Anti-CD22 immunotoxin RFB4(dsFv)-PE38 (BL22) for CD22-positive hematologic malignancies of childhood: preclinical studies and phase I clinical trial

Abstract

Purpose: Although most children with B-lineage acute lymphoblastic leukemia (ALL) and non-Hodgkin lymphoma are cured, new agents are needed to overcome drug resistance and reduce toxicities of chemotherapy. We hypothesized that the novel anti-CD22 immunotoxin, RFB4(dsFv)-PE38 (BL22, CAT-3888), would be active and have limited nonspecific side effects in children with CD22-expressing hematologic malignancies. We conducted the first preclinical and phase I clinical studies of BL22 in that setting.

Experimental design: Lymphoblasts from children with B-lineage ALL were assessed for CD22 expression by flow cytometry and for BL22 sensitivity by in vitro cytotoxicity assay. BL22 was evaluated in a human ALL murine xenograft model. A phase I clinical trial was conducted for pediatric subjects with CD22+ ALL and non-Hodgkin lymphoma.

Results: All samples screened were CD22+. BL22 was cytotoxic to blasts in vitro (median IC(50), 9.8 ng/mL) and prolonged the leukemia-free survival of murine xenografts. Phase I trial cohorts were treated at escalating doses and schedules ranging from 10 to 40 microg/kg every other day for three or six doses repeated every 21 or 28 days. Treatment was associated with an acceptable safety profile, adverse events were rapidly reversible, and no maximum tolerated dose was defined. Pharmacokinetics were influenced by disease burden consistent with rapid drug binding by CD22+ blasts. Although no responses were observed, transient clinical activity was seen in most subjects.

Conclusions: CD22 represents an excellent target and anti-CD22 immunotoxins offer therapeutic promise in B-lineage hematologic malignancies of childhood.

Fig. 1

Activity of BL22 against CD22-expressing human ALL blasts. (A) CD22 was expressed by primary patient blasts. Anti-CD22 antibody binding capacity per cell quantified in 54 primary patient samples revealed an average CD22 density of 451 – 16,523 sites per cell (median, 4,062). (B) BL22 was cytotoxic against primary patient samples in vitro in a dose responsive manner. A representative cytotoxicity curve is presented. (C) Murine xenografts treated with BL22 had significant prolongation of leukemia free survival in a dose responsive fashion (p < 0.05). PBS, phosphate-buffered saline; RFB4, anti-CD22 MoAb; LMB2, anti-CD25-PE immunotoxin

Fig. 2

Adverse events attributed to BL22. Maximum toxicity grade per patient per cycle (N=23 subjects, 30 cycles). AST, aspartate aminotransferase; ALT, alanine aminotransferase; ANC, absolute neutrophil count; WBC, white blood count; PT, prothrombin time; PTT, partial thromboplastin time

Fig. 3

The impact of disease burden on BL22 pharmacokinetics. Cycle 1 BL22 clearance correlated with the absolute peripheral blood blast count (Spearman correlation r = 0.73, 95% confidence interval, 0.45 – 0.88; p < 0.0001)

Fig. 4

Hematologic improvement after BL22. (A) Improvement in the absolute neutrophil count (ANC) and platelet count during therapy. Arrows represent BL22 treatment days. (B) [Olympus BX51 100× magnification] and (C) [Olympus BX51 1,000× oil magnification]: Bone marrow biopsies reveal decrease in blast infiltration and increase in normal hematopoietic precursors. TdT (terminal deoxynucleotidyl transferase) immunohistochemistry (blasts stain brown). (Subject #20)