Current Drug Targets - Volume 10, Issue 2, 2009

The concept of targeted tumor therapeutics and the “magic bullet” was formulated by Paul Ehrlich already at the end of the 19th century. The directed delivery of drugs to diseased cells without harming healthy cells is the final goal of targeted therapies. Especially tumor therapy is in need of magic bullets, which would demonstrate successful drug targeting. The integration of antibodies to mediate specific targeting to tumor cells was a major step forward in the field of targeted tumor therapeutics. However, after initial enthusiasm several problems appeared and led to disenchantment of magic bullets. Since then a number of novel ideas contributed to the optimization of targeted drugs and the development of Ontak, the first approved chimeric targeted cancer drug. A number of other antibody-based therapies for the treatment of cancer have now been approved lending strong support for these types of molecules, and the development of new drugs is ongoing. Conjugated targeted toxins utilize antibodies, cytokines or growth factors for specific binding to overexpressed tumor-specific structures on the cell surface and then induce cell death by different mechanisms dependent on the toxin. Besides antibodies, organelle-like liposomes and other carriers are used to deliver toxic substances to tumor cells. The main objective of all these targeted therapies is the selective killing of neoplastic cells. As conventional therapies against cancer do not achieve complete remission of tumors, the continued development of new and improved therapies, such as targeted tumor therapies, is of great interest. Many excellent groups are currently working on the optimization of existing therapies and to gain new insights into the mechanisms underlying the successful elimination of tumor cells by chimeric toxins and other targeted therapies. This special issue of Current Drug Targets aims to present reviews on different topics concerning the field of targeted tumor therapies. The selected authors of this issue describe relevant work in the corresponding area as well as the latest results of some groups working on targeted tumor therapeutics. This review issue will be published as a contribution to the Fabisch- Symposium on Targeted Tumor Therapies held from 1-3 April 2009 in Berlin, Germany. We organized this second symposium on Targeted Tumor Therapies after the great success of the first symposium in 2006. The first symposium summarized ongoing efforts on the development of anti-tumorigenic targeted drugs and presented the success of recent clinical studies with targeted anti-tumor drugs. Thus, the second symposium aims to tie up to the success of the first symposium. This effort will be greatly supported by this special review issue of Current Drug Targets.

Conventional tumor therapy is usually based on surgery, radiation and chemotherapy. Treatment with chemotherapeutics is often impeded by dose-limiting toxicities. Therefore, medical scientists sought for tools to improve chemotherapy by directly coupling targeting molecules to cytotoxic substances. This review provides a general overview on the development of targeted drugs designed for tumor therapy. Further carrier-based delivery systems of antitumorigenic drugs will not be described here. The targeting moiety is usually an antibody or a fragment thereof. Growth factors, cytokines and ligands are also used as targeting moiety. The targeting moiety is coupled to the toxic moiety either chemically or both components were combined as fusion proteins. In addition to those targeted molecules containing conventional chemotherapeutics, more sophisticated targeted drugs were developed containing protein toxins, such as diphtheria toxin or Pseudomonas exotoxin. Only a small number of these protein toxins inside tumor cells results in efficient killing of the target cell. Several of these targeted toxins are currently in clinical trials. Another targeting mechanism utilizes the activation of formerly harmless substances in the vicinity of tumor cells. This mechanism is referred to as directed enzyme prodrug therapy.

Antibody-based therapeutic approaches are yielding more and more of the promise they have held since the conception of the ‘magic bullet’ theory by Paul Ehrlich. The beneficial effect of antibody-based therapies is directly related to antibody-dependent functions, such as neutralization and antibody-dependent cellular cytotoxicity, but in many cases also relies on the delivery of toxic compounds to cancerous cells. However, the clinical utility of toxic antibody conjugates can be significantly hampered by side effects. Ideal effector compounds are inactive ‘en route’, but gain full activity once the antibody conjugate has bound to cancerous cells. Of significant potential in this respect are the pro-apoptotic ligands Tumor Necrosis Factor (TNF), fibroblast-associated cell-surface ligand (FasL) and TNF-related apoptosisinducing ligand (TRAIL). TNF ligands are normally present as homotrimeric transmembrane proteins, but can also be processed into a soluble trimeric form. Compared to their corresponding transmembrane counterpart, soluble TNF, FasL and TRAIL have a strongly reduced capacity to activate TNF receptor 2, Fas and TRAIL receptor 2. However, all sequence information required for full activation of these receptors is latently retained in these soluble ligands and can be unmasked by oligomerization or cell surface immobilization. The latter provides a clear rationale for the use of these ligands as effectors in antibody-based therapy. The antibody-targeted ligand will be in a relatively inactive soluble form while en route. However, once bound to the targeted cancer cell the soluble TNF ligand fusion proteins will be converted into fully active membrane ligand-like molecules. Here we will, after briefly detailing the biology of TNF, TRAIL and FasL, focus on the promises and pitfalls of targeted TNF ligand fusion proteins in achieving a ‘magic bullet’ with maximum cancer selective activity and minimal side effects.

