In this study, we showed that the serum ADA levels, including non-trough values, but excluding values obtained within 3 days of ADA administration, reflect the subsequent clinical course and endoscopic outcome. Our study demonstrated that ADA continuity can be expected if the non-trough serum ADA level (except within 3 days of ADA administration) is ≥ 9.2 µg/mL and that endoscopic remission can be expected if the level is ≥ 11.1 µg/mL. A previous study on trough level of ADA during maintenance therapy reported that 5‒5.8 and 4.9‒7.1 µg/mL were the lower limits of the therapeutic range when clinical remission and mucosal healing were the desired targets, respectively; both limits are approximately 4 µg/mL lower than our non-trough level. As these are the lower limits of the therapeutic range and the variation in the levels over 2 weeks between successive ADA administrations is approximately 3 µg/mL [16], our results are valid.
In other studies, AAA positivity was estimated to be 12.7%‒30.9% [9, 11, 15, 17], which is consistent with our results in this study (23.1%). Nakase et al. reported that AAA and clinical remission were significantly correlated in patients with CD receiving ADA [18]. In contrast, Robin et al. showed that AAA positivity was higher in patients with clinically active disease than in patients with clinical remission (13.5 vs. 9.8 ng/mL, respectively) [17]. In addition, AAA positivity was lower in patients who achieved mucosal healing than in those who did not (6.5 vs. 14.1 ng/mL; P = 0.06). Our study also showed that AAA did not correlate with treatment continuity or endoscopic outcomes; hence, the importance of measuring AAA level in clinical practice should be investigated further.
Reports on the benefits of the combined use of immunomodulators with ADA are controversial [19–24]. Although there was no association between the concomitant use of immunomodulators with ADA versus only ADA in this study (P = 0.252), we found that AAA positivity was significantly lower in the immunomodulator combination group (1/27 [3.7%] vs. 11/25 [44%], P < 0.05, Mann–Whitney’s U-test). However, there was no significant difference in treatment continuity with or without immunomodulators (P = 0.40). Using immunomodulators suppressed AAA production, but it did not improve any clinical outcomes in our study. However, a recent study reported that treatment continuity was higher with azathioprine when a second anti-TNFα agent was used in cases where the use of the first anti-TNFα agent resulted in immune-mediated LOR [25]. Further research is needed to determine whether it is necessary to use an immunomodulator with ADA in real-world clinical settings. In our study, because we found no other factors related to ADA level, we concluded that ADA is clinically effective if the serum ADA level is high, regardless of sex, weight, disease duration and IFX treatment history. Therefore, to predict the therapeutic efficacy of ADA, actual measurement of ADA level is important, and not the timing of blood collection to specifically measure the trough level (unless collected within 3 days of ADA administration).
A limitation of this study was that the time of measurement of ADA level differed for each patient. There are a few studies on the pharmacokinetics of ADA in patients with IBD, but information about the changes in drug level after administration is limited. A post-hoc study on the pharmacokinetics of 65 patients with CD receiving ADA reported that the half-life of ADA was 22 days in AAA-negative patients [26]. A prospective observational study in which 19 patients with CD treated with ADA underwent TDM at multiple timepoints indicated that there was negligible change in the serum ADA level over 9 days after administration [15]. Another prospective study on the pharmacokinetics of seven patients with CD undergoing remission maintenance therapy with ADA reported a minimal difference between the peak and trough blood ADA levels during ADA therapy [27]. From these studies, we inferred that the difference in the timing of TDM administration would have negligible effect on the serum ADA level in patients repeatedly receiving ADA. This is because the ADA level in the blood increases and disappears relatively slowly because of the subcutaneous formulation, and the administration interval is 2 weeks, which is shorter than other drugs.