T-ICAHT: Grading and Prognostic Impact of Thrombocytopenia After CAR T-cell Therapy

• Based on duration and timing of thrombocytopenia, we introduce T-ICAHT as a novel grading that expands on the existing N-ICAHT system.

• Severe (Grade 3+) T-ICAHT is strongly associated with increased transfusion burden and inferior survival following CAR T-cell therapy.

Immune effector cell-associated hematotoxicity (ICAHT) was recently introduced as a distinct toxicity category of CAR-T therapy. While a grading system based solely on neutrophil counts was proposed (hereafter termed N-ICAHT), the prevalence and prognostic impact of thrombocytopenia remains poorly defined. In this multicenter observational study, we systematically examined patterns of thrombocytopenia in 744 patients treated with commercial CD19 CAR-T for B-cell Non-Hodgkin Lymphoma (B-NHL). We developed a grading system termed T-ICAHT with thresholds that closely aligned with N-ICAHT - based on depth, duration and timing of thrombocytopenia. In the core NHL dataset, 43% of patients developed any-grade early T-ICAHT (day 0-30), with 23% developing severe (G3+) manifestations. Late T-ICAHT (day 31-100) was observed in 42% (G3+: 13%). While T-ICAHT and N-ICAHT gradings showed some correlation, considerable discordance was noted. On multivariate analysis, bridging therapy, poor performance status, and high Hematotox scores were associated with increased risk of severe early T-ICAHT. Patients with higher T-ICAHT grades showed increased platelet and red blood cell transfusion burden (p<0.001) and more bleeding events (p=0.01). T-ICAHT grades were inversely associated with overall survival (OS), with landmarked 2-year estimates ranging from 67% (G0), to 48% (G1-2) and 35% (G3+). In multivariable Cox regression analysis, the independent prognostic capacity of T-ICAHT for OS was confirmed. Finally, we validated T-ICAHT's clinical and prognostic utility in 3 external cohorts spanning an additional 599 pediatric and adult patients (NHL, MM, B-ALL), confirming its broad applicability. These findings support integrating T-ICAHT into the ICAHT framework to standardize thrombocytopenia grading in CAR-T recipients.