A Study to Assess the Pharmacokinetics, Pharmacodynamics, Safety, and Tolerability of Debio 4126 in Participants With Acromegaly or Functioning Gastroenteropancreatic Neuroendocrine Tumors (GEP-NETs)

Main Inclusion Criteria:

For Participants with Acromegaly:

• Treatment with octreotide LAR (≤30 mg dose once in 4 weeks [Q4W] IM) or lanreotideATG (≤120 mg Q4W or 120 mg once in 6 weeks [Q6W] to once in 8 weeks [Q8W] as deep SCinjection) for at least 6 months overall, and for at least 2 months at a stable doseas monotherapy for acromegaly treatment prior to entering Run-in (Day -28).Octreotide doses of 10, 20, and 30 mg are considered similar to lanreotide doses of 60, 90, and 120 mg. Thus, a switch between similar doses of the two products will beconsidered as the patient remaining on a stable dose, unless due to efficacy orsafety

• Diagnosis of acromegaly by historical evidence of (persistent or recurrent)acromegaly will be carried out

• IGF-1 ≤1.3 x upper limit of normal (ULN) assessed centrally at screening

For Participants with GEP-NETs:

• Treatment with octreotide LAR (≤ 30 mg dose Q4W IM) or lanreotide ATG (≤ 120 mg Q4Wor 120 mg Q6W to Q8W as deep SC injection) for at least 6 months overall, and for atleast 2 months at a stable dose as monotherapy for study disease treatment prior toentering Run-in (Day -28). Octreotide doses of 10, 20, and 30 mg are consideredsimilar to lanreotide doses of 60, 90, and 120 mg. Thus, a switch between similardoses of the two products will be considered as the participant remaining on astable dose, unless due to efficacy or safety

• Participants with functioning, well-differentiated (Grade 1 or Grade 2) GEP-NET withsymptoms of carcinoid syndrome which are controlled by Sandostatin LAR, SomatulineATG, or equivalent medications; sporadic use of rescue medication for symptomcontrol, e.g., bowel movements and/or flushing, is allowed

Main Exclusion Criteria:

For Participants with Acromegaly and GEP-NETs:

• Known ongoing gallbladder or bile duct disease or acute or chronic pancreatitis

• Hypothyroidism not adequately treated with thyroid hormone replacement therapy

• Diabetic participants whose blood glucose is poorly controlled despite adequatetherapy, as evidenced by glycated hemoglobin (HbA1c) >8.0% at screening

• Cardiology:

1. Known left ventricular ejection fraction <50%, left ventricular hypertrophy,ventricular arrhythmias, bradycardia (heart rate <50 beats per minute [bpm]),cardiomyopathy

2. New York Heart Association Class ≥3 heart failure

3. Congenital long QT syndrome or

4. Known family history of long QT syndrome or sudden cardiac death before the ageof 50

5. Symptomatic Pulmonary embolism

6. QT interval corrected for heart rate according to Fridericia's formula (QTcF)at screening >450 milliseconds (msec) for males and >470 msec for females,based on the average of a triplicate ECG

For Participants with Acromegaly:

• Participants who received pituitary irradiation <2 years prior to enrollment asstereotactic radiotherapy or <3 years prior to enrollment for conventionalradiotherapy

• Participants who received medical treatment with pasireotide (within 6 months priorto screening), pegvisomant (within 3 months prior to screening), dopamine agonists (within 3 months prior to screening)

• Participants who have undergone pituitary surgery within 6 months prior to screening

For Participants with GEP-NETs:

• Participants with short-bowel syndrome

• Participants with poorly differentiated neuroendocrine carcinoma and/or high-gradeneuroendocrine carcinoma

• Participants who have received any previous therapy with interferons, targetedtherapies (e.g., everolimus, sunitinib, bevacizumab), chemotherapy or otheranti-neoplastic systemic therapies administered for more than 1 month and within 12weeks prior to the start of the Run-in period

• Participants having history of hepatic embolization, hepatic arterialchemoembolization, and/or selective internal radiation (SIR) therapy within lessthan 6 months prior to screening

• Participants who have received Peptide receptor radionuclide therapy (PRRT) therapyduring the last 12 months prior to screening

[Note: Other inclusion/exclusion criteria mentioned in the protocol may apply.]