The primary objective is to evaluate the safety and tolerability of AMX0035 over 108 weeks of open label treatment for participants previously enrolled in Study A35-004 (PHOENIX).

All participants will receive open-label treatment with AMX0035, starting on Day 1 with twice a day oral dosing (once in the morning and once in the evening) for the duration of the study. After the Baseline Visit (Day 1), enrolled participants will complete visits approximately every 12 weeks (± 2 weeks), until Week 108 or the end of treatment (EOT) visit, followed by a safety follow-up approximately 28 days after the last dose. A survival follow-up assessment will be completed every 12 weeks following the EOT visit until time of death or end of study (EOS).

Inclusion Criteria:

1. Previous participation in Study A35-004 (PHOENIX), including completion of the randomized controlled phase through Week 48 (this timepoint may be upcoming at the time of screening). Participants who do not complete randomized-controlled phase through Week 48 for medical reasons may be included on a case-by-case basis, in consultation with the Sponsor;

2. Capable of providing informed consent;

3. Capable and willing to follow trial procedures including visits to the trial clinic, remote visits, and survival status reporting requirements;

4. Women of childbearing potential (WOCBP; e.g., not post-menopausal for at least one year or surgically sterile must agree to use adequate birth control for the duration of the trial and 3 months after the last dose of AMX0035;

   1. 12 months of spontaneous amenorrhea or 6 months of spontaneous amenorrhea with serum Follicle-stimulating hormone (FSH) levels > 40 mIU/ml (milli-international units per milliliter) or 6 weeks postsurgical bilateral oophorectomy with or without hysterectomy.

   2. Acceptable contraception methods for use in this trial are:

      • Hormonal methods, such as birth control pills, patches, injections, vaginal ring, or implants;
      • Barrier methods (such as a condom or diaphragm) used with a spermicide (a foam, cream, or gel that kills sperm);
      • Intrauterine device (IUD);
      • Abstinence (no heterosexual sex);
      • Unique partner who is surgically sterile (men) or not of childbearing potential (female).

5. Women must not be pregnant or planning to become pregnant for the duration of the trial and 3 months after last dose of AMX0035;

6. Men must agree to practice contraception for the duration of the trial and for at least 3 months after last dose of AMX0035;

7. Men must not plan to father a child or to provide sperm for donation for the duration of the trial and 3 months after the last dose of AMX0035

Exclusion Criteria:

1. History of known allergy to phenyl butyrate or bile salts;
2. Abnormal liver function defined as bilirubin levels and/or aspartate aminotransferase (AST) and/or alanine aminotransferase (ALT) > 5 times the upper limit of the normal (obtained within 12 weeks from first dose);
3. Renal insufficiency as defined by estimated glomerular filtration rate (eGFR) <60 mL/min/1.73m2 normal (obtained within 12 weeks from first dose);
4. Pregnant women or women currently breastfeeding;
5. Current severe biliary disease which may result in the Investigator medical judgement in biliary obstruction including for example active cholecystitis, primary biliary cirrhosis, sclerosing cholangitis, gallbladder cancer, gangrene of the gallbladder, abscess of the gallbladder;
6. History of Class III/IV heart failure (per New York Heart Association - NYHA);
7. Participant under severe salt restriction where the added salt intake due to treatment would put the participant at risk, in the Investigator clinical judgment;
8. Presence of unstable psychiatric disease, cognitive impairment, dementia or substance abuse that would impair ability of the participant to provide informed consent, according to Investigator judgment;
9. Clinically significant unstable medical condition (other than ALS) (e.g., cardiovascular instability, systemic infection, untreated thyroid dysfunction, severe laboratory test anomaly or clinically significant electrocardiogram [ECG] changes) that would pose a risk to the participant if he/she were to participate in the trial, according to Investigator judgment;
10. Currently enrolled in another trial (excluding Study A35-004 (PHOENIX)) involving use of an investigational therapy (or within 5 plasma half-lives) prior to first dose at Baseline Visit;
11. Implantation of Diaphragm Pacing System (DPS);
12. Currently or previously treated within the last 30 days (or 5 half-lives, whichever is longer) from first dose at the Baseline Visit or planned exposure during the treatment period to any prohibited medications listed in Section 6.7 of the protocol.