68Ga-Radiolabeling and Pharmacological Characterization of a Kit-Based Formulation of the Gastrin-Releasing Peptide Receptor (GRP-R) Antagonist RM2 for Convenient Preparation of [68Ga]Ga-RM2.

Adrien Chastel,Sacha Bodin,Stéphane Chaignepain,Marion Zerna,Elif Hindié,Stephane Claverol,Delphine Vimont,Clément Morgat,Mathias Berndt

doi:10.3390/pharmaceutics13081160

Abstract

Background: [68Ga]Ga-RM2 is a potent Gastrin-Releasing Peptide-receptor (GRP-R) antagonist for imaging prostate cancer and breast cancer, currently under clinical evaluation in several specialized centers around the world. Targeted radionuclide therapy of GRP-R-expressing tumors is also being investigated. We here report the characteristics of a kit-based formulation of RM2 that should ease the development of GRP-R imaging and make it available to more institutions and patients. Methods: Stability of the investigated kits over one year was determined using LC/MS/MS and UV-HPLC. Direct 68Ga-radiolabeling was optimized with respect to buffer (pH), temperature, reaction time and shaking time. Conventionally prepared [68Ga]Ga-RM2 using an automated synthesizer was used as a comparator. Finally, the [68Ga]Ga-RM2 product was assessed with regards to hydrophilicity, affinity, internalization, membrane bound fraction, calcium mobilization assay and efflux, which is a valuable addition to the in vivo literature. Results: The kit-based formulation, kept between 2 °C and 8 °C, was stable for over one year. Using acetate buffer pH 3.0 in 2.5–5.1 mL total volume, heating at 100 °C during 10 min and cooling down for 5 min, the [68Ga]Ga-RM2 produced by kit complies with the requirements of the European Pharmacopoeia. Compared with the module production route, the [68Ga]Ga-RM2 produced by kit was faster, displayed higher yields, higher volumetric activity and was devoid of ethanol. In in vitro evaluations, the [68Ga]Ga-RM2 displayed sub-nanomolar affinity (Kd = 0.25 ± 0.19 nM), receptor specific and time dependent membrane-bound fraction of 42.0 ± 5.1% at 60 min and GRP-R mediated internalization of 24.4 ± 4.3% at 30 min. The [natGa]Ga-RM2 was ineffective in stimulating intracellular calcium mobilization. Finally, the efflux of the internalized activity was 64.3 ± 6.5% at 5 min. Conclusion: The kit-based formulation of RM2 is suitable to disseminate GRP-R imaging and therapy to distant hospitals without complex radiochemistry equipment.

Highlights

The Gastrin Releasing Peptide-Receptor (GRP-R) is a G-protein coupled receptor of the bombesin receptor family that show great potential for imaging and therapy of many cancers [1]
GRP-R is overexpressed in prostate cancer, with low or no expression in healthy tissues
The RM2 kits were subjected to LC/MS/MS analysis, which allow qualification and quantification of the chemical species present in the kit over time

Summary

Introduction

The Gastrin Releasing Peptide-Receptor (GRP-R) is a G-protein coupled receptor of the bombesin receptor family that show great potential for imaging and therapy of many cancers [1]. Its expression in prostate cancer is higher in carcinomas of lower. Pharmaceutics 2021, 13, 1160 cancers [1]. GRP-R is overexpressed in prostate cancer, with low or no expression in healthy tissues. Its expression in prostate cancer is higher in carcinomas of of 14 lower Gleason score, lower tumor size and lower prostate specific antigen (PSA)2value [2,3]. In pre-clinical studies and in pilot studies in humans, GRP-R imaging appears complimentary to that of imaging prostate-specific membrane antigen (PSMA) for patients with primary prostate cancer [4,5]. In pre-clinical studies and in pilot studies in humans, GRP-R imaging appears complimentary to that of imaging prostate-specific membrane antigen (PSMA) for patients with primary prostate cancer [4,5]. specific

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