Abstract
1013
Objectives: Gastrin-releasing peptide receptor (GRPR), a G-protein coupled receptor, is aberrantly expressed in solid tumors including prostate, breast and lung cancers. Herein, we radiolabeled ProBOMB1, a novel bombesin derivative, with 68Ga and 177Lu, and evaluated their pharmacokinetics and distribution in tumor-bearing mice for potential theranostic application.
Methods: ProBOMB1 (DOTA-pABzA-DIG-D-Phe-Gln-Trp-Ala-Val-Gly-His-Leu-ψ(CH2N)-Pro-NH2) was synthesized by solid-phase peptide synthesis. A p-aminomethylaniline-diglycolic acid (pABzA-DIG) linker and a 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) chelator were conjugated to the peptide N-terminus. The binding affinities of Ga-ProBOMB1 and Lu-ProBOMB1 for GRPR were determined by cell-based competition assays using [125I-Tyr4]Bombesin as the radioligand. The agonist/antagonist properties of Ga-ProBOMB1 and Lu-ProBOMB1 were determined with calcium efflux assays. ProBOMB1 was radiolabeled with 68GaCl3 or177LuCl3. PET imaging (for [68Ga]Ga-ProBOMB1) and biodistribution studies were performed in immunocompromised mice bearing PC-3 prostate cancer xenografts. For specificity/blocking experiments, mice were co-injected with [D-Phe6,Leu-NHEt13,des-Met14]Bombesin(6-14).
Results: The Ki values of Ga-ProBOMB1 and Lu-ProBOMB1 were 3.97±0.76 and 30.2 ± 3.23 nM. Ga-ProBOMB1 and Lu-ProBOMB1 are antagonists of GRPR, as neither induced calcium release in PC-3 cells. [68Ga]Ga-ProBOMB1 was obtained in 48.2±10.9% decay-corrected radiochemical yield with 121±46.9 GBq/µmol molar activity, and > 95% radiochemical purity. [177Lu]Lu-ProBOMB1 was obtained in 58% decay-corrected radiochemical yield with 263 GBq/µmol molar activity, and > 99% radiochemical purity. [68Ga]Ga-ProBOMB1 and [177Lu]Lu-ProBOMB1 were excreted primarily through the renal pathway. PC-3 tumor xenografts were clearly visualized in PET images with excellent contrast at 1 and 2 h p.i. of [68Ga]Ga-ProBOMB1. Based on biodistribution studies, the tumor uptake of [68Ga]Ga-ProBOMB1 was 8.17±2.57 percent injected dose per gram (%ID/g), and 8.31±3.88 %ID/g at 1 and 2 h p.i. The tumor-to-blood and tumor-to-muscle uptake ratios of [68Ga]Ga-ProBOMB1 were 20.6±6.79 and 106±57.7 at 1 h p.i. Co-injection with [D-Phe6,Leu-NHEt13,des-Met14]Bombesin(6-14) reduced average uptake of [68Ga]Ga-ProBOMB1 in tumors by 62% at 1 h p.i. Unlike [68Ga]Ga-ProBOMB1, [177Lu]Lu-ProBOMB1 had lower tumor uptake and no retention. The tumor uptake of [177Lu]Lu-ProBOMB1 was 3.38±1.00, 1.32±0.24, and 0.31±0.04 %ID/g at 1, 4 and 24 h p.i., respectively.
Conclusions: Our data suggest that [68Ga]Ga-ProBOMB1 is a promising PET tracer for imaging GRPR expression in cancers; however, the substitution of 68Ga with 177Lu negatively impacted binding affinity and tumoral uptake. Research Support: The Canadian Institutes of Health Research (FDN-148465) and the BC Leading Edge Endowment Fund.