The recombinant CD3 immunotoxin, A-dmDT390-bisFv(UCHT1), composed of the catalytic and translocation domains of diphtheria toxin fused to two single chain Fv fragments of an anti-CD3η monoclonal antibody was administered to five patients with cutaneous T cell lymphoma (CTCL) by eight 15 min intravenous infusions over four days. Side effects were fever, chills, nausea, hypoalbuminemia, transaminasemia and reactivation of EBV and CMV. Half-life of drug was 40 min. Anti-immunotoxin antibodies developed in all patients after two weeks. Two patients had partial remissions lasting 1 and 6+ months. The agent is undergoing further dose escalation and shows promising results in this disease.

Although therapy of CD30-positive lymphomas such as classical Hodgkin lymphoma and anaplastic large cell lymphoma has been improved considerably during the last decades, patients suffer from high toxicity of current therapeutic regimens. Since CD30 expression is very restricted, CD30-positive tumors are well suited for immunotherapeutic approaches. Several distinct immunotherapeutic approaches with chimeric, humanized, and bispecific antibodies as well as immunotoxins are already described. In this report, we give a short overview of CD30-targeting approaches in humans. Furthermore, we introduce two novel anti-CD30 fusion proteins consisting of the single chain variable fragment of the CD30 monoclonal antibody Ber-H2 and human interleukin-2, evaluate their biological activity in a human CD30-positive syngeneic murine model, and demonstrate the immunological mechanisms leading to tumor rejection by these reagents. The data indicate that there are several promising approaches in CD30-targeted immunotherapy. The findings of the anti- CD30 IL-2 constructs suggest that these fusion proteins are particularly useful to remove small, residual tumors.

Prostate cancer is the most common cancer in men from Western industrialized countries and a significant proportion of patients progress to advanced metastatic disease, for which currently no curative treatment exists. Therefore, new therapeutic approaches need to be considered. Prostate specific membrane antigen (PSMA) is an integral, non-shed type 2 membrane protein that is highly and specifically expressed on prostate epithelial cells and strongly upregulated in prostate cancer. PSMA is also present in the neovasculature of other solid tumors. These findings have spurred the development of PSMA-targeted therapies and firstgeneration products have entered clinical testing. The proposed strategies range from targeted toxins and radionuclides to immunotherapeutic agents. The present review provides an overview of these approaches.

Because primary brain tumors treated with surgery, radiation therapy, and chemotherapy have a poor prognosis, this has led investigators to develop new innovative therapies such as targeted toxins. These large molecules do not cross the blood brain barrier and must be delivered into the brain by a technique known as convection-enhanced delivery (CED). When administering these agents, there are a number of pharmacokinetic considerations that must be considered that will directly affect the volume of distribution of the drug being administered and ultimately the therapeutic effect of the agent. A number of different catheter types have been used to perform CED with a hollow fiber design offering several advantages over other variations. Specific parameters have been developed to optimize the placement of the drug delivery catheters in order to enhance drug distribution in the brain. Considerable effort has been expended to identify a reliable way to image the distribution of targeted toxins administered by CED using a combination of magnetic resonance imaging and single photon emission computed tomography. Unfortunately many infusions performed in tumor patients are unsuccessful due to ventricular/subarachnoid leak or pooling of the drug in necrotic tumor tissue. To date, no targeted toxin clinical trial has demonstrated statistically significant clinical results leading to the universal acceptance of this treatment. Other agents such as standard chemotherapy or liposomal preparations have been delivered by CED. Nonneoplastic neurological diseases are being considered for treatment by CED and treating different locations of the brain other that the cerebral hemispheres are under investigation.

