The main study objective is to assess the effect of ACT-709478 in male and female subjects with photosensitive epilepsy following single dose administration

Inclusion Criteria:

• Signed informed consent in the local language prior to any study-mandated procedure
• Male and female subjects aged between 18 and 60 years (inclusive) at screening
• Photosensitive epilepsy and a generalized PPR in response to IPS of at least 4 units on a SPR in at least 1 condition (eye closure, eyes closed, or eyes open) on 2 occasions at screening with at least 1 hour interval and reproducible on Day -1 (less than 3 units difference in SPR between screening and Day -1)
• Healthy on the basis of physical examination, cardiovascular assessments and laboratory tests
• Women of childbearing potential must have a negative serum pregnancy test at screening and a negative urine pregnancy test on Day -1. They must consistently and correctly use a reliable method of contraception with a failure rate of <1% per year, be sexually inactive, or have a vasectomized partner. Hormonal contraceptives must be initiated at least 1 month before first study treatment administration

Exclusion Criteria:

• Lactating women
• Known hypersensitivity to any of the excipients of the study treatment formulation
• History or clinical evidence of any disease other than epilepsy and/or existence of any surgical or medical condition, which might interfere with the absorption, distribution, metabolism, or excretion of the study treatment (appendectomy and herniotomy allowed, cholecystectomy not allowed)
• Relevant history of a suicide attempt or suicidal behavior. Any recent suicidal ideation within the last 6 months (categories 4 or 5), or any suicidal behavior within the last 2 years, except for "Non-Suicidal Self-Injurious Behavior" (item also included in the Suicidal Behavior section), as judged by the investigator using the Columbia-Suicide Severity Rating Scale (C-SSRS) at screening
• History of status epilepticus during the last 12 months
• History of non-epileptic seizures that cannot be differentiated from the participant's epileptic seizures
• History of generalized tonic-clonic seizures triggered by IPS
• Previous history of repeated fainting, syncope, orthostatic hypotension, or vasovagal reactions in the past 5 years
• Any circumstances or conditions, which, in the opinion of the investigator, may affect full participation in the study or compliance with the protocol