• Confirmed diagnosis of MS according to the 2017 McDonald criteria .

• Age 18 years to age 75 years.

• Confirmed clinical diagnosis of non-active SPMS, for 2 years prior to the screening visit. The baseline study visit and/or the baseline MRI may not be used to confirm the diagnosis but will be used to establish a study baseline and for interpretation of the PET scan data. The second qualifying MRI must be within 3 months of the study screening MRI.

• MRI imaging consistent with a diagnosis of MS at any time point.

• Score on the Expanded Disability Status Scale (EDSS) of 2.5-6.5.

• Have failed standard-of-care treatment with disease-modifying therapies for at least 2 years with continued accumulation of disability as evaluated by treating neurologist. Treatment failure/progression can be defined as EDSS worsening despite best treatment efforts:

‣ EDSS increase by 1.0 (if baseline EDSS between 1.0-5.5) in the past 2 years.

⁃ EDSS increase by 0.5 (if baseline EDSS \>6.0) in the past 2 years.

• Screening clinical laboratory studies are within the normal ranges or within the parameters specified below, and clinically acceptable in the opinion of the Investigator. Exceptions must be approved by the CRO or Sponsor's Medical Monitor.

‣ Adequate hematologic parameters without ongoing transfusion support:

• Hemoglobin (Hb) ≥9 g/dL

∙ Platelets ≥100 × 10\^9 cells/L

⁃ Creatinine ≤1.5 × the upper limit of normal (ULN), or calculated creatinine clearance ≥60 mL/minute × 1.73 m\^2 per the Cockcroft-Gault formula.

⁃ Total bilirubin ≤1.5 times the upper limit of normal (ULN) unless due to Gilbert's disease.

⁃ Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) ≤2.0 times ULN.

• Negative serum pregnancy test at screening and negative urine pregnancy test within 7 days prior to the first dose of study therapy for women of child-bearing potential, defined as a sexually mature woman who has not undergone a hysterectomy or who has not been naturally postmenopausal for at least 24 consecutive months (i.e., who has had menses any time in the preceding 24 consecutive months).

• Sexually active women of child-bearing potential and male patients must agree to use 2 effective methods to avoid pregnancy (oral, injectable, or implantable hormonal contraceptives; tubal ligation; intra-uterine device; barrier contraceptive with spermicide; or vasectomized partner) throughout the study, and for 90 days after the completion of study treatment.

⁃ Immunizations are current and up-to-date as adjusted for disease status and prior/current treatments, and documented by the patient's treating neurologist.

⁃ Ability and willingness to provide written informed consent.