This 12-week substudy of the RA-BUILD clinical trial investigated 8 biomarkers of joint tissue turnover and inflammation in patients with active RA who had inadequate response or intolerance to csDMARDs. Biomarkers consisted of a set of novel neo-epitope biomarkers released during tissue processing, reflecting pathological events in the inflamed tissue, as well as the established inflammation marker CRP. Also included in the analysis were the established bone resorption marker CTX-I and bone formation marker osteocalcin. We characterized the biomarker profiles in response to either 2- or 4-mg baricitinib or placebo and showed a significant reduction in C1M, C3M, and C4M, the biomarkers associated with tissue remodeling of the synovium, in patients receiving both the 2-mg and 4-mg doses of baricitinib, compared to the placebo group. These results are in line with a smaller prospective study that included patients receiving another JAK inhibitor, tofacitinib, where C1M, C3M, and C4M levels were also found to be decreased after 48–60 weeks of treatment compared to each patient’s baseline [19]. Our study is the first randomized, placebo-controlled clinical trial to demonstrate rapid and sustained changes in a soft joint tissue turnover in patients with RA as a result of being treated with a JAK inhibitor.
The reduction of the biomarkers associated with tissue remodeling of the synovium found in this study is in line with the pathogenesis of RA, which likely begins with inflammation in the synovial tissue. Although these interstitial collagens type I and III are expressed in multiple tissues throughout the body, preclinical studies have shown a specific association with tissue turnover in synovial membranes. In an ex vivo synovial membrane model, both C1M and C3M are released upon cytokine challenge and synovitis and inhibited by anti-inflammatory inhibitors such as the syk inhibitor fostamatinib [20]. Type IV collagen is the major constituent of the basement membrane [21], and C4M has been linked to remodeling of the basement membrane in animal models of liver fibrosis [9]. Clinically, C4M has been associated with increased mortality in patients with atherosclerosis, suggesting an association with endothelial remodeling. Although the synovial membrane lacks a regular basement membrane, type IV collagen is found in the intercellular synovial lining and in vascular basement membranes of the normal lining layer [21, 22]. In addition, several studies have argued that a majority of fragments of type IV collagen originate from the RA synovium, showing that fragments are more abundant in patients with active RA [8, 23]. In RA specifically, several studies have investigated the association of biomarkers of collagen tissue turnover with treatment response, disease activity, and progression. Serum C2M, C3M, and C4M are increased in patients with RA compared to healthy donors [8, 24], in line with the observation in the current study that patients’ levels of these markers are in the upper part of the normal range for healthy controls as indicated by the individual assay. In clinical studies investigating the effect of the IL-6 receptor inhibitor tocilizumab, C1M, C3M, and C4M were associated with disease activity and response to treatment, and early reductions were predictive of a treatment response [16, 19]. C1M is associated with bone changes. Baseline C1M levels have been shown to be associated with structural progression measured by mTSS, indicating that C1M is also a marker of bone turnover [25, 26]. In the current study, a reduction in circulating biomarkers associated with tissue destruction and synovial inflammation in patients with RA was observed in baricitinib-treated patients, suggesting that baricitinib inhibits key pathological processes at the site of inflammation in RA.
While markers of the synovial tissue inflammation were reduced, baricitinib showed no effect on C2M. This indicates that baricitinib treatment, in the timespan of the current study, provided limited modulation of cartilage turnover. C2M is a type II collagen fragment released from hyaline cartilage upon proteolytic cleavage by MMPs [27]. C2M is increased in patients with RA compared to healthy controls [24] and can be modulated by anti-inflammatory treatment [17, 20]. Surprisingly, in the current study, we observed no effect on C2M. A possible explanation for the lack of effect may be that cartilage is a low turnover tissue and the limited study timeframe may be too short to allow changes to occur.
CRP levels in the blood decreased significantly upon baricitinib administration in line with previous studies showing a dramatic reduction in the acute phase reactant upon inhibition of inflammatory pathways [19]. JAK1/2 is downstream of the IL-6R and as such it is expected that CRP (as a proxy for IL-6 activity) would decrease. Moreover, patients in this trial were selected as having elevated CRP (≥ 3.6 mg/L), whereas no selection for elevated levels of the tissue-specific markers was made.
An important finding in the current study was that both baseline and change in biomarkers with baricitinib treatment were associated with disease activity parameters and clinical improvement. This clearly links quantitative tissue changes directly with the clinical outcome and shows that biomarkers of tissue remodeling may be used to characterize disease activity and response to treatment.
The limitations of this study include a limited sample size as only a subpopulation of patients from the full RA-BUILD study were included in these analyses. Additionally, the analysis is over a relatively short period of time (12 weeks), unlike previous biomarker studies that had longer follow-up (up to 52 weeks; 14); however, patients in the current study could have received rescue treatment at week 16 and would not have been on their randomized treatment at the time of analysis if it had gone beyond 12 weeks.
Additionally, we would like to see an early reduction in markers be predictive of long-term clinical benefit, more specifically to assess whether changes in biomarkers at week 4 were associated with clinical outcomes at week 12. Improvement in soft tissue biomarkers (C1M, C3M, C4M) at week 4 indeed predicted clinical outcome measures (ACR scores) at week 12.
Baricitinib is approved in many countries for treatment of patients with moderate to severe RA with inadequate response to conventional and/or biologic DMARDs. The current analysis investigates the use in csDMARD inadequate responders, but not biological DMARD naïve patients. The fact that we did not observe any statistically significant difference in changes in biomarkers between the two doses suggests that 2 mg is sufficient for benefit in joint tissue turnover in biological DMARD naïve patients. The current study did not analyze the difference between 2-mg and 4-mg doses in TNFi inadequate responders. Therefore, these biomarkers should be tested in both dosage arms in a biological DMARD inadequate responder study to expand the current findings.