This is a Phase 3, double-blind, randomized, placebo-controlled trial of Reldesemtiv in patients aged 18 to 80 with ALS.

A drug named "Reldesemtiv" is being investigated as a potential new therapy for the improvement of skeletal muscle function in conditions associated with muscle weakness or fatigue such as Amyotrophic Lateral Sclerosis (“ALS”).

The main purpose of this research study is to evaluate the efficacy and safety of Reldesemtiv.

If you agree to participate, your participation in the study will be for about 52 weeks. There will be up to eight clinic visits, accompanied by a Forced Vital Capacity (FVC) pulmonary function test done at home.

Nine remote assessment visits with home laboratory/blood draw, most of which will also include a phone call/video appointment with clinic staff.

In-clinic visits can be converted to remote visits in the event the clinic is closed to patient care, or if the study doctor believes your health would be best served by replacing the in-clinic visit with a remote one.

Screening and Day 1 visits can only be done in the clinic and cannot be done remotely.

This study has two parts:
1. A double-blind, placebo-controlled part, where patients are randomized (2:1) to receive the following dose of Reldesemtiv or placebo (no drug) for the first 24 weeks (double-blind, placebo-controlled part):
• 300 mg Reldesemtiv twice a day for a 600 mg total daily dose (2 tablets twice a day)
• Placebo twice daily (2 tablets twice a day)

2. An active drug period, where at the end of the 24-week double-blind, placebo-controlled part, all study participants who completed the first 24-week period (this includes participants who were assigned either Reldesemtiv or placebo) will receive Reldesemtiv for the next 24 weeks. You will not know what you received during the first 24 weeks until the entire study is over. If for any reason you were down-titrated to 1 tablet twice a day of study drug during the first 24 weeks of the study, you will take Reldesemtiv 150 mg twice a day for a 300 mg total daily dose during the active drug period.

You will take the study drug twice daily, morning and afternoon (at least 8 hours apart). All doses of the study drug should be taken with food or within 2 hours following eating.

You will be reimbursed for certain expenses related to your participation in this research study (for example, parking and mileage expenses), and your study doctor can provide more information. Study staff will be available via phone or email to answer any questions you may have if you are interested in participating in the trial or would like to learn more about the study.