PET/CT imaging with a novel ⁶⁸Ga-labelled GRP-receptor antagonist, Sarabesin 3. First clinical data in patients with prostate and breast cancer

Objectives Gastrin-releasing Peptide Receptors (GRP-R) are expressed at high levels in prostate- and breast cancer, thereby providing attractive targets for GRP-R specific imaging and therapy. In the past, GRP-R agonists have been applied for good retention of radioactivity in tumor lesions. GRP-R agonists elicited side effects; however, GRP-R antagonists are expected to have no adverse effects. In our study we applied a novel ⁶⁸Ga-labelled GRP-R antagonist, Sarabesin 3, and evaluated biodistribution, pharmacokinetics and tumor binding characteristics in patients with prostate and breast cancer.

Methods [⁶⁸Ga]Sarabesin 3 (283 ± 91 MBq, 45 ± 14 µg) was injected in 23 patients (8 with breast cancer, 9 with prostate cancer and 6 with other cancers) with disseminated disease and a history of previous therapies (e.g. anti-hormonal therapy). PET/CT fusion images were acquired 60-115 min post injection. Maximum standardized uptake values (SUV_max) were measured in different organs and in 66 tumor lesions. Pathological uptake of [⁶⁸Ga]Sarabesin 3 was defined as focal accumulation besides physiological uptake.

Results SUV_max mean values (and standard deviation) were: pancreas 43.8 ± 19.6, Kidneys 5.4 ± 1.2, blood pool 2.6 ± 0.7 and liver 1.8 ± 0.5. Tumor SUV_max mean values (and range) were 3.6 (0.7-17.8). Four (50%) out of 8 patients with breast cancer and 5 (55%) out of 9 patients with prostate cancer had pathological uptake on [⁶⁸Ga]Sarabesin 3 scan. None of the tumors in 6 patients with other cancers could be visualized. No adverse effects of [⁶⁸Ga]Sarabesin 3 were observed.

Conclusions For the first time we present the biodistribution of [⁶⁸Ga]Sarabesin 3 in patients with disseminated prostate and breast cancer. [⁶⁸Ga]Sarabesin 3 can be helpful for diagnostic and possibly therapeutic applications in a selected group of patients with prostate or breast cancer.