Assessment of drug levels as a biomarker in rheumatoid arthritis.

In this viewpoint, we focused on 2 examples of the assessments of drug levels as a biomarker in rheumatoid arthritis (RA), based on our experiences in longitudinal studies and clinical trials. Among a long list of potential biomarkers in rheumatology, the measurements of drug levels for the efficacy and toxicity of the drugs have been extensively investigated. One example is the levels of methotrexate (MTX)-polyglutamates (PGs), and the second one is that of biological disease-modifying antirheumatic drugs (DMARDs) in patients with RA. Recently, meta-analyses and population pharmacokinetics-pharmacodynamics studies have been published, confirming the previous reports. After these publications, we experienced the new information about the measurements of MTX-PGs in a prospective multicenter clinical trial. In addition, we also experienced the measurements of drug levels of a unique bDMARD, ozoralizumab, a bispecific Variable domain of Heavy chain of Heavy chain antibody (VHH) molecule, which is directed against TNFα and albumin in clinical trials. We introduced our experiences and explored the viewpoint of the measurements of drug levels as a biomarker in RA.