Safety, Pharmacokinetics, and Dosimetry of a Long-Acting Radiolabeled Somatostatin Analog 177Lu-DOTA-EB-TATE in Patients with Advanced Metastatic Neuroendocrine Tumors

The theranostic promise for Neuroendocrine Tumors in the late 2010s - Where do we stand, where do we go?

More than 25 years after the first peptide receptor radionuclide therapy (PRRT), the concept of somatostatin receptor (SSTR)-directed imaging and therapy for neuroendocrine tumors (NET) is seeing rapidly increasing use. To maximize the full potential of its theranostic promise, efforts in recent years have expanded recommendations in current guidelines and included the evaluation of novel theranostic radiotracers for imaging and treatment of NET. Moreover, the introduction of standardized reporting framework systems may harmonize PET reading, address pitfalls in interpreting SSTR-PET/CT scans and guide the treating physician in selecting PRRT candidates. Notably, the concept of PRRT has also been applied beyond oncology, e.g. for treatment of inflammatory conditions like sarcoidosis. Future perspectives may include the efficacy evaluation of PRRT compared to other common treatment options for NET, novel strategies for closer monitoring of potential side effects, the introduction of novel radiotracers with beneficial pharmacodynamic and kinetic properties or the use of supervised machine learning approaches for outcome prediction. This article reviews how the SSTR-directed theranostic concept is currently applied and also reflects on recent developments that hold promise for the future of theranostics in this context.

Somatostatin receptor PET ligands - the next generation for clinical practice

Somatostatin receptors (SSTRs) are variably expressed by a variety of malignancies. Using radiolabeled somatostatin analogs (SSAs), the presence of SSTRs on tumor cells may be exploited for molecular imaging and for peptide receptor radionuclide therapy. ¹¹¹In-DTPA-octreotide has long been the standard in SSTR scintigraphy. A major leap forward was the introduction of gallium-68 labeled SSAs for positron emission tomography (PET) offering improved sensitivity. Tracers currently in clinical use are ⁶⁸Ga-DOTA-Tyr³-octreotide (⁶⁸Ga-DOTATOC), ⁶⁸Ga-DOTA-Tyr³-octreotate (⁶⁸Ga-DOTATATE) and ⁶⁸Ga-DOTA-1-NaI³-octreotide (⁶⁸Ga-DOTANOC), collectively referred to as ⁶⁸Ga-DOTA-peptides. ⁶⁸Ga-DOTA-peptide PET has superseded ¹¹¹In-DTPA-octreotide scintigraphy as the modality of choice for SSTR imaging. However, implementation of ⁶⁸Ga-DOTA-peptides in routine clinical practice is often limited by practical, economical and regulatory factors related to the use of the current generation of ⁶⁸Ge/⁶⁸Ga generators. Centralized production and distribution is challenging due to the low production yield and relatively short half-life of gallium-68. Furthermore, gallium-68 has a relatively long positron range, compromising spatial resolution on modern PET cameras. Therefore, possibilities of using other PET radionuclides are being explored. On the other hand, new developments in SSTR PET ligands are strongly driven by the need for improved lesion targeting, especially for tumors with low SSTR expression. This may be achieved by using peptide vectors having a higher affinity for the SSTR or a broader affinity profile for the different receptor subtypes or by using compounds recognizing more binding sites, such as SSTR antagonists. This review gives an overview of recent developments leading to the next generation of clinical PET tracers for SSTR imaging.

Peptide Receptor Radionuclide Therapy in Grade 3 Neuroendocrine Neoplasms: Safety and Survival Analysis in 69 Patients

