The Safety and Efficacy of CD33 CAR-T Therapy for RR AML after HSCT

Introduction：Acute myeloid leukemia(AML) is a heterogeneous diseasea,although the prognosis has been significantly improved with the advancement of chemotherapy and targeted drugs, it still had no a safe and effective treatment for relapsed and refractory(R/R) AML,especially after allogeneic hematopoietic stem cell transplantation(allo-HSCT). Recently chimeric antigen receptor (CAR) T-cell therapy has developed rapidly on lymphoid tumors, but CAR-T cell therapy for AML still remains unsatisfactory. So, we conducted an exploration of humanized CD33 CAR-T（hCD33 CAR-T） cell therapy for R/R AML after allo-HSCT.

Methods：A total of 12 patients who received hCD33 CAR-T treatment from May 8, 2023 to March 14, 2024 were included. The leukemia cells of all patients expressed CD33 before treatment and after excluding contraindications to CAR-T cell therapy, chemotherapy based on fludarabine was given as a pre-treatment regimen, then transfused hCD33 CAR-T cells collected from the donor or the patients with 6.2 × 10⁴/kg～6.15 × 10⁵/kg. To observe the safety and efficacy after 15-30 days of CAR-T cell refusion by bone marrow（BM） and cerebrospinal fluid （CSF）examination.

Results：12pts：3 females and 9 males with a median age of 36 years (14～51 years). 1 pts relapsed after a second allo-HSCT, another 11 pts relapsed after a first allo-HSCT; 1 pts diagnosed APL, 1 pts diagnosed MDS to AML after HSCT, 10 pts were primary AML. Before CAR-T refused, 8 pts presented with 5.0% ～85.5% leukemia blasts in BM/peripheral blood(PB) and 4 pts were minimal residual disease (MRD) – positived by Flow cytometry or Real time quantitative PCR for fusion gene(FCM+/FG+), 4 pts had central nervous system leukemia（CSF+）. After CD33 CAR-T treatment, 5 (5/12, 41.67%) pts achieved BM complete remission （CR）(1 was MRD+CR, 4 was MRD- (FCM/FG)),；4 CSF+ pts:1 pts FG-MRD-of CSF, 1 pts FCM-MRD-, 1 pts MRD of CSF became negative before CAR-T refused and the other 1 pts were not examined. ALL of 12 pts, 2 pts were ineffective with no CRS response, and 1 of them had liver function damage suspected to be related to infection or leukemia; 9 pts had CRS 1, 1 pts had CRS 2, and no ICANS occurred in all patients; 4 pts experienced liver function damage after CAR-T cell transfusion, which may be related to GVHD or CRS; 2 pts effectively developed sepsis, 1 pts died from septic shock due to long-term granulocyte deficiency, and 1 pts was controlled after anti infective treatment. Follow up until June 2024（4m～1y），1 pts were CMR without bridging a second allo-HSCT，5 pts survived bridging a second allo-HSCT，another 6 pts were died due to infection or disease progression.

Conclusions：As a salvage treatment for rr AML including those who relapse after allo-HSCT，our hCD33 CAR-T resulted in CR rate of 41.67% which already was better than other treatment such as chemotherapy,donor lymphocyte reinfusion （DLI）even other CAR-Ts ；On the other hand, our experience , although we recommend patients to bridging secondary transplantation or donor stem cell reinfusion to support hematopoiesis after CD33 CAR-T, but actually CD33 CAR-T side effects can be controlled, and some patients can survive without bridging the secondary transplantation. So for rr AML patients even after allo-HSCT, CD33 CAR-T is still recommended.