Intent-to-Treat Results of a Phase I Trial of CD19 Chimeric Antigen Receptor Engineered T Cells Using a Consistent Treatment Regimen Reveals a 67% Complete Response Rate in Relapsed, Refractory Acute Lymphoblastic Leukemia

Relapsed or refractory acute lymphoblastic leukemia (ALL) remains a difficult therapeutic challenge. We developed a platform where T cells are collected, transduced via retrovirus with a chimeric antigen receptor (CAR) targeting CD19 and incorporating CD3ζ and CD28 domains and reinfused in 11 days. Our recently completed Phase I clinical trial (NCT01593696) in patients age 1-30 years with pre-B ALL or B-cell non-Hodgkin lymphoma (NHL) established a maximally tolerated dose (MTD), revealed cytokine release syndrome (CRS) as the dose-limiting toxicity, demonstrated clearance of CNS leukemia without intrathecal (IT) chemotherapy, and resulted in an intent-to-treat complete response (CR) rate of 67%.

CAR T cells were administered after fludarabine (25 mg/m2/day Days -4, -3, -2) and cyclophosphamide (900 mg/m2/day Day -2). All patients received either 1 x 10⁶ CAR+ T cells/kg (dose level 1), 3 x 10⁶CAR+ T cells/kg (dose level 2), or the maximum number of cells generated if the total available dose fell below the assigned dose level. In the latter case, patients were not evaluable for toxicity but remained evaluable for all other aspects.

We enrolled and treated 20 ALL patients (2 CNS2 leukemia, 6 primary refractory) and 1 NHL. Two of 21 CAR T cell products did not reach the dose level assigned (90% feasibility), but these were still infused.

We determined the MTD to be 1 x 10⁶CAR+ T cells/kg as 2/4 patients at dose level 2 had grade (Gr) 3 or 4 CRS. Dose level 1 was then expanded (n=15) to gain more experience and Gr 3 (n=2; 13%) and Gr 4 (n=2; 13%) CRS occurred. In total, 4 patients received the anti-IL6 receptor antibody, tocilizumab, for severe CRS, 2 of whom also required steroids. Neurotoxicities occurred even in patients without CNS disease and consisted of Gr 1 visual hallucinations (5/21; 24%) and transient Gr 3 dysphasia. No seizures occurred. All CRS and neurotoxicities resolved to baseline. No graft-versus-host disease was seen despite collecting donor-derived T cells directly from patients with prior hematopoietic stem cell transplant (HSCT). B cell aplasia occurred in 12/14 responding patients (86%) but was transient.

Using intent-to-treat analysis, the CR rate was 67% with overall survival of 51.6% (median f/u 10 mths). In the 20 ALL patients, the CR rate was 70% with 12/20 (60%) achieving minimal residual disease negative (MRD−) CR. Of these 12, the leukemia free survival is 78.8% beginning at 4.8 months. Ten had subsequent HSCT since this is standard of care for refractory, relapsed ALL in MRD− remission. The 2 patients who did not have a second HSCT relapsed with CD19− disease. In 2 patients with CNS2 leukemia CSF blasts cleared without IT chemotherapy coincident with rise in CSF CAR T cells. 61% of patients had CAR T cells in the CSF, and absolute CSF CAR T cells correlated with neurotoxicity (p=0.0039).

Peak CAR T cell expansion occurred in blood (PB) at Day 14 and was not detected beyond Day 68, though 10 patients underwent subsequent HSCT complicating interpretation of this data. Expansion of PB CAR T cells correlated with response (p=0.0042). Gr 3 or 4 CRS correlated with disease burden (p=0.0039), total CAR T cell expansion (p=0.0011), total CD8+ CAR T cells (p=0.0087), and effector memory CD8+ (p=0.0087) and CD4+ (p=0.026) CAR T cells in vivo. Maximum fold change in IL-6 and INFγ correlated with Gr 3/4 CRS (p=0.0002 for both) as did peak C-reactive protein (p=0.0015). Three responding patients received second infusions without additional benefit. No evidence of human anti-mouse antibodies were found. But, T cells in 6/11 patients tested proliferated in response to the infused CAR product >3 times that to autologous non-transduced cells (p=0.030). Importantly, all of these patients had complete responses, hence the significance of this finding remains unclear.

Our results demonstrate for the first time a high intent-to-treat feasibility and response rate in a uniformly treated patient population and also provide biomarkers for response, CRS and neurologic toxicities. We conclude that this CD19 CAR platform provides an effective bridge to transplant for patients with refractory and relapsed ALL and is highly active in primary chemorefractory ALL, inducing MRD− remission in 6/6 such patients. Future studies will expand eligibility for patients with CNS leukemia and incorporate a reinduction regimen for patients with high disease burden in an attempt to increase response rates and diminish severity of CRS.