Physiological 68Ga-RM2 Uptake in Patients with Biochemically Recurrent Prostate Cancer: An Atlas of Semi-quantitative Measurements

The discovery and validation of novel molecular targets useful in various cancers is of great importance, even more so when they can be used for both diagnosis and therapy (theragnostic). ⁶⁸Ga-labeled DOTA-4-amino-1-carboxymethyl-piperidine-D-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH2 (⁶⁸Ga-RM2) is a synthetic bombesin receptor antagonist that targets gastrin-releasing peptide receptors (GRPR). GRPR proteins are highly overexpressed in several human tumors, including prostate and breast cancers. Knowing the biodistribution and variations of ⁶⁸Ga-RM2 uptake in normal is very important for physicians to accurately assess benign vs. malignant uptake of the radiotracer, particularly as more clinical trials are planned or ongoing.

Here, we evaluated 95 patients with biochemically recurrent prostate cancer who had ⁶⁸Ga-RM2 PET/MRI. Three different readers looked at images and measured the uptake level (SUV_max and SUV_mean) in 24 different organs by drawing a 2D region of interest in the organ itself. An arbitrary scale was used and uptake in various tissues was classified into 4 categories. SUV_mean was also correlated to some factors such as BMI, age, injected dose and uptake time, to look for any possible influence of these factors on the uptake. We found out that age and BMI influenced the uptake of certain organs. However, the influence of BMI decreased consistently when the lean body mass was used to normalize the SUVs calculation instead of body weight. By improving the understanding of ⁶⁸Ga-RM2 biological behavior, this atlas will help increase the investigators’ confidence when using GRPR-targeting radiopharmaceuticals in various indication.

Written by:
Lucia Baratto, MD, Postdoctoral Research Fellow, Division of Nuclear Medicine and Molecular Imaging, Department of Radiology, Stanford University Medical Center, Stanford, California, USA
Andrei H. Iagaru, MD, FACNM, Professor of Radiology - Nuclear Medicine, Chief, Division of Nuclear Medicine and Molecular Imaging, Director, Nuclear Medicine Residency Program, Co-Director, PET-MRI Research Program, Stanford University, Stanford, California, USA

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