Characterization of a saporin mitotoxin specifically cytotoxic to cells bearing the granulocyte-macrophage colony-stimulating factor receptor

Diphtheria Toxin Fused to Granulocyte-Macrophage Colony-Stimulating Factor Eliminates Acute Myeloid Leukemia Cells With the Potential to Initiate Leukemia in Immunodeficient Mice, But Spares Normal Hemopoietic Stem Cells

We studied the cell kill induced by granulocyte-macrophage colony-stimulating factor (GM-CSF ) fused to Diphtheria Toxin (DT-GM-CSF ) in acute myeloid leukemia (AML) samples and in populations of normal primitive hemopoietic progenitor cells. AML samples from three patients were incubated in vitro with 100 ng/mL DT-GM-CSF for 48 hours, and AML cell kill was determined in a proliferation assay, a clonogenic assay colony-forming unit-AML (CFU-AML) and a quantitative long-term bone marrow (BM) culture ie, the leukemic-cobblestone area forming cell assay (L-CAFC). To measure an effect on cells with in vivo leukemia initiating potential DT-GM-CSF exposed AML cells were transplanted into immunodeficient mice. In two out of three samples it was shown that all AML subsets, including those with long-term abilities in vivo (severe combined immunodeficient mice) and in vitro (L-CAFC assay) were reduced in number by DT-GM-CSF. Cell kill induced by DT-GM-CSF could be prevented by coincubation with an excess of GM-CSF, demonstrating that sensitivity to DT-GM-CSF is specifically mediated by the GM-CSF receptor. Therefore, binding and internalization of GM-CSF probably occur in immature AML precursors of these two cases of AML. The third AML sample was not responsive to either GM-CSF or DT-GM-CSF. The number of committed progenitors of normal bone marrow (burst-forming unit-erythroid, colony-forming unit granulocyte- macrophage, and cobble stone area forming cell [CAFC] week 2) and also the number of cells with long-term repopulating ability, assayed as week 6 CAFC, were unchanged after exposure to DT-GM-CSF (100 ng/mL, 48 hours). These studies show that DT-GM-CSF may be used to eliminate myeloid leukemic cells with long-term potential in vitro and in immunodeficient mice, whereas normal hemopoietic stem cells are spared.

Destruction of neurokinin-1 receptor expressing cells in vitro and in vivo using substance P-saporin in rats

Substance P (SP) acts on neurons through the neurokinin-1 (NK-1) receptor. Conjugation of SP to the ribosome inactivating protein, saporin (SAP), produces a cytotoxin selective for cells that express the NK-1 receptor. SP-SAP cytotoxicity was inhibited by pre-treating the toxin to reduce the disulfide bond connecting SP to SAP or by pre-incubation with anti-SP antiserum or by SP analog showing that SP-SAP acts through binding of the SP moiety to NK-1 receptors. Injection of SP-SAP into the striatum selectively destroyed NK-1 receptor expressing interneurons. These results show that SP-SAP will be useful for studying the function of NK-1 receptor expressing neurons.

Recombinant Toxins Containing Human Granulocyte-Macrophage Colony-Stimulating Factor and Either Pseudomonas Exotoxin or Diphtheria Toxin Kill Gastrointestinal Cancer and Leukemia Cells

The granulocyte-macrophage colony-stimulating factor receptor (GM-CSFR) is a potential target for toxin-directed therapy, because it is overexpressed on many leukemias and solid tumors and apparently not on stem cells. To investigate the potential therapeutic use of GM-CSF toxins, we fused human GM-CSF to truncated forms of either Pseudomonas exotoxin (PE) or diphtheria toxin (DT) and tested the cytotoxicity of the resulting GM-CSF–PE38KDEL and DT388–GM-CSF on human gastrointestinal (GI) carcinomas and leukemias. Toward gastric and colon cancer cell lines, GM-CSF–PE38KDEL was much more cytotoxic than DT388–GM-CSF, with IC50s (concentration resulting in 50% inhibition of protein synthesis) of 0.5 to 10 ng/mL compared with 4 to 400 ng/mL, respectively. In contrast, toward leukemia lines and fresh bone marrow cells DT388–GM-CSF was more cytotoxic than GM-CSF–PE38KDEL. The cytotoxicity of both GM-CSF–PE38KDEL and DT388–GM-CSF toward the human cells was specific, because it could be competed by an excess of GM-CSF. Binding studies indicated that human GM-CSF receptors were present on all of the human GI and leukemic cell lines tested, at levels of 540 to 3,700 sites per cell (kd = 0.2 to 2 nmol/L), and the number of sites per cell did not correlate with the cell type. A similar pattern of cytotoxicity was found with recombinant immunotoxins binding to the transferrin receptor, in that anti-TFR(Fv)–PE38KDEL was much more cytotoxic than DT388–anti-TFR(Fv) toward GI cells, but both were similar in their cytotoxic activity toward leukemia cells. The fact that PE is more effective than DT in killing GI but not leukemic tumor cells targeted by GM-CSF indicates a fundamental difference in the way PE or DT gains access to the cytosol in these cells. GM-CSF–PE38KDEL and DT388–GM-CSF deserve further evaluation as possible treatments for selected tumors.

