This is a study of PIPE-791, an investigational study drug to treat progressive multiple sclerosis (MS) and idiopathic pulmonary fibrosis (IPF). The purpose of this study is to find out how much of the study drug gets into the brain and lung, and what the side effects and blood levels of the study drug are in healthy volunteers and patients.

Participants will:

• Take a single dose of the study drug
• Give many samples of blood and urine
• Have multiple PET scans

Main Inclusion Criteria

• All parts: male volunteers or female volunteers of non-childbearing potential; agree to follow the contraception requirements of the trial, and able to give fully informed written consent.
• Part A: normotensive volunteers, deemed healthy on the basis of a clinical history, medical examinations, ECG, vital signs, and laboratory tests of blood and urine.
• Part B: volunteers with a diagnosis of PPMS or SPMS by a neurologist, according to the 2017 Revised McDonald Criteria.
• Part C: volunteers with a diagnosis of IPF by a pulmonologist, according to the 2018 American Thoracic Society (ATS)/European Respiratory Society (ERS)/Japanese Respiratory Society (JRS)/Latin American Thoracic Society (ALAT) Clinical Practice Guideline, within 7 years before screening.

Main Exclusion Criteria

• Positive tests for hepatitis B and C, human immunodeficiency virus (HIV)
• Severe adverse reaction to any drug; sensitivity to trial medication
• Drug or alcohol abuse
• Smoking or use of tobacco or nicotine-containing products from 7 days before screening, until the final visit
• Use of a prescription medicine (except hormone replacement therapy [HRT] in women, and medications for participants with PrMS and IPF at the discretion of the investigator), or any substance known to interact with cytochrome p450 (CYP)3A enzymes (including St. John's wort and foodstuffs such as grapefruit juice) during the 28 days before the first dose of PIPE-791
• Use of any other over-the-counter medicine, with the exception of acetaminophen (paracetamol) during the 7 days before the first dose of PIPE-791, or received vaccine against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), during the 7 days before screening
• Participation in other clinical trials of unlicensed medicines, or loss of more than 400 mL blood, within the 3 months before the baseline PET scan
• Vital signs or ECGs outside the acceptable range at screening
• Clinically relevant abnormal findings at the screening assessments, including C-SSRS and MRI (Parts A [brain PET imaging arm] and B only)
• Acute or chronic illness (except PrMS and IPF in Parts B and C, respectively)
• Clinically relevant abnormal medical history or concurrent medical condition
• Possibility that volunteer will not cooperate
• Contraindications to MRI (Parts A [brain PET imaging arm] and B only), computed tomography (CT), PET, or arterial cannulation procedures
• Significant exposure to research-related radiation or other radiation exposure (exceeding 10 mSv when added to exposure from this study) within the previous 12 months