Protein therapeutics, such as antibodies, enzymes and toxins, are very promising reagents for the treatment of human disease. However, many therapeutic proteins are known to elicit immune responses when administered to humans. Certain antibodies work by neutralization; others reduce drug efficacy. It is clear that helper T cells are an important factor in the development of class-switched and affinity-maturated anti-therapeutic protein antibodies. Elimination of the T cell epitope seems reasonable, but it is probably impossible to remove all T cell epitopes from protein drugs because T cell epitopes are closely related to the major histocompatibility complex (MHC) molecules, which are known to be highly polymorphic. Accordingly, a possible practical approach for reducing immunogenicity involves the removal of B cell epitopes. In this case, reducing the affinity between the antigen and the B cell receptor may reduce B cell activation, even though the T cell will still be activated. Also a B cell epitope is not restricted by MHC class II molecules. This review seeks to address the identification and the characterization of B cell epitopes, and reports on the development of strategies for reducing immune response with modified B cell epitopes.

Saponins are plant glycosides that consist of a steroid, steroid alkaloid or triterpenoid aglycone and one or more sugar chains that are covalently linked by glycosidic binding to the aglycone. Glucose, galactose, glucuronic acid, xylose and rhamnose are commonly bound monosaccharides. Saponins are found in all organs of a variety of higher plants. Due to the great variability of their structures, diverse functions have been described for distinct saponins; including foaming and pore forming properties as well as selective removal of protozoa from the rumen. The most interesting properties are, however, favorable anti-tumorigenic effects. Several saponins inhibit tumor cell growth by cell cycle arrest and apoptosis with half maximal inhibitory concentrations of down to 0.2 μM. A drawback of saponins in tumor therapy is the nontargeted spreading throughout the whole body. Surprisingly, certain saponins were identified that drastically enhance the efficacy of targeted chimeric toxins bearing the ribosome-inactivating protein saporin as cell-killing moiety. It was demonstrated that this effect is substantially more pronounced on target cells than on non-target cells, thus not only preserving the target specificity of the chimeric toxin but also broadening the therapeutic window with simultaneous dose lowering. This review describes the role of saponins as drug in general, their use as single drug treatment in tumor therapy, their combination with conventional tumor treatment strategies and the synergistic effects with particular targeted tumor therapies that are based on recombinant proteins.

Antibody-directed enzyme prodrug therapy aims to restrict the action of a cytotoxic drug to cancer sites. An enzyme that has no human analogue is delivered to cancer sites by attachment to an antibody directed at a tumour associated antigen. In a second step an antibody or other agent inactivates and clears enzyme from blood. The third step is administration of a low toxicity prodrug that is a substrate for the enzyme thus generating a potent cytotoxic agent at cancer sites. Encouraging results were obtained with this system in small scale clinical trials using unrefined agents. During the past 10 years attempts have been made to reduce the system to two components. Although these have met with some success it is now accepted that future progress requires all three components.

Almost ever since their invention, monoclonal antibodies have held the promise of cancer-specific drug targeting - Paul Ehrlich's “magic bullet” - but only during the past decade have a modest number of anti-cancer antibodies received approval for clinical use. These, however, have proven largely successful, with very different kinds of conventional or recombinant, murine, humanized, recombinant fully human and fusion constructs, and mechanisms of action as diverse as complement or antibody dependent cytotoxicity, anti-angiogenesis, and growth factor inhibition. In these latter two mechanisms of action, antibodies compete with novel small-molecule drugs. This review tries to elucidate current trends in those antibody-based therapeutics that are currently in clinical development. With more than 400 such molecules registered for clinical trials, it is far from a chance to be complete. Still, from those antibodies selected for a closer view, two large trends can be distilled: The movement towards increasingly molecularly defined recombinant constructs, and away from classical antibody effector functions in immune activation towards additional mechanisms of action - either by stimulation or (more often) inhibition of a molecular target function, or by additional functional moieties attached to the antibody scaffold. While these trends probably mark the future of antibody development for cancer therapy and clinical applications in general, a considerable number of more conventional - hybridoma generated or recombinantly chimerized or humanized - Fc receptor-activating antibodies, originally generated a decade or longer ago, continue to make their way through clinical trials, some with remarkable success.