Current Pharmaceutical Design - Volume 12, Issue 3, 2006

Acquired drug resistance is a major limitation for successful treatment of cancer. Resistance emerges due to drug exclusion, drug metabolism and alteration of the drug target by mutation or overexpression. Depending on therapy, the type of cancer and its stage, one or several genetic or epigenetic alterations are necessary to confer resistance to treatment. The fundamental question is the following: if a genetically diverse population of replicating cancer cells is subjected to chemotherapy that has the potential to eradicate it, what is the probability of emergence of resistance? Here, we review a general mathematical framework based on multi-type branching processes designed to study the dynamics of escape of replicating organisms from selection pressures. We apply the general model to evolution of resistance of cancer cells and discuss examples for diverse mechanisms of resistance. Our theory shows how to estimate the probability of success for any treatment regimen.

The intrinsic or acquired resistance to anticancer drugs remains one of the most significant factors impeding the progress of cancer chemotherapy. This phenomenon often involves simultaneous resistance to other anticancer drugs that differ in their chemical structure and mode of action and are not even used in chemotherapy. This phenotype has been called multidrug resistance (MDR). Although the cellular basis underlying MDR is not fully understood, several factors mediating therapy resistance in tumors have been proposed. One of the mechanisms leading to chemoresistance of tumor cells is the increased activity of transporter proteins. The best-characterized transporter protein is MDR1/P-glycoprotein, and a number of clinical investigations have suggested that its intrinsic or acquired overexpression resulted in a poor clinical outcome of chemotherapy. Various types of compounds and techniques for the reversal of MDR1/P-glycoproteinmediated MDR have been developed, and efforts have concentrated on the inhibition of function and suppression of expression. This review summarizes the current state of knowledge of MDR1/P-glycoprotein and the modulation of MDR by targeting MDR1/P-glycoprotein.

The multidrug resistance (MDR) proteins are member of the ATP-binding cassette superfamily and are present in a majority of human tumors. Their activity is a crucial factor leading to therapeutic failure. It is likely that compounds which inhibit the function of the MDR-efflux proteins such as MDR1 will improve the cytotoxic action of anticancer chemotherapy. Therefore, a search for MDR reversing compounds was conducted among three classes of plant derived compounds such as diterpenes, triterpenes and carotenoids in a hope to find inhibitors without adverse effects in these natural compounds. The inhibition of efflux activity was determined by measuring the accumulation of substrate analogues such as rhodamine in tumor cells in the presence of potential inhibitors. Thus we determined the effect of structurally unrelated diterpenes, triterpenes and carotenoids on reversal of multidrug resistance in MDR-1 gene-transfected L1210 mouse lymphoma cells and MDR mediated multidrug resistance of human breast cancer cells MDA-MB-231 (HTB-26) and MCF-7. The majority of diterpenes, cycloartane triterpenes and carotenoids isolated from vegetables and medicinal plants were able to enhance rhodamine 123 accumulations of MDR-cells. Synergistic interaction was found between epirubicine and resistance modifier terpenoids in vitro. It is supposed that these MDR modulators bind into transmembrane domains and the action of ABC transporters is inhibited by induced conformational changes.

Acquired and intrinsic drug resistance in cancer is the major obstacle to long-term, sustained patient response to chemotherapy. Irinotecan (CPT-11) is a widely-used potent antitumor drug that inhibits mammalian DNA topoisomerase I (Topo I). However, overexpression of ABCG2 (BCRP/MXR/ABCP) reportedly confers cancer cells resistance to SN-38, the active form of CPT-11. To circumvent the ABCG2-associated drug resistance, we have synthesized and characterized a total of fourteen new camptothecin (CPT) analogues with respect to both the inhibition of Topo I and the substrate specificity of ABCG2. While the lactone E ring is a prerequisite for anticancer activity, modifications of the A or B rings do not significantly affect Topo I inhibition activity. In this context, we have synthesized new CPT analogues with different substitutions at positions 10 or 11 of the A ring. All of the tested CPT analogues strongly inhibited the Topo I activity in a cell-free system. Accordingly, we have examined ATP-dependent transport of those CPT analogues by using plasma membrane vesicles prepared from ABCG2-overexpressing cells. Based on the substrate specificity of ABCG2 thus evaluated, it is strongly suggested that CPT analogues with a hydroxyl group at position 10 or 11 of the A ring are good substrates for ABCG2 and therefore effectively extruded from cancer cells. Thus, hydrogen bond formation is considered to be involved in substrate recognition and/or transport processes of ABCG2. The present study provides a practical approach to discover new CPT-based drugs for the chemotherapy of drug-resistant human cancer.

