Replacing a single amino acid (Thz14→Pro14) in bombesin-based peptide tracers produced a GRPR antagonist ([68Ga]Ga-ProBOMB5) with strong tumor uptake (12.4%ID/g) and minimal pancreas accumulation (0.60-1.37%ID/g) — solving a critical safety issue for clinical translation.
Pancreas uptake: 0.60-1.37%ID/gvs. typically >10%ID/g for other GRPR tracers — a dramatic reduction that removes the major safety barrier to clinical use, while maintaining tumor uptake of 12.4%ID/g
What the researchers found
The bombesin peptide modification Thz14→Pro14 produced [68Ga]Ga-ProBOMB5 with high tumor uptake (12.4±1.35%ID/g), strong binding affinity (Ki: 12.2 nM), and minimal pancreas accumulation (0.60-1.37%ID/g) — a major improvement over existing GRPR tracers.
Why it matters
GRPR-targeted imaging could transform prostate cancer diagnosis and treatment monitoring, but high pancreas radiation has been a deal-breaker for most tracers. ProBOMB5's minimal pancreas uptake removes this barrier, potentially enabling safe, routine use of GRPR PET imaging to detect and monitor cancers that express this receptor.
The numbers in context
One antagonist (ProBOMB5) and two agonists (LW02056, LW02057) synthesized; labeled with 68Ga and 177Lu; Pro14 substitution replacing Thz14 residue.
How the study worked
Synthesized three [Pro14]bombesin(8-14) derivatives (one antagonist, two agonists). Performed 68Ga and 177Lu radiolabeling, in vitro binding affinity assays (Ki), PET imaging and biodistribution in PC-3 tumor-bearing mice at 1h post-injection, and longitudinal SPECT imaging comparing [177Lu]Lu-ProBOMB5 with [177Lu]Lu-RM2.
Who was studied
Preclinical evaluation of GRPR-targeted radiopharmaceuticals
What this study cannot tell us
Mouse study — human biodistribution may differ. The therapeutic lutetium-177 version showed faster tumor clearance than RM2, limiting its radiation dose to tumors for therapy. Only one tumor model (PC-3) tested. Clinical trials needed to confirm the favorable pancreas profile in humans.
How to read the evidence
Preliminary — preclinical proof-of-concept with promising PET imaging data in a single mouse tumor model. Clinical translation appears feasible for the imaging application but needs human trials.
When this study was published
Published in 2024, advancing GRPR-targeted radiopharmaceutical development toward clinical readiness.
The bigger picture
This work directly addresses the biggest obstacle to clinical adoption of GRPR-targeted radiopharmaceuticals. With minimal pancreas uptake solved, ProBOMB5 joins a growing arsenal of peptide-based cancer imaging agents alongside PSMA-targeted tracers. For patients whose cancers express GRPR but not PSMA, this could be the PET tracer that enables precision diagnosis and treatment planning.
Questions still open
- Can the [177Lu]Lu-ProBOMB5 structure be further modified to slow tumor clearance and improve its therapeutic potential?
- Does the favorable pancreas profile hold in humans, where GRPR expression patterns may differ from mice?
- Could ProBOMB5 be combined with PSMA-targeted tracers for comprehensive prostate cancer imaging?
Common questions
Why is pancreas accumulation such a big problem for cancer imaging tracers?
What cancers could this tracer help detect?
Read the original research
Synthesis and Evaluation of 68Ga- and 177Lu-Labeled [Pro14]bombesin(8-14) Derivatives for Detection and Radioligand Therapy of Gastrin-Releasing Peptide Receptor-Expressing Cancer.
Molecular pharmaceutics, 21(12), 6385-6397
Citation
Wang, Lei; Kuo, Hsiou-Ting; Chapple, Devon E; Chen, Chao-Cheng; Kurkowska, Sara; Colpo, Nadine; Uribe, Carlos; Bénard, François; Lin, Kuo-Shyan. (2024). Synthesis and Evaluation of 68Ga- and 177Lu-Labeled [Pro14]bombesin(8-14) Derivatives for Detection and Radioligand Therapy of Gastrin-Releasing Peptide Receptor-Expressing Cancer.. Molecular pharmaceutics, 21(12), 6385-6397. https://doi.org/10.1021/acs.molpharmaceut.4c00952