Similarities in the biodistribution of iodine-labeled anti-Tac single-chain disulfide-stabilized Fv fragment and anti-Tac disulfide-stabilized Fv fragment

Exogenous Molecular Probes for Targeted Imaging in Cancer: Focus on Multi-modal Imaging

Cancer is one of the major causes of mortality and morbidity in our health care system. Molecular imaging is an emerging methodology for the early detection of cancer, and the development of exogenous molecular probes that can be labeled for multi-modality imaging is critical to this process. Today, molecular imaging is at crossroad, and new targeted imaging agents are expected to broadly expand our ability to detect pre-malignant lesions. This integrated imaging strategy will permit clinicians to not only localize lesions within the body, but also to visualize the expression and activity of specific molecules. This information is expected to have a major impact on diagnosis, therapy, drug development and understanding of basic cancer biology. At this time, a number of molecular probes have been developed by conjugating various labels to affinity ligands for targeting in different imaging modalities. This review will describe the current status of exogenous molecular probes for optical, nuclear and MRI imaging platforms. Furthermore, we will also shed light on how these techniques can be used synergistically in multi-modal platforms and how these techniques are being employed in current research.

Molecular probes for the in vivo imaging of cancer

Advancements in medical imaging have brought about unprecedented changes in the in vivo assessment of cancer. Positron emission tomography, single photon emission computed tomography, optical imaging, and magnetic resonance imaging are the primary tools being developed for oncologic imaging. These techniques may still be in their infancy, as recently developed chemical molecular probes for each modality have improved in vivo characterization of physiologic and molecular characteristics. Herein, we discuss advances in these imaging techniques, and focus on the major design strategies with which molecular probes are being developed.

Targeted Receptor Trafficking Affects the Efficiency of Retrovirus Transduction

We describe the development of an experimental system to test the hypothesis that the efficiency of retrovirus transduction is dependent on the pathway of virus entry into the host cell and the intracellular trafficking itinerary of the cellular receptor with which it interacts. The experimental system consists of three model target cell lines, derived from HeLa cells, that stably express one of three interleukin-2 receptor alpha chain (CD25) chimeras, TAC, TAC-CD16, and TAC-DKQTLL, which have identical extracellular domains but different intracellular trafficking itineraries, and a targeted amphotropic murine leukemia retrovirus whose envelope proteins were modified to include a binding site for TAC at their N-termini. We found that the efficiency of retrovirus transduction was affected by the distribution and trafficking itinerary of the TAC receptors. Transduction of cells that expressed TAC-DKQTLL was nearly 4-fold lower than transduction of control cells that did not express any of the TAC receptors. In contrast, transduction of cells that expressed TAC was 1.6-fold higher than transduction of control cells, whereas transduction was not significantly affected by the expression of TAC-CD16. Our results suggest that in the course of designing a targeted retrovirus it may be prudent to target only those receptors that internalize retroviruses via pathways that most efficiently support post-binding steps of infection.

Optimizing the generation of recombinant single-chain antibodies against placental alkaline phosphatase

Recombinant technologies to engineer ordinary hybridoma monoclonal antibodies (MAbs) to single-chain fragment variable (scFv) may cause loss of antibody affinity, increased tendency to aggregate, increased temperature sensitivity, and low yield of active protein. In the present investigation, the well-characterized MAb H7 against placental alkaline phosphatase (PLAP), used as a model antibody, was engineered to improve solubility and stability of scFv with retained high affinity. The original procedure to generate single-chain antibodies with a 10-amino acid linker between VH and VL yielded an almost insoluble product. By site-directed mutagenesis, four selective sequence substitutions were made in the VL fragment and one in the VH fragment to improve solubility. The importance of the linker length was investigated, and a 25/30 amino acid linker was found to improve solubility. In order to further increase the stability of the single-chain antibody, an additional covalent -S-S- bond was introduced between amino acid 100 in the VL fragment and amino acid 44 in the VH region, to make a single-chain disulphide stabilized variable fragment (scdsFv). Altogether five different antibody constructs were produced and compared in terms of solubility, stability, affinity, and production properties. Immunospecificity was tested by enzyme-linked immunosorbent assay (ELISA) against the target antigen, temperature sensitivity by exposing the purified scFv to higher temperatures. All the new constructs retained almost equal activity and high affinity for their target antigen, placental alkaline phosphatase (PLAP), compared to the intact MAb H7, up to +42 degrees C as evaluated by ELISA. The overall affinity K(A) > 10(9) (M(1)) of the new antibodies could be maintained in the same order of magnitude as the original one (H7), when evaluated by Biacore technology. The best final single-chain antibody was obtained by performing the specific site-directed mutations and introducing a linker of 30 amino acids, but not by additional stabilizing disulphide bonds. The yield of the final antibody was improved approximately 10-fold by the modifications. This antibody could easily be expressed in a bacterial system using the PET-32a TrxA vector and the Escherichia coli strain BL21 Origami B (DE3). Purified antibody, which could be kept at concentrations up to 0.8 mg/mL, was obtained, which is sufficient for clinical testing of therapeutic applications.

