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Objective: A dose–response relationship for hydroxychloroquine (HCQ), in terms of the proportion of patients achieving the Paulus 20% criteria for improvement, had previously been observed in patients with rheumatoid arthritis (RA) receiving a 6‐week loading regimen of 400, 800, or 1,200 mg HCQ daily. This present retrospective analysis was performed to investigate possible relationships between the blood HCQ and HCQ‐metabolite concentrations and measures of efficacy and toxicity. In addition, we sought to ascertain whether further investigation of HCQ/HCQ‐metabolite levels might lead to testing of one of these substances as a new antirheumatic drug.Methods: Patients with active RA (n = 212) began a 6‐week, double‐blind trial comparing 3 different doses of HCQ at 400, 800, or 1,200 mg/day, followed by 18 weeks of open‐label HCQ treatment at 400 mg/day. Patients were repeatedly evaluated for treatment efficacy and toxicity. Blood samples were available from 123 patients for analysis of HCQ, desethylhydroxychloroquine (DHCQ), desethylchloroquine (DCQ), and bisdesethylchloroquine (BDCQ) levels using high‐performance liquid chromatography. Achievement of the modified Paulus 20% improvement criteria for response in RA was used as the primary efficacy parameter. Spontaneously reported adverse events were categorized and analyzed as toxicity outcome variables. The relationship between response (efficacy and toxicity) and drug levels was evaluated using logistic regression analysis.Results: The subset of patients with blood concentration data was equivalent to the larger study population in all demographic and outcome characteristics. The mean HCQ, DHCQ, and DCQ elimination half‐lives were 123, 161, and 180 hours, respectively. There was a positive correlation between the Paulus 20% improvement criteria response and blood DHCQ concentrations during weeks 1–6 (P < 0.001). A potential relationship between ocular adverse events and BDCQ levels was found (P = 0.036). Logistic regression analysis of adverse events data showed that adverse gastrointestinal events were associated with higher HCQ levels (P = 0.001–0.021) during weeks 1, 2, and 3.Conclusion: There is a weak, but predictable, relationship between blood DHCQ concentrations and efficacy of treatment with HCQ. In addition, there is an association between gastrointestinal adverse events and elevated blood HCQ concentrations. Further investigation of these relationships is warranted to see if DHCQ may be introduced as a new antirheumatic drug.

Biologic therapies that target pathogenic cytokines such as TNF, IL-1β or IL-6 have greatly improved the treatment of RA. Unfortunately, not all RA patients respond to current biologic therapies and responses are not always maintained, suggesting that there are alternative drivers of RA pathogenesis that might serve as promising therapeutic targets. Discovery of the new Th17 subset of Th cells, and their role in autoimmune disease development, has implicated the proinflammatory IL-12 and IL-17 families of cytokines in RA disease pathogenesis. Members of these cytokine families are elevated in the blood and joints of RA patients and have been shown to remain elevated in patients who do not respond to current biologics. In addition, these cytokines have been shown to play roles in joint destruction and erosion. A new subclass of biologics that target the IL-12 and/or IL-17 signalling pathways are under development. Here we review evidence for a role of Th17 cells as well as IL-12 and IL-17 cytokines in RA pathogenesis as the rationale for a subsequent discussion of the ongoing and completed clinical trials of newly emerging biologic therapies directed at IL-12 or IL-17 pathway inhibition.