Conventional synthetic DMARDs
The backbone of current maintenance therapy for RA continues to be csDMARDs, including methotrexate, sulfasalazine, leflunomide and hydroxychloroquine. Hydroxychloroquine and sulfasalazine have perhaps the best safety profile and are not thought to be associated with infection risk.
Recent data have provided information about infection risk with methotrexate. Results from systematic reviews and meta-analyses of observational studies and randomised trials have shown conflicting results; some have found no elevated risk of infection, serious infection or OI, but others have demonstrated a modest increase in risk.8 9 The recently published Cardiovascular Inflammation Reduction Trial enrolled 9300 patients for secondary prevention of cardiovascular disease using 15–20 mg methotrexate compared with placebo and showed no difference in the risk of serious infection (2.2 vs 2.5/100 person-years (pyrs), p=0.5) and a modest increase in general infection risk (16.5 vs 14.4/100 pyrs, p=0.02).10 11 Taken together, this trial and observational data suggest that there may be a small increase in non-serious infections with methotrexate, but minimal increase in the risk of severe infections. The risk of infection with leflunomide is not well described, but a Cochrane review of RCTs for leflunomide demonstrated no significant difference in infection risk between placebo, methotrexate, sulfasalazine or leflunomide.12 Combination therapy with a biologic and csDMARD does not appear to be associated with additional serious infectious risks compared with biological monotherapy.13 14
Biologics
A continually expanding array of biological DMARDs target a variety of cytokines and cell signalling pathways. Despite the differences in the targets of these drugs, they are often grouped in the literature. In a large-pooled network meta-analysis looking at infection risk for all biologics, there was a 1% absolute risk increase for serious infection compared with placebo, but different mechanisms of action lead to different risks.1 This section will focus on the data for the originator biologics approved for RA, as there are limited data on the long-term safety for biosimilar compounds. Although large biosimilar safety studies have not been performed, infection risk with biosimilars is expected to be similar. The following sections will review infection risk with TNF inhibitors (TNFi) and then examine data for other biological therapies.
TNF inhibitors
The most detailed safety data exist for TNFi. A large observational cohort study comparing patients initiating TNFi therapy to those initiating a csDMARD showed an increased risk of serious infection with TNFi (HR of 1.9, 95% CI 1.3 to 2.8), with the highest risk within the first 6 months of therapy initiation.15 A safety review of 49 observational studies found that patients on a TNFi in general had a higher risk of serious infections compared with csDMARDs (HR ranging from 1.1 to 1.8).16 Although no randomised head-to-head studies exist, some observational studies have suggested a higher risk for infection with infliximab compared with other TNFi and a lower risk with etanercept, although these results are not consistent across studies and the potential for confounding remains.17–19
While individual trials are under-powered for safety, meta-analyses of these trials have been done to look at safety data and risks of severe infection. Meta-analyses of the relevant trial data for the individual TNFi for RA have been performed and suggest an HR of 1.31 (95% CI 1.09 to 1.58) for standard-dose biological DMARDs, corresponding to an increase of approximately one serious infection for every 100 patients treated for 1 year.1
Cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) fusion proteins
Abatacept is the first generation CTLA-4 fusion protein approved for RA. In a Cochrane meta-analysis of trial data, there was an increase in serious infections seen in patients treated with abatacept compared with control patients (OR 1.91, 95% CI 1.07 to 3.42), but after removing patients co-treated with etanercept the results were no longer statistically significant (OR 1.82, 95% CI 1.00 to 3.32).20 A retrospective cohort study comparing serious infection risk across biologics using Medicare data found the lowest incidence rates with abatacept at 13.1 per 100 pyrs versus 15.9 per 100 pyrs with etanercept (adjusted HR 1.24, 95% CI 1.07 to 1.45) and 17.0/100 pyrs with infliximab (adjusted HR 1.39, 95% CI 1.21 to 1.60).18 Taken together, abatacept may have a favourable risk profile compared with other biologics for serious infectious risk, although differences are likely small.
