Background: The efficacy and safety of cladribine tablets 3.5 mg/kg (cumulative dose over 2 years) have been shown in patients with relapsing forms of multiple sclerosis (MS) in Phase 3 trials. However, there is limited real-world data on the effect of cladribine tablets 3.5 mg/kg in patients switching from another disease-modifying therapy (DMT). CLICK-MS (NCT03933215) examines effectiveness, safety, and patient-reported outcomes (PROs) of cladribine tablets 3.5 mg/kg in patients with relapsing-remitting MS (RRMS) or active secondary progressive MS (SPMS) who had suboptimal response with a prior injectable DMT in the real world. Objectives: To evaluate the safety and PROs of cladribine tablets 3.5 mg/kg from a subset of enrolled patients six months after treatment initiation. Methods: CLICK-MS is an ongoing US-based, single arm, observational, 30-month, Phase 4 study. Adult patients with RRMS or active SPMS with suboptimal response to a prior injectable DMT, and who meet the criteria to receive cladribine tablets 3.5 mg/kg per the US Prescribing Information are eligible for enrollment. This first interim analysis of the CLICK-MS trial primarily focuses on safety in a subset of enrolled patients at the 6-month time point. The subset population comprises the first 30 patients enrolled into the study. This analysis is being conducted ahead of the full baseline analysis of the total patient population. The trial is expected to be completed by November 2023. Results: First interim analysis of data for the first 30 patients enrolled is in progress. Results will be shown in the presentation. Baseline data will include patient demographic and disease characteristics, prior injectable DMT, and PROs in relation to the prior DMT. PROs include scores for the following: 14-Item Treatment Satisfaction Questionnaire for Medication, 36-Item Short Form Health Survey, Patient Determined Disease Steps scale, Beck-Depression Inventory Fast Screen, Modified Fatigue Impact Scale 5-item version, and Work Productivity Activity Impairment Multiple Sclerosis. The endpoints evaluated six months after treatment with cladribine tablets 3.5 mg/kg are adverse events, treatment adherence, and PROs. Conclusions: The results from the first interim analysis of the CLICK-MS trial will provide a preview of safety, adherence, and PROs in the first 30 patients with RRMS or active SPMS who switched treatment from injectable DMTs to cladribine tablets 3.5 mg/kg. Future interim analyses will include full baseline demographics and 6-, 12-, and 24-month data on the entire patient population.