Development of ThisCART371 Targeting CLL1 for the Treatment of Relapsed/Refractory Acute Myeloid Leukemia
Authors: Zhang Xiaomei , Xiao Xia , Lv Hairong , Bai Xue , Liu Pengjiang , Pu Yedi , Meng Juanxia , Zhu Haibo , Wang Zhao , Zhang Huan , He Xiaoyuan , Zhang Yu , Lv Cuicui , Zhao Yifan , Guo Shujing , Liu Jile , Zhao Mohan , Li Jun , Zhao Mingfeng 
Published: December 2024

Abstract
While CAR-T cells have demonstrated significant therapeutic benefits in relapsed/refractory Acute myeloid leukemia (r/r AML), the current use of autologous T cells presents limitations. Therefore, there is a need to develop an off-the-shelf allogeneic cell product. Gene-editing technologies can efficiently deplete endogenous TCR, but also leads to off-target edits and chromosomal abnormality. We developed a novel non-gene-editing platform named ThisCART371 targeting CLL1. The safety and efficacy of this product were reported in 3 patients of our center.
Methods
ThisCAR371 was manufactured based on the intracellular retention of TCRαβ/CD3 complex, allowing for allogeneic CAR-T production with a single lentiviral vector without genetic depletion of TCR. The construct contains a CLL1-targeted CAR and a KDEL-tagged anti-CD3 single chain antibody (scFv) which prevents TCRαβ/CD3 from being secreted from the endoplasmic reticulum (ER) in order to avoid graft-verse-host disease (GVHD). Finally, the product was being used as a compassionate drug in the hematology department of Tianjin First Central Hospital. Data were obtained from three patient case reports. This study represents the first evaluation of both safety and efficacy for this product. All patients received FCE lymphocyte clearance regimen (cyclophosphamide at a dose of 60mg/kg for 2 days +fludarabine at a dose of 30mg/m2 for 5 days +etoposide at a dose of 100mg for 2 days) followed by a single infusion of 3×106/kg ThisCART371.
Results
Patient 1 and 2 developed grade 3 cytokine release syndrome (CRS) characterized by fever, hypoxemia, and circulatory dynamic instability. Following CAR-T infusion, the levels of IL-6, TNF-α, CRP and ferritin significantly increased. Dexamethasone, tocilizumab and plasma exchange were utilized to mitigate inflammation. Patient 3 experienced a grade 2 CRS reaction which was promptly controlled with oxygen inhalation and symptomatic treatment using tocilizumab. In addition, patient 1 presented with grade 1 immune effector cell-associated neurotoxicity syndrome (ICANS) on day 5 post-CAR-T infusion, manifesting as headache without signs of cerebral edema on brain imaging or other neurological symptoms. Intrathecal dexamethasone administration gradually decreased alongside cytokine level reduction. No evidence of neurotoxicity was observed in the remaining two patients. No GVHD or tumor-lysis syndrome occurred in these 3 patients during the study period.
The disease response was evaluated in 3 patients. Patient 1 underwent a bone marrow examination on day 11, revealing the presence of 0.88% residual tumor cells without the expression of CLL1. Subsequently, the patient received allogeneic HSCT on day 17 without myeloablative conditioning regimen and achieved MRD-CR. Unfortunately, he experienced a relapsed again on day 13 post-transplantation and discharged from the hospital, ultimately passing away 32 days after transplantation. Patient 2 underwent a bone marrow examination on day 9, with no evidence of MRD, however, she eventually succumbed to a central nervous system infection. Patient 3 had a bone marrow examination on day 14 which revealed the detection of 2.7% tumor residue. Subsequently, allogeneic HSCT transplantation was recommended but declined by the patient, who was discharged from the hospital and unfortunately passed away due to severe neutropenia.
Robust expansion of CAR-T cells was observed in all three patients. Early prophylaxis with dexamethasone does not appear to impact the efficacy of CAR-T cell expansion and therapy. This suggests that hormone drugs may be used for routine prevention in general CAR-T applications in the future. However, due to various reasons, it is currently not feasible to detect the persistence of universal CAR-T cells, and patients may experience severe hemocytopenia following ThisCART371 treatment. In order to prevent infections resulting from severe immune deficiency, patients who have achieved remission after CAR-T cell therapy should undergo immediate allogeneic HSCT if possible, in order to prolong their survival.
Conclusions
This study highlights the potential of the “off-the-shelf” CLL1 CAR-T cell product for the treatment of r/r AML. Nevertheless, the occurrence of CRS is relatively severe, thus suggesting a potential use of hormones for CRS prevention and management in future applications.