Inclusion Criteria: Relapsing forms of Multiple Sclerosis (RMS) - Cohort 1 - Participants must have an Expanded Disability Status Scale (EDSS) of ≥ 3.0 and ≤ 5.5. - Participants must have a diagnosis of Multiple Sclerosis (MS) with relapsed/refractory MS or conversion to active secondary progressive multiple sclerosis (aSPMS), and worsening of disease within 12 months prior to Screening and while on treatment with a high-efficacy DMT for at least 6 months. Progressive forms of MS - Cohort 2 - Participants must have an EDSS ≥ 3.0 and ≤ 6.0. - Participants must have a diagnosis of primary progressive multiple sclerosis (PPMS) that is treatment-resistant or diagnosis of inactive secondary progressive multiple sclerosis (iSPMS). Exclusion Criteria: - Participants that cannot complete the 9-Hole Peg Test (9-HPT) for each hand in <240 seconds. - Participants that cannot perform a Timed 25-Foot Walk Test (T25FWT) in < 150 seconds. - Participants must not have MS lesions or symptoms that may place patients at increased risk of neurotoxicity, including, but not limited to, tumefactive lesion (3 cm or greater within 5 years prior to Screening) or decreased level of consciousness, and/or presence of active, clinically significant concomitant central nervous system pathology other than MS that may confound the ability to interpret study results or complicate identification or evaluation of neurotoxicity.