Biodistribution and radiation dosimetry of [68Ga]Ga-RM2

Ziel/Aim:

[⁶⁸Ga]Ga-RM2 is a potent GRPR antagonist that has shown promising results in patients with primary or metastatic castration resistant prostate carcinoma (mCRPC). Therefore, this study aims to analyze biodistribution and radiation dosimetry of [⁶⁸Ga]Ga-RM2.

Methodik/Methods:

3 mCRPC patients (mean age 70.7 ± 4.2 y) were injected with 161 – 165 MBq of [⁶⁸Ga]Ga-RM2 i.v. Biodistribution was assessed with dynamic (30 min, list mode with pancreas and kidneys in field of view) and static (head to mid-thigh) PET/CT acquisitions using a Siemens Biograph 2, collection of 1 – 2 ml blood samples at 8 time points and urine samples at 2 time points. For dosimetric analysis, dynamic study was reconstructed as 6 images of 300 seconds. Volumes of interest were drawn on CT images for source organs (kidneys, pancreas, stomach, liver, spleen, urinary bladder) and the whole body. The kBq counts/ml from corresponding PET images was multiplied with CT volumes to determine activity in source organs. % injected activity in source organs, blood samples and urine samples were determined. Bone marrow self-dose was determined using indirect blood based method. OLINDA/EXM software was used to determine residence times, organ absorbed dose (OAD) and equivalent doses.

Ergebnisse/Results:

Physiological uptake in kidneys, urinary bladder, pancreas, spleen and liver was seen. It was observed that early and rapid clearance of activity from kidneys into urinary bladder was accompanied by fast blood clearance and high activity accumulation in urinary bladder. Urinary bladder wall with highest mean organ absorbed dose (0.293 mSv/MBq) was the critical organ followed by pancreas (0.126 mSv/MBq), stomach wall (0.094 mSv/MBq), kidneys (0.047 mSv/MBq) and red marrow (0.009 mSv/MBq). The mean effective dose was found to be 0.031 mSv/MBq. It was observed that mean OAD in stomach was high in contrast to literature, the reason may be very high uptake in patient no1 with high pancreatic uptake. Organ absorbed dose in kidney was found lower to other ⁶⁸Ga-labeld bombesin and PSMA ligands.

Schlussfolgerungen/Conclusions:

[⁶⁸Ga]Ga-RM2 is found to be a safe diagnostic agent with lower kidney dose as compared to [⁶⁸Ga]Ga-PSMA. This is also promising for using RM2 labeled with Lu-177 for therapeutic issues.