Targeting CD22 in Childhood B-Precursor Acute Lymphoblastic Leukemia (Pre-B ALL): Pre-Clinical Studies and Phase I Trial of the Anti-CD22 Immunotoxin CAT-3888 (BL22).

Although most children with pre-B ALL are cured, treatment is associated with multiple toxicities and outcome after relapse is poor. New therapies are needed to overcome drug resistance and reduce non-specific toxicities of chemotherapy. We evaluated the B-lineage differentiation antigen CD22 and the anti-CD22 immunotoxin RFB4(dsFv)-PE38 (CAT-3888 or BL22) in childhood ALL.

Methods: Blasts were obtained from children with pre-B ALL. Flow cytometry was used to determine CD22 expression and anti-CD22 antibody binding using the BD Quantibrite System. 72 hour in vitro protein synthesis cytotoxicity assays were conducted. Murine xenografts (human ALL cell line EU-1 into CB17 SCID mice) were treated with varying doses of CAT-3888. A pediatric Phase I trial of CAT-3888 was initiated.

Results: All cases (n=69) were CD22 positive. Median CD22 site density (studied in 28 cases) was 4,099 sites/cell (range, 451 – 9,380). In vitro cytotoxicity to CAT-3888 was assessed in 43 cases and the median IC₅₀ was 10 ng/ml. Murine xenografts treated with CAT-3888 had significant prolongation of leukemia free survival in a dose responsive fashion (p<0.05). 21 subjects (18 ALL, 3 lymphoma; 3–22 years of age) were treated on a Phase I trial of CAT-3888 at doses of 10–40 mcg/kg QOD for 3–6 doses with cycles repeated every 21–28 days. Treatment was well tolerated and adverse events attributed to CAT-3888 were reversible. Pharmacokinetic parameters appeared to be influenced by disease burden consistent with rapid drug binding by CD22 positive blasts. All subjects were heavily pre-treated and had progressive high burden disease at the time of treatment. Transient clinical activity was observed in 17 of 21 subjects as evidenced by decreased peripheral blood blast count (8), decreased marrow infiltration (6), decreased extramedullary infiltration (3), increased platelet count (2), increased neutrophil count (1), increased reticulocyte count (1), improved PET scan (1), decreased tumor-associated pain (2), and/or peripheral blast count stabilization (5). A dose response was apparent with 6 of 12 (50%) achieving disease stabilization at or above 30 mcg/kg (vs. 0/9 at lower doses). Three subjects developed high-titer neutralizing antibodies, 1 of which had pre-existing low titer antibodies from prior pseudomonas infection.

Conclusions: CD22 represents a relevant target for childhood pre-B ALL. All cases tested were CD22+ and most blasts were highly sensitive to CAT-3888 in vitro at concentrations below clinically achievable levels. CAT-3888 showed a dose response effect and prolonged leukemia free survival in murine ALL xenografts. CAT-3888 is well tolerated in children and activity has been seen in most subjects treated on the Phase I trial. A new higher affinity anti-CD22 immunotoxin (CAT-8015 or HA22) is in development.