Background Biological drugs exhibit excellent efficacy and continuity in the treatment of rheumatoid arthritis (RA) and play an important role in RA treatment. Blood concentration is an important factor in the efficacy of biological drugs, particularly antibody drugs. Infliximab (IFX) is an antibody drug against TNF-α and is reported to require a blood concentration of ≥1 μg/mL to be effective.
Objectives To investigate whether clinical efficacy can be predicted based on blood concentrations at the fourth dose of IFX in patients with RA.
Methods This study included 56 patients with RA who were treated with IFX. Patients included 13 men and 43 women aged from 26 to 81 years (mean, 60.3 years). The IFX concentration was measured immediately before administering the fourth IFX dose (8 weeks after administering the third dose). We then investigated the relationship between subsequent IFX efficacy and IFX concentration immediately before the fourth dose of IFX in these patients with RA. Concentrations were measured in stored frozen serum by using the ELISA method.
Results The IFX concentration immediately before the fourth dose was ≥1 μg/mL in 32 patients (≥1 μg/mL group) and <1 μg/mL in 24 patients (<1 μg/mL group). At the fourth dose, IFX was effective in 30 patients (93.8%) in the ≥1 μg/mL group, at a mean concentration of 5.18 μg/mL, while the mean concentration was 5.69 μg/mL for the remaining 2 non-responders. IFX was also effective in 21 patients (87.5%) in the <1 μg/mL group but did not elicit any response in the other 3 patients. At this point, all 5 non-responsive patients were primary non-responders. Of all 51 responders, 58.8% were in the ≥1 μg/mL group and 41.2% were in the <1 μg/mL group. Based on the data, we observed no relationship between efficacy and IFX concentration. After 1 year of IFX treatment, 36 of the 56 patients were responsive and 20 were non-responsive. In the 2 groups, 26 responsive patients (63.9%) and 9 non-responsive patients (45.0%) had an IFX concentration of ≥1 μg/mL immediately before the fourth dose.
Conclusions At the fourth dose, many of the patients with an IFX concentration of <1 μg/mL were also responsive to the treatment, so future efficacy was difficult to predict based on IFX concentration. In other words, during clinical evaluation, measurement of IFX concentrations is not necessary in responsive patients. However, IFX concentrations should be measured in non-responsive patients or patients with a diminished response. If the concentration is <1 μg/mL, IFX efficacy should be restored by increasing the dose or shortening the administration interval.
Disclosure of Interest None declared