Stability Evaluation and Stabilization of a Gastrin-Releasing Peptide Receptor (GRPR) Targeting Imaging Pharmaceutical

The prostate-specific membrane antigen (PSMA) and gastrin-releasing peptide receptor (GRPR) are identified as important targets on prostate cancer. Receptor-targeting radiolabeled imaging pharmaceuticals with high affinity and specificity are useful in studying and monitoring biological processes and responses. Two potential imaging pharmaceuticals, AMBA agonist (where AMBA = DO3A-CH2CO-G-[4-aminobenzyl]- Gln-Trp-Ala-Val-Gly-His-Leu-Met-NH2) and RM1 antagonist (where RM1 = DO3A-CH2CO-G-[4-aminobenzyl]-D-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH2), have demonstrated high binding affinity (IC50) to GRP receptors and high tumor uptake. Antagonists, despite the poor tumor cell internalization properties, can show clearer images and pharmacokinetic profiles by virtue of their higher tumor uptake in animal models compared to agonists. For characterization, development, and translation of a potential imaging pharmaceutical into the clinic, it must be evaluated in a series of tests, including in vitro cell binding assays, in vitro buffer and serum stability studies, the biodistribution of the radiolabeled material, and finally imaging studies in preclinical animal models. Data related to acetate buffer, mouse, canine, and human sera stability of Lu-177-labeled RM1 are presented here and compared with the acetate buffer and sera stability data of AMBA agonist. The samples of Lu-177-labeled RM1 with a high radioconcentration degrade faster than low-radioconcentration samples upon storage at 2-8 degrees C. Addition of stabilizers, ascorbic acid and gentisic acid, improve the stability of Lu-177-labeled RM1 significantly with gentisic acid being more efficient than ascorbic acid as a stabilizer. The degradation kinetics of Lu-177-labeled AMBA and RM1 in sera follow the order (fastest to slowest): mouse > canine > human sera. Finally, Lu-177-labeled RM1 antagonist is slower to degrade in mouse, canine, and human sera than Lu-177-labeled AMBA agonist, further suggesting that an antagonist is a more promising candidate than agonist for the positron emission tomography (PET) imaging and therapy of prostate cancer patients.