GRPr Antagonist 68Ga-SB3 PET/CT Imaging of Primary Prostate Cancer in Therapy-Naïve Patients.

The gastrin-releasing peptide receptor (GRPr) is overexpressed in prostate cancer (PCa) cells, making it an excellent tool for targeted imaging. The 68Ga-labeled GRPr antagonist SB3 has shown excellent results in preclinical and clinical studies and was selected for further clinical investigation. The aims of this phase I study were to investigate 68Ga-SB3 PET/CT imaging of primary PCa tumors and assess safety. More aims included an investigation of biodistribution and dosimetry and a comparison with pathology and GRPr expression. Methods: Ten therapy-naïve, biopsy-confirmed PCa patients planned for prostatectomy were included. A 3-h extensive PET/CT imaging protocol was performed within 2 wk before prostatectomy. Prostate tissue was evaluated for tumor localization and Gleason score, and in vitro autoradiography was performed to determine GRPr expression. Available MRI scans performed within 3 mo before the study were matched. For dosimetry, residence times were estimated and effective dose to the body as well as absorbed doses to organs were calculated using the IDAC dose model, version 2.1. Results: Administration of 68Ga-SB3 (187.4 ± 40.0 MBq, 40 ± 5 µg) was well tolerated; no significant changes in vital signs or laboratory results were observed. 68Ga-SB3 PET/CT showed lesions in 8 of 10 patients. Pathologic analysis revealed a total of 16 tumor lesions, of which PET/CT showed 14, resulting in a sensitivity of 88%. 68Ga-SB3 PET/CT imaging showed uptake in 2 large prostatic intraepithelial neoplasia foci, considered a precursor to PCa, resulting in an 88% specificity. Autoradiography of tumor lesions revealed heterogeneous GRPr expression and was negative in 4 patients. Both PET/CT-negative patients had a GRPr-negative tumor. In autoradiography-positive tumors, the level of GRPr expression showed a significant correlation to tracer uptake on PET/CT. Dosimetry calculations estimated the effective dose to be 0.0144 mSv/MBq, similar to other 68Ga-labeled radiopeptides. The highest absorbed dose was detected in the physiologic GRPr-expressing pancreas (0.198 mGy/MBq), followed by the bladder wall and kidneys. Conclusion:68Ga-SB3 PET/CT is a safe imaging method and a promising tool for early PCa imaging.

Optimal Timing of Prostate Specific Membrane Antigen Positron Emission Tomography/Computerized Tomography for Biochemical Recurrence after Radical Prostatectomy.

PURPOSE: We developed a model predicting the probability of detecting prostate cancer recurrence outside the prostatic fossa on prostate specific membrane antigen positron emission tomography/computerized tomography in patients with biochemical recurrence after radical prostatectomy. MATERIALS AND METHODS: We retrospectively included 419 consecutive patients with biochemical recurrence (prostate specific antigen less than 2.0 ng/ml) after radical prostatectomy who underwent 68Ga-prostate specific membrane antigen-11 positron emission tomography/computerized tomography to guide salvage therapy. Patients receiving androgen deprivation therapy between radical prostatectomy and prostate specific membrane antigen positron emission tomography/computerized tomography were excluded from the study. We used multivariable logistic regression to assess predictors for the detection of prostate cancer recurrence outside the prostatic fossa on prostate specific membrane antigen positron emission tomography/computerized tomography. We minimized overfitting of the model and used decision curve analysis to determine clinical utility. RESULTS: Median prostate specific antigen at scanning was 0.40 ng/ml (IQR 0.30-0.70). Overall 174 (42%) patients had prostate cancer recurrence outside the prostatic fossa. Prostate specific antigen at time of scanning, and grade group, N stage and surgical margin status at radical prostatectomy specimen were significant predictors for detecting prostate cancer recurrence outside the prostatic fossa. The bootstrapped AUC of this model was 0.75 (IQR 0.73-0.77). The decision curve analysis showed a net benefit by a model based probability from 16%. Limitations include the retrospective design and the missing histological correlation of positive lesions. CONCLUSIONS: Next to the prostate specific antigen at time of scanning, grade group, N stage and surgical margin status at radical prostatectomy specimen are significant predictors for detecting prostate cancer recurrence outside the prostatic fossa on prostate specific membrane antigen positron emission tomography/computerized tomography. The presented model is implemented in a dashboard to assist clinicians in determining the optimal time to perform 68Ga-prostate specific membrane antigen-11 positron emission tomography/computerized tomography in patients with biochemical recurrence after radical prostatectomy.

