Evaluation of a Fluorescent and Radiolabeled Hybrid Somatostatin Analog In Vitro and in Mice Bearing H69 Neuroendocrine Xenografts

Abstract

In the treatment of neuroendocrine tumors (NETs), complete surgical removal of malignancy is generally desirable, because it offers curative results. Preoperative guidance with radiolabeled somatostatin analogs, commonly used for NET diagnosis and preoperative planning, is limited by its low resolution, with the risk that tumor margins and small metastases will be incompletely resected with subsequent recurrence. A single hybrid probe combining radiotracer and optical dye would enable high-resolution optical guidance, also during surgery. In the current study, the hybrid labeled somatostatin analog Cy5-DTPA-Tyr 3 -octreotate (DTPA is diethylene triamine pentaacetic acid) was synthesized and evaluated for its ability to specifically trace NET cells in vitro and in an animal model. The performance of the hybrid tracer was compared with that of octreotate with only radiolabel or only optical label. Methods: The binding affinity and internalization capacity of Cy5-DTPA-Tyr 3 -octreotate were assessed in vitro. Biodistribution profiles and both nuclear and optical in vivo imaging of Cy5-111 In -DTPA-Tyr 3 -octreotate were performed in NET-bearing mice and compared with the performance of 111 In-DTPA-Tyr 3 -octreotate. Results: In vitro studies showed a low receptor affinity and internalization rate for Cy5-DTPA-Tyr 3 -octreotate. The dissociation constant value was 387.7 ± 97.9 nM for Cy5-DTPA-Tyr 3 -octreotate, whereas it was 120.5 ± 18.1 nM for DTPA-Tyr 3 -octreotate. Similarly, receptor-mediated internalization reduced from 33.76% ± 1.22% applied dose for DTPA-Tyr 3 -octreotate to 1.32% ± 0.02% applied dose for Cy5-DTPA-Tyr 3 -octreotate. In contrast, in vivo and ex vivo studies revealed similar tumor uptake values of Cy5-111 In-DTPA-Tyr 3 -octreotate and 111 In -DTPA-Tyr 3 -octreotate (6.93 ± 2.08 and 5.16 ± 1.27, respectively). All organs except the kidneys showed low background radioactivity, with especially low activities in the liver, and high tumor-to-tissue ratios were achieved-both favorable for the tracer's toxicity profile. Hybrid imaging in mice confirmed that the nuclear and fluorescence signals colocalized. Conclusion: The correlation between findings with the optical and the nuclear probes underlines the potential of combining SPECT imaging with fluorescence guidance and shows the promise of this novel hybrid peptide for preoperative and intraoperative imaging of NET.

Hybrid intraoperative imaging techniques in radioguided surgery: present clinical applications and future outlook

“…Hybrid optical–gamma tracers are in development to target prostate [ 53 , 54 ], renal cell carcinoma [ 55 , 56 ] and colorectal [ 57 ], neuroendocrine [ 58 ] and breast [ 59 , 60 ] cancers in addition to glioblastoma [ 61 , 62 ] and even bacterial infections [ 63 ]. The development of these tracers opens up the possibility of the Guided Hybrid intraOperative Specific Targeting (GHOST) technique [ 30 ] for a range of cancers.…”

“…Targeted probe development is split fairly evenly between the use of β-emitting tracers (used for PET) [ 53 , 59 – 61 , 64 – 67 ] and gamma-emitting tracers (used for SPECT) [ 54 – 58 , 63 , 67 – 70 ]. Most of these probes have been developed with a view to using the gamma component preoperatively and the NIR fluorescence component intraoperatively, but in principle they could also be used for hybrid intraoperative detection.…”

Specific Targeting of Somatostatin Receptor Subtype-2 for Fluorescence-Guided Surgery

“…A major limitation in NET research is the lack of robust orthotopic or patient-derived xenograft animal models that accurately represent human disease. For further agent evaluation, we selected NCI-H69 cells, which are known to endogenously express moderate to low levels of SSTR2 (15) and have been widely used for assessment of SSTR2-targeted agents (16)(17)(18)(19)(20). As anticipated, in vitro results demonstrated specific binding and internalization in NCI-H69 cells ( Supplementary Figs.…”

Recent advances in nuclear and hybrid detection modalities for image-guided surgery

“…Here it should be noted that efforts towards the realization of hybrid radiopharmaceuticals are likely to promote the use of multiplexing approaches. Examples of potential future hybrid procedures are: prostate cancer (PSMA I&F) [248], neuroendocrine tumors (Cy5- 111 In-DTPA-Tyr3-octreotate) [249], breast cancer ([ 111 In]-DTPA-trastuzumab-IRDye800) [250] and renal cancer ( 111 Ingirentuximab-IRDye800CW) [251].…”