This is a multicenter, double-blind, placebo-controlled, parallel-group study consisting of a screening period of up to 4 weeks (with the exception of participants on primidone at baseline who will be allowed 6 weeks of screening to allow for safe discontinuation). Screening results from all patients meeting the eligibility requirements will be further assessed by the sponsor medical personnel for final approval of suitability for inclusion in the study. Randomized participants will enter a 4 week double-blind dose-titration treatment period, followed by a 1 week safety follow-up period following the last dose of study medication, and a scheduled follow-up safety telephone call one week later.~Participants will be randomized to one of two treatment groups. Group A will receive titrating doses of CX-8998 up to 10 mg BID and Group B will receive placebo. Participant randomization will be stratified by presence or absence of a single concomitant anti-tremor medication and by site-type (sub-study vs non sub-study).~Tremor will be assessed via The Essential Tremor Rating Assessment Scale (TETRAS) and accelerometry. To reduce the potential for bias in the assessments of efficacy, all participants will be videotaped during the TETRAS performance scale testing according to a consistent script. The videotapes will be rated in a blinded manner by qualified, independent raters. A subset of participants will participate in a digital biomarker sub-study.~Participants will be screened up to 4 weeks prior to initiation of dosing. Participants taking primidone at screening who are otherwise deemed eligible for participation and are willing to discontinue primidone will be allowed an additional 2 weeks of screening (a total of 6 weeks) to ensure safe primidone discontinuation. At Baseline, participants will undergo safety and tremor assessments prior to dosing, will receive their first dose of study drug and will be monitored for safety for one hour following dosing. For one week participants will receive 4 mg (or matching placebo) twice daily. Participants will return to the clinic on Day 8 for safety monitoring and dose up-titration to 8 mg (or matching placebo) twice daily. At Day 15 (Week 3) participants will return to clinic for safety and efficacy assessments and final dose up-titration to 10 mg (or matching placebo) twice daily. The final efficacy visit will occur at Day 28 (Week 4). A final safety visit will occur at Day 35 (Week 5). Should participants experience intolerable adverse events (AEs) at 4 mg BID, 8 mg BID or 10 mg BID, the dose may be decreased at Day 8 or Day 15 to the next lowest dose one time (or 2 mg BID in the case of the 4 mg BID dose.~Randomized participants will have the opportunity to participate in an optional exploratory sub-study designed to assess the utility of digital assessment tools for tremor (sub-study addendum #1). Participants with Parkinson's disease tremor will be enrolled in a second exploratory sub-study (addendum #2).
This is a multicenter, double-blind, placebo-controlled, parallel-group study consisting of a screening period of up to 4 weeks (with the exception of participants on primidone at baseline who will be allowed 6 weeks of screening to allow for safe discontinuation). Screening results from all patients meeting the eligibility requirements will be further assessed by the sponsor medical personnel for final approval of suitability for inclusion in the study. Randomized participants will enter a 4 week double-blind dose-titration treatment period, followed by a 1 week safety follow-up period following the last dose of study medication, and a scheduled follow-up safety telephone call one week later.