Radio-guided surgery in neuroendocrine tumors

Lessons learned in application driven imaging agent design for image-guided surgery

To meet the growing demand for intraoperative molecular imaging, the development of compatible imaging agents plays a crucial role. Given the unique requirements of surgical applications compared to diagnostics and therapy, maximizing translational potential necessitates distinctive imaging agent designs. For effective surgical guidance, exogenous signatures are essential and are achievable through a diverse range of imaging labels such as (radio)isotopes, fluorescent dyes, or combinations thereof. To achieve optimal in vivo utility a balanced molecular design of the tracer as a whole is required, which ensures a harmonious effect of the imaging label with the affinity and specificity (e.g., pharmacokinetics) of a pharmacophore/targeting moiety. This review outlines common design strategies and the effects of refinements in the molecular imaging agent design on the agent's pharmacological profile. This includes the optimization of affinity, pharmacokinetics (including serum binding and target mediated background), biological clearance route, the achievable signal intensity, and the effect of dosing hereon.

Radio-Guided Surgery with a New-Generation β-Probe for Radiolabeled Somatostatin Analog, in Patients with Small Intestinal Neuroendocrine Tumors

BACKGROUND

Radio-guided surgery (RGS) holds promise for improving surgical outcomes in neuroendocrine tumors (NETs). Previous studies showed low specificity (SP) using γ-probes to detect radiation emitted by radio-labeled somatostatin analogs.

OBJECTIVE

We aimed to assess the sensitivity (SE) and SP of the intraoperative RGS approach using a β-probe with a per-lesion analysis, while assessing safety and feasibility as secondary objectives.

METHODS

This prospective, single-arm, single-center, phase II trial (NCT05448157) enrolled 20 patients diagnosed with small intestine NETs (SI-NETs) with positive lesions detected at 68Ga-DOTA-TOC positron emission tomography/computed tomography (PET/CT). Patients received an intravenous injection of 1.1 MBq/Kg of 68Ga-DOTA-TOC 10 min prior to surgery. In vivo measurements were conducted using a β-probe. Receiver operating characteristic (ROC) analysis was performed, with the tumor-to-background ratio (TBR) as the independent variable and pathology result (cancer vs. non-cancer) as the dependent variable. The area under the curve (AUC), optimal TBR, and absorbed dose for the surgery staff were reported.

RESULTS

The intraoperative RGS approach was feasible in all cases without adverse effects. Of 134 specimens, the AUC was 0.928, with a TBR cut-off of 1.35 yielding 89.3% SE and 86.4% SP. The median absorbed dose for the surgery staff was 30 µSv (range 12-41 µSv).

CONCLUSION

This study reports optimal accuracy in detecting lesions of SI-NETs using the intraoperative RGS approach with a novel β-probe. The method was found to be safe, feasible, and easily reproducible in daily clinical practice, with minimal radiation exposure for the staff. RGS might potentially improve radical resection rates in SI-NETs.

CLINICAL TRIALS REGISTRATION

68Ga-DOTATOC Radio-Guided Surgery with β-Probe in GEP-NET (RGS GEP-NET) [NCT0544815; https://classic.

CLINICALTRIALS

gov/ct2/show/NCT05448157 ].

eTFC-01: a dual-labeled chelate-bridged tracer for SSTR2-positive tumors

BACKGROUND

Integrating radioactive and optical imaging techniques can facilitate the prognosis and surgical guidance for cancer patients. Using a single dual-labeled tracer ensures consistency in both imaging modalities. However, developing such molecule is challenging due to the need to preserve the biochemical properties of the tracer while introducing bulky labeling moieties. In our study, we designed a trifunctional chelate that facilitates the coupling of the targeting vector and fluorescent dye at opposite sites to avoid undesired steric hindrance effects. The synthesis of the trifunctional chelate N3-Py-DOTAGA-(tBu)3 (7) involved a five-step synthetic route, followed by conjugation to the linear peptidyl-resin 8 through solid-phase synthesis. After deprotection and cyclization, the near-infrared fluorescent dye sulfo-Cy.5 was introduced using copper free click chemistry, resulting in eTFC-01. Subsequently, eTFC-01 was labeled with [111In]InCl3. In vitro assessments of eTFC-01 binding, uptake, and internalization were conducted in SSTR2-transfected U2OS cells. Ex-vivo biodistribution and fluorescence imaging were performed in H69-tumor bearing mice.

