4.4%ID/g; and [177Lu]4: 4.8 ± 1.6 to 13.7 ± 3.8%ID/g.ConclusionsThis study has shown that amide-to-triazole substitutions in 177Lu-DOTA-PEG4-[Nle14]BBN(7-14) induced minor effects on bioavailability and tumor uptake in mice models, whereas in-situ NEP-inhibition(s) by PA impressively improved in vivo profiles.