Therapeutic Drug Monitoring of Tocilizumab in Rheumatoid Arthritis

Rationale: A wide range of serum trough concentrations is observed in tocilizumab-treated rheumatoid arthritis (RA) patients, while 1 mg/L tocilizumab is sufficient to block systemic interleukin-6 receptor. A substantial proportion of patients has higher serum tocilizumab concentrations and is likely to be overexposed. We expect that patients can at least reduce the dose aiming for a concentration of 5 mg/L without reducing efficacy. Objective: To evaluate the feasibility of the study after 20 weeks of follow-up, this includes the evaluation of the dose-reduction algorithm in tocilizumab-treated patients with RA. Study design: Double-blind randomized controlled pilot study with a follow up of 20 weeks. Study population: Consecutive RA patients that are treated with tocilizumab intravenously every four weeks for at least 24 weeks. Patients are screened for tocilizumab concentration after signing informed consent. Intervention: Patients with a concentration below 5 mg/L will continue the dose. Those patients with a tocilizumab concentration above 5 mg/L are randomly assigned (2:1) to dose reduction or to continuation of the standard care tocilizumab dose. In the intervention group, the precise dose-reduction is calculated per patient in order to achieve a tocilizumab concentration of 5 mg/L (range 4-6 mg/L). Main study parameters/endpoints: The feasibility of the study logistics is evaluated according to the dropout rate and patients opinion about the study. Second, the proportion of patients achieving the targeted tocilizumab concentration after dose reduction is evaluated. Nature and extent of the burden and risks associated with participation, benefit and group relatedness: Dose-reduction will lead to lower drug costs and possibly to reduce the risk of adverse events. Since we lower the tocilizumab concentration in a proportion of the patients, risk of a exacerbation of the disease exists. In this case, patients will receive their original dose. Previous st