Amendment 1.1 - Applicable for Germany only, incorporating changes requested by German regulatory authorities. All changes made were incorporated into Amendment 2 for all countries.

Amendment 2 Schedule of Events and Assessments: Clarified the window for randomization as 28 to 30 days after screening and clarified the window for treatment as within 8 days after randomization. Schedule of Events and Assessments footers: Further clarified the process and requirement of review by the medical monitor prior to randomization, clarified the window within which dosing must occur due to drug not typically being onsite at time of randomization, updated the timeframe of postdose vital sign measurement from 4 hours postdose to 2 hours postdose to be consistent with a shorter postdose observation period, updated timeframe of postdose the ECG procedure from 4 hours postdose to 2 hours postdose to be consistent with shorter postdose observation period, and postdose observation period after dosing completion was shortened from 4 hours to 2 hours based on review of safety data. Section 8.2: Clarified that exclusion of temporomandibular disorders must be acute or active. Added clarification to exclusionary headache and migraine types (unusual migraine subtypes such as hemiplegic migraine [sporadic and familial], ophthalmoplegic migraine and migraine with neurological accompaniments that are not typical of migraine aura [eg, diplopia, altered consciousness, or long duration]). Section 8.4.1: Added that study treatment must be discontinued with pregnancy or suicidal ideation or behavior and specified the action to be taken for subjects discontinued due to suicidal ideation and/or suicidal behavior. Section 9.4: Added restriction for hormonal therapy during the study (must remain stable through Week 32). Section 10.2.4: Clarified the scope of the screening physical examination to be comprehensive and appropriate to determine the overall physical health of each subject and clarified that examination of the genitourinary system and rectum may be deferred by the investigator if the subject’s related medical history and review of systems are negative.

Amendment 3a Protocol Synopsis: Changes were made to update the following key secondary endpoints (50% migraine responder rate [Weeks 1-12], percentage of subjects with a migraine on the day after dosing, and reduction in migraine prevalence from baseline to Week 4) and other secondary endpoints (acute migraine medication usage and change in frequency of migraine days [Weeks 1-24]) to ensure consistency across the development program for ALD403 and to ensure endpoints that were important in understanding the efficacy of ALD403 were appropriately highlighted. Protocol Synopsis: A change was made to the sample size section to clarify that 350 subjects per group provides at least 90% power for the primary endpoint and not for the change from baseline tests. Protocol Synopsis: Changes were made to the statistical analysis section to ensure the text addressed the primary and additional key secondary endpoints. It was clarified that statistical inferential testing of the primary efficacy endpoint and key secondary endpoints will be performed while maintaining a study-wide type I error rate of 2-sided 5%, and not just a testing of the change from baseline in migraine days and responder rate. Section 5.3.2: Table 5.3 was updated to reflect the status of ALD403 clinical studies Section 5.4: Clarification was made to confirm the safety findings to date.