Granulocyte-macrophage colony-stimulating factor receptor-targeted therapy of chemotherapy- and radiation-resistant human myeloid leukemias

Comprehensive approaches to preclinical evaluation of monoclonal antibodies and their next-generation derivatives

Biotherapeutics hold great promise for the treatment of several diseases and offer innovative possibilities for new treatments that target previously unaddressed medical needs. Despite successful transitions from preclinical to clinical stages and regulatory approval, there are instances where adverse reactions arise, resulting in product withdrawals. As a result, it is essential to conduct thorough evaluations of safety and effectiveness on an individual basis. This article explores current practices, challenges, and future approaches in conducting comprehensive preclinical assessments to ensure the safety and efficacy of biotherapeutics including monoclonal antibodies, toxin-conjugates, bispecific antibodies, single-chain antibodies, Fc-engineered antibodies, antibody mimetics, and siRNA-antibody/peptide conjugates.

Recombinant toxins in haematologic malignancies and solid tumours

Recombinant toxins constitute a new modality for the treatment of cancer, since they target cells displaying specific surface-receptors or antigens. They are fusion proteins, which contain toxin and ligand regions, and are produced in Escherichia coli. The ligand may be a growth factor or a fragment of an antibody, and the toxin is usually one of the two bacterial toxins: Pseudomonas exotoxin and diphtheria toxin. Compared to the earlier generation chemical conjugates of ligands and toxins, recombinant toxins have many advantages, including homogeneity with respect to the connection between the ligand and toxin, ease and yield of production and small size. A variety of chemotherapy-resistant haematologic and solid tumours have been targeted with recombinant toxins, and clinical trials with many of them have recently demonstrated their effectiveness. Moreover, their unwanted toxic effects are different from those of most chemotherapeutic agents, supporting the expectation that they can be combined with existing modalities to improve the clinical resources available to treat cancer in humans.

IBC's international Conference on Novel Therapeutic Proteins for Oncology (drug discovery and development)

It is estimated that 564,800 Americans will die of cancer in 1998. Even though earlier detection and healthier lifestyles may have led to some improvements in cancer mortality, new agents are urgently needed to treat incurable cancers. At the recent International Conference on Novel Therapeutic Proteins for Oncology, a wide range of new agents and strategies for targeting tumour cells were discussed. Many of these will be reviewed here.

Diphtheria toxin-murine granulocyte-macrophage colony-stimulating factor–induced hepatotoxicity is mediated by Kupffer cells

DT388GMCSF, a fusion toxin composed of the NH2-terminal region of diphtheria toxin (DT) fused to human granulocyte-macrophage colony-stimulating factor (GMCSF) has shown efficacy in the treatment of acute myeloid leukemia. However, the primary dose-limiting side effect is liver toxicity. We have reproduced liver toxicity in rats using the rodent cell-tropic DT-murine GMCSF (DT390mGMCSF). Serum aspartate aminotransferase and alanine aminotransferase were elevated 15- and 4-fold, respectively, in DT390mGMCSF-treated rats relative to controls. Histologic analysis revealed hepatocyte swelling; however, this did not lead to hepatic necrosis or overt histopathologic changes in the liver. Immunohistochemical staining showed apoptotic cells in the sinusoids, and depletion of cells expressing the monocyte/macrophage markers, ED1 and ED2, indicating that Kupffer cells (KC) are targets of DT390mGMCSF. In contrast, sinusoidal endothelial cells seemed intact. In vitro, DT390mGMCSF was directly cytotoxic to primary KC but not hepatocytes. Two related fusion toxins, DT388GMCSF, which targets the human GMCSF receptor, and DT390mIL-3, which targets the rodent IL-3 receptor, induced a less than 2-fold elevation in serum transaminases and did not deplete KC in vivo. In addition, DTU2mGMCSF, a modified form of DT390mGMCSF with enhanced tumor cell specificity, was not hepatotoxic and was significantly less toxic to KC in vivo and in vitro. These results show that DT390mGMCSF causes liver toxicity by targeting KC, and establish a model for studying how this leads to hepatocyte injury. Furthermore, alternative fusion toxins with potentially reduced hepatotoxicity are presented.

