Treatment of Human B-Cell Precursor Leukemia in SCID Mice by Using a Combination of the Anti-CD19 Immunotoxin B43-PAP with the Standard Chemotherapeutic Drugs Vincristine, Methylprednisolone, and L-Asparaginase

We have compared the antileukemic activity of the investigational biotherapeutic agent B43-PAP to the antileukemic activities of the standard chemotherapeutic drugs vincristine (VCR), methylprednisolone (PDN), L-asparaginase (L-ASP) as single agents as well as in a 3-drug combination regimen (“VPL”) using a SCID mouse model of human B-cell precursor (BCP) acute lymphoblastic leukemia (ALL). When mice (N = 95) were challenged with 1 × 106 NALM-6 leukemia cells, all of them died of disseminated leukemia with a median event-free survival (EFS) of 47 ± 6 days. B43-PAP was more active than VCR, PDN, or L-ASP and the two-drug combinations VCR+B43-PAP, PDN+B43-PAP, or L-ASP + B43-PAP were not significantly more active than B43-PAP. The 120 days EFS outcome results were 46 ± 13% for B43-PAP (Median EFS = 92 ± 22 days), 0 ± 0% for VCR (Median EFS = 49 ± 1 days), 40 ± 22% for PDN (Median EFS = 100 ± 20 days), 0 ± 0% for L-ASP (Median EFS = 41 ± 1 days), 60 ± 22% for VCR + B43-PAP (Median EFS = >120 days), 60 ± 22% ...