Preclinical Comparison of Al18F- and 68Ga-Labeled Gastrin-Releasing Peptide Receptor Antagonists for PET Imaging of Prostate Cancer

Gastrin-releasing peptide receptor (GRPR) is overexpressed in human prostate cancer and is being used as a target for molecular imaging. In this study, we report on the direct comparison of 3 novel GRPR-targeted radiolabeled tracers: Al18F-JMV5132, 68Ga-JMV5132, and 68Ga-JMV4168 (JMV5132 is NODA-MPAA-βAla-βAla-[H-d-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH2], JMV4168 is DOTA-βAla-βAla-[H-d-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH2], and NODA-MPAA is 2-[4-(carboxymethyl)-7-{[4-(carboxymethyl)phenyl]methyl}-1,4,7-triazacyclononan-1-yl]acetic acid). Methods: The GRPR antagonist JMV594 (H-d-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH2) was conjugated to NODA-MPAA for labeling with Al18F. JMV5132 was radiolabeled with 68Ga and 18F, and JMV4168 was labeled with 68Ga for comparison. The inhibitory concentration of 50% values for binding GRPR of JMV4168, JMV5132, natGa-JMV4168, and natGa-JMV5132 were determined in a competition-binding assay using GRPR-overexpressing PC-3 tumors. The tumor-targeting characteristics of the compounds were assessed in mice bearing subcutaneous PC-3 xenografts. Small-animal PET/CT images were acquired, and tracer biodistribution was determined by ex vivo measurements. Results: JMV5132 was labeled with 18F in a novel 1-pot, 1-step procedure within 20 min, without need for further purification and resulting in a specific activity of 35 MBq/nmol. Inhibitory concentration of 50% values (in nM) for GRPR binding of JMV5132, JMV4168, natGa-JMV5132, natGa-JMV4168, and AlnatF-JMV5132 were 6.8 (95% confidence intervals [CIs], 4.6–10.0), 13.2 (95% CIs, 5.9–29.3), 3.0 (95% CIs, 1.5–6.0), 3.2 (95% CIs, 1.8–5.9), and 10.0 (95% CIs, 6.3–16.0), respectively. In mice with subcutaneous PC-3 xenografts, all tracers cleared rapidly from the blood, exclusively via the kidneys for 68Ga-JMV4168 and partially hepatobiliary for 68Ga-JMV5132 and Al18F-JMV5132. Two hours after injection, the uptake of 68Ga-JMV4168, 68Ga-JMV5132, and Al18F-JMV5132 in PC-3 tumors was 5.96 ± 1.39, 5.24 ± 0.29, 5.30 ± 0.98 (percentage injected dose per gram), respectively. GRPR specificity was confirmed by significantly reduced tumor uptake of the 3 tracers after coinjection of a 100-fold excess of unlabeled JMV4168 or JMV5132. Small-animal PET/CT clearly visualized PC-3 tumors, with the highest resolution observed for Al18F-JMV5132. Conclusion: JMV5132 could be rapidly and efficiently labeled with 18F. Al18F-JMV5132, 68Ga-JMV5132, and 68Ga-JMV4168 all showed comparable high and specific accumulation in GRPR-positive PC-3 tumors. These new PET tracers are promising candidates for future clinical translation.

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