New strategies for cell type-specific delivery need to be developed if RNA interference is to realize its full therapeutic potential. One possible approach is the use of aptamers to deliver siRNAs selectively to tumor cells with appropriate antigens displayed on the surface. We used an aptamer that binds specifically to PSMA, a cell surface glycoprotein found in abundance on prostate cancer cells, and joined its 3' end to a siRNA specific for Eukaryotic Elongation Factor 2 mRNA (EEF2). This is an attractive target for cancer therapy because inhibiting EEF2 causes the rapid arrest of protein synthesis, inducing apoptosis and leading ultimately to cell death. In order to enhance the therapeutic efficacy of the aptamer-siRNA, we increased the valency of the construct by rational design. Two anti-PSMA aptamers were designed such that each binding sequence could fold independently into its active conformation. Here we show specific cytotoxicity resulting from siRNA-induced silencing of EEF2, as well as specific delivery to PSMA-expressing prostate cancer cells. Increasing the valency of the aptamer resulted in enhanced cytotoxicity compared with the monovalent constructs. The results presented here demonstrate the usefulness of multivalent aptamer-based delivery vehicles for siRNA therapeutics.

Death-associated protein kinase 2 (DAPK2) is a calcium/calmodulin-regulated proapoptotic serine/threonine kinase that acts as a tumor suppressor. Here we show that DAPK2 is down-regulated in Hodgkin lymphoma-derived tumor cell lines and that promoter-region hypermethylation is one mechanism for DAPK2 inactivation. To determine whether selective reconstitution of DAPK2 catalytic activity in these cells could induce apoptosis, we created a fusion protein comprising a human CD30 ligand conjugated to a human DAPK2 calmodulin-deletion mutant. Thus, recombinant immunokinase DAPK2'-CD30L has a constitutive kinase activity with enhanced proapoptotic function. We show that this immunokinase fusion protein inhibits cell proliferation and induces apoptotic cell death specifically in CD30/DAPK2-negative tumor cell lines. This proof-of-concept study provides the first demonstration of therapeutic strategies based on the restoration of a defective, tumor-suppressing kinase activity by a novel class of recombinant immunotherapeutics.

Conventional cancer treatments lack specificity and often cause severe side effects. Targeted therapeutic approaches are therefore preferred, including the use of immunotoxins (ITs) that comprise cell-binding and cell death-inducing components to allow the direct and specific delivery of pro-apoptotic agents into malignant cells. The first generation of ITs consisted of toxins derived from bacteria or plants, making them immunogenic in humans. The recent development of human cytolytic fusion proteins (hCFP) consisting of human effector enzymes offers the prospect of highly-effective targeted therapies with minimal side effects. One of the most promising candidates is granzyme B (GrB) and this enzyme has already demonstrated its potential for targeted cancer therapy. However, the clinical application of GrB may be limited because it is inactivated by the overexpression in tumors of its specific inhibitor serpin B9 (PI-9). It is also highly charged, which means it can bind non-specifically to the surface of non-target cells. Furthermore, human enzymes generally lack an endogenous translocation domain, thus the endosomal release of GrB following receptor-mediated endocytosis can be inefficient. In this review we provide a detailed overview of these challenges and introduce promising solutions to increase the cytotoxic potency of GrB for clinical applications.

RNA interference (RNAi) is a powerful endogenous process initiated by short double stranded RNAs, which results in sequence-specific posttranscriptional gene silencing. The ability to block the expression of any disease-causing or disease-related protein emphasizes the huge therapeutic potential of this technology. In a clinical setting, however, the use of RNAi-based therapeutics is limited by their short serum half lives and poor uptake into cells. In this review, we provide an overview of recent patents in the field of short interfering RNA (siRNA) delivery and discuss recent progress in the development of efficient siRNA delivery vehicles enhancing the pharmacokinetic properties of RNAi-based therapeutics and promoting cellular uptake.

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