Radiometal-Dependent Biological Profile of the Radiolabeled Gastrin-Releasing Peptide Receptor Antagonist SB3 in Cancer Theranostics: Metabolic and Biodistribution Patterns Defined by Neprilysin.

Recent advances in oncology involve the use of diagnostic/therapeutic radionuclide-carrier pairs that target cancer cells, offering exciting opportunities for personalized patient treatment. Theranostic gastrin-releasing peptide receptor (GRPR)-directed radiopeptides have been proposed for the management of GRPR-expressing prostate and breast cancers. We have recently introduced the PET tracer 68Ga-SB3 (SB3, DOTA- p-aminomethylaniline-diglycolic acid-DPhe-Gln-Trp-Ala-Val-Gly-His-Leu-NHEt), a receptor-radioantagonist that enables the visualization of GRPR-positive lesions in humans. Aiming to fully assess the theranostic potential of SB3, we herein report on the impact of switching 68Ga to 111In/177Lu-label on the biological properties of resulting radiopeptides. Notably, the bioavailability of 111In/177Lu-SB3 in mice drastically deteriorated compared with metabolically robust 68Ga-SB3, and as a result led to poorer 111In/177Lu-SB3 uptake in GRPR-positive PC-3 xenografts. The peptide cleavage sites were identified by chromatographic comparison of blood samples from mice intravenously receiving 111In/177Lu-SB3 with each of newly synthesized 111In/177Lu-SB3-fragments. Coinjection of the radioconjugates with the neprilysin (NEP)-inhibitor phosphoramidon led to full stabilization of 111In/177Lu-SB3 in peripheral mouse blood and resulted in markedly enhanced radiolabel uptake in the PC-3 tumors. In conclusion, in situ NEP-inhibition led to indistinguishable 68Ga/111In/177Lu-SB3 profiles in mice emphasizing the theranostic prospects of SB3 for clinical use.

In Vivo Stabilized SB3, an Attractive GRPR Antagonist, for Pre- and Intra-Operative Imaging for Prostate Cancer.

PURPOSE: The gastrin-releasing peptide receptor (GRPR), overexpressed on various tumor types, is an attractive target for receptor-mediated imaging and therapy. Another interesting approach would be the use of GRPR radioligands for pre-operative imaging and subsequent radio-guided surgery, with the goal to improve surgical outcome. GRPR radioligands were successfully implemented in clinical studies, especially Sarabesin 3 (SB3) is an appealing GRPR antagonist with high receptor affinity. Gallium-68 labeled SB3 has good in vivo stability, after labeling with Indium-111; however, the molecule shows poor in vivo stability, which negatively impacts tumor-targeting capacity. A novel approach to increase in vivo stability of radiopeptides is by co-administration of the neutral endopeptidase (NEP) inhibitor, phosphoramidon (PA). We studied in vivo stability and biodistribution of [111In]SB3 without/with (-/+) PA in mice. Furthermore, SPECT/MRI on a novel, state-of-the-art platform was performed. PROCEDURES: GRPR affinity of SB3 was determined on PC295 xenograft sections using [125I]Tyr4-bombesin with tracer only or with increasing concentrations of SB3. For in vivo stability, mice were injected with 200/2000 pmol [111In]SB3 -/+ 300 µg PA. Blood was collected and analyzed. Biodistribution and SPECT/MRI studies were performed at 1, 4, and 24 h postinjection (p.i.) of 2.5 MBq/200 pmol or 25 MBq/200 pmol [111In]SB3 -/+ 300 µg PA in PC-3-xenografted mice. RESULTS: SB3 showed high affinity for GRPR (IC50 3.5 nM). Co-administration of PA resulted in twice higher intact peptide in vivo vs [111In]SB3 alone. Biodistribution studies at 1, 4, and 24 h p.i. show higher tumor uptake values with PA co-administration (19.7 ± 3.5 vs 10.2 ± 1.5, 17.6 ± 5.1 vs 8.3 ± 1.1, 6.5 ± 3.3 vs 3.1 ± 1.9 % ID/g tissue (P < 0.0001)). Tumor imaging with SPECT/MRI clearly improved after co-injection of PA. CONCLUSIONS: Co-administration of PA increased in vivo tumor targeting capacity of [111In]SB3, making this an attractive combination for GRPR-targeted tumor imaging.

