LM3 is a novel somatostatin receptor antagonist, while Gallium-68 DOTATATE is a typical somatostatin receptor agonist, This study is to evaluate the safety, biodistribution, dosimetry, and lesion detection ability of Gallium-68 labeled somatostatin receptor antagonist LM3 for the diagnostic imaging of metastatic, well-differentiated neuroendocrine tumors using positron emission tomography / computed tomography (PET/CT).

The results will be compared between antagonist Gallium-68 labeled LM3 and agonist Gallium-labeled DOTATATE in the same group of patients.

It will also be compared between the two different antagonists, Gallium-68 DOTA-LM3 and Gallium-68 NODAGA-LM3, in two parallel-designed arms.

Patients with histologically confirmed metastatic, well-differentiated neuroendocrine tumors will be recruited in this study.

All patients will be randomized into two groups: Gallium-68 NODAGA-LM3 group and Gallium-68 DOTA-LM3 group.

The study will be divided into the following 2 parts:

Part ONE, which will enroll 16 patients (8 in each group), focuses on the safety evaluation, biodistribution, and dosimetry. In Part A, patients will undergo serial whole-body PET/CT scans at multiple time points (5m, 10m, 20m, 40m, 1h, 2h) after administering 40ug/150-200MBq Gallium-68 NODAGA-LM3 or Gallium-68 DOTA-LM3 (according to their group).

Part TWO, which will enroll 24 patients (12 in each group) and follows Part A study, focuses on lesion detection ability. In Part B, patients will undergo one whole-body PET/CT scan at 1 hour after administering 40ug/150-200MBq Gallium-68 NODAGA-LM3 or Gallium-68 DOTA-LM3 (according to their group).

All patients need to do a Gallium-68 DOTATATE PET/CT scan (40ug/150-200MBq, 1h post-injection) for comparison on the next day.

Inclusion Criteria:

• Written informed consent.
• Patients of either gender, aged ≥ 18 years.
• Histologically confirmed diagnosis of Metastatic, well-differentiated neuroendocrine tumor.
• A diagnostic computed tomography (CT) or magnetic resonance imaging (MRI) of the tumor region within the previous 6 months prior to dosing day is available.
• At least 1 measurable lesion based on RECIST v1.1.
• Blood test results as follows (White blood cell: ≥ 3*10^9/L, Hemoglobin: ≥ 8.0 g/dL, Platelets: ≥ 50x10^9/L, Alanine aminotransferase / Aspartate aminotransferase / Alkaline phosphatase: ≤ 5 times upper limit od normal (ULN), Bilirubin: ≤ 3 times ULN)
• Serum creatinine: within normal limits or < 120 μmol/L for patients aged 60 years or older.
• Calculated Glomerular filtration rate (GFR) ≥ 45 mL/min.

Exclusion Criteria:

• Known hypersensitivity to Gallium-68, to NODAGA, to DOTA, to LM3, to TATE or to any of the excipients of Gallium-68 DOTA-LM3, Gallium-68 NODAGA-LM3 or Gallium-68 DOTATATE.
• Presence of active infection at screening or history of serious infection within the previous 6 weeks.
• Therapeutic use of any somatostatin analog, including long-acting Sandostatin (within 28 days) and short-acting Sandostatin (within 2 days) prior to study imaging. If a patient is on long-acting Sandostatina, then a wash-out phase of 28 days is required before the injection of the study drug. If a patient is on short-acting Sandostatin, then a wash-out phase of 2 days is required before the injection of the study drug.
• Any neuroendocrine tumor-specific treatment between antagonist and agonist scans.
• Prior or planned administration of a radiopharmaceutical within 8 half-lives of the radionuclide used on such radiopharmaceutical including at any time during the current study.
• Pregnant or breast-feeding women.
• Current history of any malignancy other than neuroendocrine tumor; patients with a secondary tumor in remission of > 5 years can be included.
• Any mental condition rendering the patient unable to understand the nature, scope and possible consequences of the study, and/or evidence of an uncooperative attitude.

The details of patient groups will be sealed in sequentially numbered, opaque, sealed envelopes generated by Xuezhu Wang from the nuclear medicine department, Peking Union Medical College Hospital, who will not participate in other parts of the study.