RESULTS

eTFC-01 demonstrated a two-fold higher IC50 value for SSTR2 compared to the gold standard DOTA-TATE. Labeling of eTFC-01 with [111In]InCl3 gave a high radiochemical yield and purity. The uptake of [111In]In-eTFC-01 in U2OS.SSTR2 cells was two-fold lower than the uptake of [111In]In-DOTA-TATE, consistent with the binding affinity. Tumor uptake in H69-xenografted mice was lower for [111In]In-eTFC-01 at all-time points compared to [111In]In-DOTA-TATE. Prolonged blood circulation led to increased accumulation of [111In]In-eTFC-01 in highly vascularized tissues, such as lungs, skin, and heart. Fluorescence measurements in different organs correlated with the radioactive signal distribution.

CONCLUSION

The successful synthesis and coupling of the trifunctional chelate to the peptide and fluorescent dye support the potential of this synthetic approach to generate dual labeled tracers. While promising in vitro, the in vivo results obtained with [111In]In-eTFC-01 suggest the need for adjustments to enhance tracer distribution.

Feasibility of 177Lu activity quantification using a small portable CZT-based gamma-camera

BACKGROUND

In image processing for activity quantification, the end goal is to produce a metric that is independent of the measurement geometry. Photon attenuation needs to be accounted for and can be accomplished utilizing spectral information, avoiding the need of additional image acquisitions. The aim of this work is to investigate the feasibility of 177Lu activity quantification with a small CZT-based hand-held gamma-camera, using such an attenuation correction method.

METHODS

A previously presented dual photopeak method, based on the differential attenuation for two photon energies, is adapted for the three photopeaks at 55 keV, 113 keV, and 208 keV for 177Lu. The measurement model describes the count rates in each energy window as a function of source depth and activity, accounting for distance-dependent system sensitivity, attenuation, and build-up. Parameter values are estimated from characterizing measurements, and the source depth and activity are obtained by minimizing the difference between measured and modelled count rates. The method is applied and evaluated in phantom measurements, in a clinical setting for superficial lesions in two patients, and in a pre-clinical setting for one human tumour xenograft. Evaluation is made for a LEHR and an MEGP collimator.

RESULTS

For phantom measurements at clinically relevant depths, the average (and standard deviation) in activity errors are 17% ± 9.6% (LEHR) and 2.9% ± 3.6% (MEGP). For patient measurements, deviations from activity estimates from planar images from a full-sized gamma-camera are 0% ± 21% (LEHR) and 16% ± 18% (MEGP). For mouse measurements, average deviations of - 16% (LEHR) and - 6% (MEGP) are obtained when compared to a small-animal SPECT/CT system. The MEGP collimator appears to be better suited for activity quantification, yielding a smaller variability in activity estimates, whereas the LEHR results are more severely affected by septal penetration.

CONCLUSIONS

Activity quantification for 177Lu using the hand-held camera is found to be feasible. The readily available nature of the hand-held camera may enable more frequent activity quantification in e.g., superficial structures in patients or in the pre-clinical setting.

Radioguided Surgery for Gastroenteropancreatic Neuroendocrine Tumours: a Systematic Literature Review

BACKGROUND

Radioguided surgery (RGS) for gastroenteropancreatic neuroendocrine tumours (GEP-NETs) has been suggested as a way to improve intraoperative lesion detection. This systematic literature review of reports of the use of RGS for GEP-NETs was performed to determine if there is a benefit.

METHODS

A literature search was conducted using Google Scholar and PubMed, and snowballing from any relevant literature. Full-text studies were included if they were published in the English language and reported outcomes of RGS on human subjects with GEP-NETs. Qualitative data synthesis was performed.

