In vivo efficacy of B43 (anti-CD19)-pokeweed antiviral protein immunotoxin against human pre-B cell acute lymphoblastic leukemia in mice with severe combined immunodeficiency

Abstract

A highly aggressive subclone of the human CALLA+Cμ+ pre-B acute lymphoblastic leukemia (ALL) cell line NALM-6 (designated NALM-6-UM1) caused disseminated and fatal leukemia in CB.17 mice with severe combined immunodeficiency (SCID). An intravenous challenge with 1 x 106 (NALM-6-UM1 cells caused 15 of 27 (56%) SCID mice to become paraplegic at 31 ± 2 days (median = 33 days) and 27 of 27 (100%) mice to die of disseminated leukemia at 38 ± 1 days (median = 39 days). We used this SCID mouse model of aggressive human pre-B ALL to evaluate the in vivo antileukemic efficacy of B43 (anti-CD19)-pokeweed antiviral protein (PAP) immunotoxin. A 3-day treatment with nontoxic doses of B43-PAP markedly reduced the incidence of paraplegia and improved event-free survival (EFS) in SCID mice challenged with 1 x 106 NALM-6-UM1 pre-B ALL cells, as reflected by significantly higher cumulative proportions of mice free of paraplegia or alive at 1 to 7 months, as compared with phosphate-buffered saline (PBS) treated control mice. The Kaplan-Meier estimates and standard errors of the probability of developing paraplegia after inoculation of 1 x 106 NALM-6-UM1 cells was 64% ± 10% for PBS-treated mice (median time to paraplegia = 37 days) (N = 27), 18% ± 8% for mice treated with 15 μg B43-PAP (5 μg/mouse/d x 3 days) (N = 23) and 5% ± 5% for mice treated with 30 μg B43-PAP (10 μg/mouse/d x 3 days) (N = 21). While 27 of 27 PBS-treated control SCID mice died of leukemia at 38 ± 1 days (range = 24 to 54 days), only 16 of 44 B43-PAP-treated mice developed leukemia at 74 ± 12 days (range = 30 to 182 days), consistent with ≥6 logs kill of clonogenic NALM-6-UM1 cells in 64% of SCID mice. The Kaplan-Meier estimates and standard errors of the probability of long-term EFS after inoculation of 1 x 106 NALM-6-UM1 cells were 65% ± 10% for mice treated with 15 μg B43-PAP and 60% ± 11% for mice treated with 30 μg B43-PAP with a median survival time of greater than 7 months for both groups. In contrast, neither unconjugated B43 monoclonal antibody nor the anti-T-cell immunotoxin G17.2 (anti-CD4)-PAP decreased the incidence of paraplegia or improved EFS. Six long-term survivors in the B43-PAP-treated groups (four mice treated with 15 μg and two mice treated with 30 μg B43-PAP) were electively killed at 7 months to assess their leukemia burden. None of these mice had pre-B ALL as indicated by histopathologic examination or flow cytometry, while PCR provided molecular evidence of occult leukemia in three of four mice treated with 15 μg B43-PAP but none of the two mice treated with 30 μg B43-PAP. To our knowledge, this report is the first to examine the in vivo antileukemic efficacy of an immunotoxin against CALLA+Cμ+ human pre-B ALL in an SCID mouse model system. © 1992 by The American Society of Hematology.