Comparing Gly(11)/dAla(11)-Replacement vs. the in-Situ Neprilysin-Inhibition Approach on the Tumor-targeting Efficacy of the (111)In-SB3/(111)In-SB4 Radiotracer Pair

Background: The GRPR-antagonist (68)Ga-SB3 visualized prostate cancer lesions in animal models and in patients. Switching radiometal from (68)Ga to (111)In impaired tumor targeting in mice, but coinjection of the neprilysin (NEP)-inhibitor phosphoramidon (PA) stabilized (111)In-SB3 in circulation and remarkably increased tumor uptake. We herein report on the biological profile of (111)In-SB4: (111)In-[dAla(11)]SB3. Methods: The biological responses of (111)In-SB3/SB4 were compared in PC-3 cells and animal models. Results: Gly(11)/dAla(11)-replacement deteriorated GRPR-affinity (SB4 IC(50): 10.7 ± 0.9 nM vs. SB3 IC(50): 4.6 ± 0.3 nM) and uptake in PC-3 cells ((111)In-SB4: 1.3 ± 0.4% vs. (111)In-SB3 16.2 ± 0.8% at 1 h). (111)In-SB4 was more stable than (111)In-SB3, but PA-coinjection stabilized both radiotracers in peripheral mice blood. Unmodified (111)In-SB3 showed higher uptake in PC-3 xenografts (8.8 ± 3.0%ID/g) vs. (111)In-SB4 (3.1 ± 1.1%ID/g) at 4 h pi. PA-coinjection improved tumor uptake, with (111)In-SB3 still showing superior tumor targeting (38.3 ± 7.9%ID/g vs. 7.4 ± 0.3%ID/g for (111)In-SB4). Conclusions: Replacement of Gly(11) by dAla(11) improved in vivo stability, however, at the cost of GRPR-affinity and cell uptake, eventually translating into inferior tumor uptake of (111)In-SB4 vs. unmodified (111)In-SB3. On the other hand, in-situ NEP-inhibition turned out to be a more efficient and direct strategy to optimize the in vivo profile of (111)In-SB3, and potentially other peptide radiotracers.