Anti-CD6-blocked ricin: an anti-pan T-cell immunotoxin

Immunotoxins and anticancer drug conjugate assemblies: the role of the linkage between components

Immunotoxins and antibody-drug conjugates are protein-based drugs combining a target-specific binding domain with a cytotoxic domain. Such compounds are potentially therapeutic against diseases including cancer, and several clinical trials have shown encouraging results. Although the targeted elimination of malignant cells is an elegant concept, there are numerous practical challenges that limit conjugates’ therapeutic use, including inefficient cellular uptake, low cytotoxicity, and off-target effects. During the preparation of immunoconjugates by chemical synthesis, the choice of the hinge component joining the two building blocks is of paramount importance: the conjugate must remain stable in vivo but must afford efficient release of the toxic moiety when the target is reached. Vast efforts have been made, and the present article reviews strategies employed in developing immunoconjugates, focusing on the evolution of chemical linkers.

Immunotoxin therapy of hematologic malignancies

Patients with chemotherapy relapsed or refractory hematologic malignancies may be effectively treated with allogeneic or autologous stem cell transplants. However, many patients cannot be transplanted due to age, comorbidities, or lack of suitable donors. Further, a fraction of patients relapse post-transplant. Novel therapeutic agents that can kill multidrug-resistant malignant stem cells and are not myelosuppressive are needed. One class of such agents is immunotoxins. Immunotoxins consist of cell-selective ligands covalently linked to peptide toxins. The ligand delivers the molecule to specific cell surface receptors on malignant cells. The toxin triggers cell death either by reaching the cytosol and catalytically inactivating vital cell processes or by modifying the tumor cell surface membrane. We have synthesized immunotoxins for therapy of chemoresistant hematologic diseases. In this review, we will detail the synthesis of a number of these drugs and describe their preclinical and clinical activity. Several of these agents have shown dramatic antitumor effects in patients with hematologic neoplasms, and one immunotoxin has been approved for use by the US Food and Drug Administration (FDA). Over the next several decades, a growing number of these agents should reach the clinic.

Immunotoxins for targeted cancer therapy

Immunotoxins constitute a new modality for the treatment of cancer, since they target cells displaying specific surface-receptors or antigens. Immunotoxins contain a ligand such as a growth factor, monoclonal antibody, or fragment of an antibody which is connected to a protein toxin. After the ligand subunit binds to the surface of the target cell, the molecule internalizes and the toxin kills the cell. Bacterial toxins which have been targeted to cancer cells include Pseudomonas exotoxin and diphtheria toxin, which are well suited to forming recombinant single-chain or double-chain fusion toxins. Plant toxins include ricin, abrin, pokeweed antiviral protein, saporin and gelonin, and have generally been connected to ligands by disulfide-bond chemistry. Immunotoxins have been produced to target hematologic malignancies and solid tumors via a wide variety of growth factor receptors and antigens. Challenges facing the clinical application of immunotoxins are discussed.

Recent developments in immunotoxin therapy

Immunotoxin (IT) research has been ongoing for 15 years. During the past 2 years, work has focused on several areas: on improvements and developments in first- and second-generation ITs; the preparation of new immunotoxin constructs with anti-tumor activity; novel animal models for preclinical evaluation of immunotoxins; and clinical trials, which are now entering Phase II or III in humans.