Recombinant Toxins Containing Human Granulocyte-Macrophage Colony-Stimulating Factor and Either Pseudomonas Exotoxin or Diphtheria Toxin Kill Gastrointestinal Cancer and Leukemia Cells

The granulocyte-macrophage colony-stimulating factor receptor (GM-CSFR) is a potential target for toxin-directed therapy, because it is overexpressed on many leukemias and solid tumors and apparently not on stem cells. To investigate the potential therapeutic use of GM-CSF toxins, we fused human GM-CSF to truncated forms of either Pseudomonas exotoxin (PE) or diphtheria toxin (DT) and tested the cytotoxicity of the resulting GM-CSF–PE38KDEL and DT388–GM-CSF on human gastrointestinal (GI) carcinomas and leukemias. Toward gastric and colon cancer cell lines, GM-CSF–PE38KDEL was much more cytotoxic than DT388–GM-CSF, with IC50s (concentration resulting in 50% inhibition of protein synthesis) of 0.5 to 10 ng/mL compared with 4 to 400 ng/mL, respectively. In contrast, toward leukemia lines and fresh bone marrow cells DT388–GM-CSF was more cytotoxic than GM-CSF–PE38KDEL. The cytotoxicity of both GM-CSF–PE38KDEL and DT388–GM-CSF toward the human cells was specific, because it could be competed by an excess of GM-CSF. Binding studies indicated that human GM-CSF receptors were present on all of the human GI and leukemic cell lines tested, at levels of 540 to 3,700 sites per cell (kd = 0.2 to 2 nmol/L), and the number of sites per cell did not correlate with the cell type. A similar pattern of cytotoxicity was found with recombinant immunotoxins binding to the transferrin receptor, in that anti-TFR(Fv)–PE38KDEL was much more cytotoxic than DT388–anti-TFR(Fv) toward GI cells, but both were similar in their cytotoxic activity toward leukemia cells. The fact that PE is more effective than DT in killing GI but not leukemic tumor cells targeted by GM-CSF indicates a fundamental difference in the way PE or DT gains access to the cytosol in these cells. GM-CSF–PE38KDEL and DT388–GM-CSF deserve further evaluation as possible treatments for selected tumors.

A recombinant GM-CSF-PE40 ligand toxin is functionally active but not cytotoxic to cells

A granulocyte/macrophage colony-stimulating factor (GM-CSF)-Pseudomonas exotoxin (PE) 40 fusion protein was constructed for potential use in the treatment of myeloid leukaemias, as a conditioning agent prior to allogeneic bone marrow transplantation or for ex vivo purging of malignant cells prior to autologous bone marrow transplantation. The GM-CSF-PE40 fusion protein successfully binds to the GM-CSF receptor and is capable of initiating a mitogenic signal similar to native GM-CSF in the GM-CSF-dependent TF1 cell line. The toxin component also appears to be fully functional as determined by an in vitro adenosine diphosphate-ribosylation assay. The GM-CSF-PE40 fusion protein, however, was not cytotoxic to a number of myeloid leukaemia cell lines. It is suggested that the mechanism of internalization of the GM-CSF receptor is not appropriate for the translocation of PE to the cytosol where it can fulfil its cytotoxic potential.

Immunotoxins: an update

The use of immunotoxins (ITs) in the therapy of cancer, graft-vs-host disease (GvHD), autoimmune diseases, and AIDS has been ongoing for the past two decades. ITs contain a targeting moiety for delivery and a toxic moiety for cytotoxicity. Theoretically, one molecule of a toxin, routed to the appropriate cellular compartment, will be lethal to a cell. Newly developed MoAbs, toxins, and molecular biological technologies have enabled researchers to construct ITs that can effectively kill many different cell types. In fact, phase I/II clinical trials have given promising results. Although nonspecific toxicity and immunogenicity still limit the use of IT therapy, these agents hold enormous promise in an optimal setting to treat minimal disease.