RESULTS

Twenty-six papers including a total of 209 patients were included. The tracers used were predominantly indium-111 pentetreotide, gallium-68 DOTA-peptides, and technetium-99m EDDA/HYNIC-peptides. Heterogeneous protocols make comparisons difficult, but most papers reported a benefit from the use of RGS in tumours in the gastrointestinal tract; utility in localisation of pancreatic tumours was less clear. Time between tracer administration and operation varied: from 16 h to 8 days with indium-111, 0-24 h with technetium-99m, and 19-193 min with gallium-68. Eight teams reported the thresholding technique used for discrimination-four used a ratio, four statistical methods, and one looked at the sensitivity and specificity of different cut-offs. Six teams performed follow-up of 24 patients (three pancreas, eight gastrinoma, 13 gastrointestinal tract) for between 3 months and 3 years. Two patients relapsed (one pancreas, one gastrinoma) between 6 and 12 months post-surgery.

CONCLUSIONS

RGS appears to aid in localisation of gastrointestinal NETs, but the benefit is more equivocal in pancreatic NETs. Further work into outcomes is warranted.

Interventional nuclear medicine: a focus on radioguided intervention and surgery

Within interventional nuclear medicine (iNM) a prominent role is allocated for the sub-discipline of radioguided surgery. Unique for this discipline is the fact that an increasing number of clinical indications (e.g. lymphatic mapping, local tumor demarcation and/or tumor receptor targeted applications) have been adopted into routine care. The clinical integration is further strengthened by technical innovations in chemistry and engineering that enhance the translational potential of radioguided procedures in iNM. Together, these features not only ensure ongoing expansion of iNM but also warrant a lasting clinical impact for the sub-discipline of radioguided surgery.

Radioguided Surgery With Gallium 68 Dotatate for Patients With Neuroendocrine Tumors

Importance

Neuroendocrine tumors (NETs) express somatostatin receptors, which can be targeted with radiolabeled peptides. In a variety of solid tumors, radioguided surgery (RGS) has been used to guide surgical resection. Gallium 68 (68Ga) dota peptides have been shown to be more accurate than other radioisotopes for detecting NETs. A pilot study previously demonstrated the feasibility and safety of 68Ga-dotatate RGS for patients with NETs.

Objective

To evaluate what intraoperative techniques and thresholds define positive lesions that warrant resection during 68Ga-dotatate RGS.

Design, Setting, and Participants

This prospective cohort study, conducted between October 23, 2013, and February 14, 2018, included 44 patients with NETs who underwent 68Ga-dotatate RGS.

Intervention

Gallium 68-dotatate RGS.

Main Outcomes and Measures

The in vivo and ex vivo tumor to background ratio (TBR) was assessed for resected lesions and correlated with the histopathologic findings.

Results

Forty-four patients (22 women and 22 men; mean [SD] age, 51.0 [13.7] years) had 133 lesions detected on preoperative imaging scans, with a diagnosis of a pancreatic NET (19 of 44 [43%]), gastrointestinal NET (22 of 44 [50%]), and pheochromocytoma or paraganglioma (3 of 44 [7%]). The TBR was obtained by normalizing to the omentum (106 of 133 [79.7%]) or other solid organs (27 of 133 [20.3%]). The omentum had a significantly lower mean (SD) count than other solid organs for background count activity 3 hours after injection (22.1 [17.0] vs 34.5 [39.0]; P < .001). The lesions containing NETs had a higher TBR than those that did not contain NETs (18.9 vs 4.4; P < .001). On a receiver operating characteristic curve analysis, a TBR of 2.5 had a sensitivity of 90% and a specificity of 25%, and a TBR of 16 had a sensitivity of 54% and a specificity of 81%.

Conclusions and Relevance

A TBR of 2.5 or greater is a highly sensitive threshold for indicating a lesion to be consistent with a NET on histologic findings and thus warranting surgical resection. The omentum should be used as the background count activity for 68Ga-dotatate RGS for patients with abdominal NETs.

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

Introduction: Radioguided surgery is an ever-evolving part of nuclear medicine. In fact, this nuclear medicine sub-discipline actively bridges non-invasive molecular imaging with surgical care. Next to relying on the availability of radio- and bimodal-tracers, the success of radioguided surgery is for a large part dependent on the imaging modalities and imaging concepts available for the surgical setting. With this review, we have aimed to provide a comprehensive update of the most recent advances in the field. Areas covered: We have made an attempt to cover all aspects of radioguided surgery: 1) the use of radioisotopes that emit γ, β⁺, and/or β^- radiation, 2) hardware developments ranging from probes to 2D cameras and even the use of advanced 3D interventional imaging solutions, and 3) multiplexing solutions such as dual-isotope detection or combined radionuclear and optical detection. Expert opinion: Technical refinements in the field of radioguided surgery should continue to focus on supporting its implementation in the increasingly complex minimally invasive surgical setting, e.g. by accommodating robot-assisted laparoscopic surgery. In addition, hybrid concepts that integrate the use of radioisotopes with other image-guided surgery modalities such as fluorescence or ultrasound are likely to expand in the future.

