Study to Evaluate the Efficacy and Safety of BBP-418 (Ribitol) in Patients With Limb Girdle Muscular Dystrophy 2I (LGMD2I) (Fortify)

Participants must meet all the following criteria to be enrolled: Have a genetically confirmed diagnosis of LGMD2I/R9 (including review of records of previous molecular genetic testing) and be clinically affected (defined as demonstrating clinical weakness on bedside evaluation in either a limb-girdle pattern, or in a distal extremity). Male or female participants 12 to 60 years of age (inclusive). Have a body weight >30 kg. The participant (or parent/guardian) who signs the ICF understands the study procedures and the participant agrees to participate in the study by giving informed consent (and assent, if <18 years of age). Female participants of childbearing potential and male participants of reproductive potential must be willing to use a highly effective method of contraception from time of consent through 12 weeks after last dose. Willing and able to complete all study procedures, including biopsies, according to the Schedule of Assessments (see Appendix 1). Participants must not meet any of the following criteria to be enrolled: Evidence of clinically significant concomitant disease, including: Any significant concomitant medical condition, including mental, cardiac, renal, pulmonary, hepatic, or endocrine disease other than that associated with LGMD2I/R9. Moderate to severe renal impairment (estimated glomerular filtration rate [eGFR] of < 60 mL/min/1.73 m2 based on cystatin C [CysC]), as calculated by the central laboratory. Any other laboratory, vital sign, ECG abnormality, clinical history, or finding that, in the Investigator's opinion, is likely to unfavorably alter the risk-benefit of study participation, confound study results, or interfere with study conduct or compliance. Surgery for scoliosis or other indication that will significantly impact the participant's ability to execute clinical assessments planned or expected to be required to manage curvature within 12 months following the Screening Visit. A participant with a score of zero on any one or more of the primary or key secondary endpoints at the time of screening. (Participants who previously completed participation in Study MLB-01-001 and would be excluded due to this criterion may enroll in this study provided all inclusion and no other exclusion criteria are met.) If pregnant and/or breastfeeding or planning to conceive children within the projected duration of the study through 12 weeks after the last dose of study treatment. Use of ribose or other sugar alcohol-containing supplement within 90 days of the Screening Visit. Use of a systemic corticosteroid for the treatment of muscular dystrophy within 90 days of the Screening Visit. (An inhaled corticosteroid or bronchodilator for reactive airway disease is allowed if the participant is on a stable dose for 30 days prior to study entry.) Previously received gene therapy to treat LGMD2I/R9. Participants with active suicidal ideation as measured by Columbia-Suicide Severity Rating Scale during screening with most severe suicide ideation score of 4 (Active Suicidal Ideation with Some Intent to Act, without Specific Plan) or 5 (Active Suicidal Ideation with Specific Plan and Intent). Presence of a platelet disorder, bleeding disorder, or other contraindication to muscle biopsy. Actively on an experimental therapy or device or was on an experimental therapy or device within 90 days of the Screening Visit. In the judgment of the Investigator or Medical Monitor, has any clinically important ongoing medical condition or laboratory abnormality or condition that might jeopardize the participant's safety, increase their risk from participation, or interfere with the study. For COVID-19 infections, Investigator should refer to local guidance.