Clinical Activity and Persistence of Anti-CD22 Chimeric Antigen Receptor in Children and Young Adults with Relapsed/Refractory Acute Lymphoblastic Leukemia (ALL)

Background: Adoptive cellular therapy with genetically modified T-cells using viral-based vectors to express chimeric antigen receptors targeting the CD19 molecule have demonstrated dramatic clinical responses in patients ALL. However, not all patients respond and CD19-negative escape has been observed. Thus, additional targets are needed. CD22 is a B-lineage-restricted, transmembrane phosphoglycoprotein of the Ig superfamily that is widely expressed on B-precursor ALL. Therefore, CD22 represents a promising target. This phase I dose-escalation study represents the first in human testing of anti-CD22 CAR adoptive cell therapy. The primary objectives were to determine the feasibility of producing anti-CD22 CAR cells meeting established release criteria and to assess the safety of administering escalating doses of anti-CD22-CAR engineered T cells in children and young adults with relapsed or refractory CD22-expressing B cell malignancies following a cyclophosphamide/fludarabine preparative regimen. Secondary objectives include determination of anti-leukemia effects, measurement of persistence of adoptively transferred anti-CD22 CAR T cells, and evaluation of cytokine profiles.

Design: Children and young adults with relapsed/refractory CD22-expressing hematologic malignancies were eligible. Study endpoints included toxicity, feasibility, and antigen-specific immune and clinical responses. All enrolled subjects underwent autologous leukopheresis for peripheral blood mononuclear cells. Cells were then CD3+ enriched and cultured in the presence of anti-CD3/-CD28 beads followed by lentiviral vector supernatant containing the anti-CD22 (M971BBz) CAR, with culture duration of 7-10 days. On Day -4 (cell infusion=Day 0), subjects began induction chemotherapy with fludarabine 25 mg/m² on Days –4, –3 and –2 and cyclophosphamide 900 mg/m² on day –2. The first dose level started at 3 x 10⁵ transduced T-cells/recipient weight (kg).

Results: 6 subjects, aged 7-22 years, with ALL have been treated to date. All enrolled subjects had previously undergone at least one prior allogeneic hematopoietic stem cell transplant and all had received treatment with CD19 directed CAR-T cell therapy. Five subjects had a CD19 negative antigen escape, and one subject was a non-responder to prior CD19 CAR therapy. All subjects had demonstration of CD22 expression on > 99% of their ALL, although the antigen binding capacity had variability from < 900 to > 13,000 sites/cells. All subjects underwent successful culture, expansion and infusion of anti-CD22 CAR T-cells at the first dose level. The second subject enrolled met criteria for dose-limiting toxicity by virtue of grade 3 diarrhea which led to dose expansion at the first dose-level to treat a total of 6 subjects. Two subjects had grade 1 cytokine release syndrome (CRS), one subject had grade 2 CRS and in two subjects CRS was not seen. Evidence for CAR-T cell expansion was seen in peripheral blood, bone marrow and cerebrospinal fluid (Table). Clinical responses were evaluated at day 28 (+/- 4 days) post-infusion and  included two subjects who had disease progression, two with disease stabilization and one subject who attained a minimal residual disease (MRD) negative complete remission. Flow cytometric CAR persistence was detected out to 47 days post-infusion in the responding patient with remission maintained for 3 months post-infusion. One patient is actively undergoing treatment and is too early to evaluate.

Conclusions: This first-in-human anti-CD22 CAR T-cell therapy is safe, feasible and clinically active in patients who have undergone previous CAR therapy. Understanding mechanisms which may determine clinical efficacy are being explored. Accrual to the next dose level at 1 x 10⁶ transduced T cells/kg is planned.

#	Age/Sex	Prior HCT	Prior anti-CD19 CAR	CD19 neg relapse?	CD22 site density	Pre-HCT disease burden (% leukemia in aspirate)	Maximum CD22 CAR expansion (flow)			CRS	Best Response
							PB	Marrow	CSF
1	22/M	Y	Y	Y	2084	95-100%	0	0	n/a	None	PD
2	20/F	Y (2)	Y	Y	13452	5%	52.3%	19.5%	0%	1	MRD neg CR
3	22/M	Y	Y	Y	846	>90%	73%	36%	32%	1	SD
4	22/M	Y	Y	N	2589	95%	6%	1%	0%	2	SD
5	7/F	Y	Y	Y	2839	32%	0%	1.3%	0%	None	PD
6	17/F	Y	Y	Y	2185	1%	n/a	n/a	n/a	n/a	n/a
SD: stable disease; PD: progressive disease