Current Cancer Drug Targets - Volume 9, Issue 8, 2009

Some cytokines (interleukin (IL)-2, IL-11, transforming growth factor(TGF)β) stimulate, while others (IL-12, IL-18, Interferons (IFNs)) inhibit breast cancer proliferation and/or invasion. So far IL-2, IFNα, IFNβ and occasionally IFNγ, IL-6, IL-12 have been used for the treatment of advanced breast cancer either to induce or increase hormone sensitivity and/or to stimulate cellular immunity. Only two long term pilot studies suggest that IL-2 and IFNβ can improve clinical benefit and/or overall survival of metastatic breast cancer patients with minimal residual disease after chemotherapy or with disseminate disease non progressing during endocrine therapy. These results have been interpreted assuming that tumour microenvironment impairs the immune system of the host. Consequently, minimal disease or intense cytostatic effects following chemo or endocrine therapy, respectively, permit the patient's immune system to respond to the stimulatory effect of the cytokines. Therefore a prospective, phase III, randomised, simple blind trial has been planned. The aim is to assess whether the addition of IFNβ and IL-2 to standard hormone therapy in postmenopausal patients with metastatic breast cancer and positive or unknown positive receptors prolongs the clinical benefit and survival since the metastatic diagnosis and the beginning of first line salvage antiestrogen therapy, compared with the results achieved with standard hormone therapy alone. If this immunotherapy prolongs survival of endocrine dependent metastatic breast cancer patients, IL-2 and IFNβ can also be evaluated as adjuvant treatment of patients with positive estrogen receptors.

Membrane ion channels participate in cancerous processes such as proliferation, migration and invasion, which contribute to metastasis. Increasing evidence indicates that voltage-dependent K+ (Kv) channels are involved in the proliferation of many types of cells, including tumor cells. Kv channels have generated immense interest as a promising tool for developing new anti-tumor therapies. Therefore, the identification of potential biomarkers and therapeutic targets in specific cancers is an important prerequisite for the treatment. Since Kv1.3 and Kv1.5 are involved in the proliferation of many mammalian cells, we aimed to study the expression of Kv1.3 and Kv1.5 in a plethora of human cancers. Thus, tissues from breast, stomach, kidney, bladder, lung, skin, colon, ovary, pancreas, brain, lymph node, skeletal muscle and some of their malignant counterparts have been analyzed. Whereas Kv1.3 expression was either decreased or did not change in most tumors, Kv1.5 was overexpressed. However, the presence of Kv1.3 was mostly associated with inflammatory lymphoplasmocytic cells. Independent of the suitability of individual channels as therapeutic targets, the identification of a Kv phenotype from tumor specimens could have a diagnostic value of its own. Our results demonstrate that Kv1.5, and to some extent Kv1.3, are aberrantly expressed in a number of human cancers. These channels could serve both as novel markers of the metastatic phenotype and as potential new therapeutic targets. The concept of Kv channels as therapeutic targets or prognostic biomarkers attracts increasing interest and warrants further investigation.

The growth arrest and DNA damage-inducible 45 (Gadd45) proteins are a group of critical signal transducers that are involved in regulations of many cellular functions. Accumulated data indicate that all three Gadd45 proteins (i.e., Gadd45α, Gadd45β, and Gadd45γ) play essential roles in connecting an upstream sensor module, the transcription Nuclear Factor-κB (NF-κB), to a transcriptional regulating module, mitogen-activated protein kinase (MAPK). This NF-κBGadd45( s)-MAPK pathway responds to various kinds of extracellular stimuli and regulates such cell activities as growth arrest, differentiation, cell survival, and apoptosis. Defects in this pathway can also be related to oncogenesis. In the first part of this review, the functions of Gadd45 proteins, and briefly NF-κB and MAPK, are summarized. In the second part, the mechanisms by which Gadd45 proteins are regulated by NF-κB, and how they affect MAPK activation, are reviewed.

Epidermal growth factor inhibition (EGFR) is emerging as an important treatment modality in several epithelial malignancies, including head and neck squamous cell carcinoma (HNSCC). Despite some notable successes, less than 20% of patients respond to EGFR inhibition due to intrinsic and acquired resistance. Since EGFR inhibition is already used for lung, colorectal, breast and pancreas cancers in addition to HNSCC, overcoming treatment resistance would have a major impact on outcome. When the mechanisms of intrinsic resistance are identified, including mutations in the EGFR receptor, alternative therapeutic approaches should be employed. Mechanisms of acquired resistance that may be amenable to pharmacological therapies include dysregulation of EGFR degradation, constitutive activation of overlapping signal transduction pathways, especially cMET/HER3, the PI3K/Akt resistance pathway, angiogenesis and epithelial to mesenchymal transition. COX-2 is another promising target for HNSCC and preclinical data suggest that COX-2 inhibitors can affect most of the described acquired EGFR resistance pathways. Several combined EGFR and COX-2 inhibition trials have been completed and demonstrate promise for HNSCC. Combinatorial strategies of combined inhibition of EGFR and acquired resistance pathways in combination with radiation or chemotherapy are warranted.

