3-part Study to Assess Safety, Tolerability, Pharmacokinetics and Pharmacodynamics of EXS21546

Inclusion Criteria: * BMI 18.0 to 30.0 kg/m2 * Weight ≥60 kg * Must adhere to contraception requirements Exclusion Criteria: * Subjects who have received any IMP in a clinical research study within the 90 days prior to Day 1 * Subjects who have previously been administered IMP in this study. * Evidence of current SARS-CoV-2 infection * History of any drug or alcohol abuse in the past 2 years * Regular alcohol consumption in males \>21 units per week * A confirmed positive alcohol breath test at screening or admission * Current smokers and those who have smoked within the last 12 months. A confirmed breath carbon monoxide reading of greater than 10 ppm at screening or admission * Current users of e-cigarettes and nicotine replacement products and those who have used these products within the last 12 months * Subjects with pregnant or lactating partners * Positive hepatitis B surface antigen (HBsAg), hepatitis C virus antibody (HCV Ab) or human immunodeficiency virus (HIV) antibody results * Regimen L only: History of sleep apnea * Subjects with a history of cholecystectomy or gall stones * Serious adverse reaction or serious hypersensitivity to any drug or the formulation excipients * Presence or history of clinically significant allergy requiring treatment, as judged by the investigator. Hay fever is allowed unless it is active * Donation of blood or plasma within the previous 3 months or loss of greater than 400 mL of blood