The purpose of this study is to evaluate the effects of multiple dose regimens of relamorelin on vomiting episodes, gastric emptying and gastroparesis symptoms in participants with Type 1 and Type 2 diabetes mellitus and gastroparesis. Study drug (relamorelin and placebo) will be administered subcutaneously in a blinded fashion.

Inclusion Criteria:

• Type 1 diabetes mellitus (T1DM) or Type 2 diabetes mellitus (T2DM) with stable glycemic control and Hemoglobin A1c (HbA1c) ≤11% at screening.
• Diabetic gastroparesis (DG), defined as at least a 3-month history of symptoms suggestive of gastroparesis on an ongoing basis (e.g., vomiting, nausea, early satiety, bloating, or epigastric or abdominal pain).
• Gastroparesis Cardinal Symptom Index Daily Diary (GCSI-DD) score ≥2.6 at least once during the Screening Period (Visits 1-2).
• At least 2 vomiting episodes during the ~2 weeks prior to the first screening visit (Visit 1), as ascertained by patient history.
• Delayed Gastric Emptying (GE) confirmed at screening by abnormal Gastric Emptying Breath Test (GEBT), defined as GE half-time (t1/2) ≥79 minutes (the 80th percentile of normative data). At least 50% of patients enrolled will have a t1/2 ≥97 minutes (i.e., the 95th percentile).
• Stable concomitant medications, defined as no changes in regimen for at least 2 weeks prior to Visit 2 (daily adjustments of insulin doses are permitted).
• No use of metoclopramide, erythromycin, domperidone, or other gastrointestinal (GI) motility agents, or anti-emetics for at least 2 weeks prior to Visit 2, and willingness to remain off these medications (except as used as part of protocol-specific rescue medication) during the course of the clinical trial.
• Body mass index >18 kg/m2.
• If female, has a negative serum or urine pregnancy test and is not lactating. For females able to bear children, a hormonal (i.e., oral, implantable, or injectable) and single-barrier method, or a double-barrier method of birth control must be used throughout the study. Female patients unable to bear children must have this documented in the electronic case report form (eCRF) (i.e., tubal ligation, hysterectomy, or post-menopausal [defined as a minimum of 1 year since the last menstrual period]). Post-menopausal status will be confirmed by measurement of follicle stimulating hormone (FSH).
• Able to provide written informed consent prior to any study procedures and willing and able to comply with study procedures.

Additional inclusion criteria for randomization after the 2-week single-blind placebo run-in period:

• Compliance with the completion of the Diabetic Gastroparesis Symptom Severity Diary (DGSSD) and study drug injections, defined as approximately 80% diary completions and approximately 80% administration of injections, during the 2-week single-blind placebo run-in period. For those patients whose compliance is measured to be <80%, the final decision to randomize a patient will be made by the Investigator and the Sponsor (or designee).
• At least one vomiting episode at any time during the 2-week single-blind placebo run-in period, as recorded in the DGSSD.

Exclusion Criteria:

• Currently receiving parenteral feeding or presence of a nasogastric or other enteral tube [e.g., Percutaneous Endoscopic Gastrostomy (PEG) tube] for feeding or decompression.
• History of gastric surgery such as fundoplication, gastrectomy, gastric pacemaker placement, vagotomy, or bariatric procedure. (A history of diagnostic endoscopy is not exclusionary.)
• History of pyloric injection of botulinum toxin within 6 months of screening.
• Patients with clinical suspicion of upper GI obstruction (e.g., peptic stricture) must have been evaluated per standard of care and obstruction ruled out before screening.
• Currently taking opiates, or expecting to use opiates during the course of the clinical trial.
• Currently taking Glucagon-like peptide-1 (GLP-1) agonists, Sodium-glucose co-transporter 2 (SGLT2) inhibitors or pramlintide.
• Allergic or intolerant of egg, wheat, milk, or algae, as these are components of the Gastric emptying breath test (GEBT) study meal. (Gluten-free crackers can be provided.)
• History of anorexia nervosa, binge-eating, or bulimia within 5 years of screening.
• Alanine aminotransferase (ALT) or aspartate aminotransferase (AST) > 2 × upper limit of normal (ULN) at Visit 1.
• History of intestinal malabsorption or pancreatic exocrine disease.
• Requires hemodialysis or has end-stage renal disease.
• History of human immunodeficiency virus (HIV) infection.
• Clinically significant neurologic or psychiatric disorders that are likely to impact compliance with protocol requirements.
• Poor venous access or inability to tolerate venipuncture.
• Participation in a clinical study within the 30 days prior to dosing in the present study.
• Any other reason that, in the Investigator's opinion, would confound proper interpretation of the study or expose a patient to unacceptable risk, including renal, hepatic or cardiopulmonary disease, or significant acute electrocardiogram (ECG) abnormalities.