Lowering of pI by acylation improves the renal uptake of 99mTc-labeled anti-Tac dsFv: effect of different acylating reagents

Anti-Tac disulfide-stabilized variable region fragment (dsFv) was labeled with 99mTc by a preformed chelate approach using 99mTc-MAG3-trifluorophenyl (TFP) ester. Simultaneously it was acylated with TFP-lactate or succinic anhydride to decrease the isoelectric point of dsFv (pI 10). Acylation of dsFv (0.04 mM) with the lactate at a 73 times molar excess reduced the pI to 5.0-6.7, whereas acylation with succinic anhydride at a 30 times molar excess reduced the pI to 4.9-8.7. Comparative biodistribution studies performed in mice (n = 5) showed the reduced renal accumulation of the 99mTc proportional to the pI reduction. The effect of the pI on the reduced renal uptake was especially pronounced at 15 min postinjection. The reduced renal uptake was also reflected in the reduced whole-body retention, indicating that lowering the pI inhibited the tubular reabsorption of the labeled dsFv.

Recombinant immunotoxins for cancer therapy

Recombinant immunotoxins consist of Fv regions of tumour-selective antibodies fused to toxins found in bacteria, plants or fungi. These toxins must be modified to remove normal-tissue binding sites but to retain all other functions of cytotoxicity. The recombinant antibody fragments target the modified toxin to cancer cells which are killed, either by direct inhibition of protein synthesis, or by concomitant induction of apoptosis. Cells that are not recognised by the antibody fragment because they do not carry the tumour antigen, are spared. Many factors influence the in vivo antitumour activity of recombinant immunotoxins. Among them are considerations of which types of cancer may be the best targets for immunotoxin therapy as well as tumour specificity of the antigen that is targeted by the recombinant antibody. Other relevant issues are the affinity of immunotoxins and their ability to enter and penetrate into tissues and tumours, which in turn is dependent on the size of the protein. A great deal of protein-engineering is required to stabilise the recombinant antibody moiety of immunotoxins, since stability of the molecules is crucial for good clinical efficacy. Excellent activity and specificity can be observed for many recombinant immunotoxins in in vitro assays using cultured cancer cells as well as in animal tumour models. Ongoing clinical trials provide examples where the promising preclinical data correlate with successful results in experimental cancer therapy.

Immunotoxins

Immunotoxins are molecules which contain a protein toxin connected to a targeting antibody. The goal of therapy is for the molecule to bind selectively to cancer cells, or to cells mediating autoimmune disease, internalise and then for the toxin to kill the cell. Several immunotoxins meeting this definition are in preclinical and clinical development, but none are approved yet for use in general practice. One close relative of immunotoxins is the growth factor fusion toxin, wherein the targeting antibody is replaced with a growth factor that selectively binds and this ligand is fused in a recombinant fashion to a protein toxin. One such molecule, containing human interleukin-2 (IL-2) fused to truncated diphtheria toxin (DT), has recently been approved under the name Ontak, and others are under development. A newer class of immunotoxins, termed recombinant immunotoxins, contain the variable or antigen binding domains of an antibody fused in recombinant fashion to a toxin. Recombinant immunotoxins, like growth factor fusion toxins, can be produced efficiently from bacteria and have a defined structure with respect to the linkage between the toxin and the ligand. However, they can, like conventional immunotoxins, be directed to antigens other than growth factor receptors, including receptor subunits. Several recombinant immunotoxins are under clinical testing and major responses have been reported, particularly in haematological malignancies. Some of these molecules may enter clinical practice in the future as targeted therapy, which is a modality distinct from those of chemotherapy, surgery and radiation therapy.

Radioimmunotherapy: designer molecules to potentiate effective therapy

The evolution of monoclonal antibody forms for radioimmunotherapy and other antibody-based applications has been driven by a series of problems that each new form has introduced. Ehrlich was the first to present the concept that antibodies could be exploited in such a manner. Four decades were required before technological advances allowed the exploration of the potential of antibodies for radioimaging and radioimmunotherapeutic applications. Advances in DNA technology have led to the ability to tailor and manipulate the immunoglobulin molecule for specific functions and in vivo properties. This article discusses the use of monoclonal antibodies for radiotherapy with an emphasis on the problems that have been encountered and the subsequent solutions.