Patient description: The patient is a 57-year-old man with abdominal pain and vomiting. The pain started gradually about 20 hours ago in the epigastric and periumbilical regions, radiating to his back. He drinks around 60 units of alcohol per week and smokes 22 cigarettes per day. He is healthy with no history of allergies or using any medications. His family history is positive for type 2 diabetes (his father and sister). He lives alone and has no children. The abdomen is tender and soft. His bowel sounds are normal. His heart rate is 115/min and blood pressure 110/75 mmHg. The lab results are remarkable for leukocytosis (19.5), urea of 8.5, high CRP (145), high amylase (1200) and Glc level of 15. Cross-sectional imaging was negative for obstructive pancreatitis.

Clinical trial eligibility criteria: eligible ages (years): 18.0-65.0,  Postoperative Nausea and Vomiting Healthy subjects 2. Age 18 to 65 years of age at time of signing ICF 3. Body mass index (BMI) of 18 to 30 kg/m2 4. Must be willing and able to communicate and participate in the whole study 5. Must provide written informed consent 6. Must agree to use an adequate method of contraception  Subjects who have received any investigational medicinal product (IMP) in a clinical research study within the 3 months prior to IMP administration on this study 2. Subjects who are study site employees, or immediate family members of a study site or sponsor employee 3. Subjects who have previously been enrolled in this study 4. Women who are pregnant or breastfeeding 5. Subjects who have received amisulpride for any indication within the previous 4 weeks 6. Allergy to amisulpride or any of the excipients of APD421 or ondansetron 7. History of any drug or alcohol abuse in the past 2 years 8. Regular alcohol consumption >21 units per week 9. Current smokers and those who have smoked within the last 12 months; this includes cigarettes, e-cigarettes and nicotine replacement products (current smoking may be assessed by a validated technique such as urine or serum cotinine levels) 10. Subjects who do not have suitable veins for multiple venepunctures/cannulation as assessed by the investigator at screening 11. History of epilepsy 12. History of clinically significant syncope 13. Family history of sudden death 14. Family history of premature cardiovascular death 15. Clinically significant history or family history of congenital long QT syndrome (e.g. Romano-Ward syndrome, Jervell and Lange-Nielson syndrome) or Brugada's syndrome 16. History of clinically significant arrhythmias or ischaemic heart disease (especially ventricular arrhythmias, atrial fibrillation (AF), recent conversion from AF or coronary spasm) 17. Conditions predisposing the volunteer to electrolyte imbalances (e.g. altered nutritional states, chronic vomiting, anorexia nervosa, bulimia nervosa) 18. Any clinically important abnormalities in rhythm, conduction or morphology of resting ECG that may interfere with the interpretation of QTc interval changes. This includes subjects with any of the following at screening Absence of regular supraventricular rhythm Clinically significant PR (PQ) interval prolongation Intermittent second or third degree AV block Incomplete or complete bundle branch block Abnormal T-wave morphology Prolonged QTcB >450 ms or shortened QTcB < 350 ms or family history of long QT syndrome Subject with borderline deviations from these may be included if the deviations do not pose a safety risk, as judged by the investigator 19. Clinically significant abnormal biochemistry, haematology or urinalysis at screening as judged by the investigator, especially Creatinine clearance (estimated using Cockcroft-Gault formula) < 60 mL/min Alanine aminotransferase (ALT) > 1.5 x upper limit of normal or bilirubin > 3 x upper limit of normal 20. Positive drugs of abuse test result 21. Positive hepatitis B surface antigen (HBsAg), hepatitis C virus antibody (HCV Ab) or human immunodeficiency virus (HIV) results at screening 22. Donation or loss of greater than 100 mL of blood within the 3 months prior to screening or planned blood donation during the study until after final visit 23. Subjects who are taking, or have taken, any prescribed or over-the-counter drug (other than 4 g per day paracetamol) or herbal remedies in the 14 days before IMP administration 24. Failure to satisfy the investigator of fitness to participate for any other reason