Insight into the mechanisms of primary or acquired drug resistance of (hematological) malignancies is critical for the development of new treatment strategies. This review will focus on Bcl-2 and the mevalonate pathway as targets for reversal of drug resistance in multiple myeloma. The Bcl-2 protein is highly expressed in myeloma patients and in vitro studies have shown its role in the regulation of chemosensitivity, which makes Bcl-2 an attractive target for treatment. Statins are widely used for the treatment of hypercholesteremia. Several in vitro studies have shown that statins may also kill hematological malignant cells including myeloma cells. We found that lovastatin induced apoptosis in myeloma and lymphoma cells by inhibition of geranylgeranylation and subsequent down regulation of Mcl-1, probably the most important anti-apoptotic protein in myeloma. Phase 1 and 2 studies have been performed with Bcl-2 antisense oligonucleotides and high dose simvastatin in combination with chemotherapy in heavily pre-treated myeloma patients. Encouraging results from these studies may provide the framework for the future application of new treatment strategies for myeloma

The median survival of patients with glioblastoma treated by surgery, radiotherapy and chemotherapy is in the range of 12 months. These limits in the efficacy of current treatment modalities call for the development of novel therapeutic approaches targeting the specific biological features of this type of cancer. Glioblastomas are a rich source of immunosuppressive molecules which may interfere with immune recognition and rejection as well as clinical strategies of active immunotherapy. The most prominent glioblastoma-associated immunosuppressant is the cytokine, transforming growth factor (TGF)β, a multifunctional cytokine which not only interferes with multiple steps of afferent and efferent immune responses, but also stimulates migration, invasion and angiogenesis. The complex regulation of TGFβ bioavailability includes its synthesis as a proprotein, proteolytic processing by furin-like proteases, assembly in a latent complex, and finally liberation from latency by multiple effector mechanisms, a process collectively referred to as activation. Several in vitro paradigms and rodent glioma models have been used to demonstrate that the antagonism of TGFβ holds promise for the treatment of glioblastoma, employing antisense strategies, inhibition of pro-TGFβ processing, scavenging TGFβ by decorin, or blocking TGFβ activity by specific TGFβ receptor (TGFβR) I kinase antagonists. Moreover, the local application of TGFβ2 antisense oligonucleotides is currently evaluated in a randomized clinical trial for recurrent malignant glioma. In summary, we propose that TGF- β-antagonistic treatment strategies are among the most promising of the current innovative approaches for glioblastoma, particularly in conjunction with novel approaches of cellular immunotherapy and vaccination.

For the immune system to mount an effective antitumor T-cell response, an adequate number of T-cells specific for the antigens expressed by the malignancy must be activated [1]. Since most antigens expressed by tumors are "self"- antigens, tumor antigens often lack endogenous immunogenicity and thus do not sufficiently activate T-cells to levels that can mediate tumor eradication. In addition, virtually all solid tumor cells lack the costimulatory molecules necessary to activate tumor-specific T-cells. Approaches that stimulate immune responses to these tumor antigens have the potential to alter this poor responsiveness. This theory has promoted the use of active immunotherapy to generate immune responses against tumor-associated antigens (TAAs) for the treatment of cancer. As one such vaccine strategy, we have utilized poxviruses as delivery vehicles for TAAs in combination with T-cell costimulatory molecules. Initial studies have demonstrated that the insertion of costimulatory molecule trangenes into viral vectors, along with a TAA transgene, greatly enhances the immune response to the antigen. Using this approach, a TRIad of COstimulatory Molecules (TRICOM; B7-1, ICAM-1 and LFA-3) has been shown to enhance T-cell responses to TAAs to levels far greater than any one or two of the costimulatory molecules in combination. In this article, preclinical findings and recent clinical applications of TRICOMbased vaccines as a cancer immunotherapy are reviewed.

