Preclinical comparison of Al18F- and 68Ga-labeled gastrin-releasing peptide receptor antagonists for PET imaging of prostate cancer

Journal of Nuclear Medicine, 2014, Volume: 55, Issue: 12, Pages: 2050-2056,  DOI: 10.2967/jnumed.114.141143

K. l. L. S. Chatalic, G. M. Franssen, W. M. van Weerden, W. J. McBride,  P. Laverman, E. de Blois, B. Hajjaj, L. Brunel, D. M. Goldenberg, J.-A. Fehrentz,  J. Martinez, O. C. Boerman, M. de Jong,


Gastrin-releasing peptide receptor (GRPR) is overexpressed in human prostate cancer and is being used as a target for mol. 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 concn. of 50% values for binding GRPR of JMV4168, JMV5132, natGa-JMV4168, and natGa-JMV5132 were detd. in a competition-binding assay using GRPR-overexpressing PC-3 tumors.  The tumor-targeting characteristics of the compds. were assessed in mice bearing s.c. PC-3 xenografts.  Small-animal PET/CT images were acquired, and tracer biodistribution was detd. 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 purifn. and resulting in a specific activity of 35 MBq/nmol.  Inhibitory concn. 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), resp.  In mice with s.c. 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 g), resp.  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 resoln. obsd. 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-pos. PC-3 tumors.  These new PET tracers are promising candidates for future clin. translation.