68Ga/177Lu-NeoBOMB1, a Novel Radiolabeled GRPR Antagonist for Theranostic Use in Oncology.

Because overexpression of the gastrin-releasing peptide receptor (GRPR) has been reported on various cancer types, for example, prostate cancer and breast cancer, targeting this receptor with radioligands might have a significant impact on staging and treatment of GRPR-expressing tumors. NeoBOMB1 is a novel DOTA-coupled GRPR antagonist with high affinity for GRPR and excellent in vivo stability. The purpose of this preclinical study was to further explore the use of NeoBOMB1 for theranostic application by determining the biodistribution of 68Ga-NeoBOMB1 and 177Lu-NeoBOMB1. METHODS: PC-3 tumor-xenografted BALB/c nu/nu mice were injected with either approximately 13 MBq/250 pmol 68Ga-NeoBOMB1 or a low (∼1 MBq/200 pmol) versus high (∼1 MBq/10 pmol) peptide amount of 177Lu-NeoBOMB1, after which biodistribution and imaging studies were performed. At 6 time points (15, 30, 60, 120, 240, and 360 min for 68Ga-NeoBOMB1 and 1, 4, 24, 48, 96, and 168 h for 177Lu-NeoBOMB1) postinjection tumor and organ uptake was determined. To assess receptor specificity, additional groups of animals were coinjected with an excess of unlabeled NeoBOMB1. Results of the biodistribution studies were used to determine pharmacokinetics and dosimetry. Furthermore, PET/CT and SPECT/MRI were performed. RESULTS: Injection of approximately 250 pmol 68Ga-NeoBOMB1 resulted in a tumor and pancreas uptake of 12.4 ± 2.3 and 22.7 ± 3.3 percentage injected dose per gram (%ID/g) of tissue, respectively, at 120 min after injection. 177Lu-NeoBOMB1 biodistribution studies revealed a higher tumor uptake (17.9 ± 3.3 vs. 11.6 ± 1.3 %ID/g of tissue at 240 min after injection) and a lower pancreatic uptake (19.8 ± 6.9 vs. 105 ± 13 %ID/g of tissue at 240 min after injection) with the higher peptide amount injected, leading to a significant increase in the absorbed dose to the tumor versus the pancreas (200 pmol, 570 vs. 265 mGy/MBq; 10 pmol, 435 vs. 1393 mGy/MBq). Using these data to predict patient dosimetry, we found a kidney, pancreas, and liver exposure of 0.10, 0.65, and 0.06 mGy/MBq, respectively. Imaging studies resulted in good visualization of the tumor with both 68Ga-NeoBOMB1 and 177Lu-NeoBOMB1. CONCLUSION: Our findings indicate that 68Ga- or 177Lu-labeled NeoBOMB1 is a promising radiotracer with excellent tumor uptake and favorable pharmacokinetics for imaging and therapy of GRPR-expressing tumors.