Chimeric antigen receptor (CAR) T-cell therapy causes serious side effects including cytokine release syndrome (CRS).
CRS-related coagulopathy is associated with hypofibrinogenemia that has up to now been considered the result of disseminated
intravascular coagulation (DIC) and liver dysfunction. We investigated the incidence and risk factors for hypofibrinogenemia
in 41 consecutive adult patients with hematologic malignancies (median age 69 years, range 38-83 years)
receiving CAR T-cell therapy between January 2020 and May 2023 at the University Medical Center Regensburg. CRS occurred
in 93% of patients and was accompanied by hypofibrinogenemia already from CRS grade 1. Yet DIC and liver dysfunction
mainly occurred in severe CRS (≥ grade 3). After an initial increase during CRS, fibrinogen levels dropped after administration
of tocilizumab in a dose-dependent manner (r = -0.44, P=0.004). In contrast, patients who did not receive tocilizumab
had increased fibrinogen levels. Logistic regression analysis identified tocilizumab as an independent risk factor for hypofibrinogenemia
(odds ratio = 486, P<0.001). We thus hypothesize that fibrinogen synthesis in CRS is up-regulated in an
interleukin-6-dependent acute phase reaction compensating for CRS-induced consumption of coagulation factors. Tocilizumab
inhibits fibrinogen upregulation resulting in prolonged hypofibrinogenemia. These observations provide novel insights
into the pathophysiology of hypofibrinogenemia following CAR T-cell therapy, and emphasize the need for close fibrinogen
monitoring after tocilizumab treatment of CRS.