To date, limited data are available concerning peptide receptor radionuclide therapy (PRRT) of grade 3 (G3) neuroendocrine neoplasms (NENs) with a Ki-67 proliferation index of greater than 20%. The purpose of this study was to analyze the long-term outcome, efficacy, and safety of PRRT in patients with somatostatin receptor (SSTR)-expressing G3 NENs. Methods: A total of 69 patients (41 men; age, 28-81 y) received PRRT with ¹⁷⁷Lu- or ⁹⁰Y-labeled somatostatin analogs (DOTATATE or DOTATOC). Twenty-two patients had radiosensitizing chemotherapy. Kaplan-Meier analysis was performed to calculate progression-free survival (PFS) and overall survival (OS), defined from the start of PRRT, including a subgroup analysis for patients with a Ki-67 index of less than or equal to 55% and a Ki-67 index of greater than 55%. Treatment response was evaluated according to RECIST 1.1 as well as molecular imaging criteria (European Organization for Research and Treatment of Cancer). Short- and long-term toxicity was documented (Common Terminology Criteria for Adverse Events, v 5.0) using a structured database (comprising >250 items per patient) and retrospectively analyzed. Results: Forty-six patients had pancreatic NENs, 11 had unknown primary cancer, 6 had midgut NENs, 3 had gastric NENs, and 3 had rectal NENs. The median follow-up was 94.3 mo. The median PFS was 9.6 mo, and the median OS was 19.9 mo. For G3 NENs with a Ki-67 index of less than or equal to 55% (n = 53), the median PFS was 11 mo and the median OS was 22 mo. Patients with a Ki-67 index of greater than 55% (n = 11) had a median PFS of 4 mo and a median OS of 7 mo. For patients with positive SSTR imaging but no ¹⁸F-FDG uptake, the median PFS was 24 mo and the median OS was 42 mo. A significant difference was found for both PFS and OS, with median PFS of 16 mo and 5 mo and median OS of 27 mo and 9 mo for an SUV_max of greater than 15.0 and an SUV_max of less than or equal to 15.0, respectively, on SSTR PET. In the group with ¹⁸F-FDG uptake scored as 3 or 4, the median PFS was 7.1 mo and the median OS was 17.2 mo. In the group with ¹⁸F-FDG uptake scored as 0-2, the median PFS was 24.3 mo and the median OS was 41.6 mo. PRRT was well tolerated by all patients; no grade 3 or grade 4 hematotoxicity occurred, and no clinically significant decline in renal function was observed. There was no hepatotoxicity. Conclusion: PRRT was tolerated well, without significant adverse effects, and was efficacious in G3 NENs; the clinical outcome was promising, especially in patients with a Ki-67 index of less than or equal to 55% and even in patients for whom chemotherapy had failed. Baseline ¹⁸F-FDG along with SSTR molecular imaging was useful for stratifying G3 NEN patients with high uptake on SSTR PET/CT and no or minor ¹⁸F-FDG avidity-a mismatch pattern that was associated with a better long-term prognosis.

Novel Structural Modification Based on Evans Blue Dye to Improve Pharmacokinetics of a Somastostatin-Receptor-Based Theranostic Agent

The development of somastatin (SS) peptide analogues for the detection and treatment of neuroendocrine tumors has been successful with the recent FDA approval of ⁶⁸Ga-DOTA-TATE and ¹⁷⁷Lu-DOTA-TATE. The structure of these peptide constructs contains the peptide binding motif that binds to the receptor with high affinity, a chelator to complex the radioactive metal, and a linker between the peptide and chelator. However, these constructs suffer from rapid blood clearance, which limits their tumor uptake. In this study, this design has been further improved by incorporating a modification to control the in vivo pharmacokinetics. Adding a truncated Evans Blue (EB) dye molecule into the construct provides a prolonged half-life in blood as a result of its low micromolar affinity to albumin. We compared ¹⁷⁷Lu-DOTA-TATE to the modified ¹⁷⁷Lu Evans Blue compound (¹⁷⁷Lu-DMEB-TATE), in vitro and in vivo in mice bearing A427-7 xenografts. The tumor uptake of ¹⁷⁷Lu-DMEB-TATE was significantly greater than the uptake of ¹⁷⁷Lu-DOTA-TATE in the biodistribution and SPECT-imaging studies. The therapeutic effect of the ¹⁷⁷Lu-DMEB-TATE construct was superior to the that of the ¹⁷⁷Lu-DOTA-TATE construct at the doses evaluated.

Radioligand Therapy of Prostate Cancer with a Long-Lasting Prostate-Specific Membrane Antigen Targeting Agent 90Y-DOTA-EB-MCG

Several radioligands targeting prostate-specific membrane antigen (PSMA) have been clinically introduced as a new class of radiotheranostics for the treatment of prostate cancer. Among them, ((( R)-1-carboxy-2-mcercaptoethyl)carbamoyl)-l-glutamic acid (MCG) has been successfully labeled with radioisotopes for prostate cancer imaging. The aim of this study is to conjugate MCG with an albumin binding moiety to further improve the in vivo pharmacokinetics. MCG was conjugated with an Evans blue (EB) derivative for albumin binding and a DOTA chelator. PSMA positive (PC3-PIP) and PSMA negative (PC3) cells were used for both in vitro and in vivo studies. Longitudinal PET imaging was performed at 1, 4, 24, and 48 h post-injection to evaluate the biodistribution and tumor uptake of ⁸⁶Y-DOTA-EB-MCG. DOTA-EB-MCG was also labeled with ⁹⁰Y for radionuclide therapy. Besides tumor growth measurement, tumor response to escalating therapeutic doses were also evaluated by immunohistochemistry and fluorescence microscopy. Based on quantification from ⁸⁶Y-DOTA-EB-MCG PET images, the tracer uptake in PC3-PIP tumors increased from 22.33 ± 2.39%ID/g at 1 h post-injection (p.i.), to the peak of 40.40 ± 4.79%ID/g at 24 h p.i. Administration of 7.4 MBq of ⁹⁰Y-DOTA-EB-MCG resulted in significant regression of tumor growth in PSMA positive xenografts. No apparent toxicity or body weight loss was observed in all treated mice. Modification of MCG with an Evans blue derivative resulted in a highly efficient prostate cancer targeting agent (EB-MCG), which showed great potential in prostate cancer treatment after being labeled with therapeutic radioisotopes.

