Future designs of BB2r-targeted agents should include a careful consideration of the effect linking group functionality has upon tumor targeting and retention. Furthermore, this investigation clearly demonstrates the significance of the linking group upon not only the in vivo clearance of the radiopharmaceutical, but on the in vivo uptake and retention of the BB2r-targeted agent in tumor tissue.
In conclusion, our studies indicate that radioconjugates incorporating aromatic linking groups, of the type investigated, generally demonstrated enhanced retention in BB2r expressing tissues in comparison to either the hydrocarbon or ether linking moieties. tract retention than the hydrocarbon or ether linking moieties. In both biodistribution and Micro-SPECT/CT imaging studies, the radioconjugates containing aromatic linking groups typically exhibited significantly higher G.I. Fused Micro-SPECT/CT imaging studies performed at 24 hr post-injection revealed substantial accumulation of radioactivity in the tumor tissue for all radioconjugates. By 24 hr post-injection, the radioconjugates containing aromatic groups exhibited the highest percentage tumor retention with 11.4, 19.8, 26.6, 25.8 and 25.5% relative to the 15 min values remaining in the tumor tissue for the 8-AOC, 8-ADS, AMBA, Gly-AMBA and Gly-AM2BA radioconjugates, respectively. For PC-3 tumor bearing SCID mice, the tumor uptake was found to be 6.66 ± 2.00, 6.21 ± 1.57, 6.36 ± 1.60, 4.46 ± 0.81 and 7.76 ± 1.19 %ID/g for the 8-AOC, 8-ADS, AMBA, Gly-AMBA and Gly-AM2BA radioconjugates, respectively, at 15 min post-injection. In CF-1 mice, the BB2r uptake in the pancreas of radioconjugates containing aromatic linking groups was found to be significantly higher at 1 hr post-injection than the radioconjugates with ether linker moieties.
All of the natIn-conjugates demonstrated nanomolar binding affinities to the BB2r. Specifically, we synthesized six radioconjugates with the structure 111In-DOTA- X-BBN(7-14)NH 2, where X = 8-aminooctanoic acid (8-AOC), 5-amino-3-oxapentyl-succinamic acid (5-ADS), 8-amino-3,6-dioxaoctyl-succinamic acid (8-AOS), p-aminobenzoic acid (AMBA), Gly-AMBA and Gly- p-aminomethylbenzoic acid (Gly-AM2BA). The goal of this study was to systematically evaluate the pharmacokinetic profile of aliphatic hydrocarbon, aromatic and polyethylene glycol (ether) functional groups in order to obtain a better understanding of the in vivo properties of these pharmacokinetic modifiers. Understanding the in vivo properties of the various pharmacokinetic modifying linking groups is crucial for developing BB2r-targeted agents with improved targeting and clearance characteristics. Development of BB2r-targeted agents, based on the bombesin (BBN) peptide, has largely involved the use of the bifunctional chelate approach in which the linking group serves several key roles including pharmacokinetic modification. The high incidence of BB2 receptor (BB2r) expression in various cancers has prompted investigators to pursue the development of BB2r-targeted agents for diagnostic imaging, chemotherapy and radiotherapy.
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