A Two Part Study to Assess Safety, PK, PD, and Food Effect of Oral HTL0016878

Inclusion Criteria: * Normotensive 18-55 year old (Parts 1a, 1b and 2a only) or 65+ year old (Parts 1c and 2b) male (all parts) or female (parts 1b, 2a and 2b only) volunteers with a body mass index 18-32kg/m². * Healthy on the basis of a clinical history, physical examination, electrocardiogram (ECG), vital signs, heart rate (part 1a only), exercise history (part 1a only), and laboratory tests of blood and urine. * Willingness to comply with requirements or the trial, including contraception requirements. * Able to give fully informed consent. Exclusion Criteria: * Positive tests for hepatitis B \& C, HIV * severe adverse reaction to any drug * sensitivity to trial medication * drug or alcohol abuse * smoking * use of medication that inhibits CYP2D6 within previous 21 days or other prescribed and over-the-counter medication and herbal remedies within previous 21 days before dosing (with the exception of acetaminophen, contraceptive medications and hormone replacement therapy), unless the principal investigator (PI) considers that it would not interfere with trial * participation in other clinical trials of unlicensed medicines in the previous 3 months, or regularly take part in more than 4 studies a year * loss of more than 500 mL blood in the previous 3 months * vital signs, QTcF interval or laboratory values outside the acceptable range * poor metabolizers of CYP2D6 (apart from one optional cohort in Part 1a, which may enrol poor metabolizers only) * clinically relevant abnormal findings at the screening assessment * acute or chronic illness * history of epilepsy or seizures * clinically relevant abnormal medical history or concurrent medical condition * disease associated with cognitive impairment and/or psychosis * recent history of suicidal thoughts or ideation, or insomnia * excessive use of caffeine containing beverages, exceeding 8 cups of coffee or equivalent/day and the inability to refrain from the use of caffeine containing beverages whilst on the ward * consumption of cranberry, pomegranate, star fruit, grapefruit, pomelos, exotic citrus fruits or Seville oranges (including marmalade and juices made from these fruits) within 3 days before admission; possibility that volunteer will not cooperate * pre-menopausal females who are pregnant or lactating, or who are sexually active and not using a reliable method of contraception * Objection by the GP.