Response to Single Low-dose 177Lu-DOTA-EB-TATE Treatment in Patients with Advanced Neuroendocrine Neoplasm: A Prospective Pilot Study

Objective:¹⁷⁷Lu-DOTA-EB-TATE is a theranostic agent based on octreotate that uses an Evans blue structure to bind albumin to improve the pharmacokinetics and pharmacodynamics. This pilot study aims to evaluate the efficacy of a single low-dose treatment using ¹⁷⁷Lu-DOTA-EB-TATE in patients with advanced neuroendocrine neoplasm (NEN).

Methods: With IRB approval and informed consent, 4 NEN patients were enrolled to undergo ¹⁷⁷Lu-DOTA-EB-TATE treatment with a single low dose of 0.66 ± 0.06 GBq (17.8 ± 1.7 mCi); 3 other NEN patients were enrolled as controls to undergo ¹⁷⁷Lu-DOTA-TATE treatment with administered activity of 3.98 ± 0.17 GBq (107.6 ± 4.6 mCi). One primary tumor and 62 metastatic lesions in the 7 patients were evaluated by ⁶⁸Ga-DOTA-TATE PET/CT immediately before and one or three months after the treatment. Maximum SUV (SUV_max) of the tumors ≥2.0 cm in diameter were measured and percentage of change (ΔSUV) after treatment were calculated.

Results: All 4 patients subjected to ¹⁷⁷Lu-DOTA-EB-TATE treatment tolerated the administered activity without significant adverse effects and showed symptomatic remission. Among the patients, 40 tumors were found with diameter ≥2.0 cm, with the baseline SUV_max varied from 1.5-82.9 (35.9 ± 21.0) and the ΔSUVs before and three months after the treatment from -75.1-26.3% (-38.9 ± 25.5%). Twenty-nine (72.5%) of the tumors showed >15% decrease of SUV_max (ΔSUV = -75.1%--17.1%). There was a significant negative correlation between the baseline SUV_max and the ΔSUV after treatment (r = -0.852, P < 0.001). Compared with the control ¹⁷⁷Lu-DOTA-TATE therapy, the ¹⁷⁷Lu-DOTA-EB-TATE treatment using approximately 1/6 the dose showed no significant difference in ΔSUV (-7.9 ± 5.4% vs. -5.8 ± 3.9%, P = 0.189) as demonstrated by the tumors with comparable baseline SUV_max from 10.0-35.0.

Conclusion: A single low-dose ¹⁷⁷Lu-DOTA-EB-TATE treatment appears to be safe and effective in the treatment of NENs with high ⁶⁸Ga-DOTA-TATE uptake. This pilot study merits further investigation with increased dose and frequency of ¹⁷⁷Lu-DOTA-EB-TATE administration with potential advantages over ¹⁷⁷Lu-DOTA-TATE.

The clinical value of scintigraphy of neuroendocrine tumors using (99m)Tc-HYNIC-TOC

PURPOSE

To assess the value of whole body scintigraphy using (99m)Tc-HYNIC-TOC (Tektrotyd) and with single photon emission computerized tomography (SPECT) in the detection of primary and metastatic neuroendocrine tumors (NETs).

METHODS

Thirty patients with different neuroendocrine tumors, mainly gastroenteropancreatic (GEP), were investigated. Whole body scintigraphy was performed 2 h (if necessary 10 min and 24h) after i.v. administration of 740 Mbq (99m)Tc-Tektrotyd, Polatom. In cases of unclear findings obtained by whole body scintigraphy, investigation was followed by SPECT.

RESULTS

From 12 patients with NETs of unknown origin, there were 10 true positive (TP), and 2 false negative (FN) findings. Diagnosis was made with SPECT in 6 patients. From 8 patients with gut carcinoids, there were 4 TP, 2 true negative (TN), one FN, and one false positive (FP) finding. Diagnosis was made with SPECT in 2 patients. From 7 patients with neuroendocrine pancreatic carcinomas there were 4 TP and 3 TN findings. Diagnosis was made with SPECT in 2 patients. From 3 patients with gastrinomas there were 2 TP findings and one TN findings. Diagnosis was made with SPECT findings in 2 patients. Sensitivity of (99m)Tc-HYNIC-TOC was 87%, specificity 86%, positive predictive value 95%, negative predictive value 67% and accuracy 87%.

CONCLUSION

We concluded that scintigraphy with (99m)Tc-Tektrotyd is an useful method for diagnosis, staging and follow up of the patients with NETs.