Interleukin-6 (IL-6) inhibitors
Tocilizumab and sarilumab are potent IL-6 receptor inhibitors. Initial trials were not powered to detect safety but did report cases of severe Epstein-Barr virus reactivation, zoster and limb abscess.21 With initial trials showing elevated LDL cholesterol, a cardiovascular safety trial randomised 3080 patients to tocilizumab or etanercept. While there was no difference in cardiovascular outcomes, there was a significantly higher risk of serious infections with tocilizumab, with 4.5 serious infections/100 pyrs for tocilizumab compared with 3.2/100 pyrs for etanercept (HR 1.39, 95% CI 1.08 to 1.79).22 One observational study from insurance databases showed a small difference in the rate of a composite outcome including serious bacterial infection, diverticulitis, and skin and soft tissue infections in tocilizumab compared with TNFi (HR 1.19, 95% CI 1.07 to 1.33), and showed larger differences compared with abatacept (1.40, 95% CI 1.2 to 1.63).23 A separate observational study showed similar risk of serious infection with tocilizumab compared with TNFi.18 Overall the risk for infection with IL-6 inhibitors seems similar or perhaps slightly greater than the risk with TNFi, although more data are needed to better quantify this risk.
CD20 targeting for B cell depletion
Rituximab is a monoclonal antibody targeting CD20 which leads to B cell depletion. The data regarding the risk of serious infections in rituximab are generally favourable. Pooled analysis of cohort and randomised studies showed no significant differences between rituximab and other treatment groups both in overall infections and in serious infections (4.1% vs 4.6%; OR 1.05; 95% CI 0.84 to 1.31).24 A Cochrane review of eight trials in rituximab in conjunction with methotrexate for RA compared with methotrexate alone showed no significant difference in the risk of all infections (relative risk [RR] 1.1, 95% CI 0.95 to 1.30) or serious infections (RR 0.68, 95% CI 0.42 to 1.10).25 One observational study suggested a higher risk of infection with rituximab compared with other biologics, although this has not been found in other observational studies.18 19 26 27 The risk for serious infection with rituximab does not seem substantially different from other biologics, with some studies even suggesting a risk similar to csDMARDs.
JAK inhibitors
The most recent additions to the armamentarium for RA are JAK inhibitors, including tofacitinib, baricitinib and upadacitinib. As newer agents, less safety data exist; even meta-analyses of existing trials are underpowered to assess serious infection risk, although the rates of infection were overall similar to those seen in trials of other RA therapies (2–3 serious infections/100 pyrs).28 In a head-to-head trial of upadacitinib versus abatacept for RA, there were no significant differences in serious infection (1.0% vs 0.3%) or OIs (1.3% vs 0.3%), although the study was not powered to detect differences in these outcomes.2 A multi-database cohort study using insurance databases including more than 130 000 patients with RA sought to compare the risk of infections with tofacitinib compared with biological DMARDs. The serious infection risk for tofacitinib was similar to TNFi and abatacept.29 Overall, current data support similar risk of serious infection with JAK inhibitors compared with biological DMARDs, although continued safety assessments will be needed over time.
Glucocorticoids
Despite the rapid expansion of effective therapeutics in RA, GC remain a common and important treatment. Although most guidelines recommend limiting use when possible to short-term bridging therapy in patients starting or changing DMARDs, 30%–60% of patients with RA remain on GC long term, although this practice is highly variable among rheumatologists.14 30–32 Risk of infection with GC is dependent on both dose and duration of use, with long-term use of doses >10 mg/day known to be associated with a more than doubling of the risk of serious infections.7 13 14 33 34 Risk of higher doses of GC appears to be substantially greater than that seen with csDMARDs, biologics or JAK inhibitors.
Accumulating data has also suggested that even low-dose therapy ≤5 mg/day is associated with a clinically meaningful increase in serious infection risk. Existing randomised trials of GC are not powered to assess for serious infection risk with low-dose GC, but several different observational studies have found similar risks with low-dose therapy. A case-control study by Dixon et al, modelled recent and prior GC exposure and found a significant risk of infection with long-term doses ≤5 mg/day (HR 1.32, 95% CI 1.06 to 1.63).34 Several observational cohort studies have demonstrated similar associations.6 35 A recent observational study examining patients with RA on stable DMARDs in two large administrative databases found that use of ≤5 mg/day of GC was associated with significant serious infection risk in both cohorts (HR 1.29, 95% CI 1.25 to 1.34 and HR 1.32, 95% CI 1.18 to 1.47), representing approximately 1–2 additional serious infections among 100 patients treated for 1 year—a small but clinically meaningful effect.14 Notably, this infection risk is similar in magnitude to infection risk with TNFi.1 14 GC risks were similar in patients receiving biologics and in patients receiving csDMARDs.14