Ixekizumab Treatment Persistence in Psoriatic Arthritis: Response to Joaquin et al

We extend our sincere thanks to Joaquin et al for their interest in our work and for their commentary—though, if we may, somewhat predictable—on our recent study regarding the treatment persistence of ixekizumab in psoriatic arthritis.

We would first like to clarify a fundamental methodological point: our study was not based on administrative data, but rather on prospectively collected real-world clinical data from a multicenter observational cohort. This distinction is crucial, as administrative databases—often criticized for their limited clinical depth—were not involved in our analyses.^1-3

Regarding the observation on adherence, we are fully aware of the conceptual distinction between adherence and persistence. However, we believe it is important to emphasize what is well-established in the rheumatological literature: persistence is routinely assessed through the Drug Retention Rate (DRR), which represents the accepted and validated measure in observational studies of biologic therapies. The DRR reflects a complex and clinically relevant interplay of factors—efficacy, safety, tolerability, clinical decision-making, and patient choice—and it is precisely this multidimensionality that makes it such a valuable indicator of therapeutic performance in real-world practice.

Certainly, one may always wish to incorporate additional layers of behavioral detail—such as pharmacy records, electronic monitoring devices, or other technological refinements—but in a real-world multicenter clinical research context, such tools are not only impractical but also unnecessary for the specific purpose of a persistence analysis. It may be worth recalling that virtually all major published studies on the topic adopt precisely this methodological approach.

In other words, we fully agree that adherence is an interesting topic—but to suggest that its absence invalidates persistence data is somewhat like criticizing a thermometer for not measuring blood pressure. Different tools serve different purposes, and our study was designed, as clearly stated, to evaluate treatment persistence—not to conduct a behavioral pharmacology investigation.

We welcome the opportunity to clarify these points and are pleased that our work has stimulated reflection on the nuances of treatment evaluation. We remain convinced that the methodological rigor, real-world multicenter design, and consistency of our findings with the international literature make our data a reliable reference for understanding ixekizumab persistence in routine clinical practice.