The recent developments in the field of recombinant DNA, protein engineering and cancer biology, have let us gain insight into many cancer-related mechanisms. Moreover, novel techniques have facilitated tools allowing unique distinction between malignantly transformed cells, to regular ones. This understanding has paved the way for the rational design of a new age of pharmaceuticals; monoclonal antibodies and their fragments. Antibodies can select antigens on both a specific and high affinity account, and further implementation of these qualities is used to target cancer cells by specifically identifying exogenous antigens of cancer cell populations. The structure of the antibody provides plasticity resonating from its functional sites. Upon binding to the Fc Receptor on effector cells, the crystallisable fragment (Fc) region elicits the onslaught of Antigen Dependant Cell-mediated Cytotoxicity (ADCC) and the plasma-native Complement Dependant Cytotoxicity (CDC) response and apoptosis. The progenitor form of the antibody can evolve in to a tailored therapeutic molecule with the help of recombinant DNA technology. Recombinant antibodies may be linked to potent toxins or radio-labeled fragments, conferring a high killing capability. Other recombinant techniques such as ADEPT, conjugate the specificity of antibodies to a prodrug-catalytic subunit thus creating a high local concentration of an activated chemotherapeutic. Antibodies can be used to recruit the adaptive immune response by binding the antibody fragment to a recombinant MHC molecule displaying a highly immunogenic peptide. Apart from their therapeutic capabilities antibodies are powerful detection tools as observed in the operating theater in a procedure known as Radio-immunoguided Surgery (RIGS).

Treating malignant tumor through the induction of cell differentiation has been an attractive concept, but clinical development of differentiation-inducing agents to treat malignant tumor, especially for solid tumors has been limited to date. Nerve growth factor, all trans retinoic acid, dimethyl sulfoxide, active form vitamin D3, peroxisome proliferatoractivated receptorg, 12-0-tetradecanoylphorbol 13-acetate, hexamethylene-bis-acetamide, transforming growth factor-β, butyric acid, cAMP, and vesnarinone are known to have a differentiation-inducing capability on solid tumors in vitro and/or in vivo. Moreover some of the differentiation-inducing agents have been used for treating patients with solid tumor, but the therapeutic effect of the differentiation-inducing agents on solid tumor is not strong when compared with that of conventional chemotherapeutic agents. However, because most of the differentiation-inducing agents can potentiate the effect of conventional chemotherapy or radiation therapy, combination of differentiation-inducing therapy with conventional chemotherapy or radiation therapy might be used as a second- or third-line therapy in patients with advanced cancer. Furthermore, analysis of the molecular mechanisms of the tumor differentiation therapy might provide selective and targeted molecules for novel cancer therapy.

Vascular endothelial growth factor (VEGF)-mediated angiogenesis is thought to play a critical role in tumor growth and metastasis. Consequently, anti-VEGF therapies are being actively investigated as potential anti-cancer treatments, either as alternatives or adjuncts to conventional chemo or radiation therapy. Among the techniques used to block the VEGF pathway are: 1) neutralizing monoclonal antibodies against VEGF or its receptor, 2) small molecule tyrosine kinase inhibitors of VEGF receptors, 3) soluble VEGF receptors which act as decoy receptors for VEGF, and 4) ribozymes which specifically target VEGF mRNA. Recent evidence from phase III clinical trials led to the approval of bevacizumab, an anti-VEGF monoclonal antibody, by the FDA as first line therapy in metastatic colorectal carcinoma in combination with other chemotherapeutic agents. However, may challenges still remain, and the role of anti-VEGF therapy in the treatment of other solid tumors remains to be elucidated. The aim of this article is to review the progress of clinical investigations involving VEGF inhibitors in the treatment of different types of solid tumors.