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

Abstract

Purpose: Clinically available intraoperative imaging tools to assist surgeons in identifying occult lesions are limited and partially responsible for the high rate of disease recurrence in patients with neuroendocrine tumors (NET). Using the established clinical efficacy of radiolabeled somatostatin analogs as a model, we demonstrate the ability of a fluorescent somatostatin analog to selectively target tumors that overexpress somatostatin receptor subtype-2 (SSTR2) and demonstrate utility for fluorescence-guided surgery (FGS). Experimental Design: A multimodality chelator (MMC) was used as a "radioactive linker" to synthesize the fluorescently labeled somatostatin analog, 67/68 Ga-MMC(IR800)-TOC. In vivo studies were performed to determine the pharmacokinetic profile, optimal imaging time point, and specificity for SSTR2-expressing tissues. Meso-and microscopic imaging of resected tissues and frozen sections were also performed to further assess specific binding, and binding to human NETs was examined using surgical biospecimens from patients with pancreatic NETs. Results: Direct labeling with 67 Ga/ 68 Ga provided quantitative biodistribution analysis that was in agreement with fluorescence data. Receptor-mediated uptake was observed in vivo and ex vivo at the macro-, meso-, and microscopic scales. Surgical biospecimens from patients with pancreatic NETs also displayed receptor-specific agent binding, allowing clear delineation of tumor boundaries that matched pathology findings. Conclusions: The radioactive utility of the MMC allowed us to validate the binding properties of a novel FGS agent that could have a broad impact on cancer outcomes by equipping surgeons with real-time intraoperative imaging capabilities.

Development of a drug–device combination for fluorescence-guided surgery in neuroendocrine tumors

“…2.2 67 Ga labeling of MMC(IR800)-TOC MMC(IR800)-TOC was radiolabeled with 67 Ga (t 1∕2 ¼ 3.3 d) using cation exchange chromatography and purified as previously described. 31…”

“…SSTR2 overexpressing HCT116 (HCT116-SSTR2) cells were cultured as previously described. 31 For all procedures, mice were anesthetized with 1% to 2% isoflurane. For xenografting, 6 to 8 week old mice were subcutaneously injected with 5 × 10 6 HCT116-SSTR2 cells in Matrigel (Corning):PBS (1:1) in the shoulder (n ¼ 24).…”

“…Suspected residual tumor tissues identified during intraoperative FGS were cryo-conserved, sectioned, and stained with hematoxylin & eosin (H&E) and immunohistochemistry as previously described. 31…”

“…Based on our previous results, we selected 2 nmol as the injection dose and imaged at 48-h post-injection. 31 Mice with HCT116-SSTR2 xenografts were imaged longitudinally using the OnLume imager and representative images are shown in Fig. 3 (cohort shown in Fig.…”

“…Consistent SSTR2 specificity was observed at the multiscale level, which included cancer cells, xenografts, and biospecimens obtained from patients with pancreatic NETs. 31 Direct comparison of 68∕67 Ga-MMC(IR800)-TOC to 68 Ga-DOTA-TOC was critical to benchmark the imaging properties of our agent against a clinical gold standard and indicated excellent potential for translational FGS studies. 26 Preclinical evaluation of tumor-specific FGS drugs is commonly performed using imagers optimized for conditions that do not recapitulate factors found in the operating room.…”