Copper-64 Radiopharmaceuticals for Oncologic Imaging

The positron emitting radionuclide (64)Cu has a radioactive half-life of 12.7 hours. The decay characteristics of (64)Cu allow for PET images that are comparable in quality to those obtained using (18)F. Given the longer radioactive half-life of (64)Cu compared with (18)F and the versatility of copper chemistry, copper is an attractive alternative to the shorter-lived nuclides for PET imaging of peptides, antibodies, and small molecules that may require longer circulation times. This article discusses a number of copper radiopharmaceuticals, such as Cu-ATSM, that have been translated to the clinic and new developments in copper-based radiopharmaceuticals.

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

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.

Laparoscopic resection of gastrointestinal neuroendocrine tumors with special contribution of radionuclide imaging

The surgical treatment of neuroendocrine tumors (NETs) draws on experience and guidelines more than on prospective randomized trials. The incidence of NET is increasing in all parts of the gastrointestinal tract. A variety of classifications introduced over the last decade may have led to difficulties in judging clinical relevance and determining the right surgical strategy. The North American Neuroendocrine Tumor Society and the European Neuroendocrine Tumor Society have developed usable guidelines from the available literature. For more than 20 years laparoscopy has developed as the gold standard for various surgical indications. Nevertheless, few trials have compared open and laparoscopic surgery with regard to NET. This review summarizes the recent literature on surgery for NET and incorporates the evidence on laparoscopy for cancer which might be also applied for NET.

[Radioguided surgery in neuroendocrine tumors. A review of the literature]

Radioguided surgery can be a useful technique in the localization of neuroendocrine tumors. It detects more and smaller lesions compared to pre-surgical imaging and intraoperative digital palpation by the surgeon. It detects residual lesions and also indicates the shortest access route to the lesion. Nevertheless, its use has not become widespread because of technical difficulties. There is a limited number of published series, a lack of standardized protocol because of the great variability regarding type of radiopharmaceutical, dose of radiotracer, timing between injection and surgery. In this paper, we review these issues, describing the experience of different authors in diverse tumors.

Intraoperative somatostatin receptor detection after peptide receptor radionuclide therapy with (177)Lu- and (90)Y-DOTATOC (tandem PRRNT) in a patient with a metastatic neuroendocrine tumor

AIM

The aim of this chapter is to present the results of the first intraoperative somatostatin receptor detection after peptide receptor radionuclide therapy (PRRNT) with (90)Y- and (177)Lu-DOTATOC using a handheld gamma probe and comparison with the findings of preoperative (68)Ga-DOTATOC PET/CT in a patient with a metastatic neuroendocrine tumor (NET) of the ileum.

MATERIALS AND METHODS

A 56-year-old female patient, treated twice by PRRNT, was admitted for the third cycle and subsequent surgery. Before operation, the following studies were performed for restaging after the second cycle of PRRNT: (99m)Tc-MAG3 + TER, (99m)Tc-DTPA + GFR, abdominal ultrasonography, MRI of the abdomen, (68)Ga-DOTATOC PET/CT as well as (18)F-fluoride PET/CT. Serum tumor markers were measured before and after PRRNT. Tandem PRRNT was performed using 3000 MBq (90)Y-DOTATOC and 6000 MBq (177)Lu-DOTATOC. Whole-body scintigrams were obtained at 23 and 43 h. Five days after PRRNT, the patient was operated using a handheld gamma probe. Immunohistochemistry and histopathology of the resected tissue were performed.

