A Long-Term Safety Study of Elismetrep (K-304) in the Acute Treatment of Migraine

Inclusion Criteria: 1. Participant completing an acute treatment trial of elismetrep (Study K-304 P003; attending the EoS Visit 3 within the allowed visit window of the acute treatment study), treated a qualifying migraine in that study, was at least 80% compliant with the daily e-Diary 'check-ins' in the base study and completed the 2-hour and 4-hour post-dose assessments in the eDiary. Note: If a subject treats their qualifying migraine in the base study within 5 days of randomization (i.e. ≤ 4 daily check-ins should have been completed in the eDiary), they are eligible for this study if they missed 1 daily check-in, even though compliance was \<80%; 2. Participant voluntarily agrees to participate in the study by giving written informed consent; 3. Participant is able to read, understand, and complete the study questionnaires and diary; 4. Participant is willing and able to comply with the study schedule of visits, all trial procedures and restrictions; 5. Participant is willing to use his/her own personal, qualified smartphone to download any specific eDiary applications for use for the entire duration of the study; 6. For females of reproductive potential (as determined in the base study), continues to agree to remain abstinent from heterosexual activity or a. Agrees to use (or have their partner use) a birth control method that is acceptable from the first dose of study drug until the EoS visit. Acceptable methods of birth control are: i. Combined (estrogen and progestogen containing) hormonal contraception associated with inhibition of ovulation: oral, intravaginal, transdermal. ii. Progestogen-only hormonal contraception associated with inhibition of ovulation: oral; injectable; implantable; intrauterine device; intrauterine hormone-releasing system; bilateral tubal occlusion; vasectomized partner; sexual abstinence\*; progestogen-only oral hormonal contraception, where inhibition of ovulation is not the primary mode of action; male or female condom with or without spermicide; cap, diaphragm or sponge with spermicide; a combination of male condom with either cap, diaphragm or sponge with spermicide (double barrier methods). * Abstinence can be used as the sole method of contraception if it is in line with the participant's preferred and usual lifestyle and if considered acceptable by local regulatory agencies and ethics committees. Periodic abstinence (e.g., calendar, ovulation, sympto-thermal, post-ovulation methods, etc.) and withdrawal are not acceptable methods of contraception. Exclusion Criteria: 1. Is a female who is pregnant, breast-feeding, or intends to become pregnant during the planned course of the study. Note: Participants must have a negative urine pregnancy result at the base study EoS/Visit 3 (i.e., pre-randomization for this study); 2. Has, at the time of the base study EoS/Visit 3, had any change in medical history or a laboratory finding since screening in the acute treatment study that in the opinion of the investigator renders the participant unsuitable for this study (e.g., development of a finding that would have precluded participation in the base acute treatment study); 3. Participant is at imminent risk of self-harm, based on clinical interview and responses on the Columbia Suicidality Severity Rating Scale (C-SSRS) taken during the base study E0S/Visit 3, or of harm to others in the opinion of the investigator. Participants must be excluded if they report suicidal ideation with intent, with or without a plan (i.e., Type 4 or 5 on the C-SSRS) in 'since last version' version of the C-SSRS; 4. Participant is currently in violation of study requirements for prohibited and permissible concomitant medications or treatments (not already specified in other criteria) or is anticipated to violate these requirements during trial participation) as assessed for the base study E0S/Visit 3; 5. Has any major surgery scheduled for the duration of the trial; 6. Has any other unwillingness or inability, not covered by any of the other exclusion criteria, which in the investigator's opinion, might jeopardize the participant's safety or compliance with the protocol, or otherwise interfere with the interpretation of efficacy and/or safety results; 7. At Visit 1, has an open/ongoing adverse event from the base study EoS visit; 8. Experienced a serious adverse event in the base study; 9. Has previous participation in this trial. Participation is defined as signed informed consent following completion of the base study EoS/Visit 3 procedures.