The inability of the host immune system to control tumor growth appears to result from dominant mechanisms of immune suppression that prevent the immune system from effectively responding in a way that consistently results in tumor rejection. Among the many possible mediators of tumoral immune escape, the immunoregulatory enzyme, indoleamine 2,3-dioxygenase (IDO), has recently gained considerable attention. IDO is a heme-containing, monomeric oxidoreductase that catalyzes the first and rate-limiting step in the degradation of the essential amino acid tryptophan to N-formyl-kynurenine. Tryptophan depletion as well as the accumulation of its metabolites results in a strongly inhibitory effect on the development of immune responses by blocking T cell activation, inducing T cell apoptosis and promoting the differentiation of naïve T cells into cells with a regulatory phenotype (Tregs). Recent data obtained from preclinical tumor models demonstrate that IDO inhibition can significantly enhance the antitumor activity of various chemotherapeutic and immunotherapeutic agents. These results, coupled with data showing that increased IDO expression is an independent prognostic variable for reduced overall survival in cancer patients, suggest that IDO inhibition may represent an effective strategy to treat malignancies, either alone or in combination with chemotherapeutics or other immune based therapies. This review will focus on the role of IDO as a mediator of peripheral immune tolerance, evidence that IDO becomes dysregulated in human cancers, and the latest progress on the development of IDO inhibitors as a novel anti-cancer therapy.

Poly(ADP-ribose) glycohydrolase (Parg) is the main enzyme for degradation of poly(ADP-ribose) by splitting ribose-ribose bonds. Parg-deficient (Parg+/- and Parg-/-) mouse ES cell lines have been established by disrupting both alleles of Parg exon 1 through gene-targeting. A transcript encoding a full length isoform of Parg was eliminated and only low amounts of Parg isoforms were detected in Parg-/- embryonic stem (ES) cells. Poly(ADP-ribose) degradation activity was decreased to one-tenth of that in Parg+/+ ES cells. Parg-/- ES cells exhibited the same growth rate as Parg+/+ ES cells in culture. Sensitivity of Parg-/- ES cells to various DNA damaging agents, including an alkylating agent dimethyl sulfate, cisplatin, gemcitabine, 5-fluorouracil, camptothecin, and γ-irradiation was examined by clonogenic survival assay. Parg-/- ES cells showed enhanced lethality after treatment with dimethyl sulfate, cisplatin and γ-irradiation compared with wildtype (Parg+/+) ES cells (p<0.05, respectively). In contrast, a sensitization effect by Parg-deficiency was not observed with gemcitabine and camptothecin. These results suggest the possibility that functional inhibition of Parg leads to sensitization of tumor cells to some chemo- and radiation therapies.

Histone proteins are subject to a diverse range of post-translational modifications which, along with DNA methylation, play a major role in controlling gene expression, cell division, survival and differentiation. Alterations in these chromatin modifications are thought to contribute to important human diseases including cancer. Inhibition of the enzymes that introduce and remove these chromatin modifications is proving an effective approach to cancer therapy and inhibitors of histone deacetylases and DNA methyltransferases have been approved for use in haematological malignancies. Here we provide a background to the biology of chromatin modifications and review some of the evidence validating histone deacetylases and DNA methyltransferases as targets for anti-cancer drug discovery. We then focus on two of the key issues in this field; the identification of novel inhibitors to overcome shortcomings of first generation agents and the potential role of histone deacetylase and DNA methyltransferase inhibitors in combination therapies for oncology. Finally, we highlight some of the challenges that will need to addressed to further progress the development of epigenetic-based therapies for cancer.

The requirements for a cell surface molecule to be suitable as an antibody-drug conjugate target are stringent. The notion that antibodies-directed toward targets on the surface of malignant cells could be used for drug delivery is not new. The history of antibody-drug conjugates has been marked by hurdles identified and overcome. Early conjugates used mouse antibodies, drugs that were either not sufficiently potent, were immunogenic (proteins) or were too toxic and linkers that were not sufficiently stable in circulation. Three main avenues have been explored using antibodies to target cytotoxic species to malignant cells, antibody-protein toxin conjugates (or antibody fragment-protein toxin fusion proteins), antibody-small molecule drug conjugates and antibody-enzyme conjugates administered along with small molecule prodrugs requiring metabolism by the conjugated enzyme to release the activate species. This review focuses on cell surface proteins that have been targeted primarily by antibody-small molecule drug conjugates and briefly discusses 34 targets being investigated. While only one antibody-drug conjugate has reached regulatory approval, there are nearly 20 of these in clinical trial. The time may have come for this technology to become a major contributor to improving treatment for cancer patients.