Background Formation of anti-drug antibodies (ADA) might be responsible for sub-therapeutic serum drug levels resulting in a lack of clinical response. The need to test ADA levels in patients treated with a TNF inhibitor is the subject of discussion (1, 2).
Objectives ADA levels were measured in patients treated with ADL or ETN or IFX to check immunogenicity in patients receiving various TNF inhibitors and clarify whether ADA measurement reflects treatment output.
Methods Frozen serum samples from patients with rheumatoid arthritis (RA), spondylarthritis (SpA) and psoriasis arthritis (PsA) and treated with adalimumab (ADL; n=41) or etanercept (ETN; n=42) or infliximab (IFX; n=42) were selected. All patients were receiving continuous care of one of the authors. Anti-ADL, anti-ETN or anti-IFX levels were tested with commercially available assays (Grifols Deutschland GmbH) according to the manufacturer's instructions. Drug levels were also determined with assays from the same manufacturer.
Results Among ADL-treated patients we found 8/41 (20%) to be positive for ADA. In the ETN group elevated ADA levels were found in 3/42 (7%) patients; in the IFX group 8/42 (19%) patients had positive ADA levels. High ADA concentrations did not always correlate with diminished therapeutic response. Among ADA positive patients 3/8 in the ADL group (1 of each disease), 3/3 in the ETN group (RA 2, SpA 1) and 5/8 in the IFX group (RA 1, PsA 3, SpA 2) continued the given therapy with good response. The three positive ETN patients showed only borderline elevated ADA levels.
Conclusions ADA levels were more often found in patients treated with ADL or IFX than with ETN. These results agree with those of previous studies (3) and might indicate differences in immunogenicity between these three TNF inhibitors. Nevertheless positive ADA levels did not always indicate failure of therapy. The low incidence of ADA levels, the high cost of ADA assays and the absence of a need to change therapy based on ADA levels have to be considered before ADA analysis is ordered within a routine control.
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Acknowledgement This study was supported by PFIZER within an unlimited industrial grant.
Disclosure of Interest None declared