A Pharmacokinetics (PK), Pharmacodynamics (PD), Safety and Tolerability Study of Fenebrutinib in Children and Adolescents With Relapsing Multiple Sclerosis (RMS)

Inclusion Criteria: * A diagnosis of RMS in accordance with the International Pediatric Multiple Sclerosis Study Group (IPMSSG) criteria for pediatric MS, Version 2012, and the revised 2017 McDonald Criteria and one or more of the following: at least one MS relapse during the previous year or two MS relapses in the previous 2 years or evidence of at least one Gd enhancing lesion on MRI within 6 month * Expanded Disability Status Scale (EDSS) at screening from 0 to 5.5 points, inclusive * Children and adolescents must have received all childhood vaccinations as per local/national recommendations for childhood vaccination against infectious diseases Exclusion Criteria: * A diagnosis of primary progressive multiple sclerosis (PPMS) or non-active secondary progressive multiple sclerosis (SPMS) * Co-morbid Conditions: * Potentially confounding neurological, somatic, or metabolic disorders * Current clinically significant psychiatric or medical illness * History of cancer, transplants, or bleeding disorders * Inability to complete an MRI scan or get gadolinium * Abnormal liver function tests or blood counts * Sensitivity or intolerance to any ingredient (including excipients) of fenebrutinib tablets * Active, recurrent, or chronic infections * Recent or anticipated use of prohibited medications/treatments: * Certain disease-modifying therapy (DMT) and other immunosuppressants * Drugs interacting with fenebrutinib (Cytochrome P450 3A4 \[CYP3A4\] inhibitors) * Any other investigational therapy, anticoagulants, certain vaccines * A score of 4 or 5 on the "last 6 months" section of the screening SI section or "yes" on any item of the "last 6 months" Suicidal Behavior (SB) section of the C-SSRS or a positive answer on Question 9 of the Patient Health Questionnaire-9 Modified for Adolecents (PHQ-A) or significant risk of suicide, in the investigator's judgment