RESULTS

Tandem PRRNT was very well tolerated by the patient. Before PRRNT, (68)Ga-DOTATOC PET/CT revealed the primary tumor in the ileocoecal region as well as bilobular liver metastases and a right iliac bone lesion (osteoblastic on (18)F-fluoride PET/CT). Compared with the previous findings, there was good therapy response (partial remission of the tumor lesions). No nephrotoxicity was observed. Serum chromogranin A (836 μg/l, n < 100) and serotonin (852 μg/l, n < 200) were strongly elevated. Posttherapy scans showed intense uptake in metastases and the primary tumor. Intraoperative gamma probe detected-in addition to the known lesions-bilateral ovarian metastases not visualized by (68)Ga-DOTATOC PET/CT. Bilateral adnexectomy, right hemicolectomy, excision of hepatic metastases in S3, and partial resection of peritoneum were performed. Histopathology confirmed metastases in both ovaries.

CONCLUSIONS

Gamma probe-guided surgery after (177)Lu PRRNT is feasible and appears to be more sensitive than (68)Ga-DOTATOC PET/CT. This technique might aid the surgeon in achieving more complete tumor resection through intraoperative detection of very small lesions (<5 mm) directly after PRRNT.

Radioguided surgery in neuroendocrine tumors using Ga-68-labeled somatostatin analogs: a pilot study

BACKGROUND

Previous studies of the intraoperative use of a hand-held gamma probe to localize metastases and primary tumors have shown improved assessment of tumor spread and changes in surgical management based on additional information gained by radioguided surgery (RGS).

PURPOSE

The aim was to test the feasibility and advantages of doing RGS using a gallium-68 labeled with somatostatin receptor analogs in the intraoperative detection of neuroendocrine tumors.

METHODS

Ga-68 somatostatin receptor PET/CT imaging was performed preoperatively in 9 patients with gastroenteropancreatic neuroendocrine tumors. Statistical analyses were performed to find out the correlation between the pathologic size of the tumor lesions and the maximum standardized uptake value on PET/CT as well as the target/nontarget ratio (T/NT) of gamma probe counts. Thereafter, the impact of the planned operation procedure and the lesion-based sensitivity of tumor detection (surgical palpation vs. PET/CT vs. gamma probe) had been observed.

RESULTS

Overall, 72 locations in 9 patients were examined intraoperatively using gamma probes. The gamma probe detected 94% of the whole histologically quantified lesions, whereas the PET/CT allocated 69% and surgical palpation, 50%. RGS resulted in change in the operative procedure in 56%. There was a significant correlation between the maximum standardized uptake value and tumor size (0.74; P < 0.005).

CONCLUSION

Hand-held gamma probe surgery using gallium-68-labeled somatostatin analogs is a feasible and an attractive option for real-time detection of small metastases and primaries of neuroendocrine tumors.

Neoadjuvant peptide receptor radionuclide therapy for an inoperable neuroendocrine pancreatic tumor

Pancreatic endocrine tumors are rare but are among the most common neuroendocrine neoplasms of the abdomen. At diagnosis many of them are already advanced and difficult to treat. We report on an initially inoperable malignant pancreatic endocrine tumor in a 33-year-old woman, who received neoadjuvant peptide receptor radionuclide therapy (PRRT) as first-line treatment. This resulted in a significant downstaging of the tumor and allowed its subsequent complete surgical removal. Follow-up for eighteen months revealed a complete remission. This is the first report on neoadjuvant PRRT in a neuroendocrine neoplasm with subsequent successful complete resection.

New applications of radioguided surgery in oncology

OBJECTIVE

To report oncological cases (excluding those related to breast cancer) for which radioguided surgery has been used in combination with the Radioguided Occult Lesion Localization technique.

INTRODUCTION

Radioguided surgery enables a surgeon to identify lesions or tissues that have been preoperatively marked with radioactive substances. The Radioguided Occult Lesion Localization technique has been widely used to identify the sentinel lymph node and occult lesions in patients with breast cancer. However, few studies have reported the use of this technique for non-breast cancer pathologies.

METHODOLOGY

In all cases, injection of Technecium-99m sulfur colloid was performed, directly inside or near by the suspicious lesion, guided by ultrasound or computed tomography, up to 36 hours prior to the surgical procedure. Intraoperative lesion detection was carried out using a gamma-probe.

RESULTS

We report five oncology cases in which preoperative markings of the lesions were carried out using the Radioguided Occult Lesion Localization technique. The patients presented with the following: recurrence of renal cell carcinoma, cervical recurrence of papillary carcinoma of the thyroid, recurrence of retroperitoneal sarcoma, lesions of the popliteal fossa, and recurrence of rhabdomyosarcoma of a thigh. In each case, the lesions that were marked preoperatively were ultimately successfully excised.

