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Abstract Background The NLRP3 inflammasome drives release of pro‐inflammatory cytokines including interleukin (IL)‐1β and IL‐18 and is a potential target for ulcerative colitis (UC). Selnoflast (RO7486967) is an orally active, potent, selective and reversible small molecule NLRP3 inhibitor. We conducted a randomized, placebo‐controlled Phase 1b study to assess the safety, tolerability, pharmacokinetics (PK) and pharmacodynamics (PD) of selnoflast. Methods Nineteen adults with previous diagnosis of UC and current active moderate to severe disease were randomized 2:1 to selnoflast or placebo for 7 days. A dose of 450 mg QD (once daily) was selected to achieve 90% IL‐1β inhibition in plasma and colon tissue. Consecutive blood, sigmoid colon biopsies and stool samples were analyzed for a variety of PD markers. Safety and PK were also evaluated. Results Selnoflast was well‐tolerated. Plasma concentrations increased rapidly after oral administration, reaching Tmax 1 h post‐dose. Mean plasma concentrations stayed above the IL‐1β IC90 level throughout the dosing interval (mean Ctrough on Day 1 and Day 5: 2.55 μg/mL and 2.66 μg/mL, respectively). At steady state, post‐dose selnoflast concentrations in sigmoid colon (5‐20 μg/g) were above the IC90. Production of IL‐1β was reduced in whole blood following ex vivo stimulation with lipopolysaccharide (LPS) (in the selnoflast arm). No changes were observed in plasma IL‐18 levels. There were no meaningful differences in the expression of an IL‐1‐related gene signature in sigmoid colon tissue, and no differences in the expression of stool biomarkers. Conclusions Selnoflast was safe and well‐tolerated. Selnoflast 450 mg QD achieved plasma and tissue exposure predicted to maintain IL‐1β IC90 over the dosing interval. However, PD biomarker results showed no robust differences between treatment arms, suggesting no major therapeutic effects are to be expected in UC. The limitations of this study are its small sample size and indirect assessment of the effect on IL‐1β in tissue. Trial registration ISRCTN16847938

T cell engagers represent a novel promising class of cancer-immunotherapies redirecting T cells to tumor cells and have some promising outcomes in the clinic. These molecules can be associated with a mode-of-action related risk of cytokine release syndrome (CRS) in patients. CRS is characterized by the rapid release of pro-inflammatory cytokines such as TNF-α, IFN-γ, IL-6 and IL-1β and immune cell activation eliciting clinical symptoms of fever, hypoxia and hypotension. In this work, we investigated the biological mechanisms triggering and amplifying cytokine release after treatment with T cell bispecific antibodies (TCBs) employing an in vitro co-culture assay of human PBMCs or total leukocytes (PBMCs + neutrophils) and corresponding target antigen-expressing cells with four different TCBs. We identified T cells as the triggers of the TCB-mediated cytokine cascade and monocytes and neutrophils as downstream amplifier cells. Furthermore, we assessed the chronology of events by neutralization of T-cell derived cytokines. For the first time, we demonstrate the contribution of neutrophils to TCB-mediated cytokine release and confirm these findings by single-cell RNA sequencing of human whole blood incubated with a B-cell depleting TCB. This work could contribute to the construction of mechanistic models of cytokine release and definition of more specific molecular and cellular biomarkers of CRS in the context of treatment with T-cell engagers. In addition, it provides insight for the elaboration of prophylactic mitigation strategies that can reduce the occurrence of CRS and increase the therapeutic index of TCBs.