Study to Evaluate the Efficacy and Safety of [177Lu]Lu-DOTA-TATE in Patients With Grade 1 and Grade 2 Advanced GEP-NET

Inclusion Criteria:

* Presence of metastasized or locally advanced, unresectable (curative intent), histologically proven, well differentiated Grade 1 or Grade 2 (Ki-67 \<10%) gastroenteropancreatic neuroendocrine tumor (GEP-NET) diagnosed within 6 months prior to screening.
* Participants with high disease burden in the Investigator's opinion. Following criteria should be used as the guiding principle for determining high disease burden:

  * Primary tumor or a metastatic lesion \> 4 cm
  * More than one tumor or metastatic lesions measuring \> 2 cm
  * Elevated alkaline phosphatase \> 2.5 X upper limit of normal (ULN)
  * Presence of bone metastasis
  * Presence of peritoneal metastasis
  * Symptoms due to tumor volume such as pain, fatigue, weight loss, anorexia etc.
  * Symptoms due to hormone excess requiring active management
  * Additionally, participants who, in the Investigator's opinion, have high disease burden due to their disease characteristics not specified above could also be considered eligible.
* Participants ≥ 12 years of age.
* RLI somatostatin receptor (SSTR) uptake on all target lesions (defined by RECIST v1.1 criteria) at least as high as normal liver uptake assessed within 3 months prior to randomization. Any of the RLI modalities as available (some examples are listed below) can be used as per local practice:

  * \[68Ga\]Ga-DOTA-TOC PET/CT or PET/MRI
  * \[68Ga\]Ga-DOTA-TATE PET/CT or PET/MRI
  * \[64Cu\]Cu-DOTA-TATE PET/CT or PET/MRI
  * Somatostatin receptor scintigraphy (SRS) (planar and/or SPECT/CT) with \[111In\]In-pentetreotide
  * SRS (planar and/or SPECT/CT) with \[99mTc\]Tc-octreotide.
* Adequate bone marrow and organ function as defined by the following laboratory values prior to receiving the first study treatment:

  * White blood cell (WBC) count ≥ 2 x 109/L
  * Platelet count ≥ 75 x 109/L
  * Hemoglobin (Hb) ≥ 8 g/dL
  * Creatinine clearance \> 40 mL/min calculated by the Cockcroft Gault method
  * Total bilirubin ≤ 3 x ULN
  * Potassium within normal limits. Potassium level of up to 6.0 millimoles per liter (mmol/L) is acceptable at study entry if associated with creatinine clearance within normal limits calculated using Cockcroft-Gault formula. Mild decrease (grade 1) below lower limit of normal (LLN) is acceptable at study entry if considered not clinically significant by Investigator.
* ECOG performance status 0-1.
* Presence of at least 1 measurable site of disease.

Exclusion Criteria:

* Prior administration of a therapeutic radiopharmaceutical for GEP-NET at any time prior to randomization in the study.
* Any previous therapy with interferons, mTOR-inhibitors, chemotherapy or other systemic therapies except somatostatin analogues (SSAs) of GEP-NET. If as per Investigator's opinion a participant is candidate for such therapies, such participant must not be enrolled.
* Participant who received more than 4 cycles of prior SSAs (e.g., octreotide long-acting release) are not eligible. In addition, any participant receiving treatment with short-acting octreotide, which cannot be interrupted for 24 h before the administration of \[177Lu\]Lu-DOTA-TATE, or any participant receiving treatment with SSAs, which cannot be interrupted for at least 4 weeks before the administration of \[177Lu\]Lu-DOTA-TATE.
* Documented RECIST v1.1 progression during previous SSA treatments for the current GEP-NET at any time prior to randomization.
* Any previous radioembolization, chemoembolization and radiofrequency ablation for GEP-NET.
* Any major surgery within 12 weeks prior to randomization in the study.
* Known brain metastases.
* Participant with known intolerance to CT scans with intravenous (i.v.) contrast due to allergic reaction or renal insufficiency. If such a participant can be imaged with MRI, then the participant would not be excluded.
* Hypersensitivity to any somatostatin analogues, to the Investigational Medicinal Products (IMPs) active substance or to any of the excipients.
* Active severe urinary incontinence, severe voiding dysfunction, or urinary obstruction requiring an indwelling/condom catheter that, in the judgment of the Investigator, could prevent adhering to radiation safety instructions.

Other protocol-defined Inclusion/Exclusion criteria may apply.