This study is an evaluation of the safety and tolerability of XEN-D0103 in healthy subjects. Part 1 assesses the safety, tolerability and blood levels of the ascending single doses of XEN-D0103. Blood levels of XEN-D0103 will be assessed when administered with food and also without food in Part 2. The safety, tolerability and blood levels of XEN-D0103 will be assessed following dosing for 10 days in Part 3.

Inclusion Criteria:

• For Parts 1 and 3 healthy, male subjects aged 18 to 45 years, inclusive. For Part 2 healthy, male and female subjects aged 45 to 65 years, inclusive.
• Females must be of non-childbearing potential

Exclusion Criteria:

• Subject has a known heart disease.
• Subject has any of QTcF > 450 msec, PR ≥ 210 msec or QRS ≥ 120 msec at screening (determined from median of three readings).
• For Part 1 only: subject has resting heart rate outside of the range 45 to 80 beats per minute (bpm) at screening (determined from median of three readings).
• Subject has any other condition which, in the investigator's opinion will interfere with the trial.
• Subject has a systolic blood pressure (BP) <80 or >160 mmHg at screening or a diastolic BP >90 or <45 mmHg at screening.
• Subject has a clinically significant abnormal laboratory test result at screening.
• Female subject who is pregnant or breast feeding and female subjects of childbearing potential.