CONCLUSIONS

Radioguided surgery has proven to be a safe and effective alternative for the management of oncology patients. The Radioguided Occult Lesion Localization technique can be useful in selected cases where suspect lesions may be difficult to identify intraoperatively, due to their dimensions or anatomical location. The procedure allows for more conservative excisions and reduces the surgery-related morbidity.

A comprehensive overview of radioguided surgery using gamma detection probe technology

The concept of radioguided surgery, which was first developed some 60 years ago, involves the use of a radiation detection probe system for the intraoperative detection of radionuclides. The use of gamma detection probe technology in radioguided surgery has tremendously expanded and has evolved into what is now considered an established discipline within the practice of surgery, revolutionizing the surgical management of many malignancies, including breast cancer, melanoma, and colorectal cancer, as well as the surgical management of parathyroid disease. The impact of radioguided surgery on the surgical management of cancer patients includes providing vital and real-time information to the surgeon regarding the location and extent of disease, as well as regarding the assessment of surgical resection margins. Additionally, it has allowed the surgeon to minimize the surgical invasiveness of many diagnostic and therapeutic procedures, while still maintaining maximum benefit to the cancer patient. In the current review, we have attempted to comprehensively evaluate the history, technical aspects, and clinical applications of radioguided surgery using gamma detection probe technology.

Design and in vitro characterization of highly sst2-selective somatostatin antagonists suitable for radiotargeting

Radiolabeled sst 2 and sst 3 antagonists are better candidates for tumor targeting than agonists with comparable binding characteristics (Ginj, M.; Zhang, H.; Waser, B.; Cescato, R.; Wild, D.; Erchegyi, J.; Rivier, J.; Mäcke, H. R.; Reubi, J. C. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 16436-16441.). Because most of the neuroendocrine tumors express sst 2, we used the known antagonists acetyl- pNO 2Phe (2)- c[ dCys (3)-Tyr (7)- dTrp (8)-Lys (9)-Thr (10)-Cys (14)]- dTyr (15)-NH 2 ( 1) (Bass, R. T.; Buckwalter, B. L.; Patel, B. P.; Pausch, M. H.; Price, L. A.; Strnad, J.; Hadcock, J. R. Mol. Pharmacol. 1996, 50, 709-715. Bass, R. T.; Buckwalter, B. L.; Patel, B. P.; Pausch, M. H.; Price, L. A.; Strnad, J.; Hadcock, J. R. Mol. Pharmacol. 1997, 51, 170; Erratum.) and H-Cpa (2)- c[ dCys (3)-Tyr (7)- dTrp (8)-Lys (9)-Thr (10)-Cys (14)]-2Nal (15)-NH 2 ( 7) (Hocart, S. J.; Jain, R.; Murphy, W. A.; Taylor, J. E.; Coy, D. H. J. Med. Chem. 1999, 42, 1863-1871.) as leads for analogues with increased sst 2 binding affinity and selectivity. Among the 32 analogues reported here, DOTA- pNO 2Phe (2)- c[ dCys (3)-Tyr (7)- dAph (8)(Cbm)-Lys (9)-Thr (10)-Cys (14)- dTyr (15)-NH 2 ( 3) and DOTA-Cpa (2)- c[ dCys (3)-Aph (7)(Hor)- dAph (8)(Cbm)-Lys (9)-Thr (10)-Cys (14)]- dTyr (15)-NH 2 ( 31) had the highest sst 2 binding affinity and selectivity. All of the analogues tested kept their sst 2 antagonistic properties (i.e., did not affect calcium release in vitro and competitively antagonized the agonistic effect of [Tyr (3)]octreotide). Moreover, in an immunofluorescence-based internalization assay, the new analogues prevented sst 2 internalization induced by the sst 2 agonist [Tyr (3)]octreotide without being active by themselves. In conclusion, several analogues (in particular 3, 31, and 32) have outstanding sst 2 binding and functional antagonistic properties and, because of their DOTA moiety, are excellent candidates for in vivo targeting of sst 2-expressing cancers.