4.1. Clinical characteristics of T-HFIB
In present study, 221 patients treated with tocilizumab were included for analysis, and 54.75% (121/221) of patients developed T-HFIB. It is reported that the probability of T-HFIB ranges from 29–76.47% [10, 12, 18, 19]. In present study, the median time of T-HFIB was 6 days, 90.91% (110/121) occurred within 30 days after tocilizumab administration, and most of them were mild or moderate hypofibrinogenemia patients (85.6%, 104/121).
An et al. [19] and Souri et al. [20] did not observe bleeding symptoms in rheumatoid arthritis patients with H-FIB. However, Imamura et al. [9] pointed out that rheumatoid arthritis patients treated with tocilizumab lower fibrinogen levels compared to those without treatment, and had higher blood loss after total knee arthroplasty operation. Our study found that 9 patients (52.9%, 9/17) with severe and life-threating hypofibrinogenemia had bleeding, which was significantly higher than that in mild and moderate hypofibrinogenemia patients (20.2%, 21/104). Therefore, it is necessary to monitor the serum fibrinogen levels and other coagulation markers in clinical tocilizumab administration to prevent fatal bleeding event. In addition, 11.6% (153/1314) of patients were excluded since baseline fibrinogen below 2 g/L, and 71.5% (940/1314) due to fibrinogen levels data missing. This suggesting that the drug-induced hypofibrinogenemia has not been fully recognized in clinical practice and it is necessary to strengthen the routine monitoring of serum fibrinogen levels.
Among the patients, 76.9% (170/221) of them were combined with CAR-T therapy, rheumatic diseases and COVID-19. The incidence rate of T-HFIB in patients with COVID-19 and CAR-T therapy (74/103) was significantly higher than that in patients with rheumatic diseases (25/67) (χ2 = 12.90, p < 0.001). While there was no significant difference in the incidence rate of T-HFIB between patients with COVID-19 and CAR-T therapy. Patients both in case group (4.05 (2.97–5.19) g/L) and the control group (4.60 (3.32–5.67) g/L) have a high level of serum baseline fibrinogen, which may be related to the original diseases. The vast majority (95.0%, 114/121) of patients developed hypofibrinogenemia after the 1–3 administration of tocilizumab, consistent with previous reports that T-HFIB mainly occurred during the 1–4 administration [11, 18, 19]. Uskudar Cansu et al. [11] speculated that the occurrence of hypofibrinogenemia may be affected by the cumulative dose of tocilizumab, but we did not observe the difference in the cumulative dose between the T-HFIB and the controls.
4.2. Related risk factors of T-HFIB
We found that combined with infection, COVID-19, CAR-T therapy, antirheumatic drugs, glucocorticoids, and baseline fibrinogen level were the influencing factors of T-HFIB. Clinical studies have reported that patients with severe COVID-19 pneumonia have high serum baseline fibrinogen levels (median 5.20 g/L, IQR 4.36 ~ 7.14), and fibrinogen levels decreased after 10 days of tocilizumab treatment (median 2.17 g/L, IQR 1.50 ~ 2.85) [21]. Consistently, Tomasiewicz et al. [22] found that the serum fibrinogen concentrations of severe COVID-19 patients were decreased significantly after treatment with tocilizumab (p < 0.001), but the incidence was not mentioned.
Cytokine release syndrome (CRS), a major complication of CAR-T therapy, is a systemic inflammatory reaction associated with the release of cytokines, such as IL-6, IFN-γ, and TNF-α [23]. CRS-related coagulopathy is associated with hypofibrinogenemia. In adult hematologic malignancies patients accepting CAR-T therapy, serum fibrinogen levels elevated at early stage of CRS and dropped significantly after administration of tocilizumab in a dose dependent manner (p = 0.004), while patients who did not receive tocilizumab had increased fibrinogen levels [24]. The mechanism of T-HFIB still unclear. In acute phase reaction, fibrinogen biosynthesis is positively regulated by IL-6-mediated transcription of the fibrinogen mRNA [25]. As an inhibitor of IL-6R, tocilizumab may inhibit the expression of fibrinogen by blocking the IL-6 signaling pathway and lead to prolonged hypofibrinogenemia. Therefore, for patients with infection, COVID-19, CAR-T therapy, it is necessary to monitor fibrinogen levels during tocilizumab treatment.
Baseline fibrinogen level has been a strong predictor of HFIB in multiple studies [26–28]. Consistently, we found that high baseline fibrinogen level was a protector factor for T-HFIB. Among the patients included in present study, about 50% of baseline serum fibrinogen levels were 3.0-5.4 g/L, the quartile value of serum fibrinogen was 4.42 g/L. The lowest baseline serum fibrinogen was 2.08 g/L and the highest value was 11.22 g/L. We choose 4.4 g/L as the inflection point to analyze the correlation between bleeding risk and baseline fibrinogen level, and found that patients with baseline level at 2-4.4 g/L were have a significantly higher risk of bleeding than those with baseline above 4.4 g/L (95%CI:1.037 ~ 3.026, p = 0.036). It is reported that the median level of baseline fibrinogen was 5.47 (3.32–7.99) g/L in systemic-onset juvenile idiopathic arthritis patients, 5.20 (4.36–7.14) g/L in COVID-19 patients, and higher than 4.0 g/L in patients after CAR-T therapy with grade 3 CRS without tocilizumab treatment [18, 21, 24]. Therefore, for those patients, serum fibrinogen should be closely monitored after tocilizumab treatment even in normal range, since they have a high risk of bleeding.
Combination of disease modifying antirheumatic drugs was identified as a protective factor T-HFIB. Since other rheumatism related indexes were not analyzed in present study, it is unclear that whether T-HFIB was associated with rheumatoid disease activity. According to previous study, in rheumatoid arthritis patients treated with tocilizumab, tender joint count and swollen joint count were independent risk factors for the occurrence of hypofibrinogenemia [19].
4.3. The prediction effect of nomogram on the risk of T-HFIB.
Taking infection, COVID-19, CAR-T therapy, combined use of antirheumatic drugs and glucocorticoids, and baseline fibrinogen as independent variables, we established a nomogram to predict the risk of hypofibrinogenemia after tocilizumab treatment. The validation results of nomogram showed a good discrimination and accuracy. This prediction model will helpful to identify the patients with high-risk of T-HFIB and provide data support for improving patient treatment safety.
This study still has some limitations. Firstly, it is a single-center retrospective study. Secondly, the sample size included in this study is limited, science only a small percentage of patients receiving tocilizumab were routinely monitored for fibrinogen. Finaly, there were significant differences in the diagnosed diseases of the included patients, including tumor, blood disease, rheumatism, infection, and the combination therapy of tocilizumab with various other drugs is relatively complex, which may lead to bias in results. In the future, a large-scale, prospective multi-center studies should be conducted to verify the results of this study and provide more reliable data for clinical therapy.