Background

Chimeric antigen receptor (CAR)-T cell therapy has demonstrated remarkable success in treating a variety of blood cancers, such as CD19 CAR-T for B-cell malignancies and BCMA CAR-T for myeloid myeloma (MM). However, similar progress has yet to be achieved with relapsed and refractory acute myeloid leukemia (R/R AML), primarily due to the heterogeneous nature of AML, making it difficult to find an ideal CAR-T target. Previous efforts have targeted single CD33, CD123, LeY, NKG2D, or CD70 receptors, but the overall response rate is much lower compared to CD19 CAR-T. Improved clinical outcome was recently reported with a dual-receptor CAR-T (CD33 and CLL1), reaching ~80% CR for R/R AML patients. However, CD33 is also expressed in normal hematopoietic stem cells (HSC) and patients need allogeneic hematopoietic stem cell transplantation (HSCT) following CAR-T therapy. Notably, AML mainly affects the elder population, and patients more than 65 years old are usually not suitable for HSCT. To address these challenges, we aim to find an effective target for AML without the need for the HSCT. In our study, CLL1 is chosen as a promising target as it is not expressed on normal HSCs, but highly expressed on AML blasts cells and leukemia stem cells (LSCs), which is a small population that plays an important role in disease progression and relapse. Here we report the results from a Phase I clinical trial to evaluate the toxicity and efficacy of the CLL1 CAR-T in the treatment of pediatric R/R AML.

Methods

We have generated a 2nd generation of CLL1 CAR-T, the extracellular scFv was derived from a murine CLL1 monoclonal antibody, which was generated by hybridoma technology. Autologous CAR-T cells were manufactured in a cGMP facility. Between Oct 2019 and Mar 2020, 3 pediatric R/R AML patients were infused. CAR-T cells were given by a dose at 0.2-1x106/kg with a single dose.

Results

Of the 3 patients infused, cytokine release syndrome (CRS) occurred in 3 patients (2 grade Ⅰ, 1grade II), no neurotoxicity occurred. All patients suffered pancytopenia, granulocytopenia and monocytopenia. All the adverse effects were resolved after treatment.

Patient 1 is a refractory AML patient with more than 95% of AML blasts being CLL1 positive. The infused CAR-T cells showed typical clonal expansion peaking at Day 8. The patient reached CR/MRD- when evaluated at Day 21 post infusion. The patient received HSCT at Day 35, and remained CR/MRD- till now (Jul 2020, 8 months post CAR-T infusion).

Patient 2 is a relapsed patient after HSCT and showed severe bone marrow necrosis (BMN). Therefore, we only generated a dose 0.35-1x106/kg CAR-T cells. The patient reached CR/MRD- when evaluated at Day 14 post infusion. The patient went through a second HSCT at Day 38. However, the patient passed away due to GVHD two months after the second HSCT.

Patient 3 is a refractory AML patient with about 85% of AML blasts being CLL1 positive. The patient reached CR/MRD+ (1%) when evaluated at Day 14 post infusion. However, AML blast increased to 11% when evaluated at Day 30, a majority of them were CLL1 negative. The patient received Azacitidine treatment at Day 30-34, and AML blast decreased to 2.8% at Day 45. Notably this patient was refractory to Azacitidine treatment previously, suggesting that CLL1 CAR-T could eradicate LSCs, making the left AML tumor cells sensitive to chemotherapy. This patient received HSCT at Day 75, and remained CR/MRD- till now (Jul 2020, 7 months post CAR-T infusion).

Conclusion

Our study suggested that CLL1 CAR-T is a therapy with high efficacy and manageable toxicity in R/R AML patients. All patients in this study could reach CR within one month and only experienced 1-2 grade CRS. For patients with CLL1 negative AML blast, the CLL1- cells may take over after CLL1 CAR-T therapy. Combination with chemotherapy like Azacitidine may help patients reach complete response.

Disclosures

No relevant conflicts of interest to declare.

Author notes

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Asterisk with author names denotes non-ASH members.