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 x 10(6) 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% for PDN + B43-PAP (Median EFS = >120 days), and 50 +/- 25% for L-ASP + B43-PAP (Median EFS = 93 +/- 27 days), When mice (N = 61) were challenged with 5 x 10(6) NALM-6 cells, all of them rapidly died of disseminated leukemia with a median EFS of 37 +/- 3 days. The 3-drug combination "VPL" (Median EFS = 75 +/- 23 days) was slightly less active than B43-PAP (Median EFS = 84 +/- 19 days) (P = 0.09). Notably, the combination of "VPL" with B43-PAP (i.e., VPLB) resulted in 100% survival. By comparison, the combination of "VPL" with daunorubicin (i.e., VPLD) (Median EFS = 69 +/- 31 days) was not more active than VPL. To our knowledge, this preclinical study is the first to demonstrate the feasibility and superb antileukemic activity of immunochemotherapy using anti-CD19 immunotoxin in combination with the standard 3-drug combination "VPL" against BCP ALL.