A urokinase-activated recombinant diphtheria toxin targeting the granulocyte-macrophage colony-stimulating factor receptor is selectively cytotoxic to human acute myeloid leukemia blasts

Novel agents to treat acute myeloid leukemia (AML) are needed with increased efficacy and specificity. We have synthesized a dual-specificity fusion toxin DTU2GMCSF composed of the catalytic and translocation domains of diphtheria toxin (DT) fused to the granulocyte-macrophage colony-stimulating factor (GM-CSF) in which the DT furin cleavage site 163RVRRSV170 is modified to a urokinase plasminogen activator (uPA) cleavage site 163GSGRSA170, termed U2. DTU2GMCSF was highly toxic to the TF1-vRaf AML cell line (proliferation inhibition assay; IC50 = 3.14 pM), and this toxicity was greatly inhibited following pretreatment with anti-uPA and anti-GM-CSF antibodies. The activity of this toxin was then tested on a larger group of 13 human AML cell lines; 5 of the 13 cell lines were sensitive to DTU2GMCSF. An additional 5 of the 13 cell lines became sensitive when exogenous pro-uPA was added. Sensitivity to DTU2GMCSF strongly correlated with the expression levels of uPA receptors (uPARs) and GM-CSF receptors (GM-CSFRs) as well as with total uPA levels. DTU2GMCSF was less toxic to normal cells expressing uPAR or GMCSFR alone, that is, human umbilical vein endothelial cells and peripheral macrophages, respectively. These results indicate that DTU2GMCSF may be a selective and potent agent for the treatment of patients with AML. (Blood. 2004;104:2143-2148)

Targeted therapies for high-risk acute myeloid leukemia

Approximately half of children with acute myeloid leukemia (AML) can be cured with contemporary chemotherapy regimens; however, various forms of drug resistance pose considerable obstacles for curing the remaining patients. Recent advances in immunology, cytogenetics, and cellular and molecular biology have provided new insights into fundamental biological differences between leukemic myeloid blasts and their normal counterparts. This article focuses on new technologies involving: (1) antibody- or growth factor-mediated targeting of antigens or growth factor receptors found on AML blasts and restricted sub-groups of normal cells, (2) pharmacologic targeting of the pathologic t(15;17) translocation of acute promyelocytic leukemia with all-trans retinoic acid, (3) pharmacologic and immunologic targeting of mutant RAS oncogenes and related aberrant signaling in AML blasts, and (4) targeting of pathological signaling of the Bcr-Abl oncoprotein and c-kit tyrosine kinase in myeloid leukemias. These advances herald an exciting new era of AML-specific therapies.

Production of recombinant DTctGMCSF fusion toxin in a baculovirus expression vector system for biotherapy of GMCSF-receptor positive hematologic malignancies

The fusion toxin DTctGMCSF has been constructed by genetically replacing the native receptor-binding domain of diphtheria toxin (DT) with human granulocyte-macrophage colony stimulating factor (GMCSF). This recombinant fusion toxin preserves the catalytic (c) and membrane translocation (t) domains of DT and includes a sterically neutral peptide linker separating the toxin and growth factor domains. Previous work using DTctGMCSF produced in Escherichia coli has shown that this chimeric toxin is selectively cytotoxic to GMCSF receptor (R)-positive acute myeloid leukemia (AML) cells both in vitro and in vivo. Its clinical development has been hampered due to very low expression levels, requirements for solubilization with guanidine hydrochloride and subsequent refolding, and concerns about bacterial endotoxin contamination. These difficulties prompted us to investigate the utility of a baculovirus/insect cell expression system for the production of DTctGMCSF. Here, we report that a soluble form of DTctGMCSF can be produced in the baculovirus expression vector system (BEVS) and purified to homogeneity by column chromatography. The BEVS-derived DTctGMCSF fusion toxin caused apoptotic death in GMCSF-R-positive human AML cells at nanomolar concentrations. In contrast to the 100 microg/L yields of purified DTctGMCSF obtained from E. coli, the BEVS allows us to routinely generate 8-10 mg/L of purified DTctGMCSF. This increased capacity provided by the BEVS for the production of DTctGMCSF makes it now possible to obtain sufficient quantities to carry out preclinical and clinical trials. To our knowledge, this is the first report of the successful utilization of the BEVS for producing a therapeutic fusion toxin.