Anti-CD19 Chimeric Antigen Receptor (CAR) T Cells Produce Complete Responses With Acceptable Toxicity But Without Chronic B-Cell Aplasia In Children With Relapsed Or Refractory Acute Lymphoblastic Leukemia (ALL) Even After Allogeneic Hematopoietic Stem Cell Transplantation (HSCT)
Authors: Lee Daniel W. , Shah Nirali N , Stetler-Stevenson Maryalice , Sabatino Marianna , Delbrook Cindy , Richards Kelly , Kochenderfer James N , Rosenberg Steven A. , Stroncek David , Wayne Alan S. , Mackall Crystal L 
Published: October 2019

Abstract
Survival after relapsed and refractory pediatric pre-B ALL and non-Hodgkin lymphoma (NHL) is poor despite intensive chemotherapy and HSCT. CAR T cells combine the specificity and MHC independence of antibodies with the cytotoxic capacity of T cells. By genetically engineering them with a CAR that links an scFv for CD19, present on most NHL and ALL blasts, with the CD3zeta activation domain of the T cell receptor and the CD28 co-stimulatory domain, cytotoxicity can be specifically and fully activated with a single interaction. We undertook a Phase I clinical trial (NCT01593696) of anti-CD19-CD28-zeta CAR T cells in children with relapsed and refractory ALL and NHL with the aim of assessing 1) feasibility of generating adequate numbers of CAR T cells from a high risk population including post-HSCT patients 2) response rate 3) CAR T cell persistence and trafficking to extramedullary sites and 4) toxicities including acute toxicity related to cytokine release syndrome (CRS) and chronic B-cell aplasia related to long-term persistence of CAR T cells.
Patients were enrolled on study, then PBMCs were collected by apheresis then immediately enriched for T cells using activating anti-CD3/CD28 beads before retroviral transduction of the CAR gene. After an 11-day manufacturing process, CAR T cells were infused fresh to patients who have received fludarabine (25 mg/m2/day Days -4, -3, -2) and cyclophosphamide (900 mg/m2/day Day -2).
We have enrolled and treated 8 patients (7 ALL, 1 NHL; 4 pre-HSCT, 4 post-HSCT) aged 10-23 years. Regarding feasibility, 6 of 8 patients had successful expansion of CAR T cells that met the assigned dose level, with transduction efficiencies of 18-87%. Expansion was insufficient to meet the target dose for 2 patients, but each still received products that were 3% and 14% of the target dose.

Response rate
The overall complete response (CR) rate is 5 of 8 (62.5%; 95% CI 29-96%) or 5 of 7 ALL patients (71.4%; 95% CI 38-105%) with 3 of these being MRD-negative, including 1 patient who was primarily refractory to chemotherapy, and who proceeded to HSCT following CD19 CAR therapy. Both patients who received cells below the target dose experienced anti-leukemic effects, one with a transient CR and the other with an MRD-negative CR.
CAR T cell expansion, persistence and trafficking: CAR T cells have been identified in blood (0.1-38%) and marrow (0.1-5%) in all responding patients. They have also been found in the CSF of 3 patients (0.3-17%), in the pleural fluid (13%) of an NHL patient with pre-existing malignant pleural effusions, and are suspected to have caused Gr 1 scrotal edema in a patient with a remote history of testicular disease. One patient with CNS2 disease at the time of enrollment cleared all CSF blasts as detected by flow cytometry without additional intrathecal chemotherapy after CAR T cells were administered (max 17% CAR T cells in CSF). The mean time to undetectable CAR T cells in any tissue in responding patients was 55 days (± 22.6; 95% CI 35-72).

Toxicity
Treatment was well tolerated. Two patients had Gr 2 CRS (Gr 3 fever, Gr 2 hypotension) that resolved with IV fluids and correlated with high IL6, GM-CSF, IFNg, TNFa, and C-reactive protein. One DLT (Gr 4 CRS) occurred (3 x 10^6 CAR+ T cells/kg) and required vasopressors for hypotension. After identifying high plasma IL6, the anti-IL6 receptor antibody, tocilizumab, was administered, and quickly reversed most toxicity from CRS. At the time of Day 28 restaging, CD19+ hematogones were detected by flow cytometry in four of five responding patients (mean 81.4% of all CD19+ cells; 95% CI 51-112%), indicating that significant antileukemic effects can be induced by CD19 CAR T cells without chronic depletion of B cell precursors. None of the patients with prior HSCT developed graft versus host (GVH) disease despite administering donor-derived activated T cells harvested from the recipient.

Conclusions
Anti-CD19-CD28-zeta CAR T cells that mediate potent antileukemic effects can be reliably generated, even from very advanced patients with or without a history of allogeneic HSCT. Using intent-to-treat reporting, CR rates are high (62.5%) in this refractory population. CD19 CAR T cells traffic to extramedullary sites and can mediate anti-tumor effects in CSF. Acute toxicity is manageable and because the anti-CD19-CD28-zeta CAR T cells do not persist at high levels for prolonged periods of time, rapid resumption of B cell lymphopoiesis occurs following therapy.

Disclosures:
Off Label Use: Anti-CD19 CAR T cells for the treatment of ALL and NHL.