This is a first in human single and multiple ascending dose study with the objective to evaluate safety, tolerability, PK, PD, and food effect of HTL0016878 in healthy younger and elderly subjects. Part 1 will assess single doses of HTL0016878 and Part 2 will assess multiple doses of HTL0016878. Part 1 will be divided into 3 sub-parts: Part 1a will assess single ascending doses (SAD) of HTL0016878 in younger adult subjects, Part 1b will evaluate the effect of food on bioavailability of HTL0016878 and part 1c will investigate the effect of age on the PK of HTL0016878. Part 1c will only proceed after review of safety, tolerability and PK data from part 1a. Part 2 will be divided into 2 sub-parts to assess multiple ascending doses (MAD) of HTL0016878 in younger adult (part 2a) and elderly adult (part 2b) subjects. Part 2 will only proceed after review of safety, tolerability and PK data from Parts 1a, b and c.