First-in-Human CLL1-CD33 Compound CAR T Cell Therapy Induces Complete Remission in Patients with Refractory Acute Myeloid Leukemia: Update on Phase 1 Clinical Trial

Background

CD19-specific chimeric antigen receptor (CAR) T cell therapy has achieved high efficacy in acute lymphoblastic leukemia patients. However, the treatment of acute myeloid leukemia (AML) has remained a particular challenge due to the heterogeneity of AML bearing cells, which renders single antigen targeting CAR T cell therapy ineffective. CLL1 and CD33 are often used as targets for AML CAR T cell therapy. CLL1 is associated with leukemia stem cells and disease relapse, and CD33 is expressed on the bulk AML disease. Previously, we demonstrated the profound anti-tumor activity of CLL1-CD33 compound CAR (cCAR) T cells. Here we present the efficacy of cCAR in preclinical study and update the success in level 1 dose escalation clinical trial on relapsed/refractory AML patients.

Methods

We engineered a cCAR comprising of an anti-CLL1 CAR linked to an anti- CD33 CAR via a self-cleaving P2A peptide and expressing both functional CAR molecules on the surface of a T-cell cell. We tested the anti-leukemic activities of CLL1-CD33 cCAR using multiple AML cell lines, primary human AML samples, human leukemia cell line (REH cells) expressing either CLL1 or CD33, and multiple mouse models. An alemtuzumab safety switch has also been established to ensure the elimination of CAR T cells following tumor eradication. Children and adults with relapsed/refractory AML were enrolled in our phase 1 dose escalation trial with primary objective to evaluate the safety of cCAR and secondary objective to assess the efficacy of cCAR anti-tumor activity.

Results

Co-culture assays results showed that cCAR displayed profound tumor killing effects in AML cell lines, primary patient samples and multiple mouse model systems. Our preclinical findings suggest that cCAR, targeting two discrete AML antigens: CLL1 and CD33, is an effective two-pronged approach in treating bulk AML disease and eradicating leukemia stem cells. Patients enrolled in the phase 1 dose escalation trial have shown remarkable response to cCAR treatment. Noticeably, a 6-yr-old female patient diagnosed with a complex karyotype AML including FLT3-ITD mutation had achieved complete remission. The patient was diagnosed with Fanconi anemia, which had progressed to juvenile myelomonocytic leukemia and eventually transformed into AML. The patient had been resistant to multiple lines of treatments, including 5 cycles of chemotherapy with FLT3 inhibitor prior to receiving cCAR. Before the treatment, patient's leukemia blasts comprised 73% of the peripheral blood mononuclear cells and 81% of the bone marrow. Patient underwent lymphodepletion therapy (Fludarabine and Cyclophosphamide) prior to cCAR infusion. Two split doses, each consisting of 1x10⁶/kg CAR T cells, were infused on day 1 and day 2 respectively. On day 12, while leukemia blast still counting up to 98% of the bone marrow (Fig. 1A), robust CAR T cell expansion was detected in both peripheral blood and bone marrow. On day 19, patient achieved MRD- complete remission with bone marrow aspirates revealing complete ablation of myeloid cells (Fig. 1B). Flow cytometry confirmed the absence of leukemia blasts and showed that CAR T cells comprised 36% of the PBMC and 60% of the bone marrow. The patient later underwent non-myeloablative hematopoietic cell transplantation with less toxicities compared to conventional total body radiation and high dose chemotherapies. Updated results on other patients enrolled in this clinical trial including adverse events will be presented.

Conclusion

Our first-in-human clinical trial demonstrates promising efficacy of cCAR therapy in treating patients with relapsed/ refractory AML. cCAR is able to eradicate leukemia blasts and leukemia stem cells, exerting a profound tumor killing effect that is superior to single target CAR T cell therapies. cCAR is also shown to induce total myeloid ablation in bone marrow, suggesting that it may act as a safer alternative to avoid the severe toxicities caused by standard bone marrow ablation regimens without sacrificing the anti-tumor efficacy. This strategy will likely benefit patients who are unable to tolerate total body radiation or high dose chemotherapies. In addition to AML, cCAR also has the potential to treat blast crisis developed from myelodysplastic syndrome, chronic myeloid leukemia, and chronic myeloproliferative neoplasm.

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